SPC AC 230553

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1253 AM CDT Sun Aug 23 2020

   Valid 231200Z - 241200Z

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS TODAY INTO
   TONIGHT ACROSS PARTS OF WESTERN SOUTH DAKOTA INTO CENTRAL AND
   SOUTHERN NORTH DAKOTA AND NORTHERN AND CENTRAL MINNESOTA....AS WELL
   AS ACROSS PARTS OF UPSTATE NEW YORK INTO VERMONT...NEW
   HAMPSHIRE...MASSACHUSETTS AND SOUTHERN MAINE...

   ...SUMMARY...
   Strong thunderstorms may impact parts of the northern Great Plains,
   as well as parts of Upstate New York into New England today,
   accompanied by at least some risk for severe weather. A risk for
   tornadoes could also develop by late tonight across far southeastern
   Louisiana coastal areas.

   ...Synopsis...
   Mid-level subtropical ridging, centered over the Four Corners
   states, appears likely to remain prominent through this period. 
   Farther east, flatter ridging over the subtropical western Atlantic
   may build westward into the south Atlantic Coast states, while a
   pair of developing tropical cyclones generally migrate around its
   southwestern periphery.  This includes Tropical Storm Laura, which
   is forecast to migrate west-northwestward across the Dominican
   Republic through the vicinity of eastern Cuba today through tonight.
    Tropical Storm Marco is forecast to intensify into a hurricane
   while migrating across the central Gulf of Mexico, before
   approaching far southeastern Louisiana coastal areas toward 12Z
   Monday.

   Within the mid-latitude westerlies, as one short wave trough
   approaches the British Columbia and Pacific Northwest coast,
   downstream ridging appears likely to build across the northern U.S.
   Rockies into the Canadian Prairies.  Another short wave trough to
   the east likely will be forced east-southeastward into an
   increasingly confluent regime across Manitoba through northwest
   Ontario, while a downstream perturbation shifts eastward across
   Quebec through the Canadian Maritimes.

   In association with this regime, steep mid-level lapse rates
   associated with warm elevated mixed-layer air will remain confined
   to the Great Plains.  There may be some suppression of this air mass
   across parts of North Dakota and Minnesota, while a cold front in
   lower-levels outpaces this cooling, reaching the Upper Midwest,
   portions of the middle Missouri Valley and Black Hills/Big Horns
   vicinity by late tonight.

   A general west-northwest through east-southeast oriented baroclinic
   zone may be maintained across Quebec into New England, but at least
   some low-level warming and moistening may take place to the lee of
   the lower Great Lakes into portions of southern New England.

   ...Northern Great Plains vicinity...
   There remains little clear signal concerning the possible convective
   evolution for this period.  However, the most substantive pre-cold
   frontal convective potential may be confined to portions of eastern
   North Dakota into northern/central Minnesota late this afternoon and
   evening.  This is where mid-level capping will be relatively weaker
   or in the process of weakening, and a lingering moist boundary layer
   may become supportive of sizable mixed-layer CAPE of 2000-3000+
   J/kg, with daytime heating beneath steep mid-level lapse rates.

   To the south of the main mid-level short wave, at least some model
   output indicates that a 30-40 kt mid-level (around 700 mb) speed
   maximum will nose across northeastern North Dakota into north
   central Minnesota by this evening.  Forcing for ascent and
   strengthening shear associated with this feature could provide
   support initially for isolated supercell development, which could
   grow upscale into a small organized convective cluster.  If this
   occurs, activity may be accompanied by a risk for severe hail and/or
   an isolated tornado, before strong wind gusts become the more
   prominent hazard.

   A more substantive signal exists for convective development in the
   wake of this feature, driven by lower/mid tropospheric warm
   advection, mainly across central/southern North Dakota this
   afternoon into tonight.  This may be rooted above a relatively
   dry/stable boundary layer, to the north of the surface front. 
   However, thermodynamic profiles characterized by sizable CAPE, steep
   mid-level lapse rates and dry sub-cloud air, coupled with moderately
   strong cloud-bearing layer shear, could become favorable for
   organized convection capable of producing severe hail and strong
   surface gusts.

   Additional scattered storms, based above a hot and deeply mixed
   boundary layer with modest CAPE, may develop south of the front,
   across western South Dakota late this afternoon and evening.  Some
   of these may be accompanied by locally strong surface gusts.

   ...Upstate New York into New England...
   Models indicate at least modest boundary-layer destabilization today
   with CAPE on the order of 1000-2000 J/kg by this afternoon. 
   Probably largely aided by orographic forcing for ascent, this may
   provide support for scattered vigorous thunderstorm development,
   which may become enhanced by shear on the southerly fringe of the
   mid-latitude westerlies.  Some of this activity may be accompanied
   by small hail and potentially damaging wind gusts, before
   diminishing this evening.

   ...Southeastern Louisiana...
   Based on the current forecast track of Marco, the center of
   circulation may still be migrating relatively quickly
   north-northwestward, toward the southeastern Louisiana Gulf coast
   late tonight.  Boundary-layer moistening/destabilization,forcing for
   ascent, and enlarging low-level hodographs in the right front
   quadrant (with respect to storm motion) may begin impacting portions
   of coastal areas by 12Z Monday, accompanied by increasing potential
   for a tornado or two.

   ..Kerr/Moore.. 08/23/2020

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