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Abstract
A commonly employed explanation for single- and multiple-banded clouds and precipitation in the extratropics is slantwise convection due to the release of moist symmetric instability (MSI), of which one type is conditional symmetric instability (CSI). This article presents a review of CSI with the intent of synthesizing the results from previous observational, theoretical, and modeling studies. This review contends that CSI as a diagnostic tool to assess slantwise convection has been, and continues to be, misused and overused. Drawing parallels to an ingredients-based methodology for forecasting deep, moist convection that requires the simultaneous presence of instability, moisture, and lift, some of the misapplications of CSI can be clarified. Many of these pitfalls have been noted by earlier authors, but are, nevertheless, often understated, misinterpreted, or neglected by later researchers and forecasters. Topics include the evaluation of the potential for slantwise convection, the relationship between frontogenesis and MSI, the coexistence of moist gravitational instability and MSI, the nature of banding associated with slantwise convection, and the diagnosis of slantwise convection using mesoscale numerical models. The review concludes with suggested directions for future observational, theoretical, and diagnostic investigation.
Abstract
A commonly employed explanation for single- and multiple-banded clouds and precipitation in the extratropics is slantwise convection due to the release of moist symmetric instability (MSI), of which one type is conditional symmetric instability (CSI). This article presents a review of CSI with the intent of synthesizing the results from previous observational, theoretical, and modeling studies. This review contends that CSI as a diagnostic tool to assess slantwise convection has been, and continues to be, misused and overused. Drawing parallels to an ingredients-based methodology for forecasting deep, moist convection that requires the simultaneous presence of instability, moisture, and lift, some of the misapplications of CSI can be clarified. Many of these pitfalls have been noted by earlier authors, but are, nevertheless, often understated, misinterpreted, or neglected by later researchers and forecasters. Topics include the evaluation of the potential for slantwise convection, the relationship between frontogenesis and MSI, the coexistence of moist gravitational instability and MSI, the nature of banding associated with slantwise convection, and the diagnosis of slantwise convection using mesoscale numerical models. The review concludes with suggested directions for future observational, theoretical, and diagnostic investigation.
Abstract
A climatology of single-banded snowfall in the central United States and the variability of processes at work in its formation are presented. Ninety-eight snowbands are identified in association with 66 cyclones over 5 yr spanning the winters from 2006/07 through 2010/11. An additional 38 cyclones featured nonbanded snowfall exceeding 4 in. (10.2 cm). Nearly twice as many bands were observed to the northeast of the surface low than to the northwest. Over each snowband’s life cycle, the median (mean) snowband lasted 4.0 (5.2) h, was 42 (45) km wide, 388 (428) km long, and had an aspect ratio of 10.2 (10.8). A common appearance exists for snowbands in different large-scale flow regimes and locations relative to the surface cyclone. The median snowband elongates during the first half of its life span, with its width remaining constant. During the second half of the median snowband’s life span, the length and width contract. Composite analysis of the synoptic and broad mesoscale environments that snowbands form in illustrates that the juxtaposition of the ingredients necessary for snowbands are similar no matter which quadrant of the surface low the band is located in, indicating that the synoptic-scale flow determines where these ingredients are organized with respect to the cyclone. The frequency of banded snowfall within each northern quadrant of the surface low, the typical snowband characteristics and their evolution, and the patterns that give rise to snowbands documented by this work can all prove useful to forecasters tasked with maintaining situational awareness in the presence of many solutions provided by ensemble numerical weather prediction.
