SPC AC 211944

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0244 PM CDT Tue Jul 21 2020

   Valid 212000Z - 221200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF THE
   CENTRAL HIGH PLAINS...AND PORTIONS OF THE MIDDLE AND UPPER
   MISSISSIPPI VALLEY...

   ...SUMMARY...
   Severe thunderstorms are possible today across parts of the central
   High Plains, and Middle and Upper Mississippi Valley regions.

   ...20Z Update...

   ...CO/WY...
   Overall forecast scenario outlined in the previous outlook remains
   valid. MCD 1303 was recently issued discussing the short-term
   forecast and Severe Thunderstorm Watch 392 was recently issued to
   coverage the severe threat.

   ...Mid MS Valley...
   Strong to isolated severe thunderstorms are possible through the
   afternoon and early evening from east-central MO into central IL.
   Locally damaging wind gusts will be the main hazard with this
   activity. More short-term forecast details are available in MCD
   1302.

   ...MN/WI...
   Scattered thunderstorm development is possible in the
   coming hours. While storm coverage is uncertain, any storms that
   develop may pose a threat for severe wind, hail, and perhaps a
   tornado. More short-term forecast details are available in MCD 1301.

   ...Central Appalachians/Mid-Atlantic...
   Cluster of thunderstorms continues to move eastward/southeastward
   along outflow across southwest/south-central PA. Occasionally strong
   updrafts have been noted within this cluster, and the expectation is
   for the cluster to continue downstream through western MD and the WV
   Panhandle.

   Farther south, air mass destabilization and moderate instability
   will continue to foster widely scattered to scattered thunderstorm
   development. The lack of vertical shear should limit storm
   organization and longevity, but occasionally strong updrafts with
   the moist air mass could result in a damaging wind gusts associated
   with water-loaded downbursts.

   ...Middle OH Valley...
   Convective line moving through central IN recently produced a 50 kt
   gust at GUS. Organized structure of this line suggests it will
   continue east-northeastward for the next few hours. However, the
   downstream air mass is only modestly unstable and the line is
   expected to gradually weaken.

   ..Mosier.. 07/21/2020

   .PREV DISCUSSION... /ISSUED 1126 AM CDT Tue Jul 21 2020/

   ...CO/WY...
   Weak flow and upper ridging dominate the weather across the southern
   United States today, with marginally strong west-northwesterly flow
   over the central/northern High Plains.  Clear skies are noted over
   most of this area, with a moist boundary-layer over eastern WY/CO
   where dewpoints are in the low/mid 50s.  Strong heating will yield
   afternoon MLCAPE values around 1500 J/kg from east-central CO
   northward into eastern WY.  It appears likely that scattered
   thunderstorms will form this afternoon over the foothills and spread
   eastward into the adjacent plains.  Coverage of storms appears
   highest over southeast WY, with more isolated cells farther south
   into CO where a weak cap might limit storms.  Forecast soundings
   indicate steep mid-level lapse rates and sufficient vertical shear
   for large hail in the strongest cells.  A deeply-mixed boundary
   layer will also support damaging wind gusts.

   ...MO/IL/IN...
   Multiple remnant MCVs and outflow boundaries are present today over
   the mid MS Valley.  Model guidance is not handling the mesoscale
   details very well, and pockets of cloud cover/precipitation further
   reduce confidence in the location/intensity of thunderstorm clusters
   later today.  Nevertheless, a band of stronger 3-6km flow over MO is
   expected to overspread the SLGT risk area this afternoon, helping to
   organize any storms that can develop.  Locally damaging winds would
   be the main threat with this activity.

   ...MN/WI...
   A weak surface over central MN will track slowly eastward today
   toward northern WI.  Forecast soundings in the warm-sector of the
   low show favorable low-level shear profiles for rotating storms
   capable of damaging winds or a tornado or two.  However,
   considerable cloud cover and affects of morning precipitation are
   lessening confidence in overall severe threat.  Will maintain the
   SLGT risk with some adjustments in case clearing from the west
   results in greater destabilization than currently anticipated.  

   ...Mid Atlantic Region...
   Hot/humid conditions over parts of PA/MD/VA/NC will lead to
   scattered afternoon thunderstorms.  Weak flow aloft indicates that
   storms will likely be disorganized, but localized gusty/damaging
   winds are possible in the stronger downdrafts.

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