SPC AC 270602

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0102 AM CDT Tue Apr 27 2021

   Valid 271200Z - 281200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS OF
   THE SOUTHERN PLAINS...AND INTO THE CENTRAL HIGH PLAINS REGION...

   ...SUMMARY...
   Isolated to scattered strong/severe thunderstorms are expected this
   afternoon and tonight across portions of the southern and central
   Plains, and across the Upper Midwest to the Upper Great Lakes.

   ...Synopsis...
   An upper low, and broad/surrounding cyclonic flow, will make slow
   eastward progress across the West today, while downstream ridging
   over the East gradually deamplifies, as several short-wave troughs
   ejecting ahead of the main low traverse the Mississippi and Ohio
   Valleys and Great Lakes region through the period.  One of these
   waves is expected to cross the Upper Mississippi Valley and Upper
   Great Lakes through the day, accompanied by showers and scattered
   thunderstorms.  A second will shift eastward/northeastward across
   Texas through the first half of the period, and later will cross the
   Mississippi Valley into the Upper Great Lakes/Ohio Valley/Mid South.
   Showers and isolated to scattered thunderstorms will likewise
   accompany the progression of this trough.

   At the surface, a frontal zone will remain roughly in place from
   southern portions of the Upper Great Lakes west-southwestward to the
   central High Plains, where it will intersect a lee trough extending
   southward across the southern High Plains region.  During the
   afternoon, a dryline will mix eastward off the southern High Plains
   into western Oklahoma/western North Texas.

   ...Southern Plains...
   Widespread clouds and weak/high-based convection will likely spread
   across much of the southern Plains through the day today, as a
   short-wave trough crosses the region.  This will likely limit
   heating to some degree which -- in conjunction with mid-level
   moisture/weak lapse rates, will temper destabilization across the
   region.

   Still, as a dryline mixes eastward into western Oklahoma and across
   the South Plains/western North Texas, isolated to scattered
   afternoon storms will likely develop.  Other storms will likely
   develop over the higher terrain southwest of the Rio Grande,
   spreading into the Edwards Plateau and parts of southern Texas
   through the afternoon.  Large hail will likely be the main risk with
   these storms.

   During the evening, as the dryline retreats, continued convective
   development is expected across western North Texas and portions of
   the Texas Panhandle, and into Oklahoma.  The convective development
   will likely persist through the evening and into the overnight
   hours, as the upper trough continues to gradually advance.  An
   increase in low-level southerly flow could permit some uptick in
   tornado potential during the evening across this area, and locally
   stronger wind gusts will also be possible.  However, large hail will
   remain the primary severe risk through the period.

   ...Central High Plains...
   Afternoon heating in the vicinity of the surface boundary progged to
   lie west-to-east across northern Kansas and into eastern Colorado,
   combined with modest low-level moisture, should permit weak
   destabilization to occur across this area.  Just to the cool side of
   the boundary, northeasterly/upslope flow may permit isolated storm
   development to occur from the Palmer Divide northward across
   northeastern Colorado, and eastward across northwest Kansas and
   adjacent southwestern Nebraska near the front.  With moderately
   strong mid-level southwesterlies atop level easterly/northeasterly
   flow on the cool side of the boundary, a couple of stronger/rotating
   storms may evolve through late afternoon/early evening, along with
   attendant severe risk.  Hail would be the most likely threat, though
   gusty/damaging winds or even a brief tornado could occur.  Risk
   should wane into the evening, with the onset of diurnal cooling.

   ...Mid Missouri Valley northeastward to the Upper Great Lakes...
   Isolated thunderstorms may be ongoing across portions of the
   southern Minnesota/northern Iowa vicinity early, which will spread
   east-northeastward across Wisconsin and upper Michigan with time.  A
   stronger storm or two could be accompanied by marginal hail.

   Meanwhile, as a short-wave trough shifts northeastward across
   Missouri and later the mid Mississippi Valley, additional/isolated
   storm development is expected.  Aided by steep lapse rates aloft,
   and ample shear, a few stronger storms may evolve within a broad
   area of isolated convection, spreading out of eastern Kansas and
   across Missouri through the evening.  Overnight, the convection will
   likely spread into/across Illinois/southern Wisconsin, and
   eventually into southern Lower Michigan by the end of the period. 
   Marginally severe hail would be the primary severe risk.

   ..Goss/Moore.. 04/27/2021

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