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- Author or Editor: David O. Blanchard x
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Abstract
An unusual severe weather event with supercell thunderstorms developed across portions of northern Arizona in the midst of the warm-season North American monsoon—a regime characteristically dominated by a subtropical upper-level high over the southwestern United States. The approach of a midlatitude, cold-core, upper-level low brought an environment of enhanced shear and increased instability supportive of supercells. This atypical system is described and how a correct interpretation of the winds and hodograph would allow a forecaster to maintain situational awareness is discussed.
Abstract
An unusual severe weather event with supercell thunderstorms developed across portions of northern Arizona in the midst of the warm-season North American monsoon—a regime characteristically dominated by a subtropical upper-level high over the southwestern United States. The approach of a midlatitude, cold-core, upper-level low brought an environment of enhanced shear and increased instability supportive of supercells. This atypical system is described and how a correct interpretation of the winds and hodograph would allow a forecaster to maintain situational awareness is discussed.
Abstract
The case presented here is submitted as an example of a previously undocumented type of interaction between a supercell thunderstorm and a frontal boundary. During the afternoon of 8 June 1995, a supercell thunderstorm formed near a quasi-stationary frontal boundary and then moved northeast across Beaver County in the Oklahoma Panhandle. Its motion took it away from the boundary and deeper into the cool air. As the storm matured and strengthened, a portion of the boundary to the south of the supercell moved northward and briefly became entrained in the low-level circulation of the storm. This northward advance of the boundary was subsequently followed by a southward motion back to near its original location. High-density spatial and temporal observations from the Oklahoma Mesonet and the Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX) Mobile Mesonetwork are presented to document the northward advance of the boundary into the supercell circulation.
Abstract
The case presented here is submitted as an example of a previously undocumented type of interaction between a supercell thunderstorm and a frontal boundary. During the afternoon of 8 June 1995, a supercell thunderstorm formed near a quasi-stationary frontal boundary and then moved northeast across Beaver County in the Oklahoma Panhandle. Its motion took it away from the boundary and deeper into the cool air. As the storm matured and strengthened, a portion of the boundary to the south of the supercell moved northward and briefly became entrained in the low-level circulation of the storm. This northward advance of the boundary was subsequently followed by a southward motion back to near its original location. High-density spatial and temporal observations from the Oklahoma Mesonet and the Verification of the Origins of Rotation in Tornadoes Experiment (VORTEX) Mobile Mesonetwork are presented to document the northward advance of the boundary into the supercell circulation.
Abstract
Comparisons of convective available potential energy (CAPE) with standard instability indices for evaluating the convective potential of the atmosphere such as the lifted index (LI) reveal only moderate correlations. This is because the LI is a measure of single-level buoyancy while CAPE is a measure of both integration depth and the buoyancy. Normalizing the CAPE values by the depth over which the integration takes place provides an index (NCAPE) that is independent of the depth and is a convenient measure of the mean parcel buoyancy. This normalization effectively distinguishes between environments with similar CAPE but exhibiting different buoyancy and integration depth. Also, because the vertical distribution of CAPE can have an important effect on convective updraft strength, it is advantageous to vertically partition CAPE and NCAPE into multiple layers. NCAPE may provide a more useful indicator of buoyancy in environments in which the depth of free convection is shallow and total CAPE is small. It is suggested that NCAPE computations be used in combination with CAPE for evaluation of convective potential.
Abstract
Comparisons of convective available potential energy (CAPE) with standard instability indices for evaluating the convective potential of the atmosphere such as the lifted index (LI) reveal only moderate correlations. This is because the LI is a measure of single-level buoyancy while CAPE is a measure of both integration depth and the buoyancy. Normalizing the CAPE values by the depth over which the integration takes place provides an index (NCAPE) that is independent of the depth and is a convenient measure of the mean parcel buoyancy. This normalization effectively distinguishes between environments with similar CAPE but exhibiting different buoyancy and integration depth. Also, because the vertical distribution of CAPE can have an important effect on convective updraft strength, it is advantageous to vertically partition CAPE and NCAPE into multiple layers. NCAPE may provide a more useful indicator of buoyancy in environments in which the depth of free convection is shallow and total CAPE is small. It is suggested that NCAPE computations be used in combination with CAPE for evaluation of convective potential.