Abstract
A climatology of single-banded snowfall in the central United States and the variability of processes at work in its formation are presented. Ninety-eight snowbands are identified in association with 66 cyclones over 5 yr spanning the winters from 2006/07 through 2010/11. An additional 38 cyclones featured nonbanded snowfall exceeding 4 in. (10.2 cm). Nearly twice as many bands were observed to the northeast of the surface low than to the northwest. Over each snowband’s life cycle, the median (mean) snowband lasted 4.0 (5.2) h, was 42 (45) km wide, 388 (428) km long, and had an aspect ratio of 10.2 (10.8). A common appearance exists for snowbands in different large-scale flow regimes and locations relative to the surface cyclone. The median snowband elongates during the first half of its life span, with its width remaining constant. During the second half of the median snowband’s life span, the length and width contract. Composite analysis of the synoptic and broad mesoscale environments that snowbands form in illustrates that the juxtaposition of the ingredients necessary for snowbands are similar no matter which quadrant of the surface low the band is located in, indicating that the synoptic-scale flow determines where these ingredients are organized with respect to the cyclone. The frequency of banded snowfall within each northern quadrant of the surface low, the typical snowband characteristics and their evolution, and the patterns that give rise to snowbands documented by this work can all prove useful to forecasters tasked with maintaining situational awareness in the presence of many solutions provided by ensemble numerical weather prediction.
Abstract
The morphology of mesocyclones associated with the regional tornado outbreak on 24 June 2003 is examined to illustrate the effects of changing vertical wind profiles. The large-scale environment supported deep moist convection, with forcing for ascent and convective instability. Postevent analysis indicated there were changes in the shear in space and time across a small geographical area. The event was separated into sectors based on both the synoptic setting and the differing shear profiles. Near the surface warm front, the vertical wind profile and mesocyclone evolution exhibited a classic appearance and produced significant tornadoes. In the warm sector, where no discernible surface boundaries were evident, classic supercells initially were favored but only produced short-lived tornadoes rated as F0 on the Fujita scale. The vertical wind profile changed as a low-level jet intensified after 0000 UTC 25 June. The majority of the vertical wind shear became located below 3 km. Meanwhile, mesocyclone elevation lowered and rotational velocity increased. As the dynamically induced low-level jet and an area of mixed-layer (ML) convective available potential energy (CAPE) became juxtaposed where the boundary layer was uncapped, strong low-level mesocyclones and 32 tornadoes developed in an area with no discernible surface boundaries. The event illustrates the need for warning meteorologists to monitor not only the amount of shear present, but also its distribution in the hodograph owing to its strong correspondence with mesocyclone morphology.
Abstract
The morphology of mesocyclones associated with the regional tornado outbreak on 24 June 2003 is examined to illustrate the effects of changing vertical wind profiles. The large-scale environment supported deep moist convection, with forcing for ascent and convective instability. Postevent analysis indicated there were changes in the shear in space and time across a small geographical area. The event was separated into sectors based on both the synoptic setting and the differing shear profiles. Near the surface warm front, the vertical wind profile and mesocyclone evolution exhibited a classic appearance and produced significant tornadoes. In the warm sector, where no discernible surface boundaries were evident, classic supercells initially were favored but only produced short-lived tornadoes rated as F0 on the Fujita scale. The vertical wind profile changed as a low-level jet intensified after 0000 UTC 25 June. The majority of the vertical wind shear became located below 3 km. Meanwhile, mesocyclone elevation lowered and rotational velocity increased. As the dynamically induced low-level jet and an area of mixed-layer (ML) convective available potential energy (CAPE) became juxtaposed where the boundary layer was uncapped, strong low-level mesocyclones and 32 tornadoes developed in an area with no discernible surface boundaries. The event illustrates the need for warning meteorologists to monitor not only the amount of shear present, but also its distribution in the hodograph owing to its strong correspondence with mesocyclone morphology.
