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  • View in gallery
    Fig. 1.

    Isochrones every 3 h of the leading 45-dBZ line of derecho A (including derechos AN and AS; green), MCS 1 (yellow), derecho BW (red), derecho BE (maroon), MCS 2 (blue), and the combined system of derecho BE and MCS 2 (purple) overlaid upon storm reports of wind (blue plus signs), hail (green dots), and tornadoes (red dots) between 1800 UTC 3 Jul and 1500 UTC 5 Jul. Isochrones appear for times when the convective systems meet classification criteria discussed in the text. The time–date labels appear in association with the first and last isochrone drawn for each system. (Source: Severe Plot v.2.0)

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    Fig. 2.

    Composite analyses consisting of every 6 h from 3 to 5 Jul 2003 of (a) mean SLP (black lines every 2 hPa) and anomalous SLP (shaded in hPa); (b) mean 925-hPa mixing ratio (black lines every 2 g kg−1), anomalous 925-hPa mixing ratio (shaded in g kg−1), mean 0–6-km shear (black barbs in m s−1 where one pennant is 25 m s−1), and anomalous 0–6-km shear (blue barbs in m s−1 where one pennant is 25 m s−1); and (c) mean 200-hPa geopotential heights (black lines every 12 dam), anomalous 200-hPa geopotential heights (shaded in dam), mean 200-hPa wind (black barbs in m s−1 where one pennant is 25 m s−1), and anomalous 200-hPa wind (green barbs in m s−1 where one pennant is 25 m s−1). (Source: 2.5° NCEP–NCAR reanalysis)

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    Fig. 3.

    Manual surface analyses of SLP (black lines every 2 hPa), potential temperature (red dashed lines every 2 K), and mixing ratio equal to 18 g kg−1 (green line) for (a) 2100 UTC 3 Jul and (b) 1800 UTC 4 Jul. The analysis is overlaid upon NOWrad composite reflectivity (shaded in dBZ), and standard ASOS observations. The blue dotted line in (b) illustrates a surface wind shift. (Source: University at Albany surface data archive and NOWrad data)

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    Fig. 4.

    (a) SLP (green lines every 4 hPa), 1000–500-hPa thickness (red dashed lines every 6 dam), and IR satellite imagery; (b) 200-hPa geopotential heights (black lines every 12 dam), 200-hPa wind (shaded in m s−1), and 850-hPa wind (barbs in m s−1 where one pennant is 25 m s−1, beginning at 7.5 m s−1); (c) 400-hPa geopotential heights (black lines every 6 dam), 400-hPa absolute vorticity (shaded in × 10−5 s−1), and 400-hPa wind (barbs in m s−1 where one pennant is 25 m s−1); and (d) most unstable CAPE (shaded in J kg−1) and 0–6-km shear (barbs in m s−1 where one pennant is 25 m s−1) for 2100 UTC 3 Jul. (Source: 20-km RUC analysis and BAMEX field catalog)

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    Fig. 5.

    As in Fig. 4, but for 0900 UTC 4 Jul. (Source: 20-km RUC analysis and BAMEX field catalog)

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    Fig. 6.

    Potential vorticity (shaded in PVU), pressure (black lines every 30 hPa), and wind (barbs in m s−1 where one pennant is 25 m s−1) on the 325-K potential temperature surface for (a) 2100 UTC 3 Jul, (b) 0900 UTC 4 Jul, and (c) 2100 UTC 4 Jul. Water vapor imagery from 2100 UTC 4 Jul is inset into the lower-left corner of (c). Line A–A′ in (a) shows the location of the cross section in Fig. 7. (Source: 20-km RUC analysis and BAMEX field catalog)

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    Fig. 7.

    Mean absolute vorticity (shaded in × 10−5 s−1), negative meridional absolute vorticity gradient (green lines at −25 × 10−11 m−1 s−1), potential temperature (black lines in K), and wind (barbs in m s−1 where one pennant is 25 m s−1) averaged every 3 h from 1800 UTC 3 Jul to 1200 UTC 5 Jul along cross-section line A–A′ in Fig. 6a. (Source: 20-km RUC analyses)

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    Fig. 8.

    As in Fig. 4, but for 2100 UTC 4 Jul. (Source: 20-km RUC analysis and BAMEX field catalog)

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    Fig. 9.

    As in Fig. 4, but for 0900 UTC 5 Jul. (Source: 20-km RUC analysis and BAMEX field catalog)

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    Fig. 10.

    As in Fig. 3, but for (a) 0600 and (b) 1800 UTC 4 Jul. The KSLB and KCNC in (b) refer to the locations of meteograms in Fig. 12. (Source: University at Albany surface data archive and NOWrad data)

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    Fig. 11.

    North–south cross sections of the previous 3-h potential temperature decrease (shaded in K), potential temperature (black lines every 3 K), and wind (barbs in m s−1 where one pennant is 25 m s−1) for (a) 0900 and (b) 1600 UTC 4 Jul. Magenta lines show the approximate horizontal extent of convection that passed through each cross section over the previous 3 h. NOWrad composite radar imagery for 0600 and 0900 UTC 4 Jul (1300 and 1600 UTC 4 Jul), are inset in the upper-left and upper-right corners of (a) [(b)], respectively. All radar images contain a black line representing the location of the associated cross section. (Source: 20-km RUC analysis and NOWrad data)

  • View in gallery
    Fig. 12.

    Meteograms from 1500 UTC 4 Jul to 0300 UTC 5 Jul of surface temperature (black lines), surface dewpoint (gray lines), and surface wind (barbs in m s−1 where one pennant is 25 m s−1) for (a) KSLB and (b) KCNC. The locations of KSLB and KCNC appear in Fig. 10b. (Source: University at Albany surface data archives)

  • View in gallery
    Fig. 13.

    Visible satellite imagery at (a) 1703 UTC, (b) 1903 UTC, and (c) 2108 UTC 4 Jul. The “IGW” in (a) and (b) refers to an inertia-gravity wave. The green circle in (a) shows the location of the MIPS profiler data shown in Fig. 14. (Source: BAMEX field catalog)

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    Fig. 14.

    MIPS wind profiler data from 1900 to 2300 UTC 4 Jul and 0–7-km in height, taken from the location shown in Fig. 13a. Wind barbs are shaded every 2.5 kt. The black arrow illustrates a wind maximum associated with an IGW crest passage. (Source: BAMEX field catalog)

  • View in gallery
    Fig. 15.

    As in Fig. 3, but for 0000 UTC 5 Jul. The black and gray hexagons show the locations of RUC soundings in Fig. 16. (Source: University at Albany surface data archives and NOWrad data)

  • View in gallery
    Fig. 16.

    RUC analysis soundings of temperature (solid lines), dewpoint (dashed lines), and wind (barbs in m s−1 where one pennant is 25 m s−1) for 0000 UTC 5 Jul at the gray hexagon (gray sounding, 41.23°N, 94.11°W) and black hexagon (black sounding, 42.52°N, 94.12°W) in Fig. 15. (Source: 20-km RUC analysis)

  • View in gallery
    Fig. 17.

    WSR-88D reflectivity from KOAX (shaded in dBZ), 850-hPa convergence (black solid lines every 3 × 10−5 s−1 less than −3 × 10−5 s−1), 850-hPa divergence (black dashed lines every 3 × 10−5 s−1 greater than 3 × 10−5 s−1), and 850-hPa wind (barbs in m s−1 where one pennant is 25 m s−1) for (a) 0200, (b) 0300, (c) 0400, and (d) 0500 UTC 5 Jul. (Source: 20-km RUC analysis and WSR–88D data)

  • View in gallery
    Fig. 18.

    As in Fig. 6, but on the 335-K potential temperature surface for (a) 0000 and (b) 1200 UTC 4 Jul. The blue arrow in (b) illustrates a region of possible PV nonconservation associated with the convection from derecho A (see Fig. 1 for track). (Source: 20-km RUC analysis)

  • View in gallery
    Fig. 19.