Mesoscale convective systems observed in the southern High Plains during the Oklahoma-Kansas Preliminary Regional Experiment for STORM-central (PRE-STORM) field program were analyzed using radar and rawinsonde data. Although radar data indicate that no two systems are identical, basic recurring mesoscale structures are evident. Based on these recurrent features, the systems have been classified into three types of mesoscale convective patterns: linear mesoscale systems, occluding mesoscale systems, and chaotic mesoscale systems. Examples of all three types are discussed. High-density rawinsonde data collected in the regions ahead of the mesoscale systems have been averaged to produce composite soundings; the composites exhibit differences in both thermodynamic and wind structure between types.
Mesoscale convective systems observed in the southern High Plains during the Oklahoma-Kansas Preliminary Regional Experiment for STORM-central (PRE-STORM) field program were analyzed using radar and rawinsonde data. Although radar data indicate that no two systems are identical, basic recurring mesoscale structures are evident. Based on these recurrent features, the systems have been classified into three types of mesoscale convective patterns: linear mesoscale systems, occluding mesoscale systems, and chaotic mesoscale systems. Examples of all three types are discussed. High-density rawinsonde data collected in the regions ahead of the mesoscale systems have been averaged to produce composite soundings; the composites exhibit differences in both thermodynamic and wind structure between types.
Abstract
Damage surveys in the aftermath of tornadoes occurring in the forested regions of the Mogollon Rim in northern Arizona have been assessed using the enhanced Fujita scale (EF scale) damage indicator (DI) and degree of damage (DOD) tables. These surveys often revealed different DODs within close proximity as well as different spatial patterns and areal extent of tree damage exhibiting the same DOD, making the determination of wind speed and EF-scale ratings challenging. A localized tornado outbreak occurred across northern Arizona on 6 October 2010, producing at least 11 tornadoes and substantial areas of forest damage. Remarkably, one of these tornadoes passed over a three-dimensional sonic anemometer. Wind data from this sensor are compared with tree damage in the adjacent forest to assess the performance of the EF-scale metrics for damage to trees.
Abstract
Damage surveys in the aftermath of tornadoes occurring in the forested regions of the Mogollon Rim in northern Arizona have been assessed using the enhanced Fujita scale (EF scale) damage indicator (DI) and degree of damage (DOD) tables. These surveys often revealed different DODs within close proximity as well as different spatial patterns and areal extent of tree damage exhibiting the same DOD, making the determination of wind speed and EF-scale ratings challenging. A localized tornado outbreak occurred across northern Arizona on 6 October 2010, producing at least 11 tornadoes and substantial areas of forest damage. Remarkably, one of these tornadoes passed over a three-dimensional sonic anemometer. Wind data from this sensor are compared with tree damage in the adjacent forest to assess the performance of the EF-scale metrics for damage to trees.
Abstract
This study of the Oklahoma–Kansas area on 10 May 1985 undertakes to explain why severe convection developed in only a small portion of northwestern Kansas despite large potential instability for surface air over the entire region and despite the approach of a mobile upper-level trough from the southwest. Special soundings from the O–K PRF-STORM program showed that a persistent thermodynamic lid above the warm moist surface boundary layer separated this layer from the middle and upper troposphere in which the instability could be realized and was almost completely effective in suppressing deep convection.
Only one of the soundings with these characteristics showed temporary removal of this lid, and the only convective storm developed near the place and time of this removal. This coincidence points to removal as the likely, although not certain, cause. Isentropic trajectories showed that adiabatic lifting was the cause, and that this lift was part of a series of mesoscale waves with wavelengths of about 200 km, vertical extent from 1 to 5 km above the ground, and crests approximately parallel to the wind shear in this layer. The shear was highly ageostrophic, representing a strong transverse circulation in the exit region of a jet streak. Thus, the jet dynamics were responsible only indirectly for the convective outbreak by providing a favorable environmental shear for the directly responsible mesoscale disturbance.
A series of prominent mesoscale oscillations of surface dewpoint along the northwestern boundary of the moist surface layer began coincidentally with the convective development and is considered to have been caused by it.
Abstract
This study of the Oklahoma–Kansas area on 10 May 1985 undertakes to explain why severe convection developed in only a small portion of northwestern Kansas despite large potential instability for surface air over the entire region and despite the approach of a mobile upper-level trough from the southwest. Special soundings from the O–K PRF-STORM program showed that a persistent thermodynamic lid above the warm moist surface boundary layer separated this layer from the middle and upper troposphere in which the instability could be realized and was almost completely effective in suppressing deep convection.
