SPC AC 031245

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0745 AM CDT Thu Sep 03 2020

   Valid 031300Z - 041200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS OVER PORTIONS
   OF THE MID-ATLANTIC REGION...

   ...SUMMARY...
   Damaging gusts and a few tornadoes should be the main threats with
   today's thunderstorms over portions of the Mid-Atlantic region.

   ...Synopsis...
   In mid/upper levels, a large cyclone over north-central Canada is
   forecast to continue dominating the pattern over much of central/
   northern North America, while pivoting across Hudson Bay through the
   period.  A broad/associated field of cyclonic flow will extend from
   the northern Great Plains across the Great Lakes and Ohio Valley to
   the Mid-Atlantic and Northeastern CONUS.  A strong/embedded
   shortwave trough was apparent in moisture-channel imagery across
   eastern MN to adjoining northwestern ON.  This feature will move
   eastward to northeastern ON, Lake Huron and northern Lower MI by
   00Z, then eject across southern QC tonight.

   Farther south, a pronounced height weakness -- between the highs
   over NV and the southern Atlantic Coast -- extended from northern MX
   across central/northeast TX to the Ozarks.  The main vorticity
   maximum should drift eastward across the Red River region of north
   TX and OK to AR and northeast TX through 12Z tomorrow.  A positively
   tilted shortwave trough and associated vorticity banner extended
   from the Ozarks area northeastward to OH, and should eject across
   the Mid-Atlantic region to western New England today.

   At the surface, the 11Z analysis showed a slow-moving, essentially
   quasistationary frontal zone across southern NJ, the Delmarva
   Peninsula, and central VA, arching across parts of OH to a low near
   SDF.  The front extended southwestward from there across northern/
   western AR, north-central TX, an the TX Permian Basin region.  The
   near-coastal portion of this boundary should retreat northward
   today, while the rest remains quasistationary with general
   weakening.  A stronger cold front -- related to the trough aloft
   near Lake Superior -- was drawn across northwestern WI, western IA,
   northwestern KS, and east-central/southeastern CO. By 00Z this front
   should reach southeastern ON, northern IN, central MO, northwestern
   OK, and northeastern NM.  By 12Z the front should extend from ME
   across PA to western KY, and eastern OK, becoming more diffuse and
   quasistationary westward across the southern High Plains.  A
   prefrontal trough should settle in the immediate lee of the
   Appalachians across the Mid-Atlantic and Carolinas.

   ...Mid-Atlantic and vicinity...
   Widely scattered to scattered thunderstorms are expected to develop
   this afternoon in a regime of prefrontal surface troughing and
   terrain-aided local lift across the western parts of the outlook
   areas.  Activity then should move eastward while organizing in both
   strength and coverage, with additional development possible across a
   well-heated coastal plain.  Damaging downdraft winds should be
   common, with some severe (50+ kt) gusts possible.  A few tornadoes
   also may occur.  The most-concentrated area of convective/severe
   potential still appears to be across a corridor from northern VA
   across the Delmarva Peninsula and southern NJ, in accordance with
   the 30%-wind/enhanced area.

   Much of this activity should occur in a field of height falls, and
   strengthening large-scale/mid-upper lift of mesoscale spatial
   coverage, ahead of the ejecting Ozarks/Ohio Valley vorticity field. 
   The associated area of vertical motion has been supporting
   non-severe convection the past several hours across portions of
   southern/eastern KY, and will move over the surface trough and
   across a diurnally destabilizing, richly moist, weakly capped
   boundary layer today.  As the favorable low-level air mass spreads
   northward up the Delaware Valley and NJ areas, a field of 1500-2000
   J/kg MLCAPE (locally/briefly higher) should develop over lower
   elevations, amidst 40-50-kt effective shear magnitudes.  Favorable
   speed shear will spread over the region as flow aloft intensifies,
   and any areas of relatively backed surface winds will yield enlarged
   low-level hodographs with 200-300 J/kg effective SRH. This supports
   an all-hazards supercell threat, with low LCL, from north-central VA
   eastward.  The bulk of convection should weaken and/or move offshore
   by around 03Z, with the severe threat decreasing markedly later in
   the evening through tonight.

   ...Lower MI...
   A narrow, strongly forced band of convection -- with at least
   isolated embedded thunderstorms -- is expected to develop near the
   surface cold front over northern Lower MI during midday to early
   afternoon.  This activity should sweep eastward across the peninsula
   this afternoon while backbuilding southward somewhat, and offering
   strong/isolated severe gusts.  Locally strong convective downdrafts
   may result in severe gusts when superimposed  on the intensifying
   ambient/gradient flow.

   As the shortwave trough approaches, winds should strengthen
   throughout most of the troposphere, along with large-scale lift/DCVA
   in midlevels.  That, along with direct frontal forcing, will offset
   the meager moisture and marginal instability enough to support the
   convective band.  Forecast soundings suggest a narrow prefrontal
   corridor of 100-500 J/kg MLCAPE is possible with surface-based
   effective-inflow parcels.  The threat should diminish after about
   22-23Z as the main area of most-vigorous forcing shifts across Lake
   Huron into adjoining ON, and the remaining warm sector farther south
   begins stabilizing diabatically in tandem with weaker lift.

   ...Central/south TX...
   Scattered to locally numerous, slow-moving thunderstorms are
   expected this afternoon, initially across parts of the Hill Country,
   and perhaps eastward off the Balcones Escarpment across the southern
   blackland prairies and interior coastal plain.  The main concern
   will be isolated severe gusts from wet downbursts, with the most
   intense convection.  Low-level and deep shear will be weak near the
   trough aloft, with less than 15 kt of flow likely throughout most of
   the troposphere.  However, subtle large-scale ascent near the
   trough, weak MLCINH, the presence of outflow/differential-heating
   boundaries for localized lift, large boundary-layer theta-e, and a
   deep troposphere will support vigorous multicellular updrafts and
   downdrafts.  Very rich low-level moisture should linger, with
   surface dew points commonly in the 70s F and PW between 1.5-2.25
   inches.  Areas of strong diurnal heating will contribute to peak
   MLCAPE in the 2500-3500 J/kg range.

   ..Edwards/Kerr.. 09/03/2020

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