    (a) Analyzed 1000–500-hPa thickness for 1800 UTC 4 Jul minus 66-h forecast 1000–500-hPa thickness for 1800 UTC 4 Jul (shaded in dam), analyzed 1000–500-hPa thickness for 1800 UTC 4 Jul (dashed purple lines every 6 dam), and 66-h forecast 1000–500-hPa thickness for 1800 UTC 4 Jul (black lines every 6 dam). (b) Analyzed 200-hPa wind speed for 1800 UTC 4 Jul minus 66-h forecast 200-hPa wind speed for 1800 UTC 4 Jul (shaded in m s−1), analyzed 200-hPa geopotential heights for 1800 UTC 4 Jul (dashed purple lines every 12 dam), and 66-h forecast 200-hPa geopotential heights for 1800 UTC 4 Jul (black lines every 12 dam). The arrow in (a) illustrates a region where the analyzed 1000–500-hPa thickness was greater than in the 66-h forecast, while the arrow in (b) illustrates a region where the analyzed 200-hPa wind speed was greater than in the 66-h forecast. (Source: 1.0° GFS)

  • View in gallery
    Fig. 20.

    Hovmöller plots averaged in the 47°–51°N latitude band from 0000 UTC 2 Jul to 0000 UTC 6 Jul and 135°–55°W of (a) the magnitude of the 200-hPa wind (shaded in m s−1), (b) analyzed 200-hPa wind speed minus forecast 200-hPa wind speed from 0000 UTC 2 Jul (shaded in m s−1), and (c) analyzed 300–200-hPa layer-average PV minus forecast 300–200-hPa layer-average PV from 0000 UTC 2 Jul (shaded in PVU). The arrows in (a),(b), and (c) illustrate a region of maximum 200-hPa wind, 200-hPa wind error, and 300–200-hPa average PV error, respectively. (Source: 1° GFS)

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Derecho and MCS Development, Evolution, and Multiscale Interactions during 3–5 July 2003

Nicholas D. MetzDepartment of Atmospheric and Environmental Sciences, University at Albany, State University of New York, Albany, New York

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Lance F. BosartDepartment of Atmospheric and Environmental Sciences, University at Albany, State University of New York, Albany, New York

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Abstract

From 3 to 5 July 2003 during the Bow Echo and Mesoscale Convective Vortex Experiment (BAMEX), multiple mesoscale convective systems (MCSs 1 and 2) and derechos (derechos AN, AS, A, BW, and BE) progressed across a preferred upper Midwest corridor. The derechos evolved in a favorable synoptic-scale environment. However, the environmental details associated with each derecho, such as the characteristics of the initial surface boundary, the formation position relative to the upper-level jet stream, the presence of an upper-level mesoscale disturbance, and the CAPE/shear environment varied from derecho to derecho.

The MCSs and derechos composed three distinct convective episodes. Multiple mesoscale interactions between the MCSs and derechos and the environment altered the character and longevity of these episodes. The first convective episode consisted of derecho A, which formed from merging derechos AN and AS (northern and southern systems, respectively). The ∼200-hPa-deep cold pool associated with derecho A decreased surface potential temperatures by 4–8 K. MCS 1 dissipated upon entering this cold pool and an inertia–gravity wave was emitted that helped to spawn MCS 2. This inertia–gravity wave connected MCSs 1 and 2 into a compound convective episode. As derecho BW (western system) approached a strong surface boundary across Iowa created by the cold pools of derecho A and MCS 1, derecho BE (eastern system) formed. The remnants of derecho BW merged with derecho BE creating another compound convective episode.

The upscale effects resulting from this active convective period directly affected subsequent convective development. Upper-level diabatic heating associated with derecho A resulted in NCEP GFS 66-h negative 1000–500-hPa thickness errors of 4–8 dam (forecast too cold) and negative 200-hPa wind errors of 10–20 m s−1 (forecast too weak). The resulting stronger than forecast 200-hPa jet stream likely increased synoptic-scale forcing for the formation and evolution of derecho BW.

Corresponding author address: Nicholas D. Metz, Department of Atmospheric and Environmental Sciences, University at Albany, State University of New York, ES-351, 1400 Washington Ave., Albany, NY 12222. Email: nmetz@atmos.albany.edu

Abstract

From 3 to 5 July 2003 during the Bow Echo and Mesoscale Convective Vortex Experiment (BAMEX), multiple mesoscale convective systems (MCSs 1 and 2) and derechos (derechos AN, AS, A, BW, and BE) progressed across a preferred upper Midwest corridor. The derechos evolved in a favorable synoptic-scale environment. However, the environmental details associated with each derecho, such as the characteristics of the initial surface boundary, the formation position relative to the upper-level jet stream, the presence of an upper-level mesoscale disturbance, and the CAPE/shear environment varied from derecho to derecho.

The MCSs and derechos composed three distinct convective episodes. Multiple mesoscale interactions between the MCSs and derechos and the environment altered the character and longevity of these episodes. The first convective episode consisted of derecho A, which formed from merging derechos AN and AS (northern and southern systems, respectively). The ∼200-hPa-deep cold pool associated with derecho A decreased surface potential temperatures by 4–8 K. MCS 1 dissipated upon entering this cold pool and an inertia–gravity wave was emitted that helped to spawn MCS 2. This inertia–gravity wave connected MCSs 1 and 2 into a compound convective episode. As derecho BW (western system) approached a strong surface boundary across Iowa created by the cold pools of derecho A and MCS 1, derecho BE (eastern system) formed. The remnants of derecho BW merged with derecho BE creating another compound convective episode.

The upscale effects resulting from this active convective period directly affected subsequent convective development. Upper-level diabatic heating associated with derecho A resulted in NCEP GFS 66-h negative 1000–500-hPa thickness errors of 4–8 dam (forecast too cold) and negative 200-hPa wind errors of 10–20 m s−1 (forecast too weak). The resulting stronger than forecast 200-hPa jet stream likely increased synoptic-scale forcing for the formation and evolution of derecho BW.

Corresponding author address: Nicholas D. Metz, Department of Atmospheric and Environmental Sciences, University at Albany, State University of New York, ES-351, 1400 Washington Ave., Albany, NY 12222. Email: nmetz@atmos.albany.edu

1. Introduction

The Bow Echo and Mesoscale Convective Vortex (MCV) Experiment (BAMEX) was based out of the Mid-America St. Louis Airport in Mascoutah, Illinois, and occurred from 20 May to 6 July 2003 (Davis et al. 2004). The participants in the field phase of BAMEX sampled multiscale aspects of bow echoes (e.g., Fujita 1978; Weisman 2001) and MCVs (e.g., Raymond and Jaing 1990; Davis et al. 2002; Davis and Galarneau 2009; Galarneau et al. 2009) as they occurred throughout the life cycle of mesoscale convective systems (MCSs). The target range for field operations occurred throughout the Midwest (see Fig. 3 from Davis et al. 2004) as this region contains a climatological maximum for bow echoes (e.g., Johns and Hirt 1987) and MCVs (e.g., Bartels and Maddox 1991). The focus of this paper will be on the active 3–5 July 2003 period during BAMEX when repeated MCS and derecho (e.g., Johns and Hirt 1987; Coniglio et al. 2004) activity affected the upper Midwest. Multiple mesoscale interactions occurred among the MCSs primarily associated with convectively generated cold pools. The bulk upscale effects of the MCSs altered the synoptic-scale environment through repeated convective heating.

In the United States to the east of the Rocky Mountains, rapidly moving MCSs that produce severe wind damage during the warm season are common. These wind-producing MCSs were first classified during the late nineteenth century as derechos, a Spanish word meaning “straight ahead” (Hinrichs 1888). Derechos account for ∼40% of the causalities associated with thunderstorm winds (Ashley and Mote 2005). Johns and Hirt (1987) identified two derecho patterns: progressive and serial. A progressive derecho often appears as a bow echo oriented perpendicular to the mean wind direction, bowing forward in the direction of the mean flow. A majority of the severe downburst activity associated with the bow echo occurs near the bow apex. The serial derecho takes the form of a squall line oriented with a small angle between the squall-line axis and the mean wind. A serial derecho can consist of an extensive squall line in which downburst activity occurs along multiple line echo wave patterns (Nolen 1959). Typically, progressive derechos occur during the warm season, while serial derechos occur during transition seasons when dynamical forcing is stronger. The derechos that occurred from 3 to 5 July 2003 generally fit the progressive classification.

MCSs, such as those during the 3–5 July 2003 period, can be regularly generated in the lee of the Rockies and progress eastward as convective episodes (e.g., Carbone et al. 1990, 2002; Tuttle and Carbone 2004; Ahijevych et al. 2004; Trier et al. 2006; Tuttle and Davis 2006; Carbone and Tuttle 2008). Ahijevych et al. (2004) demonstrated that the frequency of warm-season convective episodes in the United States is strongly associated with the distance east of the Rockies and the time of day. Their observations illustrated the long-range effects of orography on precipitation frequency and eastward-propagating long-lived convective events.