Abstract
A comparison between the climatological structure of retarded and unretarded fronts aligned parallel to the Appalachian Mountains is investigated. With the average height of the Appalachians being 1 km, retarded and unretarded fronts are common occurrences during the cold season. Because of the narrow half-width of 100 km and the 1000-km length of the mountain chain, a comparison to two- and three-dimensional numerical studies can be performed. Of the 142 cases of frontal passages over the Appalachians during the winters between October 1984 and April 1990, over 55% of all cold fronts were retarded by the mountains. Statistical analysis showed that retarded fronts have a stronger cross-front temperature gradient and a weaker cross-front pressure gradient. Composite fields of sea level pressure, 850-, 500-, and 200-mb heights; quasigeostrophic potential vorticity and its advection, and potential height (U/N) were computed for all retarded and unretarded fronts. Unretarded fronts were associated with stronger cyclones, larger potential vorticity anomalies, larger positive potential vorticity advection, and more amplified flow at all levels. There was no significant difference between the potential height fields of the two types of fronts. In addition the average potential height, for both groups of fronts, easily met the criteria for retardation. Rather than depending upon the Froude number of the flow, it is hypothesized that the strength of the synoptic-scale circulations in the middle and upper troposphere primarily determines whether or not a front will be retarded by the Appalachian Mountains.
Abstract
A comparison between the climatological structure of retarded and unretarded fronts aligned parallel to the Appalachian Mountains is investigated. With the average height of the Appalachians being 1 km, retarded and unretarded fronts are common occurrences during the cold season. Because of the narrow half-width of 100 km and the 1000-km length of the mountain chain, a comparison to two- and three-dimensional numerical studies can be performed. Of the 142 cases of frontal passages over the Appalachians during the winters between October 1984 and April 1990, over 55% of all cold fronts were retarded by the mountains. Statistical analysis showed that retarded fronts have a stronger cross-front temperature gradient and a weaker cross-front pressure gradient. Composite fields of sea level pressure, 850-, 500-, and 200-mb heights; quasigeostrophic potential vorticity and its advection, and potential height (U/N) were computed for all retarded and unretarded fronts. Unretarded fronts were associated with stronger cyclones, larger potential vorticity anomalies, larger positive potential vorticity advection, and more amplified flow at all levels. There was no significant difference between the potential height fields of the two types of fronts. In addition the average potential height, for both groups of fronts, easily met the criteria for retardation. Rather than depending upon the Froude number of the flow, it is hypothesized that the strength of the synoptic-scale circulations in the middle and upper troposphere primarily determines whether or not a front will be retarded by the Appalachian Mountains.
Abstract
In response to Sherwood’s comments and in an attempt to restore proper usage of terminology associated with moist instability, the early history of moist instability is reviewed. This review shows that many of Sherwood’s concerns about the terminology were understood at the time of their origination. Definitions of conditional instability include both the lapse-rate definition (i.e., the environmental lapse rate lies between the dry- and the moist-adiabatic lapse rates) and the available-energy definition (i.e., a parcel possesses positive buoyant energy; also called latent instability), neither of which can be considered an instability in the classic sense. Furthermore, the lapse-rate definition is really a statement of uncertainty about instability. The uncertainty can be resolved by including the effects of moisture through a consideration of the available-energy definition (i.e., convective available potential energy) or potential instability. It is shown that such misunderstandings about conditional instability were likely due to the simplifications resulting from the substitution of lapse rates for buoyancy in the vertical acceleration equation. Despite these valid concerns about the value of the lapse-rate definition of conditional instability, consideration of the lapse rate and moisture separately can be useful in some contexts (e.g., the ingredients-based methodology for forecasting deep, moist convection). It is argued that the release of potential (or convective) instability through layer lifting may occur in association with fronts, rather than with isolated convection, the terminology “convective” being an unfortunate modifier. The merits and demerits of slantwise convective available potential energy are discussed, with the hope of improving diagnostic methodologies for assessing slantwise convection. Finally, it is argued that, when assessing precipitation events, undue emphasis may appear to be placed on instability, rather than the forcing for ascent, which should be of primary importance.
