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John R. Gyakum

Abstract

Six years of daily temperature and precipitation forecasting are studied for Urbana, Illinois. Minimum temperature forecast skills, measured against a climatological control, are 57%, 48%, 34% and 20% for the respective forecast ranges of one, two, three, and four days. Maximum temperature skills are comparable. Precipitation probability skills of 29%, 19%, 6% and −2% are found for the same respective forecast ranges. However, our skill in predicting precipitation amount, given that a measurable quantity occurs, is only 17% at the first day range and negligible thereafter. An examination of objective National Weather Service (NWS) forecasts shows this guidance to be slightly less skillful than our consensus in forecasting temperature and precipitation. Some temporal improvement is found in both the consensus and guidance temperature forecasts, but none can be found in the more difficult problem of forecasting precipitation.

Significant warm and dry biases are frequently found in both our consensus and NWS guidance forecasts, especially during the summer season. These biases may be associated with the organized convective character of the precipitation in Illinois. Forecasts often miss these key events and, therefore, will often predict excessively warm maximum temperatures.

Finally, the results show that our consensus skill is comparable to the state of the art. Student or faculty individuals usually lose to our consensus, as does the NWS objective forecast guidance. This establishment of the consensus forecast as typically being superior to an individual forecast has been reported by investigators in eastern United States cities.

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Paul A. Sisson and John R. Gyakum

Abstract

Several classes of significant cold-season precipitation events occurring in Burlington, Vermont (KBTV), during the 33-yr period from 1963 to 1995, are studied with the objective of identifying large-scale circulation precursors to the more extreme events. Several physically interesting and unique features that correspond to 24-h totals of 25 to 50 mm of precipitation are found. Preferential southerly and more maritime surface geostrophic flow occur in the heavier cases, in association with a strong cyclone (anticyclone) to the west (east) of KBTV. The 1000–500-hPa positive thickness anomaly corresponds to a depth-mean virtual temperature anomaly of +10.5°C in the heavy events. Additionally, statistically significant negative thickness anomalies, responsible for triggering these significant precipitation events, can be traced westward to a position in the Pacific Ocean at least 6 days prior to the event. Significantly heavier precipitable water amounts and preferentially strong water vapor transports from maritime regions are also associated with the heavier cold-season precipitation events.

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Gary M. Lackmann and John R. Gyakum

Abstract

Warm, moist southwesterly airflow into the northwestern United States during the cold season can result in rapid snowmelt and flooding. The objectives of this research are to document characteristic synoptic flow patterns accompanying cold-season (November–March) flooding events, and isolate flow anomalies associated with the moisture transport during a representative event. The first objective is accomplished through a 46-case composite spanning the years 1962–88; the second objective is addressed through diagnosis of a flooding event that occurred on 17–18 January 1986.

The 46-case composite is constructed for a 6-day period centered at 1200 UTC on the day of heavy precipitation onset (denoted τ 0). Composite 500-hPa geopotential height anomaly fields reveal anomalous ridging over the Bering Sea preceding the precipitation event, a negative anomaly over the Gulf of Alaska throughout the composite evolution, and a positive anomaly over the southwestern Unites States and adjacent eastern Pacific Ocean during and after the event. The gulf trough and southwestern ridge lead to enhanced southwesterly geostrophic flow into the northwestern United States at τ 0. A positive temperature anomaly at the 850-hPa level advances northeastward into the northwestern United States by τ 0, and expands over much of the United States by τ +48.

Piecewise geostrophic moisture transport computations for 17–18 January 1986, based on quasigeostrophic potential vorticity inversion, demonstrate that the transport of moisture into the northwestern United States is largely associated with a duo of mobile cyclones that track from the subtropical Pacific Ocean toward British Columbia. There is also a smaller contribution from a stationary anticyclone over the southwestern United States. These results indicate that the role of the planetary-scale flow, as depicted in the composite analyses, is to provide a persistent storm track, while the moisture flow within this storm track is modulated by cyclone-scale dynamics.

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John R. Gyakum and Katherine J. Samuels

Abstract

Objective precipitation guidance has been evaluated for specific regions within the continental United States during the period 1984–85. Cold-season precipitation probability skills for seven locations range from 52% at 12–24 h forecast range to about 21% at 36–48 h range. While these skills show the probability forecasts to be generally useful, an examination of forecasts with an absolute error of greater than 0.5 reveals this smaller sample to contain a disproportionately large number of observed precipitating events. This suggests that large-error-precipitation-probability forecasts have an unexpectedly large number of essentially unforecasted precipitation events, rather than false alarms. Warm-season precipitation probability skills are generally lower and show more variability within a given forecast range, with values ranging from 38 to 6% at 24–36 h range.

