SPC AC 021726

   Day 2 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1226 PM CDT Wed Sep 02 2020

   Valid 031200Z - 041200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
   THE MID-ATLANTIC...

   ...SUMMARY...
   Scattered severe thunderstorms appear likely across parts of the
   Mid-Atlantic on Thursday. Damaging wind gusts and a few tornadoes
   should be the main threats, although isolated large hail may also
   occur.

   ...Central Appalachians into the Mid-Atlantic...
   A large-scale upper trough over the Upper Midwest and central Canada
   is forecast to move eastward across the Great Lakes, Northeast, and
   Ontario/Quebec through the period. A low-amplitude shortwave trough
   embedded within the upper trough should move from the OH
   Valley/central Appalachians to the Mid-Atlantic by Thursday
   afternoon. A belt of 40-50 kt southwesterly mid-level flow should
   accompany the shortwave trough passage. At the surface, rich
   low-level moisture characterized by upper 60s to mid 70s dewpoints
   will remain in place across much of the Mid-Atlantic to the south of
   a northward-advancing warm front.

   Confidence has increased that at least scattered storms will form
   over the higher terrain of the central Appalachians and/or Blue
   Ridge Mountains by peak afternoon heating on Thursday as large-scale
   ascent associated with the shortwave trough overspreads this area.
   Rather poor mid-level lapse rates will persist, but strong heating
   of the moist low-level airmass should still yield MLCAPE of
   1500-2500 J/kg, stronger with southward extent into VA and the
   Delmarva. The enhanced (40-50 kt) mid-level flow attendant to the
   shortwave trough will foster similar values of deep-layer shear, and
   organized storms appear likely, with both supercells and bowing line
   segments possible. This convection is expected to spread eastward
   across the Mid-Atlantic late Thursday afternoon and into the
   evening.

   A few tornadoes may occur with any initially discrete supercells, as
   a 30-40 kt southwesterly low-level jet should provide sufficient
   veering/strengthening of the low-level winds. This tornado potential
   appears to be maximized Thursday afternoon along/east of the Blue
   Ridge Mountains into the I-95 corridor of MD/DC/northern VA. At
   least scattered damaging winds also appear likely later into the
   afternoon and early Thursday evening across this region and into the
   Delmarva, as some clustering of storms should occur with eastward
   extent. Confidence in this scenario occurring has increased enough
   to include greater severe probabilities for both tornadoes and
   damaging winds. A corresponding Enhanced Risk has been introduced
   across northern VA, DC, central/eastern MD, DE, and far southern NJ
   to account for these threats.

   ...Hudson Valley into Southern New England...
   There may be some potential for strong/gusty winds to occur north of
   the more robust convection across the Mid-Atlantic Thursday
   evening/night. Low-level warm advection would be the main forcing
   mechanism. However, most 12Z guidance does not suggest strong storm
   development across the Hudson Valley into southern New England late
   in the period. Have therefore trended the Marginal Risk southward a
   bit, but still including the NYC metro and far southern New England
   in case convection develops a little farther north than currently
   forecast.

   ..Gleason.. 09/02/2020

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