In addition to episodic generation in the lee of the Rockies, MCSs can excite secondary areas of convection. Physical processes known to trigger convection (e.g., outflow boundaries, gravity waves, etc.) may act in concert with one another to develop new MCSs (e.g., Purdom 1973, 1976; Zhang and Fritsch 1988; Carbone et al. 2000). For example, the intersection point between these mesoscale outflow boundaries, gravity waves, and other features such as fronts or squall lines represents an area of likely convective development or enhancement. Deep convection initiated in response to mesoscale forcing can in turn modify the synoptic-scale environment, allowing for alterations in the convective mode (e.g., Wicker et al. 1983; Fritsch and Forbes 2001) and the geopotential height pattern via diabatically driven ridge building (e.g., Zhang and Harvey 1995; Dickinson et al. 1997). Multiscale interactions among the serial MCSs that occurred from 3 to 5 July 2003 altered the life cycles of existing convective episodes.

The goal of this paper is to use the active 3–5 July 2003 period during BAMEX to (i) illustrate the variety of derecho environments and life cycles that can occur within a short period, (ii) discuss complex interactions on the mesoscale that can alter the longevity and character of convective episodes, and (iii) elucidate the upscale effects of MCS activity that can influence subsequent MCS development within an active convective period. The remainder of this paper is organized as follows. Section 2 describes the datasets used and introduces the MCSs present during 3–5 July 2003. Section 3 details the synoptic-scale pattern and describes the evolution of each MCS. Section 4 examines mesoscale interactions among the MCSs and environmental–system interactions, while section 5 discusses synoptic-scale impacts of the MCSs. Section 6 synthesizes key results and places them within the context of previous literature.

2. Data and methods

Several datasets were used in the synthesis of results presented herein including: (i) severe weather reports [obtained from Severe Plot v.2.0; Hart (1993)], (ii) hourly Automated Surface Observing System (ASOS) observations [obtained from the University at Albany], (iii) composite National Operational Weather radar (NOWrad) imagery [obtained from the University Corporation for Atmospheric Research and the WSI Corporation], (iv) Weather Surveillance Radar-1988 Doppler (WSR-88D) level-II data (obtained from the National Climatic Data Center), (v) infrared (IR) satellite imagery [obtained from the Comprehensive Large Array-data Stewardship System (CLASS)], and (vi) water vapor, visible satellite, and Mobile Integrated Profiling System (MIPS) imagery (obtained from the BAMEX field catalog; more information is available online at http://catalog.eol.ucar.edu/bamex/index.html).

Most of the standard synoptic and mesoscale analyses and diagnostic calculations were performed using the 20-km Rapid Update Cycle (RUC) hourly analyses (Benjamin et al. 2002). The only exceptions are the synoptic-scale composites1 in section 3 that were composed using the 2.5° × 2.5° National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) 6-h reanalysis (Kalnay et al. 1996), and the forecast comparisons in section 5 that were preformed using the 1.0° × 1.0° NCEP Global Forecast System (GFS) 6-h analyses and forecasts.

All MCSs considered during 3–5 July 2003 grew to meso-α in size (200–2000 km; Orlanski 1975) and had at least 45-dBZ composite NOWrad radar reflectivities. Each MCS also was maintained for at least three consecutive hours at the aforementioned size and reflectivity thresholds and entered the BAMEX field campaign domain (a 600-km radius centered on Mid-America Airport). Three distinct convective episodes occurred during 3–5 July. The first consisted of northern derecho AN2 that merged with southern derecho AS to form derecho A (Table 1, Fig. 1). The second included MCSs 1 and 2 that did not achieve derecho status. The third comprised western derecho BW3 that weakened and merged with eastern derecho BE. Derechos BW and BE initiated from differing mechanisms but combined to form one long damage swath, with each meeting the derecho criteria. All of the MCSs and derechos during the 3–5 July period traversed a preferred upper Midwest convective corridor (e.g., Tuttle and Davis 2006) and caused numerous severe weather reports (Fig. 1).

3. Synoptic overview: 3–5 July 2003

a. Mean/anomalous flow pattern

Synoptic-scale analyses for 3–5 July 2003 reveal a favorable pattern for MCS development over the northern United States. The time-mean SLP pattern shows cyclonic flow associated with a trough in the lee of the Rockies from which negative SLP anomalies of 2–6 hPa extended toward the upper Midwest (Fig. 2a). The SLP pattern and abnormally strong lee trough favored enhanced southwesterly flow toward the Great Lakes.

During 3–5 July, time-mean 925-hPa mixing ratios in excess of 14 g kg−1 were centered on Iowa (Fig. 2b). Anomalously high mixing ratios (1.5–3.5 g kg−1 above normal) were located in a band that extended from Kansas into the northeastern United States. These anomalously high mixing ratios coincided with the aforementioned region of anomalously low SLP (Fig. 2a), and marked the corridor along which the MCSs and derechos progressed during 3–5 July (Fig. 1). Additionally, within the region of anomalously high mixing ratios that stretched from South Dakota into Wisconsin, mean 0–6-km shear values were 15–20 m s−1, which is 5–10 m s−1 above normal (Fig. 2b). Large quantities of low-level moisture contributed to high values of convective available potential energy (CAPE) and combined with above-normal shear values to increase the potential for derechos (Weisman 1993).

A 200-hPa time-mean and anomalous geopotential height and wind analysis for 3–5 July reveals a jet entrance region located along the western U.S.–Canadian border near where much of the convection during 3–5 July initiated (Fig. 2c). The jet stream core contained winds in excess of 50 m s−1 along the U.S.–Canadian border, up to 20 m s−1 above normal. This anomalously strong 200-hPa jet stream resulted from 200-hPa geopotential heights that were 5–15 dam below (above) normal over southern Canada (the northern United States), leading to an enhanced meridional geopotential height gradient.

b. Surface and upper-level analyses

1) 2100 UTC 3 July

Derecho A developed initially as two separate derechos (Fig. 1), each forming around 2100 UTC 3 July. Derecho AN formed along a surface boundary that stretched from central Montana to southern North Dakota (Fig. 3a). The potential temperature gradient across this boundary was ∼8°C (100 km)−1, and the wind was west-southwesterly (westerly) to the south (north) of the boundary. Farther to the south along the Nebraska–Colorado border, derecho AS formed along a second surface boundary that extended from east-central Wyoming to southeastern Nebraska (Fig. 3a). The potential temperature gradient across this second boundary was ∼6°C (100 km)−1, weaker than the boundary along which derecho AN formed (Fig. 3a). The winds near derecho AS were confluent across Nebraska, and mixing ratios increased rapidly from 7 g kg−1 along the Nebraska–Colorado border to 18 g kg−1 in central Nebraska.

At 2100 UTC 3 July, the initial convection associated with derecho AN was located on the Montana–North Dakota border along the poleward periphery of a 1000–500-hPa thickness ridge at 570 hPa (Fig. 4a), and slightly poleward of a 50 m s−1 200-hPa jet stream (Fig. 4b). Derecho AN was located to the southeast of a broad 400-hPa trough (Fig. 4c), in an environment of ∼500 J kg−1 of most unstable CAPE (hereafter CAPE), and 25–30 m s−1 of 0–6-km shear (Fig. 4d). Farther to the south, derecho AS was organizing near a surface trough in the lee of the Rockies at 2100 UTC 3 July (Fig. 4a). The exit region of a 10 m s−1 low-level jet (LLJ) was situated near derecho AS (Fig. 4b). Derecho AS was positioned on a significant CAPE gradient; CAPE values increased from ∼500 to above 5500 J kg−1 across west-central Nebraska within a region of 15–20 m s−1 of 0–6-km shear (Fig. 4d). This gradient in CAPE is collocated with the aforementioned gradient in surface mixing ratios (Fig. 3a).

2) 0900 UTC 4 July

By 0900 UTC 4 July, derechos AN and AS had combined into a single progressive derecho over Iowa and Wisconsin (Figs. 1 and 5a). Derecho A continued to progress eastward along the poleward periphery of a 1000–500-hPa-thickness ridge, having moved toward higher thickness values compared with 12 h previously (Figs. 4a and 5a), and was positioned along the southern flank of a 60 m s−1 200-hPa jet stream (Fig. 5b). At 0900 UTC 4 July, an anticyclonically curving 15 m s−1 850-hPa LLJ impinged on the southern portion of derecho A (Fig. 5b). Derecho A was no longer located near the 400-hPa trough that derecho AN had formed near 12 h earlier (Figs. 4c and 5c). However, derecho A was located along a zonal corridor of CAPE in excess of 2500 J kg−1 and in a region of 17.5–22.5 m s−1 of 0–6-km shear, a robust convective environment considering the time of day (Fig. 5d).