Abstract
In response to Sherwood’s comments and in an attempt to restore proper usage of terminology associated with moist instability, the early history of moist instability is reviewed. This review shows that many of Sherwood’s concerns about the terminology were understood at the time of their origination. Definitions of conditional instability include both the lapse-rate definition (i.e., the environmental lapse rate lies between the dry- and the moist-adiabatic lapse rates) and the available-energy definition (i.e., a parcel possesses positive buoyant energy; also called latent instability), neither of which can be considered an instability in the classic sense. Furthermore, the lapse-rate definition is really a statement of uncertainty about instability. The uncertainty can be resolved by including the effects of moisture through a consideration of the available-energy definition (i.e., convective available potential energy) or potential instability. It is shown that such misunderstandings about conditional instability were likely due to the simplifications resulting from the substitution of lapse rates for buoyancy in the vertical acceleration equation. Despite these valid concerns about the value of the lapse-rate definition of conditional instability, consideration of the lapse rate and moisture separately can be useful in some contexts (e.g., the ingredients-based methodology for forecasting deep, moist convection). It is argued that the release of potential (or convective) instability through layer lifting may occur in association with fronts, rather than with isolated convection, the terminology “convective” being an unfortunate modifier. The merits and demerits of slantwise convective available potential energy are discussed, with the hope of improving diagnostic methodologies for assessing slantwise convection. Finally, it is argued that, when assessing precipitation events, undue emphasis may appear to be placed on instability, rather than the forcing for ascent, which should be of primary importance.
An extratropical cyclone of unusual intensity and areal extent affected much of the Gulf and East Coasts of the United States on 12–14 March 1993. In this paper, the many effects of the storm will be highlighted, including perhaps the most widespread distribution of heavy snowfall of any recent East Coast storm, severe coastal flooding, and an outbreak of 11 confirmed tornadoes. A meteorological description of the storm is also presented, including a synoptic overview and a mesoscale analysis that focuses on the rapid development of the cyclone over the Gulf of Mexico. This is the first part of a three-paper series that also addresses the performance of the operational numerical models and assesses the forecasting decisions made at the National Meteorological Center and National Weather Service local forecast offices in the eastern United States.
An extratropical cyclone of unusual intensity and areal extent affected much of the Gulf and East Coasts of the United States on 12–14 March 1993. In this paper, the many effects of the storm will be highlighted, including perhaps the most widespread distribution of heavy snowfall of any recent East Coast storm, severe coastal flooding, and an outbreak of 11 confirmed tornadoes. A meteorological description of the storm is also presented, including a synoptic overview and a mesoscale analysis that focuses on the rapid development of the cyclone over the Gulf of Mexico. This is the first part of a three-paper series that also addresses the performance of the operational numerical models and assesses the forecasting decisions made at the National Meteorological Center and National Weather Service local forecast offices in the eastern United States.
Abstract
This is the second of two papers that examine the organization of the precipitation field during central U.S. cold-season cyclones involving inverted troughs (ITs). The first paper (Part I) used a climatology and composites to find synoptic-scale differences between storms with precipitation located ahead of the IT (ahead cases) and those with precipitation located behind the IT (behind cases). This paper expands the conclusions in Part I through the use of a comparative case study between two cyclones. The first cyclone, on 29 October 1996, was an ahead case that produced heavy rainfall and was associated with a potential vorticity (PV) anomaly moving across the central plains. The IT formed in the lee of the Rockies prior to 0600 UTC 29 October and moved east into the northern plains over the next 18 h. The trough itself was coincident with the limiting streamline, which separated moist air rising over the warm front from dry air subsiding behind the cyclone. The second cyclone, on 17–18 January 1996, had precipitation on both sides of the IT and was associated with heavy snow and blizzard conditions in the northern plains and significant ice accumulation in the western Great Lakes. The IT was associated with large frontogenesis over the snow area. The ascent was further enhanced by a jet streak moving across southern Canada. Dynamically, the IT resembled a warm front, with veering winds with height and a strong frontal inversion.
