Chase target for August 19
Chase target:
Osceola, IA (40 mi S of Des Moines).
Timing:
Storm initiation 6 PM CDT.
Storm characteristics:
Supercell storms early in evolution capable of 2†hail.
Discussion:
This morning, a number of disturbances were rotating around a positively-tilted trough in the WRN CONUS. Lead S/WV, which was lifting into WRN MN, has fired an MCS in MN. Upstream S/WV was entering the WRN Dakotas and will be associated with today’s WX in the upper Midwest. A SFC CF/trough was located from DTL to a low pressure centered between FSD and YKN, and then to JYR and to a second low south of HLC. A convectively-enhanced WF extended EWRD from the low in KS along a TOP/SPI line, while an outflow-enhanced boundary extended EWRD from the NRN low along an ULM to EAU line in SRN MN. Throughout IA, moisture depth was very shallow as a result of yesterday’s convection there, and moisture return into much of the state N of I-80 will be minimal. Basically, extensive convection over NRN MO overnight has produced a cold pool and driven the front further S then earlier guidance has forecasted. Believe the 12Z NAM has a good handle overall, although it too may be bringing the WF too far north through the early evening.
Today, the WF in MO will lift NWRD towards I-80 by early evening. Convection will fire along it by early evening as a 25kt LLJ focuses into the area. Extreme instability with MLCAPE’s to 3500J/kg, coupled with deep layer sheer to 45 kts will support a large hail threat early in storm evolution, which will then transition into a severe wind threat as storms organize into a large complex and establish a cold pool. Minimal 0-1km and 0-3km shear and helicities should minimize overall tornado threat although a brief tornado may result from storm-scale boundary interactions.
- bill