To the west of derecho A at 0900 UTC 4 July, MCS 1 was also progressing eastward along the poleward periphery of the same 1000–500-hPa-thickness ridge discussed in relation to derecho A (Fig. 5a). MCS 1 was located near the core of the aforementioned 200-hPa jet stream (Fig. 5b) and the 400-hPa trough located along the U.S.–Canadian border (Fig. 5c). MCS 1 formed along the same surface boundary that derecho AN initiated along (Fig. 3a), and progressed into an environment at 0900 UTC 4 July that generally featured CAPE values of 500–1500 J kg−1 and 0–6-km shear of 20–25 m s−1 (Fig. 5d). MCS 1 eventually dissipated in the cold pool of derecho A (see section 4b).

3) PV perspective

A PV analysis on the 325-K potential temperature surface is employed to gain insight into the formation of an intense mesoscale disturbance that directly affected derecho BW. At 2100 UTC 3 July, the 400-hPa trough located along the U.S.–Canadian border near where derecho AN and MCS 1 formed (Figs. 4c and 5c) is manifest by a zonally oriented PV streamer (e.g., Appenzeller and Davies 1992) that extended from Montana into Oregon (Fig. 6a). This PV streamer was associated with a reversal of the meridional PV gradient, a necessary (but not sufficient) condition for barotropic instability. A time-mean cross section, averaged from 1800 UTC 3 July to 1200 UTC 5 July, was constructed across this PV streamer to further investigate the presence of barotropic instability (Figs. 6a and 7). A deep column of absolute vorticity greater than 12 × 10−5 s−1 extended downward from 200 to 700 hPa (Fig. 7). The negative absolute vorticity gradient to the north of this column illustrates a reversal of the meridional absolute vorticity gradient and satisfies the necessary condition for barotropic instability. At 0900 UTC 4 July, the aforementioned PV streamer was breaking down into an isolated mesoscale disturbance over eastern Montana, although a PV streamer with a value above 1.0 PVU still connected this disturbance to the synoptic-scale trough over southern Canada (Fig. 6b). At 2100 UTC 4 July, the mesoscale disturbance was no longer connected to the high-PV reservoir (Fig. 6c), and derecho BW formed immediately to the east of this disturbance [see section 3b(4)]. Water vapor imagery shows this mesoscale disturbance and a series of waves extending from southwestern Montana westward to the Pacific Ocean along the PV streamer and likely barotropic instability (Fig. 6c).

4) 1800 and 2100 UTC 4 July

At 1800 UTC 4 July, derecho BW was organizing along the Montana–North Dakota border (Fig. 3b). The formation occurred near a northeast–southwest-oriented surface trough and wind shift boundary (Fig. 3b). Surface mixing ratios averaged ∼6 g kg−1 in the region where derecho BW formed. Although no significant surface potential temperature gradient was present, derecho BW did organize in advance of a weak thermal trough.

By 2100 UTC 4 July, derecho BW was located in western South Dakota along the poleward periphery of the same 1000–500-hPa-thickness ridge along which derecho A and MCS 1 had progressed (Figs. 4a, 5a, and 8a). Derecho BW formed near the poleward-entrance region of a 50 m s−1 200-hPa jet stream (Fig. 8b), and was located immediately to the east of a 24 × 10−5 s−1 400-hPa mesoscale vorticity maximum (Fig. 8c) that likely formed in association with barotropic instability (Figs. 6a–c). Rawinsondes and RUC soundings (not shown) revealed zero surface CAPE and minimal elevated CAPE near the initiation region of derecho BW. At 2100 UTC 4 July, the reservoir of CAPE above 500 J kg−1 was located to the southeast of derecho BW (Fig. 8d). Still, derecho BW organized in this negligible CAPE (Fig. 8d) and surface moisture (Fig. 3b) environment, in the presence of ∼20 m s−1 of 0–6-km shear (Fig. 8d). Thus, much of the initial ascent was likely dynamically driven, associated with the intense mesoscale disturbance (Fig. 8c) that subsequently helped to destabilize the atmosphere.

In addition, at 2100 UTC 4 July, derecho A, the remnants of MCS 1, and MCS 2 were apparent on IR satellite imagery (Fig. 8a). Derecho A had reached eastern Ohio (Fig. 8a), was progressing eastward away from the most favorable CAPE/shear environment (Fig. 8d), and was ∼3 h away from dissipating. MCS 2 had formed to the south of Lake Michigan following the dissipation of MCS 1 in the cold pool of derecho A (see section 4b). MCS 2 was located near a 12.5 m s−1 850-hPa LLJ (Fig. 8b), under a 400-hPa short-wave ridge (may be a reflection of the convection from MCS 2; Fig. 8c), in a region of greater than 5500 J kg−1 of CAPE, and in conjunction with 7.5–10 m s−1 of 0–6-km shear (Fig. 8d). Further discussion of MCS 2 will occur in section 4c.

5) 0900 UTC 5 July

By 0900 UTC 5 July, the cloud shields of progressive derecho BE (the evolution from derecho BW to BE will be discussed in section 4d) and MCS 2 had merged near Lake Michigan on the northeastern periphery of the persistent 1000–500-hPa-thickness ridge (Fig. 9a). Both MCSs were located along and to the anticyclonic shear side of a 50 m s−1 200-hPa jet stream and near a 10–12.5 m s−1 zonally oriented LLJ (Fig. 9b). The intense 400-hPa mesoscale disturbance that likely formed as a result of barotropic instability was located to the west of derecho BE, but the absolute vorticity had decreased to ∼20 × 10−5 s−1 (Fig. 9c). The amount of CAPE available for derecho BE and MCS 2 was 500–1500 J kg−1, significantly higher than the negligible CAPE environment in which derecho BW formed (Figs. 8d and 9d). While the 0–6-km shear was still ∼20 m s−1 near the 400-hPa mesoscale disturbance and derecho BE (Figs. 8d and 9d), the disturbance was likely not as vital to convective maintenance as it was to convective initiation, given the more favorable thermodynamic environment derecho BE was located in at 0900 UTC 5 July.

4. Interactions among MCSs and derechos

a. Cold pool created by derecho A

The evolutionary details of each MCS and derecho were affected by mesoscale interactions with the environment and mutual interactions among the other MCSs and derechos. Many of these interactions centered on the cold pool of derecho A. As derechos AN and AS progressed eastward into Minnesota and Iowa at 0600 UTC 4 July, they encountered surface mixing ratios in excess of 18 g kg−1 (Fig. 10a). Although there was little baroclinicity as measured by the surface potential temperature gradient to the east of derechos AN and AS, surface potential temperatures behind each derecho decreased by 4–8 K, a reflection of significant convectively generated cold pools (Fig. 10a). A cross section at 0900 UTC 4 July shows evidence of a surface-based cold air dome behind derecho A (Fig. 11a). Multiple isentropes intersect the ground along the southern edge of the cold pool and represent a surface quasi-stationary (QS) boundary produced by derecho A. Three-hour temperature differences along the cross section reveal that from 0600 to 0900 UTC 4 July, temperatures decreased by more than 4.5 K behind derecho A, and this cooling extended to ∼800 hPa near the center of the cold pool.

b. QS boundary and MCS 1 dissipation

The convectively induced cold pool associated with derecho A produced a zonally oriented QS boundary with a potential temperature gradient of ∼4 K (100 km)−1 over northern Indiana at 1800 UTC 4 July. Farther west, MCS 1 was dissipating over Iowa in this same cold pool (Fig. 10b). The remnant precipitation from MCS 1 created another cold pool that reinforced the QS boundary created by the cold pool of derecho A across Iowa. The combination of these two cold pools resulted in a surface potential temperature gradient across Iowa that exceeded 10 K (100 km)−1. Mixing ratios above 18 g kg−1 were located to the south of the convectively cooled air in Iowa. The cold pool produced by MCS 1 over Iowa was deeper than that of derecho A as 3-h temperature differences between 1300 and 1600 UTC 4 July reveal cooling in excess of 6 K that extended to ∼725 hPa (Figs. 11a,b). Note that the maximum cooling from MCS 1 occurred above the surface. Since MCS 1 was weakening at the time of the cross section, it is possible that the remaining precipitation did not fall all the way to the surface, resulting in an elevated region of maximum cooling. Multiple isentropes intersect the ground near where the most significant temperature decreases were observed at 1600 UTC 4 July, collocated with the QS boundary on the surface map (Figs. 10b and 11b).