The mechanism that appeared to control the different precipitation organization between the two systems was the orientation of the PV anomalies and the airstreams associated with their secondary circulations. This resulted in a differing orientation of the baroclinicity north and east of the cyclone. In the ahead case, the rising branches of the secondary circulations forced by the northern and southern anomalies remained separate. This allowed the baroclinicity to develop along the traditional warm front, while the IT never developed a thermal gradient as it moved east. In the both sides case, the southern stream anomaly helped to fix the northern anomaly-forced jet streak in place, so that a strong temperature gradient developed along the IT with strong frontogenesis and warm-air advection observed behind the IT. As the frontal circulation developed, the direct circulation associated with the right entrance region of a jet streak enhanced the ascent to the west of the IT.
A conceptual model is proposed based upon the case studies and the results of Part I. This model can be used by forecasters to differentiate between the precipitation regimes in cyclones associated with ITs.
Abstract
This is the second of two papers that examine the organization of the precipitation field during central U.S. cold-season cyclones involving inverted troughs (ITs). The first paper (Part I) used a climatology and composites to find synoptic-scale differences between storms with precipitation located ahead of the IT (ahead cases) and those with precipitation located behind the IT (behind cases). This paper expands the conclusions in Part I through the use of a comparative case study between two cyclones. The first cyclone, on 29 October 1996, was an ahead case that produced heavy rainfall and was associated with a potential vorticity (PV) anomaly moving across the central plains. The IT formed in the lee of the Rockies prior to 0600 UTC 29 October and moved east into the northern plains over the next 18 h. The trough itself was coincident with the limiting streamline, which separated moist air rising over the warm front from dry air subsiding behind the cyclone. The second cyclone, on 17–18 January 1996, had precipitation on both sides of the IT and was associated with heavy snow and blizzard conditions in the northern plains and significant ice accumulation in the western Great Lakes. The IT was associated with large frontogenesis over the snow area. The ascent was further enhanced by a jet streak moving across southern Canada. Dynamically, the IT resembled a warm front, with veering winds with height and a strong frontal inversion.
The mechanism that appeared to control the different precipitation organization between the two systems was the orientation of the PV anomalies and the airstreams associated with their secondary circulations. This resulted in a differing orientation of the baroclinicity north and east of the cyclone. In the ahead case, the rising branches of the secondary circulations forced by the northern and southern anomalies remained separate. This allowed the baroclinicity to develop along the traditional warm front, while the IT never developed a thermal gradient as it moved east. In the both sides case, the southern stream anomaly helped to fix the northern anomaly-forced jet streak in place, so that a strong temperature gradient developed along the IT with strong frontogenesis and warm-air advection observed behind the IT. As the frontal circulation developed, the direct circulation associated with the right entrance region of a jet streak enhanced the ascent to the west of the IT.
A conceptual model is proposed based upon the case studies and the results of Part I. This model can be used by forecasters to differentiate between the precipitation regimes in cyclones associated with ITs.
Abstract
The objective of this study is to provide guidance on when hail and/or wind is climatologically most likely (temporally and spatially) based on the ratio of severe hail reports to severe wind reports, which can be used by National Weather Forecast (NWS) forecasters when issuing severe convective warnings. Accordingly, a climatology of reported hail-to-wind ratios (i.e., number of hail reports divided by the number of wind reports) for observed severe convective storms was derived using U.S. storm reports from 1955 to 2017. Owing to several temporal changes in reporting and warning procedures, the 1996–2017 period was chosen for spatiotemporal analyses, yielding 265 691 hail and 294 449 wind reports. The most notable changes in hail–wind ratios occurred around 1996 as the NWS modernized and deployed new radars (leading to more hail reports relative to wind) and in 2010 when the severe hail criterion increased nationwide (leading to more wind reports relative to hail). One key finding is that hail–wind ratios are maximized (i.e., relatively more hail than wind) during the late morning through midafternoon and in the spring (March–May), with geographical maxima over the central United States and complex/elevated terrain. Otherwise, minimum ratios occur overnight, during the late summer (July–August) as well as November–December, and over the eastern United States. While the results reflect reporting biases (e.g., fewer wind than hail reports in low-population areas but more wind reports where mesonets are available), meteorological factors such as convective mode and cool spring versus warm summer environments also appear associated with the hail–wind ratio climatology.
