SPC AC 030559

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1259 AM CDT Wed Jun 03 2020

   Valid 031200Z - 041200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS
   OF PA INTO NJ AND THE NYC METRO...

   ...SUMMARY...
   Clusters of strong to severe storms are possible today and tonight
   from the central Plains through the Midwest and Mid-Atlantic. The
   most intense storms, capable of damaging winds and hail, are
   expected from parts of Pennsylvania into New Jersey and the New York
   City metro area.

   ...Synopsis...

   A broad swath of northwesterly flow will stretch from the northern
   High Plains to the Great Lakes and Northeast/Mid-Atlantic today as
   several shortwave impulses migrate through northern-stream flow. At
   the surface, 60s F dewpoints will exist from the southern/central
   Plains eastward across much of the Midwest to the Mid-Atlantic ahead
   of a southward sagging cold front. More modest boundary-layer
   moisture will exist across the northern/central High Plains a more
   diffuse cold front associated with a surface low over Saskatchewan
   develops eastward.

   ...PA into the Mid-Atlantic...

   Storms are expected to develop in the vicinity of Lake Erie by late
   morning as a cold front develops east/southeast across the
   international border. Ahead of the front, low to mid 60s dewpoints
   will be maintained on southwesterly low level flow. Seasonally warm
   temperatures in the 70s to low 80s will support MLCAPE values from
   1500-2000 J/kg. Fast midlevel flow will be in place across the
   region with 35-50 kt winds noted in the 850-700 mb layer in most
   guidance. Deep layer flow will mainly be unidirectional, but
   increasing winds with height from the surface through 700 mb will
   result in effective shear greater than 40 kt, supporting organized
   storm structures amid moderate midlevel lapse rates (7-7.5 C/km).
   Daytime heating will result in steepening low-level lapse rates, and
   PW values near 1.75 inches will further support strong downdrafts.
   Some upscale growth is expected via outflow interactions and
   possible cold pool generation. Most CAMs guidance support one or
   more bowing segments tracking east/southeast across portions of
   northern/eastern PA into NJ and the NYC Metro area from early
   afternoon into the evening hours, and a corridor of damaging winds
   is expected. Some severe hail is also possible given the
   thermodynamic and kinematic parameter space, but may be limited by
   storm mode.

   ...Midwest...

   Thunderstorms are expected to develop in the vicinity of the surface
   cold front from MO eastward through OH. Strong heating and
   mid-to-upper 60s F dewpoints beneath modestly steep midlevel lapse
   rates will support 2000-3000 J/kg MLCAPE. Deep-layer flow will be
   weaker across the Midwest as strong shortwave impulses are expected
   to remain to the west across parts of the Plains and to the east
   across the Mid-Atlantic, and effective shear will be marginal,
   around 20-30 kt. This will result in briefly intense cells capable
   of strong downburst winds and hail. 

   ...Portions of the Northern/Central Plains...

   A shortwave impulse is forecast to eject across WY and shift
   east/southeast over NE/KS toward northwest MO through the period.
   Initial high-based convection over the northern High Plains will
   pose a threat for strong downburst winds via a dry, deeply-mixed
   sub-cloud layer. As the impulse shifts east toward the central
   Plains, convection will encounter increasing moisture and a
   strengthening low level jet during the evening/overnight. Initial
   cells will be capable of large hail with midlevel lapse rates in
   excess of 8 C/km and 40+ kt effective shear across the region. By
   evening, upscale growth is expected as the low level jet increases,
   and a bowing MCS may track eastward across parts of NE and northern
   KS, posing a damaging wind threat.

   ..Leitman/Lyons.. 06/03/2020

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