Bill Schintler
EF4
Chase Target for Thursday, August 19
Chase target:
Madison, SD (33 miles northwest of Sioux Falls)
Timing and storm mode:
Storms will develop after 6 PM CDT, with supercell structures early in convective evolution. All modes of severe weather are possible before 9 PM, including a tornado or two. Later in the evening, storms will evolve into one or more large complexes and track east though southern MN.
Discussion:
By late afternoon, a shortwave trough will arrive in western MN; however, models disagree on the details of this feature. At the surface, a diffuse cold front will stall and then slowly retreat as a warm front along the SD/NE boarder. The temperature gradient across this boundary will be enhanced somewhat by differential insolation due to high clouds persisting over SD throughout the peak heating hours.
Capping should prevent surface-based convection through 00Z with a stout EML in place. Model soundings indicate a well-mixed boundary extending through 800mb, with surface dewpoints into the low 70s prevalent. 700mb temperatures in excess of 12C south of I-90 will result in a large degree of CIN until 00Z at which time low-level theta-e advection and mid-level cooling in association with the aforementioned trough overspreads the region. High-based convection should initiate first during the late afternoon hours along a pre-frontal trough from northeastern CO and west of North Platte, NE northeast to about Mitchell, SD. These storms should be primarily multicell and linear in nature, and should generally remain below severe limits other then some hail and isolated wind gusts. Later, additional storms will develop further north and east along the warm front, eventually developing eastward into MN. This area will be the primary focus for vigorous and rotating updrafts.
Once initiation occurs over southeastern SD into southwestern MN, steep lapse rates will couple with strong instability and shear parameters to favor supercells with initial storm development. Deep layer shear (SFC-6km) should locally increase to 50 kts as the shortwave overspreads the area. Backing flow with enhanced hodograph curvatures should exist along and north of the warm front in southeastern SD, especially after 00Z when the southwesterly LLJ increases to 40 kts. LCLs should slowly decrease after 00Z as the boundary layer begins to cool. A window of opportunity will exist for tornadoes as the LCLs decrease and directional shear increases, and before the boundary layer decouples. The SPC SREF ensemble indicates a significant tornado parameter in excess of 4, suggesting the possibility of strong tornadoes with storms that remain discrete.
During the evening hours, storms will expand in coverage along and north of the warm front as the LLJ noses into the area. This developing MCS will track east across southern NM overnight, with heavy rainfall the primary hazard with precipital waters of nearly two inches along with training and backbuilding of storms into the LLJ.
Bill Schintler
KD0DJG
www.twistertoursusa.com
10:28 PM CDT, 08/18/10
Chase target:
Madison, SD (33 miles northwest of Sioux Falls)
Timing and storm mode:
Storms will develop after 6 PM CDT, with supercell structures early in convective evolution. All modes of severe weather are possible before 9 PM, including a tornado or two. Later in the evening, storms will evolve into one or more large complexes and track east though southern MN.
Discussion:
By late afternoon, a shortwave trough will arrive in western MN; however, models disagree on the details of this feature. At the surface, a diffuse cold front will stall and then slowly retreat as a warm front along the SD/NE boarder. The temperature gradient across this boundary will be enhanced somewhat by differential insolation due to high clouds persisting over SD throughout the peak heating hours.
Capping should prevent surface-based convection through 00Z with a stout EML in place. Model soundings indicate a well-mixed boundary extending through 800mb, with surface dewpoints into the low 70s prevalent. 700mb temperatures in excess of 12C south of I-90 will result in a large degree of CIN until 00Z at which time low-level theta-e advection and mid-level cooling in association with the aforementioned trough overspreads the region. High-based convection should initiate first during the late afternoon hours along a pre-frontal trough from northeastern CO and west of North Platte, NE northeast to about Mitchell, SD. These storms should be primarily multicell and linear in nature, and should generally remain below severe limits other then some hail and isolated wind gusts. Later, additional storms will develop further north and east along the warm front, eventually developing eastward into MN. This area will be the primary focus for vigorous and rotating updrafts.
Once initiation occurs over southeastern SD into southwestern MN, steep lapse rates will couple with strong instability and shear parameters to favor supercells with initial storm development. Deep layer shear (SFC-6km) should locally increase to 50 kts as the shortwave overspreads the area. Backing flow with enhanced hodograph curvatures should exist along and north of the warm front in southeastern SD, especially after 00Z when the southwesterly LLJ increases to 40 kts. LCLs should slowly decrease after 00Z as the boundary layer begins to cool. A window of opportunity will exist for tornadoes as the LCLs decrease and directional shear increases, and before the boundary layer decouples. The SPC SREF ensemble indicates a significant tornado parameter in excess of 4, suggesting the possibility of strong tornadoes with storms that remain discrete.
During the evening hours, storms will expand in coverage along and north of the warm front as the LLJ noses into the area. This developing MCS will track east across southern NM overnight, with heavy rainfall the primary hazard with precipital waters of nearly two inches along with training and backbuilding of storms into the LLJ.
Bill Schintler
KD0DJG
www.twistertoursusa.com
10:28 PM CDT, 08/18/10