Meteograms to the north [Storm Lake (KSLB), Iowa] and south [Chariton (KCNC), Iowa] of the QS boundary across Iowa reveal differing environmental characteristics (Figs. 12a,b). At KSLB from 1500 UTC 4 July to 0300 UTC 5 July, the temperature (dewpoint) remained below 25°C (20°C) and after 2200 UTC 4 July winds became northerly (Fig. 12a). Cloud cover associated with the remnants of MCS 1, along and to the north of the QS boundary (Fig. 8a), kept temperatures at KSLB from warming above 25°C. Conversely, to the south of the QS boundary at KCNC, the temperature (dewpoint) remained largely above 30°C (22°C) (Fig. 12b). Winds veered to the south by 2200 UTC 4 July, and remained predominately southerly. Dewpoints at KCNC, in a region unaffected by the cold pools of derecho A and MCS 1, were 5°–8°C higher than at KSLB (Figs. 12a,b).

c. Formation of MCS 2

In addition to reinforcing the QS boundary across Iowa, MCS 1 directly influenced the formation of MCS 2. As MCS 1 dissipated in the cold pool produced by derecho A over Iowa at 1703 UTC 4 July, an inertia–gravity wave (IGW; Davis and Trier 2004) progressed eastward in the stable layer created by this cold pool (Fig. 13a). By 1903 UTC 4 July, the IGW was located in northeastern Illinois and was ∼200 km to the east of the MCS 1 remnants (Fig. 13b). Beginning at around 1900 UTC 4 July, the MIPS vertical profiler was positioned in northeastern Illinois, directly in the path of the approaching IGW (Figs. 13a, 14). At approximately 1940 UTC 4 July, the MIPS profiler recorded a maximum in near-surface winds of 25–30 kt, likely associated with the passage of the IGW crest (Fig. 14; e.g., Eom 1975; Koch and Saleeby 2001). This was coincident with the time that the IGW feature on satellite imagery would have passed the MIPS profiler (Fig. 13b). The MIPS profiler did not measure pressure, but various surface stations along the path of the IGW recorded pressure perturbations of ∼1 hPa, consistent with previous research (e.g., Uccellini 1975; Uccellini and Koch 1987). By 2108 UTC 4 July, convection associated with MCS 2 erupted to the south of Lake Michigan in a region where the IGW intersected the QS boundary created by the cold pool of derecho A (Fig. 13c; see Fig. 10b for a depiction of the QS boundary ∼3 h earlier). The formation of MCS 2 was not as tied to the synoptic scale as the other MCSs and derechos previously considered. MCS 2 formed in a region of relatively weak upper-level steering and meandered to the south and southeast of Lake Michigan (Figs. 8a and 9a). About the same time that MCS 2 formed around 2100 UTC 4 July, the MIPS profiler recorded a minimum in near-surface winds of less than 5 kt, likely associated with the passage of the IGW trough (Fig. 14).

d. QS boundary effects on derechos BW and BE

At 0000 UTC 5 July, the QS boundary remained across Iowa (Fig. 15). Winds at most stations to the north (south) of the QS boundary were easterly or northerly (southerly) in agreement with the KSLB and KCNC meteograms (Figs. 12a,b). The magnitude of the potential temperature gradient in Iowa across the QS boundary was ∼4 K (50 km)−1 (Fig. 15). Mixing ratios above 18 g kg−1 were present in the nonconvectively contaminated areas to the south of the QS boundary. Derecho BW was approaching the QS boundary from central South Dakota (Fig. 15).

Representative RUC soundings in Iowa further reveal differences across the QS boundary (Fig. 16). To the south of the QS boundary, RUC-derived temperatures were 3°–5°C warmer below 700 hPa, and dewpoint temperatures were 2°–4°C higher below 800 hPa than to the north (Fig. 16). The cooler low-levels in the northern sounding reflect the cold pool from MCS 1 that was also evident up to ∼700 hPa (Fig. 11b). The temperature–moisture profile to the south (north) of the QS boundary resulted in ∼2200 J kg−1 (∼700 J kg−1) of surface-based CAPE. Wind profiles below 850 hPa, were southerly (northwesterly) to the south (north) of the QS boundary (Fig. 16), resulting in convergence from the surface up to ∼850 hPa in the presence of the remnant cold pool from MCS 1 (not shown).

At 0200 UTC 5 July, derecho BW approached the QS boundary in Iowa and only contained a few cells with base reflectivity values above 50 dBZ on Omaha, Nebraska (KOAX) radar (Fig. 17a). A zonally oriented 850-hPa convergence band of −9 × 10−5 s−1 was located to the southeast of derecho BW and north of a 15 m s−1 850-hPa LLJ. One hour later, the 850-hPa convergence increased to −12 × 10−5 s−1 (Fig. 17b). Derecho BE formed along the 850-hPa convergence band as the southerly LLJ was forced over the remnant cold dome from derecho A and MCS 1 (Figs. 11a,b). By 0400 UTC 5 July, radar echoes associated with derecho BE had reached 65 dBZ as the 850-hPa LLJ had strengthened to 17.5–20 m s−1 to the south of the QS boundary and convergence had increased to −15 × 10−5 s−1 (Fig. 17c). At 0500 UTC 5 July, the remnants of derecho BW were located within 50 km of derecho BE (Fig. 17d). Much of the convection associated with derecho BW dissipated before the remnants merged with derecho BE around 0520 UTC 5 July (not shown). Derecho BW had been progressing eastward along with an intense 400-hPa mesoscale disturbance (Fig. 8c). Once the remnants of derecho BW and derecho BE came into close proximity, derecho BE progressed eastward along with the 400-hPa mesoscale disturbance (Figs. 9a,c) as the system tapped the increased CAPE and moisture to the south of the QS boundary (Figs. 9d and 15). Around 0900 UTC 5 July, derecho BE subsequently merged with MCS 2 (Fig. 9a). This merger began as derecho BE expanded eastward along the QS boundary until convection connected derecho BE and MCS 2 (not shown). This combined system progressed eastward, remaining a derecho until 1500 UTC 5 July (Fig. 1).

5. Upscale effects of the MCSs and derechos

The repeated MCS activity during 3–5 July 2003 resulted in bulk upscale effects such as diabatically driven, tropospheric thickness increases (e.g., Zhang and Harvey 1995; Dickinson et al. 1997), and these synoptic-scale impacts affected subsequent MCS and derecho development. Deep convection typically reduces PV above a diabatic-heating maximum in the upper troposphere, and results in increased upper-level ridging (e.g., Martin 2006). Each MCS and derecho was associated with upper-level PV nonconservation along a similar corridor (Fig. 1), one example of which appears in Figs. 18a,b. At 0000 UTC 4 July, derecho AN was located in North Dakota on a gradient of PV on the 335-K potential temperature surface, while derecho AS was located in Nebraska under a low-PV ridge at values of 0.25–0.50 PVU (Fig. 18a). The IR satellite imagery at 0900 UTC 4 July shows widespread convective cloud cover associated with derecho A extending into Canada (Fig. 5a). A significant reduction in PV on the 335-K potential temperature surface occurred by 1200 UTC 4 July from central Minnesota and Wisconsin northward to southern Ontario compared to 12 h earlier (Figs. 18a,b). Winds on the 335-K potential temperature surface, along the periphery of the low-PV ridge, were largely parallel to contours of PV and did not have a northward component at 0000 or 1200 UTC 4 July. Therefore, horizontal advection of PV was probably not a significant contributor to the observed PV reduction. Instead, this PV reduction can likely be attributed to diabatic heating. The convection associated with derecho A modified the synoptic-scale environment by creating a low-PV ridge in the upper troposphere. (Note that MCS 1 was ongoing at 1200 UTC 4 July and may have had a contribution to the diabatic ridging.)

