8/19/2005: FCST: Central Plains

Just wanted to start a thread for the central plains for today (it's late, and I don't have much time to go into too many details). The latest 0Z NAM is painting a pretty good supercell threat across IA. A strong shortwave, combined with good moisture pooling at the SFC (and CAPEs AOA 4000J/KG), in addition to deep-layer shear in excess of >45kts would lead me to believe a pretty decent risk of severe thunderstorms is probable. In addition, given the moisture pooling, LCLs will be quite low, so it would be easy for a storm to produce a tornado... However, not overly sure of the tornado threat, since we're somewhat lacking good low-level flow. Although, I'd still see a pretty good bet for supercells with VERY large hail possible, and maybe a couple tornadoes too.
 
00Z NAM showing 300-400 m2/s2 0-3 km SRH across southwestern MN/northwestern IA.... that would be along the warm front and ahead of the sfc low. With the WSW 500-mb winds....most storms would be moving in an ENE to E fashion, but if we can get any supercell to make a right turn and move along the warm front in northwestern IA, SRH may increase ahead of it on a storm-scale enough that, combined with the very strong instability and rich low-level moisture/low LCL heights, a significant tornado may be possible. Otherwise, like Nick said, with the strong instability and decent deep-layer shear profiles in place, giant hail (3+ inches) may be a big problem for much of Iowa this afternoon.
 
I would have to agree with Fred, if anything develops in NW Iowa, I wouldn't be surprised at more severe conditions. Dewpoints across west-central Iowa will aid in development as the low pushes onward. CAPE values are into the 3000 J/KG range. There doesn't seem much to limit widespread development which could cause things to go linear later on. There is a little band of higher CIN that hooks out from Sioux Falls over NW Iowa. I think I would get out ahead and if things spark closer to SD, within an hour I should be in prime location.

Preliminary virtual target: Storm Lake
Actual target: Home Run Porch at Metrodome in Minneapolis - Go Twins!
 
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 2â€￾ 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
 
Nick whats going on with all these storm in NE this rain building up , i think it will change our chances somewhat" or is it gonna clear out... ?


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