Limited-area Fine-Mesh (LFM model, cold-season, quantitative precipitation forecasts (QPFs) for specific cities show no skill beyond a 12-h forecast range. This loss of skill is associated with statistically significant overprediction of precipitation. However, to account for a coding error in the LFM model, we recomputed our statistics by halving all QPFs. The skills of these forecasts rose to respectable overall levels of 18.2, 14.8, 13.1 and 4.0% for the respective forecast ranges of 0–12, 12–24, 24–36 and 36–48 h. These revised forecasts have eliminated all suggestion of precipitation overprediction, and instead show a systematic underprediction of precipitation.

Cold-season, area-averaged QPFs taken directly from the LFM show a loss of skill against the climatological control forecast beyond 24 h. When we halved all forecasts, our area-averaged results showed, generally, more respectable overall skills of 9.3, 20.8, 16.9 and 5.2% for the respective forecast ranges.

Warm-season point and area-averaged QPFs show no skill against the climatological control forecast for any of the four forecast ranges out to 48 h. Statistically significant precipitation underprediction is found for the raw warm season QPFs. When the forecasts are halved, the skills deteriorate to even lower values and systematic underprediction of precipitation is more prevalent.

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Shawn M. Milrad, Eyad H. Atallah, and John R. Gyakum

Abstract

St. John’s, Newfoundland, Canada (CYYT), is frequently affected by extreme precipitation events, particularly in the cool season (October–April). Previous work classified precipitation events at CYYT into categories by precipitation amount and a manual synoptic typing was performed on the 50 median extreme precipitation events, using two separate methods. Here, consecutive extreme precipitation events in December 2008 are analyzed. These events occurred over a 6-day period and produced over 125 mm of precipitation at CYYT. The first manual typing method, using a backward-trajectory analysis, results in both events being classified as “southwest,” which were previously defined as the majority of the backward trajectories originating in the Gulf of Mexico. The second method of manual synoptic typing finds that the first event is classified as a “cyclone,” while the second is a “frontal” event. A synoptic analysis of both events is conducted, highlighting important dynamic and thermodynamic structures. The first event was characterized by strong quasigeostrophic forcing for ascent in a weakly stable atmosphere in association with a rapidly intensifying extratropical cyclone off the coast of North America and transient high values of subtropical moisture. The second event was characterized by primarily frontogenetical forcing for ascent in a weakly stable atmosphere in the presence of quasi-stationary high values of subtropical moisture, in association with a northeast–southwest-oriented baroclinic zone situated near CYYT. In sum, the synoptic structures responsible for the two events highlight rather disparate means to produce an extreme precipitation event at CYYT.

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Shawn M. Milrad, Eyad H. Atallah, and John R. Gyakum

Abstract

The St. Lawrence River valley (SLRV) is an important orographic feature in eastern Canada that can affect surface wind patterns and contribute to locally higher amounts of precipitation. The impact of the SLRV on precipitation distributions associated with transitioning, or transitioned, tropical cyclones that approached the region is assessed. Such cases can result in heavy precipitation during the warm season, as during the transition of Hurricane Ike (2008). Thirty-eight tropical cyclones tracked within 500 km of the SLRV from 1979 to 2011. Utilizing the National Centers for Environmental Prediction (NCEP) North American Regional Reanalysis (NARR), 19 of the 38 cases (group A) had large values of ageostrophic frontogenesis within and parallel to the SLRV, in a region of northeasterly surface winds associated with pressure-driven wind channeling. Using composite and case analyses, results show that the heaviest precipitation is often located within the SLRV, regardless of the location of large-scale forcing for ascent, and is concomitant with ageostrophic frontogenesis. The suggested physical pathway for precipitation modulation in the SLRV is as follows. Valley-induced near-surface ageostrophic frontogenesis is due to pressure-driven wind channeling as a result of the along-valley pressure gradient [typically exceeding 0.4 hPa (100 km)−1] established by the approaching cyclone. Near-surface cold-air advection as a result of the northeasterly pressure-driven channeling results in a temperature inversion, similar to what is observed in cool-season wind-channeling cases. The ageostrophic frontogenesis, acting as a mesoscale ascent-focusing mechanism, helps air parcels to rise above the temperature inversion into a conditionally unstable atmosphere, which results in enhanced precipitation focused along the SLRV.

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Shawn M. Milrad, Eyad H. Atallah, and John R. Gyakum

Abstract

Quantitative precipitation forecasting (QPF) continues to be a significant challenge in operational forecasting, particularly in regions susceptible to extreme precipitation events. St. John’s, Newfoundland, Canada (CYYT), is affected frequently by such events, particularly in the cool season (October–April).