An analysis of the GFS forecast initialized at 0000 UTC 2 July, illustrates the effects of this PV nonconservation. Both the analysis and the 66-h forecast GFS 1000–500-hPa-thickness fields valid at 1800 UTC 4 July, reveal a thermal trough axis to the lee of the Rockies with a thickness ridge axis centered in eastern North America (Fig. 19a). However, along and to the north of the preferred MCS corridor (Fig. 1) the analyzed 1000–500-hPa-thickness values at 1800 UTC 4 July, after derecho A had occurred, were 4–8 dam higher than the 66-h GFS forecast thickness values valid at the same time. One possible source of this thickness error at forecast hour sixty six may be an inadequate representation of the bulk upscale effects of the deep convection associated with derecho A.

Previous research (e.g., Zhang and Fritsch 1988; Wolf and Johnson 1995) has shown that convectively driven outflow can strengthen an upper-level jet stream. Figure 19b shows the difference between the analyzed 200-hPa winds at 1800 UTC 4 July and the 66-h forecast of 200-hPa winds verifying at 1800 UTC 4 July. The analyzed 200-hPa winds were 10–20 m s−1 stronger from Wyoming, eastward into the Dakotas and Minnesota than the winds in the 66-h GFS forecast. Not only were analyzed 200-hPa winds stronger than the 66-h-forecasted 200-hPa winds, but the core of the observed 200-hPa jet stream was farther to the west than the 66-h forecast (not shown), as the diabatic ridging associated with derecho A helped to anchor the jet entrance region farther westward. As the diabatic ridge moved poleward, a region of PV values below 0.25 PVU near the Dakotas, became collocated with PV values above 4 PVU to the north (Fig. 18b), increasing the PV gradient and the strength of the upper-level jet stream.

A Hovmöller diagram, averaged from 47° to 51°N, the approximate band of the strongest 200-hPa jet stream from 3 to 5 July 2003 (e.g., Fig. 5c), further reveals the impacts of the diabatic heating from derecho A on the wind field (Fig. 20a). Between 0600 and 1200 UTC 4 July, the 200-hPa wind speed averaged from 47° to 51°N, increased to above 51 m s−1 near 95°–90°W (Fig. 20a). This wind maximum is likely associated with the diabatic heating from derecho A at similar longitudes (Fig. 18b). In the same location as the 200-hPa wind maximum, the analyzed 200-hPa winds were ∼12 m s−1 stronger than in the 0000 UTC 2 July forecast, again suggesting that the forecast did not properly capture the effects of the diabatic heating from derecho A. Note that near 85°W beginning at 1800 UTC 4 July, a region of winds that were 12 m s−1 weaker than forecast appeared (Fig. 20b). These longitudes were located under the axis of the diabatic ridge (Fig. 18b), and the northern progression of the ridge at these longitudes displaced the jet stream poleward relative to the forecast (Fig. 19b). Thus, the choice of the latitude band for the Hovmöller results in a wind speed decrease near 85°W when compared to the forecast (Fig. 20b). The analyzed average PV in the 300–200-hPa layer was 2–3 PVU lower than in the forecast, in the same location as the aforementioned wind forecast errors (Fig. 20c). This PV error was also likely associated with diabatic ridging and progressed downstream quickly as the diabatic ridge was advected eastward and as subsequent diabatic heating occurred. Recall that derecho BW formed in southeastern Montana around 1800 UTC 4 July (Fig. 3b). Thus, the altered upper-level jet environment and the associated jet entrance region that derecho BW progressed into was likely more favorable for convective development than if the diabatic heating from derecho A never occurred.

While a comprehensive comparison of forecast errors from different models and model runs is beyond the scope of this paper, it is worth noting that in addition to the 0000 UTC 2 July GFS model run, the 1200 UTC 2 July, 0000 UTC 3 July, and 1200 UTC 3 July GFS and Eta model runs all revealed similar 1000–500-hPa thickness and 200-hPa wind errors. While these models often do not capture the precise timing and location of convective development, these runs did hone in on much of the convection that formed during 3–5 July 2003. However, these model runs did not capture the formation of derecho BW; this is likely, at least in part, due to the effects of derecho A on the synoptic-scale pattern, as all of these model runs were initialized prior to the formation of derecho A.

6. Discussion and conclusions

a. Derechos

The derechos considered from 3 to 5 July 2003 evolved in differing environments and had varying characteristics. In a 5-yr climatology, Johns and Hirt (1987) found the average derecho duration to be ∼9.2 h, with a few lasting up to 20 h. Derecho A (including derechos AN and AS) had wind reports that lasted in excess of 24 h. On the other hand, the wind reports from derechos BW and BE only lasted for 7 and 12 h, respectively, although the merging of the remnants of derecho BW with derecho BE over Iowa gave the impression one long swath of severe wind reports.

Johns and Hirt (1987) found that derecho development is typically associated with an upper-level mesoscale disturbance. However, during 3–5 July 2003, such a disturbance was not always present. Derecho AN did not form in association with a mesoscale disturbance, forming instead immediately downstream of a synoptic-scale trough (Fig. 4a). Farther south, derecho AS also did not form in association with any discernable upper-level mesoscale disturbance (Fig. 4c). Derecho BW formed immediately to the east of an intense mesoscale disturbance that likely formed in association with barotropic instability (Figs. 6a–c and 7), while derecho BE did not form in association with an upper-level disturbance. However, derecho BE transitioned from stationary to mobile as the mesoscale disturbance initially associated with derecho BW approached from the west (Fig. 9c).

Johns and Hirt (1987) and Johns (1993) showed that derechos typically form and progress along surface thermal boundaries. Derecho AN formed along a surface boundary characterized by a potential temperature gradient of ∼8 K (100 km)−1 and a distinct wind shift in eastern Montana (Fig. 3a). Farther south in western Nebraska, derecho AS formed along a surface wind shift boundary that was coincident with a surface trough and a dryline (Fig. 3a). Derecho BW formed along a surface trough and wind shift boundary in eastern Montana, but there was no associated potential temperature or moisture gradient (Fig. 3b), while derecho BE initiated along a QS boundary that was characterized by potential temperature and moisture gradients and a surface wind shift (Fig. 15). While all of the derechos initially formed along a surface boundary, each was unique in terms of thermal, moisture, and kinematic gradients across the boundary.

Additionally, previous research has shown that progressive derechos, such those described in this paper, often form in the equatorward-entrance region of an upper-level jet stream (Coniglio et al. 2004). Derecho BE did form in the equatorward-entrance region of a 50–60 m s−1 200-hPa jet stream (Figs. 8c and 9c), while derecho AS formed well to the south of the same 200-hPa jet stream but was still located in the broad equatorward entrance region (Fig. 4c). However, derechos AN and BW progressed from the cyclonic to the anticyclonic shear side of the 200-hPa jet stream (Fig. 4c, 5c, 8c, and 9c). Thus, the derechos during 3–5 July did not all form in the equatorward-entrance region of the 200-hPa jet stream, a region of favorable synoptic-scale forcing for ascent, indicating that this forcing for ascent is not always necessary for derecho formation. Instead, those that did not (derechos AN and BW) moved from the poleward entrance region to the equatorward entrance region of the 200-hPa jet stream, became better organized, and subsequently strengthened.

Past studies on derechos have shown that they tend to initiate and mature in environments of robust CAPE (>2500 J kg−1) and shear (>20 m s−1; e.g., Johns 1993; Coniglio et al. 2004). When Derecho A was well organized at 0900 UTC 4 July, it progressed eastward in an environment that contained 2500–3500 J kg−1 of CAPE and 17.5–22.5 m s−1 of 0–6-km shear, consistent with previous research (Fig. 5d). At 0900 UTC 5 July derecho BE featured somewhat lesser CAPE/shear values of 500–1500 J kg−1 and 15–17.5 m s−1, respectively (Fig. 9d). However, the CAPE/shear environment associated with the mature stage of derecho BE was only slightly below the aforementioned mean environmental values. Derecho BW was able to form and mature in an environment featuring relatively minimal CAPE and 20 m s−1 of 0–6-km shear (Fig. 8d). Granted, derecho BW never became a robust bowing MCS, typical of many progressive derechos, but the length and character of the damage path were still consistent with that of a derecho. Therefore, although mean values of atmospheric variables are helpful in determining likely regions for derecho formation, derechos can form and mature in conditions that may not initially appear favorable.

b. Mesoscale interactions and convective episodes

The active 3–5 July 2003 convective corridor is highlighted by a “parade of MCSs” at 2108 UTC 4 July as MCSs 1 and 2 and derechos A and BW appear on the same visible satellite image across the upper Midwest (Fig. 13c). Derecho A was the most intense convective episode that occurred from 3 to 5 July 2003 and it left behind a strong cold pool that created a QS boundary (Figs. 10a,b). This cold pool and QS boundary played an important role in altering the life cycles of the two subsequent convective episodes.

