SPC AC 200526

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1226 AM CDT Fri Sep 20 2024

   Valid 201200Z - 211200Z

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM SOUTHWEST
   LOWER MICHIGAN INTO PARTS OF INDIANA...ILLINOIS...AND SOUTHEAST
   MISSOURI...AND OVER PARTS OF THE CENTRAL PLAINS...

   ...SUMMARY...
   Isolated strong to severe thunderstorms are possible during the late
   afternoon over parts of Illinois, Indiana, and southwest Lower
   Michigan. Isolated strong to severe storms are also possible across
   parts of the south-central Plains.

   ...Synopsis...
   A leading shortwave trough will weaken as it moves across the upper
   Great Lakes today, while a secondary and stronger system moves into
   MT and the northern Plains late. An upper high will hold over the
   southern Plains, which will aid a strengthening midlevel height
   gradient over the central and northern Plains as the wave approaches
   from the northwest. Meanwhile, a deep upper low will affect southern
   CA and move into AZ through Saturday morning. 

   At the surface, a trough will develop over the northern High Plains
   late in the day, with high pressure over the eastern states. A
   weak/residual boundary will extend roughly from Lake MI into central
   IL, MO, and toward I-70 in KS, with a moist air mass to the south.
   Mid to upper 60s F dewpoints will return north late this evening and
   overnight as southerly winds around 850 mb increase to over 40 kt.
   Otherwise, a cold front associated with the MT disturbance will push
   across the Dakotas and into central NE overnight and through 12Z
   Saturday.

   ...Western Lower MI into IN and IL...
   Daytime heating of a moist low-level air mass will yield a
   moderately unstable and uncapped air mass near the weak surface
   boundary. Scattered thunderstorms are anticipated from western Lower
   MI southwestward into southern MO as a result. However, forecast
   soundings indicate poor midlevel lapse rates along with substantial
   midlevel dry air, which may diminish severity somewhat. That said,
   locally strong gusts will be possible, and perhaps small to marginal
   hail.

   ...Central Plains...
   Strong heating and southwest surface winds will lead to a plume of
   steep low-level lapse rates from the TX Panhandle toward the KS/OK
   border this afternoon. Meanwhile, higher dewpoints will remain near
   a stalled front over KS, which will support high-based storms
   initially over northeast NM across the Panhandles and into southwest
   KS. Given the very deep mixed layers, a few strong to severe gusts
   may occur. With time, outflow may yield downstream development with
   more robust cores and perhaps marginal hail across south-central KS.

   During the late evening, the returning boundary and associated
   theta-e advection may support a rash of storms from near the NE/KS
   border, with veering 850 mb winds over time redirecting said
   advection and lift eastward across the KC Metro. Strong wind gusts
   appear to be the main risk with such activity.

   ..Jewell/Thornton.. 09/20/2024

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