The 50 median events in the extreme (>33.78 mm during a 48-h period) precipitation event category are selected for further analysis. A manual synoptic typing is performed on these 50 events, using two separate methodologies to partition events. The first method utilizes a Lagrangian backward air parcel trajectory analysis and the second method utilizes the evolution of dynamically relevant variables, including 1000–700-hPa horizontal temperature advection, 1000–700-hPa (vector) geostrophic frontogenesis, and 700–400-hPa absolute vorticity advection.

Utilizing the first partitioning method, it is found that south cases are characterized by a strong anticyclone downstream of St. John’s, southwest events are synoptically similar to the overall extreme composite and are marked by a strong cyclone that develops in the Gulf of Mexico, while west events are characterized by a weak Alberta clipper system that intensifies rapidly upon reaching the Atlantic Ocean. The second partitioning method suggests that while cyclone events are dominated by the presence of a rapidly developing cyclone moving northeastward toward St. John’s, frontal events are characterized by the presence of a strong downstream anticyclone and deformation zone at St. John’s.

It is the hope of the authors that the unique methodology and results of the synoptic typing in this paper will aid forecasters in identifying certain characteristics of future precipitation events at St. John’s and similar stations.

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Paul J. Roebber, John R. Gyakum, and Diep N. Trat

Abstract

The inverted trough and coastal front that occurred during ERICA IOP 2 were studied in order to assess the ability of an operational mesoscale model, the Canadian Regional Finite Element Model, to provide credible forecast guidance concerning the regional distribution of precipitation associated with such events. The observed distribution of the heaviest precipitation was dependent on the orientation and inland penetration of the coastal front and its associated (geostrophic) frontogenetic forcing, although considerable local variability was evident. The observations indicate that the front penetrated inland along a line west of Boston through east-central Connecticut. The model, which performed exceptionally well in forecasting the eventual development of the main cyclone, correctly forecast the existence and general location of the inverted trough and coastal front and thereby gave indications of the development of heavy snowfall within the coastal region of New York–New England 18–24 h in advance. However, detailed modeling of the coastal front itself is still beyond the scope of current operational models. In the case studied, the position of the regional environment in which the coastal front forms was forecast incorrectly, preventing a precise regional forecast of the precipitation distribution.

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Shawn M. Milrad, Eyad H. Atallah, and John R. Gyakum

Abstract

The issue of quantitative precipitation forecasting continues to be a significant challenge in operational forecasting, particularly in regions susceptible to frequent and extreme precipitation events. St. John’s, Newfoundland, Canada, is one location affected frequently by such events, particularly in the cool season (October–April). These events can include flooding rains, paralyzing snowfall, and damaging winds.

A precipitation climatology is developed at St. John’s for 1979–2005, based on discrete precipitation events occurring over a time period of up to 48 h. Threshold amounts for three categories of precipitation events (extreme, moderate, and light) are statistically derived and utilized to categorize such events. Anomaly plots of sea level pressure (SLP), 500-hPa height, and precipitable water are produced for up to 3 days prior to the event. Results show that extreme events originate along the Gulf Coast of the United States, with the location of anomaly origin being farther to the north and west for consecutively weaker events, culminating in light events that originate from the upper Midwest of the United States and south-central Canada. In addition, upper-level precursor features are identified up to 3 days prior to the events and are mainly located over the west coast of North America.

Finally, results of a wind climatology produced for St. John’s depict a gradual shift in the predominant wind direction (from easterly to southwesterly) of both the 925-hPa geostrophic wind and 10-m observed wind from extreme to light events, inclusively. In addition, extreme events are characterized by almost exclusively easterly winds.

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Christopher D. McCray, Eyad H. Atallah, and John R. Gyakum

Abstract

Freezing rain can cause severe impacts, particularly when it persists for many hours. In this paper, we present the climatology of long-duration (6 or more hours) freezing rain events in the United States and Canada from 1979 to 2016. We identify three focus regions from this climatology and examine the archetypal thermodynamic evolution of events in each region using surface and radiosonde observations. Long-duration events occur most frequently in the northeastern United States and southeastern Canada, where freezing rain typically begins as lower-tropospheric warm-air advection develops the warm layer aloft. This warm-air advection and the latent heat of fusion released when rain freezes at the surface erode the cold layer, and freezing rain transitions to rain once the surface temperature reaches 0°C. In the southeastern United States, a larger percentage of events are of long duration than elsewhere in North America. Weak surface cold-air advection and evaporative cooling in the particularly dry onset cold layers there prevent surface temperatures from rising substantially during events. Finally, the south-central United States has a regional maximum in the occurrence of the top 1% of events by duration (18 or more hours), despite the relative rarity of freezing rain there. These events are associated with particularly warm/deep onset warm layers, with persistent low-level cold-air advection maintaining the cold layer. The thermodynamic evolutions we have identified highlight characteristics that are key to supporting persistent freezing rain in each region and may warrant particular attention from forecasters tasked with predicting these events.

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