The second convective episode began with MCS 1 that was located in the Dakotas by 0900 UTC 4 July (Fig. 5a). Before MCS 1 could strengthen and become a derecho, it encountered the significantly reduced CAPE environment that resulted from the ∼200-hPa-deep cold pool created by derecho A and completely dissipated by 2100 UTC 4 July (Figs. 8a,d and 10b). Derechos, and more generally MCSs, often decay as they progress into regions of minimal CAPE (e.g., Coniglio et al. 2004). As MCS 1 dissipated in the cold pool from derecho A, an IGW was emitted and progressed across Iowa and Illinois in the stable layer created by the cold pool from derecho A (Figs. 13a–c; Davis and Trier 2004). IGWs have long been associated with the development of new convection (e.g., Purdom 1973; Uccelini 1975; Zhang and Fritsch 1988; Carbone et al. 2002). As the IGW progressed eastward and daytime heating increased on 4 July, MCS 2 formed to the south of Lake Michigan (Fig. 13c), along the QS boundary created by derecho A (Fig. 10b). The excitation of MCS 2 associated with the IGW continued the convective episode initially associated with MCS 1. Carbone et al. (2002) point out that convective episodes typically have a greater longevity than that of a single MCS, and perhaps are made up of multiple (compound) MCS events. This event was of the compound type as an IGW “connected” distinct MCSs 1 and 2.

The third convective episode began as derecho BW approached the QS boundary created by derecho A and reinforced by MCS 1. An increasing 850-hPa LLJ by 0300 UTC 5 July caused convergence along the QS boundary and derecho BE development (Fig. 17b). Two hours later, the remnants of derecho BW had merged with derecho BE, which then progressed eastward (Fig. 17d). This merger created a compound convective episode that initially began with derecho BW and revealed another method that can prolong a convective episode beyond the time scale traditionally associated with a single MCS (e.g., Carbone et al. 2002; Tuttle and Carbone 2004).

c. Synoptic forecast implications

The repeated MCS activity resulted in bulk upscale effects on the synoptic scale through diabatic heating. Derecho A produced diabatic heating aloft (Figs. 18a,b) that likely contributed to 66-h negative forecast errors of 1000–500-hPa thickness (forecast too cold) of 4–8 dam (Fig. 19a). Maddox et al. (1981) and others have shown that MCSs can modify the midlevel mass field through latent heating. Under these conditions, geostrophic adjustment can occur with resultant jet stream intensification downstream of the heating source as the meridional temperature gradient is enhanced (e.g., Wolf and Johnson 1995; Hamilton et al. 1998). This mass field modification and jet strengthening likely occurred in association with the repeated convection from 3 to 5 July (Figs. 19b and 20b). These diabatically enhanced jet streams can contribute to additional convective outbreaks as they progress downstream (Hamilton et al. 1998). In this instance though, a favorable collocation between low-PV air associated with a diabatic ridge and higher PV air associated with a PV streamer strengthened the 200-hPa jet stream and made the synoptic-scale environment more favorable for the development of derecho BW.

Latent heat release associated with organized deep convection can lead to large forecasting errors (e.g., Dickinson et al. 1997; Bosart 1999; Zhang et al. 2002). Recently, discussion of these convectively induced forecast errors has focused on the effects of latent heating on the extratropical transition of tropical cyclones, such as increasing cyclonic vorticity advection over the surface cyclone in response to diabatically driven downstream ridging (e.g., Bosart and Lackmann 1995; Atallah and Bosart 2003; Atallah et al. 2007). However, as shown with the convection from 3–5 July across the upper Midwest, significant forecast errors of 1000–500-hPa thickness and 200-hPa wind can also be caused by organized midlatitude MCSs (Figs. 18a,b). The diabatic heating that causes these errors can impact the forecast of subsequent MCS development that occurs in the region of error. Thus, the effects of multiple MCSs can significantly affect the life cycle of subsequent MCSs not only through mesoscale interactions, but also through changes in the synoptic-scale pattern.

Acknowledgments

Funding for this research was provided by NSF Grants ATM-0233172 and ATM-0646907. The authors thank Daniel Keyser, Alan Srock, Jay Cordeira, and Tom Galarneau, for useful discussions and suggestions throughout the research process. Three anonymous reviewers supplied many ideas that improved the quality of this manuscript. The Atmospheric Radiation Program and NOAA/ESRL Physical Sciences Division provided much of the data used in this research. Kevin Knupp (University at Alabama in Huntsville) and Chris Davis (NCAR) provided the MIPS data used in Fig. 14.

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Fig. 1.
Fig. 1.

Isochrones every 3 h of the leading 45-dBZ line of derecho A (including derechos AN and AS; green), MCS 1 (yellow), derecho BW (red), derecho BE (maroon), MCS 2 (blue), and the combined system of derecho BE and MCS 2 (purple) overlaid upon storm reports of wind (blue plus signs), hail (green dots), and tornadoes (red dots) between 1800 UTC 3 Jul and 1500 UTC 5 Jul. Isochrones appear for times when the convective systems meet classification criteria discussed in the text. The time–date labels appear in association with the first and last isochrone drawn for each system. (Source: Severe Plot v.2.0)

Citation: Monthly Weather Review 138, 8; 10.1175/2010MWR3218.1

Fig. 2.
Fig. 2.

Composite analyses consisting of every 6 h from 3 to 5 Jul 2003 of (a) mean SLP (black lines every 2 hPa) and anomalous SLP (shaded in hPa); (b) mean 925-hPa mixing ratio (black lines every 2 g kg−1), anomalous 925-hPa mixing ratio (shaded in g kg−1), mean 0–6-km shear (black barbs in m s−1 where one pennant is 25 m s−1), and anomalous 0–6-km shear (blue barbs in m s−1 where one pennant is 25 m s−1); and (c) mean 200-hPa geopotential heights (black lines every 12 dam), anomalous 200-hPa geopotential heights (shaded in dam), mean 200-hPa wind (black barbs in m s−1 where one pennant is 25 m s−1), and anomalous 200-hPa wind (green barbs in m s−1 where one pennant is 25 m s−1). (Source: 2.5° NCEP–NCAR reanalysis)

Citation: Monthly Weather Review 138, 8; 10.1175/2010MWR3218.1

Fig. 3.
Fig. 3.

Manual surface analyses of SLP (black lines every 2 hPa), potential temperature (red dashed lines every 2 K), and mixing ratio equal to 18 g kg−1 (green line) for (a) 2100 UTC 3 Jul and (b) 1800 UTC 4 Jul. The analysis is overlaid upon NOWrad composite reflectivity (shaded in dBZ), and standard ASOS observations. The blue dotted line in (b) illustrates a surface wind shift. (Source: University at Albany surface data archive and NOWrad data)

Citation: Monthly Weather Review 138, 8; 10.1175/2010MWR3218.1

Fig. 4.
Fig. 4.

(a) SLP (green lines every 4 hPa), 1000–500-hPa thickness (red dashed lines every 6 dam), and IR satellite imagery; (b) 200-hPa geopotential heights (black lines every 12 dam), 200-hPa wind (shaded in m s−1), and 850-hPa wind (barbs in m s−1 where one pennant is 25 m s−1, beginning at 7.5 m s−1); (c) 400-hPa geopotential heights (black lines every 6 dam), 400-hPa absolute vorticity (shaded in × 10−5 s−1), and 400-hPa wind (barbs in m s−1 where one pennant is 25 m s−1); and (d) most unstable CAPE (shaded in J kg−1) and 0–6-km shear (barbs in m s−1 where one pennant is 25 m s−1) for 2100 UTC 3 Jul. (Source: 20-km RUC analysis and BAMEX field catalog)

Citation: Monthly Weather Review 138, 8; 10.1175/2010MWR3218.1

Fig. 5.
Fig. 5.

As in Fig. 4, but for 0900 UTC 4 Jul. (Source: 20-km RUC analysis and BAMEX field catalog)

Citation: Monthly Weather Review 138, 8; 10.1175/2010MWR3218.1

Fig. 6.
Fig. 6.