Abstract
The objective of this study is to provide guidance on when hail and/or wind is climatologically most likely (temporally and spatially) based on the ratio of severe hail reports to severe wind reports, which can be used by National Weather Forecast (NWS) forecasters when issuing severe convective warnings. Accordingly, a climatology of reported hail-to-wind ratios (i.e., number of hail reports divided by the number of wind reports) for observed severe convective storms was derived using U.S. storm reports from 1955 to 2017. Owing to several temporal changes in reporting and warning procedures, the 1996–2017 period was chosen for spatiotemporal analyses, yielding 265 691 hail and 294 449 wind reports. The most notable changes in hail–wind ratios occurred around 1996 as the NWS modernized and deployed new radars (leading to more hail reports relative to wind) and in 2010 when the severe hail criterion increased nationwide (leading to more wind reports relative to hail). One key finding is that hail–wind ratios are maximized (i.e., relatively more hail than wind) during the late morning through midafternoon and in the spring (March–May), with geographical maxima over the central United States and complex/elevated terrain. Otherwise, minimum ratios occur overnight, during the late summer (July–August) as well as November–December, and over the eastern United States. While the results reflect reporting biases (e.g., fewer wind than hail reports in low-population areas but more wind reports where mesonets are available), meteorological factors such as convective mode and cool spring versus warm summer environments also appear associated with the hail–wind ratio climatology.
Abstract
This paper is the first of two papers that examines the organization of the precipitation field in central U.S. cold-season cyclones involving inverted troughs. The first portion of the study examines the varying precipitation distribution that occurred during a 6-yr synoptic climatology of inverted trough cases. The definition of inverted trough cases has been expanded from the groundbreaking work by Keshishian et al. by 1) not requiring a closed cyclonic isobar along the frontal wave along the conventional surface front and 2) not requiring a surface thermal gradient to be present along the inverted trough. Only 8.5% of the expanded dataset produced the precipitation primarily occurring to the west of the inverted trough (“behind” cases) as seen in Keshishian et al. The largest group of cases, comprising about 40% of the cases, produced precipitation that primarily occurred between the inverted trough and the conventional warm front (“ahead” cases). A composite study compared a subset of the ahead cases with a subset of the behind cases. The ahead cases tended to be more progressive with a stronger jet stream located over the center of the parent low. Broad warm-air advection and frontogenesis in the lower troposphere were observed between the inverted trough and the surface warm front. Cold-air advection to the west of the inverted trough precluded the development of “wraparound precipitation.” In contrast, the behind cases had a stronger low-latitude wave couplet with a trough upstream of the surface low and a ridge downstream. The region of warm-air advection and frontogenesis were displaced to the west of the inverted trough and surface cyclone. In addition, the entrance region of a southwest–northeast-oriented jet streak aided the development of ascent to the west of the inverted trough while precluding the development of precipitation to the north of the conventional warm front. Thus, the inverted trough tended to act like a warm front in behind cases, as shown by Keshishian et al. Composites were also computed at both 12 and 24 h before inverted trough formation in order to generate comparisons useful to operational applications. Case study results for both ahead and behind cases will be compared with the composite cases in the companion paper.
