SPC AC 280056

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0756 PM CDT Tue Apr 27 2021

   Valid 280100Z - 281200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
   THE SOUTHERN AND CENTRAL PLAINS...

   ...SUMMARY...
   Severe thunderstorms will affect parts of the southern and central
   Plains this evening and tonight. Marginally severe storms may also
   affect parts of the mid to upper Mississippi Valley and western
   Great Lakes region.

   ...Central Plains...
   The latest radar imagery shows a cluster of strong to severe
   thunderstorms in northeastern Colorado. The storms are located to
   the north of a 996 mb surface low along an inverted trough where
   low-level convergence is maximized. The storms are also located just
   to the west of a pocket of instability with the RAP analyzing MLCAPE
   near 1000 J/kg. The storm complex should remain intact as low-level
   flow continues to increase this evening. This combined with strong
   deep-layer shear, evident on the North Platte WSR-88D VWP, will
   support a severe threat this evening. Large hail and wind damage
   will be possible with the stronger cells...see MCD 440.

   ...Southern Plains...
   The latest radar imagery shows a line of strong thunderstorms
   extending from northeast of Midland, Texas to near Lawton, Oklahoma.
   Ahead of the line, the RAP is analyzing a corridor moderate
   instability from the low Rolling Plains northeastward into
   south-central Oklahoma. WSR-88D VWPs along the corridor have 0-6 km
   shear in the 55 to 70 kt range, which will support supercell
   development this evening. Large hail will be likely with supercells,
   and hailstones greater than 2 inches in diameter will be possible
   with the strongest of updrafts. As the line moves east-northeastward
   into stronger instability, wind damage will also be possible along
   the leading edge and with the downdrafts with supercells. A tornado
   or two will also be possible with the stronger supercells. A severe
   threat may also affect parts of central Oklahoma and southeast
   Kansas as the convection moves northeastward and other storms
   develop this evening due to the strengthening low-level jet.

   Further to the south across southwest Texas, surface analysis shows
   east-southeasterly upslope flow. Strong instability is analyzed by
   the RAP from the lower Rio Grande Valley northeastward to near Del
   Rio, Texas. Convective initiation is expected to take place near of
   Del Rio around midnight as the low-level jet strengthens across the
   southern Plains. RAP forecast soundings in southwest Texas tonight
   have 50 to 60 kt of 0-6 km shear along with 500-700 mb lapse rates
   approaching 7.5 C/km. This environment will support a large hail
   threat with any supercell that can develop across southwest Texas
   tonight. An isolated tornado and/or wind damage threat will also be
   possible.

   ...Mid to Upper Mississippi Valley/Western Great Lakes...
   The latest water vapor imagery shows an anticyclonic flow pattern
   from the lower Missouri Valley into the western Great Lakes. At the
   surface, a cold front is located from central Iowa into southern
   Wisconsin and southern Lower Michigan. An axis of surface-based
   instability is analyzed by the RAP just to the south of the front,
   with elevated instability located to the north of the front across
   northern Iowa and central Wisconsin. As low-level flow strengthens
   tonight across the Upper Midwest, scattered thunderstorms are
   forecast to initiate and move eastward across the region. Most of
   the convection is expected to be elevated, remaining to the north of
   the front, where a hail threat will be possible after midnight.

   ..Broyles.. 04/28/2021

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