Only one of the soundings with these characteristics showed temporary removal of this lid, and the only convective storm developed near the place and time of this removal. This coincidence points to removal as the likely, although not certain, cause. Isentropic trajectories showed that adiabatic lifting was the cause, and that this lift was part of a series of mesoscale waves with wavelengths of about 200 km, vertical extent from 1 to 5 km above the ground, and crests approximately parallel to the wind shear in this layer. The shear was highly ageostrophic, representing a strong transverse circulation in the exit region of a jet streak. Thus, the jet dynamics were responsible only indirectly for the convective outbreak by providing a favorable environmental shear for the directly responsible mesoscale disturbance.
A series of prominent mesoscale oscillations of surface dewpoint along the northwestern boundary of the moist surface layer began coincidentally with the convective development and is considered to have been caused by it.
Abstract
An unusual low-precipitation cumulonimbus that developed in northeastern Colorado is photographically documented in some detail. The storm produced at least 12 funnels, mostly at midlevels on the north side of the main updraft. The base of the cloud consisted of a lenticular “bell” that rotated cyclonically, while a couplet of counterrotating storm-scale eddies prevailed aloft. The funnels originated in a region of enhanced shear between easterly low-level flow on the north side of the bell and westerly flow aloft on the north side of a midlevel anticyclonic eddy.
Abstract
An unusual low-precipitation cumulonimbus that developed in northeastern Colorado is photographically documented in some detail. The storm produced at least 12 funnels, mostly at midlevels on the north side of the main updraft. The base of the cloud consisted of a lenticular “bell” that rotated cyclonically, while a couplet of counterrotating storm-scale eddies prevailed aloft. The funnels originated in a region of enhanced shear between easterly low-level flow on the north side of the bell and westerly flow aloft on the north side of a midlevel anticyclonic eddy.
Abstract
All of the 0000 UTC soundings from the United States made during the year 1992 that have nonzero convective available potential energy (CAPE) are examined. Soundings are classified as being associated with nonsupercell thunderstorms, supercells without significant tornadoes, and supercells with significant tornadoes. This classification is made by attempting to pair, based on the low-level sounding winds, an upstream sounding with each occurrence of a significant tornado, large hail, and/or 10 or more cloud-to-ground lightning flashes. Severe weather wind parameters (mean shear, 0–6-km shear, storm-relative helicity, and storm-relative anvil-level flow) and CAPE parameters (total CAPE and CAPE in the lowest 3000 m with buoyancy) are shown to discriminate weakly between the environments of the three classified types of storms. Combined parameters (energy–helicity index and vorticity generation parameter) discriminate strongly between the environments. The height of the lifting condensation level also appears to be generally lower for supercells with significant tornadoes than those without. The causes for the very large false alarm rates in the tornadic/nontornadic supercell forecast, even with the best discriminators, are discussed.
Abstract
All of the 0000 UTC soundings from the United States made during the year 1992 that have nonzero convective available potential energy (CAPE) are examined. Soundings are classified as being associated with nonsupercell thunderstorms, supercells without significant tornadoes, and supercells with significant tornadoes. This classification is made by attempting to pair, based on the low-level sounding winds, an upstream sounding with each occurrence of a significant tornado, large hail, and/or 10 or more cloud-to-ground lightning flashes. Severe weather wind parameters (mean shear, 0–6-km shear, storm-relative helicity, and storm-relative anvil-level flow) and CAPE parameters (total CAPE and CAPE in the lowest 3000 m with buoyancy) are shown to discriminate weakly between the environments of the three classified types of storms. Combined parameters (energy–helicity index and vorticity generation parameter) discriminate strongly between the environments. The height of the lifting condensation level also appears to be generally lower for supercells with significant tornadoes than those without. The causes for the very large false alarm rates in the tornadic/nontornadic supercell forecast, even with the best discriminators, are discussed.
A brief overview of the 13 June 1984 Denver hailstorm is presented. This storm produced very large hail in a few locations and copious amounts of small hail over a large area. Documentation of the storm includes data from a surface mesonetwork, cooperative observers and storm spotters, dual Doppler radar, profiler winds, radiosonde, lightning detectors, and photographs of smoke tracers. Integration of these data sets provides an interesting and informative look at this destructive storm.
A brief overview of the 13 June 1984 Denver hailstorm is presented. This storm produced very large hail in a few locations and copious amounts of small hail over a large area. Documentation of the storm includes data from a surface mesonetwork, cooperative observers and storm spotters, dual Doppler radar, profiler winds, radiosonde, lightning detectors, and photographs of smoke tracers. Integration of these data sets provides an interesting and informative look at this destructive storm.
