05/05/05 FCST: Upper-Midwest

Chase Target:
Milbank, SD

Timing:
Storm initiation 4 PM CDT, with storms tracking slowly eastward into MN at 10mph.

Storm type:
Small hail and winds to 50mph. Storms should remain under severe limits.

Discussion:
12Z 300mb analysis and WV imagery showed a low-amplitude S/WV diving ESE into WRN SD/ND. This feature was embedded in broad 45-50kt WNWRLY flow. A weak reflection of this wave was seen at 700mb, with a trough axis along a HON/BIS/MIB line. Mid-level winds were weak, with 700mb winds generally under 10kts. At the surface, SSWRLY flow was in place, with a small S/WV axis along a FFM/ILL line and broader cyclonic flow in ERN SD at 15Z. An interesting N/S-oriented boundary was moving slowly WWRD through STC/AIC, apparently outflow from elevated convection to the E of the Twin Cities. Dewpoints were generally in the mid-forties, with 50F dewpoints SW of a ONL/HON line, and another moisture ridge pushing into WRN IA along a OUN/SLB line. Capping was strong, with OAX and ABR soundings showing a CT of 23-24C.

By mid-afternoon, the surface trough will approach the target area. Areas in the S should remain capped. Ongoing elevated precipitation/virga near and N of the target area should help cool the mid-levels. Also, a large temperature gradient will be in place along and N of the SD/ND border at 700mb. Went with the 12Z NAM and RUC for this forecast, as they initialized well.

- bill
 
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