SPC AC 201249

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0749 AM CDT Mon Sep 20 2021

   Valid 201300Z - 211200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS THE UPPER
   MIDWEST...

   ...SUMMARY...
   Isolated severe thunderstorms are expected across parts of the Upper
   Mississippi Valley and Upper Great Lakes southward to the Ozark
   Plateau this afternoon and evening.

   ...Upper Midwest/Upper Great Lakes to Ozarks...
   DPVA/height falls will influence the Upper Midwest via the continued
   eastward progression of a prominent late-summer trough that is
   centered over the northern High Plains and Canadian Prairies early
   today. Attendant to the upper trough, a strengthening of
   cyclonically curved westerlies aloft will occur especially this
   afternoon through tonight, focused near/behind an
   east/southeastward-advancing cold front.

   Scattered showers and thunderstorms are prevalent this morning near
   the front and up to about 100-150 miles ahead (east) of it, and the
   potential lingering influence of this activity may effectively
   spatially influence downstream convective intensification this
   afternoon via outflow/differential heating. Particularly given the
   pre-frontal cloud debris and residual precipitation, relatively
   modest destabilization (perhaps up to 1000-1250 J/kg MLCAPE) is
   expected this afternoon across southeast Minnesota and much of
   Wisconsin, with moderately stronger destabilization (up to 2000 J/kg
   MLCAPE) expected across eastern Iowa/northern Missouri/far western
   Illinois.

   As the pre-frontal boundary layer warms, convective intensification
   appears most likely to initially occur across central/northeast Iowa
   and northwest Missouri northeastward into southeast Minnesota and
   far western Wisconsin by around mid-afternoon (circa 20-21Z). While
   the strongest winds aloft/vertical shear will tend to lag the
   east/southeast-moving front, stronger deep-layer winds will extend
   somewhat (eastward) into the warm sector across Iowa into eastern
   Minnesota and Wisconsin, where some low-level southeasterly backing
   of surface winds may also occur given relative proximity to the
   frontal wave/surface low.

   Some initial supercells are possible especially within the
   aforementioned corridor, while the influence of the cold front and
   front-parallel vertical shear otherwise leads to a prevalence of
   linear bands/clusters including some bowing segments. Some initial
   severe hail is possible, but damaging winds should become the more
   common severe risk through the evening. Some tornado risk will also
   exist as well via a mixed storm mode, including a QLCS-related
   tornado potential. Overall storm intensity is likely to wane into
   the late evening and overnight hours as boundary-layer convective
   inhibition nocturnally increases.

   ..Guyer/Smith.. 09/20/2021

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