Abstract
This paper is the first of two papers that examines the organization of the precipitation field in central U.S. cold-season cyclones involving inverted troughs. The first portion of the study examines the varying precipitation distribution that occurred during a 6-yr synoptic climatology of inverted trough cases. The definition of inverted trough cases has been expanded from the groundbreaking work by Keshishian et al. by 1) not requiring a closed cyclonic isobar along the frontal wave along the conventional surface front and 2) not requiring a surface thermal gradient to be present along the inverted trough. Only 8.5% of the expanded dataset produced the precipitation primarily occurring to the west of the inverted trough (“behind” cases) as seen in Keshishian et al. The largest group of cases, comprising about 40% of the cases, produced precipitation that primarily occurred between the inverted trough and the conventional warm front (“ahead” cases). A composite study compared a subset of the ahead cases with a subset of the behind cases. The ahead cases tended to be more progressive with a stronger jet stream located over the center of the parent low. Broad warm-air advection and frontogenesis in the lower troposphere were observed between the inverted trough and the surface warm front. Cold-air advection to the west of the inverted trough precluded the development of “wraparound precipitation.” In contrast, the behind cases had a stronger low-latitude wave couplet with a trough upstream of the surface low and a ridge downstream. The region of warm-air advection and frontogenesis were displaced to the west of the inverted trough and surface cyclone. In addition, the entrance region of a southwest–northeast-oriented jet streak aided the development of ascent to the west of the inverted trough while precluding the development of precipitation to the north of the conventional warm front. Thus, the inverted trough tended to act like a warm front in behind cases, as shown by Keshishian et al. Composites were also computed at both 12 and 24 h before inverted trough formation in order to generate comparisons useful to operational applications. Case study results for both ahead and behind cases will be compared with the composite cases in the companion paper.
Abstract
Using system-relative composites, based on a dataset of significant tornadoes and null supercell events, environmental conditions associated with occurrences of significant tornadoes near discernible surface boundaries were compared to nontornadic boundary supercells, and warm sector significant tornadoes to nontornadic warm sector supercells, for a portion of the Great Plains. Results indicated that significant boundary tornadoes were associated with the exit region of a 300-hPa jet maximum, while null boundary events were in closer proximity to the 300-hPa jet entrance region. The differences at 300 hPa led to significant differences at the surface, as the null composite indicated deformation and confluence into the surface boundary and enhanced frontogenesis, while this was not present in the boundary significant tornado composite. Significant synoptic differences also were noted between the warm sector tornadoes and the warm sector null events. The warm sector significant tornadoes were associated with a much stronger, negatively tilted synoptic storm system, with the composite tornado in the 300-hPa jet exit region and downstream of increasing values of absolute vorticity. Additional thermodynamic and kinematic parameters pertaining to low-level moisture and environmental winds appeared to be important in distinguishing boundary and warm sector significant tornadoes from nontornadic supercell events. Statistical comparisons between boundary and warm sector significant tornado events showed significant differences in the climatology of their length, width, and date and time of occurrence.
Abstract
Using system-relative composites, based on a dataset of significant tornadoes and null supercell events, environmental conditions associated with occurrences of significant tornadoes near discernible surface boundaries were compared to nontornadic boundary supercells, and warm sector significant tornadoes to nontornadic warm sector supercells, for a portion of the Great Plains. Results indicated that significant boundary tornadoes were associated with the exit region of a 300-hPa jet maximum, while null boundary events were in closer proximity to the 300-hPa jet entrance region. The differences at 300 hPa led to significant differences at the surface, as the null composite indicated deformation and confluence into the surface boundary and enhanced frontogenesis, while this was not present in the boundary significant tornado composite. Significant synoptic differences also were noted between the warm sector tornadoes and the warm sector null events. The warm sector significant tornadoes were associated with a much stronger, negatively tilted synoptic storm system, with the composite tornado in the 300-hPa jet exit region and downstream of increasing values of absolute vorticity. Additional thermodynamic and kinematic parameters pertaining to low-level moisture and environmental winds appeared to be important in distinguishing boundary and warm sector significant tornadoes from nontornadic supercell events. Statistical comparisons between boundary and warm sector significant tornado events showed significant differences in the climatology of their length, width, and date and time of occurrence.
