SPC AC 210552

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1252 AM CDT Tue Jul 21 2020

   Valid 211200Z - 221200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER THE COLORADO
   FRONT RANGE AND SOUTHEAST WYOMING...

   ...SUMMARY...
   Scattered severe storms are possible across parts of the central
   High Plains with hail and wind. Isolated strong to severe
   thunderstorms are possible over parts of the Upper Mississippi
   Valley, and from the mid Mississippi Valley to the Mid-Atlantic,
   with a few damaging wind gusts.

   ...Synopsis...
   A low-amplitude shortwave trough will move east across the upper MS
   Valley during the day, and across the upper Great Lakes overnight. A
   surface low and cold front will move from MN toward Lake MI, though
   pressures will not be very low (above 1010 mb). Ahead of the low,
   modest low-level moisture return will develop as southerly 850 mb
   winds increase to 20 kt, which will aid destabilization and strong
   to severe storm potential during the day over MN and WI.

   Meanwhile, a moist and unstable air mass will exist from the central
   Plains to the Mid Atlantic, beneath generally weak winds aloft, with
   various clusters of storms likely during the day. 

   One potentially focused area will be along the CO Front Range, where
   easterly surface winds will bring 50s F dewpoints westward,
   supporting several severe storms. Another concentrated area of
   storms is likely from eastern KS across MO and into IL, as early-day
   storms and outflows evolve eastward. Finally, scattered storms are
   possible over the Mid Atlantic, with localized damaging wind gusts.

   ...Southeast WY southward across the Front Range...
   Moderate instability is forecast to develop along the Front Range by
   late afternoon, due to strong heating and ample low-level moisture
   with easterly surface winds. MUCAPE up to 2000 J/kg is possible,
   with steep lapse rates. Storms are likely to form over the higher
   terrain, then move slowly southeast across southeast WY and the
   Front Range of CO. Large hail to golf ball size is possible, with a
   report or two approaching 2.00". A few damaging gusts may also occur
   as outflows increase through evening, and storms move into the
   Plains. Low-level winds will be weak, with minimal tornado threat
   expected.

   ...MN and WI...
   Moisture will return slowly north ahead of a cold front, with storms
   forming along it over MN by midday. Capping will not be an issue,
   though forecast soundings indicate only modest lapse rates. However,
   boundary-layer RH will be high, and veering winds with height
   (effective SRH over 200 m2/s2) suggest a supercell or two will be
   possible. A relatively small area of severe potential will thus
   exist from MN into WI, but questions exist regarding instability and
   moisture quality. A small Slight Risk cannot be ruled out in later
   outlooks as more observational data is incorporated into the
   forecast.

   ...Mid Atlantic...
   Heating of a moist air mass will lead to MLCAPE to 2000 J/kg over
   WV/KY/TN, with values approaching 3000 J/kg across NC and VA. Winds
   aloft will be weak, but slow-moving storms capable of damaging
   downbursts will be possible, beginning over the Appalachians after
   20Z and persisting through 00-02Z over parts of VA and NC. Storm
   motions will largely be driven by outflow with east/southeastward
   propagation. Marginal hail cannot be ruled out.

   ..Jewell/Cook.. 07/21/2020

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