Potential vorticity (shaded in PVU), pressure (black lines every 30 hPa), and wind (barbs in m s−1 where one pennant is 25 m s−1) on the 325-K potential temperature surface for (a) 2100 UTC 3 Jul, (b) 0900 UTC 4 Jul, and (c) 2100 UTC 4 Jul. Water vapor imagery from 2100 UTC 4 Jul is inset into the lower-left corner of (c). Line A–A′ in (a) shows the location of the cross section in Fig. 7. (Source: 20-km RUC analysis and BAMEX field catalog)

Citation: Monthly Weather Review 138, 8; 10.1175/2010MWR3218.1

Fig. 7.
Fig. 7.

Mean absolute vorticity (shaded in × 10−5 s−1), negative meridional absolute vorticity gradient (green lines at −25 × 10−11 m−1 s−1), potential temperature (black lines in K), and wind (barbs in m s−1 where one pennant is 25 m s−1) averaged every 3 h from 1800 UTC 3 Jul to 1200 UTC 5 Jul along cross-section line A–A′ in Fig. 6a. (Source: 20-km RUC analyses)

Citation: Monthly Weather Review 138, 8; 10.1175/2010MWR3218.1

Fig. 8.
Fig. 8.

As in Fig. 4, but for 2100 UTC 4 Jul. (Source: 20-km RUC analysis and BAMEX field catalog)

Citation: Monthly Weather Review 138, 8; 10.1175/2010MWR3218.1

Fig. 9.
Fig. 9.

As in Fig. 4, but for 0900 UTC 5 Jul. (Source: 20-km RUC analysis and BAMEX field catalog)

Citation: Monthly Weather Review 138, 8; 10.1175/2010MWR3218.1

Fig. 10.
Fig. 10.

As in Fig. 3, but for (a) 0600 and (b) 1800 UTC 4 Jul. The KSLB and KCNC in (b) refer to the locations of meteograms in Fig. 12. (Source: University at Albany surface data archive and NOWrad data)

Citation: Monthly Weather Review 138, 8; 10.1175/2010MWR3218.1

Fig. 11.
Fig. 11.

North–south cross sections of the previous 3-h potential temperature decrease (shaded in K), potential temperature (black lines every 3 K), and wind (barbs in m s−1 where one pennant is 25 m s−1) for (a) 0900 and (b) 1600 UTC 4 Jul. Magenta lines show the approximate horizontal extent of convection that passed through each cross section over the previous 3 h. NOWrad composite radar imagery for 0600 and 0900 UTC 4 Jul (1300 and 1600 UTC 4 Jul), are inset in the upper-left and upper-right corners of (a) [(b)], respectively. All radar images contain a black line representing the location of the associated cross section. (Source: 20-km RUC analysis and NOWrad data)

Citation: Monthly Weather Review 138, 8; 10.1175/2010MWR3218.1

Fig. 12.
Fig. 12.

Meteograms from 1500 UTC 4 Jul to 0300 UTC 5 Jul of surface temperature (black lines), surface dewpoint (gray lines), and surface wind (barbs in m s−1 where one pennant is 25 m s−1) for (a) KSLB and (b) KCNC. The locations of KSLB and KCNC appear in Fig. 10b. (Source: University at Albany surface data archives)

Citation: Monthly Weather Review 138, 8; 10.1175/2010MWR3218.1

Fig. 13.
Fig. 13.

Visible satellite imagery at (a) 1703 UTC, (b) 1903 UTC, and (c) 2108 UTC 4 Jul. The “IGW” in (a) and (b) refers to an inertia-gravity wave. The green circle in (a) shows the location of the MIPS profiler data shown in Fig. 14. (Source: BAMEX field catalog)

Citation: Monthly Weather Review 138, 8; 10.1175/2010MWR3218.1

Fig. 14.
Fig. 14.

MIPS wind profiler data from 1900 to 2300 UTC 4 Jul and 0–7-km in height, taken from the location shown in Fig. 13a. Wind barbs are shaded every 2.5 kt. The black arrow illustrates a wind maximum associated with an IGW crest passage. (Source: BAMEX field catalog)

Citation: Monthly Weather Review 138, 8; 10.1175/2010MWR3218.1

Fig. 15.
Fig. 15.

As in Fig. 3, but for 0000 UTC 5 Jul. The black and gray hexagons show the locations of RUC soundings in Fig. 16. (Source: University at Albany surface data archives and NOWrad data)

Citation: Monthly Weather Review 138, 8; 10.1175/2010MWR3218.1

Fig. 16.
Fig. 16.

RUC analysis soundings of temperature (solid lines), dewpoint (dashed lines), and wind (barbs in m s−1 where one pennant is 25 m s−1) for 0000 UTC 5 Jul at the gray hexagon (gray sounding, 41.23°N, 94.11°W) and black hexagon (black sounding, 42.52°N, 94.12°W) in Fig. 15. (Source: 20-km RUC analysis)

Citation: Monthly Weather Review 138, 8; 10.1175/2010MWR3218.1

Fig. 17.
Fig. 17.

WSR-88D reflectivity from KOAX (shaded in dBZ), 850-hPa convergence (black solid lines every 3 × 10−5 s−1 less than −3 × 10−5 s−1), 850-hPa divergence (black dashed lines every 3 × 10−5 s−1 greater than 3 × 10−5 s−1), and 850-hPa wind (barbs in m s−1 where one pennant is 25 m s−1) for (a) 0200, (b) 0300, (c) 0400, and (d) 0500 UTC 5 Jul. (Source: 20-km RUC analysis and WSR–88D data)

Citation: Monthly Weather Review 138, 8; 10.1175/2010MWR3218.1

Fig. 18.
Fig. 18.

As in Fig. 6, but on the 335-K potential temperature surface for (a) 0000 and (b) 1200 UTC 4 Jul. The blue arrow in (b) illustrates a region of possible PV nonconservation associated with the convection from derecho A (see Fig. 1 for track). (Source: 20-km RUC analysis)

Citation: Monthly Weather Review 138, 8; 10.1175/2010MWR3218.1

Fig. 19.
Fig. 19.

(a) Analyzed 1000–500-hPa thickness for 1800 UTC 4 Jul minus 66-h forecast 1000–500-hPa thickness for 1800 UTC 4 Jul (shaded in dam), analyzed 1000–500-hPa thickness for 1800 UTC 4 Jul (dashed purple lines every 6 dam), and 66-h forecast 1000–500-hPa thickness for 1800 UTC 4 Jul (black lines every 6 dam). (b) Analyzed 200-hPa wind speed for 1800 UTC 4 Jul minus 66-h forecast 200-hPa wind speed for 1800 UTC 4 Jul (shaded in m s−1), analyzed 200-hPa geopotential heights for 1800 UTC 4 Jul (dashed purple lines every 12 dam), and 66-h forecast 200-hPa geopotential heights for 1800 UTC 4 Jul (black lines every 12 dam). The arrow in (a) illustrates a region where the analyzed 1000–500-hPa thickness was greater than in the 66-h forecast, while the arrow in (b) illustrates a region where the analyzed 200-hPa wind speed was greater than in the 66-h forecast. (Source: 1.0° GFS)

Citation: Monthly Weather Review 138, 8; 10.1175/2010MWR3218.1

Fig. 20.
Fig. 20.

Hovmöller plots averaged in the 47°–51°N latitude band from 0000 UTC 2 Jul to 0000 UTC 6 Jul and 135°–55°W of (a) the magnitude of the 200-hPa wind (shaded in m s−1), (b) analyzed 200-hPa wind speed minus forecast 200-hPa wind speed from 0000 UTC 2 Jul (shaded in m s−1), and (c) analyzed 300–200-hPa layer-average PV minus forecast 300–200-hPa layer-average PV from 0000 UTC 2 Jul (shaded in PVU). The arrows in (a),(b), and (c) illustrate a region of maximum 200-hPa wind, 200-hPa wind error, and 300–200-hPa average PV error, respectively. (Source: 1° GFS)

Citation: Monthly Weather Review 138, 8; 10.1175/2010MWR3218.1

Table 1.

Description of the MCSs and derechos that affected the upper Midwest during the 3–5 Jul 2003 period.

Table 1.

1

Composites were constructed in reference to the 1968–96 atmospheric mean.

2

Derechos were classified following the criteria of Johns and Hirt (1987). All derechos existed as weaker MCSs for a time subsequent to formation and prior to dissipation.

3

Derecho BW formed as a supercell and morphed into a broken convective line. While the Johns and Hirt (1987) study focused on linear–bowing MCSs, derecho BW still met all of the derecho criteria described by the authors.

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