SPC AC 031959

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0259 PM CDT Thu Sep 03 2020

   Valid 032000Z - 041200Z

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

   ...SUMMARY...
   Scattered damaging winds and a few tornadoes remain the primary
   severe hazards across portions of the Mid-Atlantic through about 10
   PM EDT.

   ...20Z Update...
   No major changes have been made to the ongoing forecast. Visible
   satellite imagery shows the strongest diurnal heating has occurred
   mostly along/south of the MD/PA border, to the south of widespread
   low to mid-level cloudiness. Current expectations are for storms to
   gradually intensify across northern VA into western/central MD and
   southeastern PA over the next couple of hours, as large-scale ascent
   associated with a shortwave trough overspreads the Mid-Atlantic
   region. Strong mid-level flow (around 50 kt at 500 mb) was observed
   with the 18Z IAD sounding, and increasing low-level flow is also
   forecast through the remainder of the afternoon and continuing into
   the evening. This veering/strengthening wind profile with height
   through mid levels should support supercell storms with a tornado
   threat initially, even through mid-level lapse rates may tend to
   temper updraft strength to some degree. Damaging straight-line winds
   should also occur with any robust storms that develop and
   subsequently move eastward through the evening given the
   increasingly favorable kinematic environment. For more information
   on the near-term severe threat across the Mid-Atlantic, see
   Mesoscale Discussion 1673 and recently issued Tornado Watch 485.

   Elsewhere, 5% wind probabilities have been removed behind a
   low-topped convective line along a cold front in Lower MI. Only
   minor changes have been made to the Marginal Risk across parts of TX
   based on latest observational trends.

   ..Gleason.. 09/03/2020

   .PREV DISCUSSION... /ISSUED 1130 AM CDT Thu Sep 03 2020/

   ...Central Appalachians into the Mid-Atlantic...
   A low-amplitude shortwave impulse over the Upper OH Valley will
   quickly progress into New England by evening, with an attendant
   southwesterly low/mid-level speed max. Pervasive cloud coverage
   immediately ahead of this impulse across the MD Panhandle into
   western/central PA should marginalize the near-term severe threat. A
   more substantial severe risk is expected later this afternoon as
   boundary-layer warming has been more pronounced east of the Blue
   Ridge across much of VA into eastern PA. 12Z soundings sampled poor
   mid-level lapse rates, but the presence of low to mid 70s surface
   dew points in the Delmarva should yield a plume of moderate buoyancy
   with MLCAPE reaching 1000-2000 J/kg from southeast PA southward.

   40-50 kt effective shear with an enlarged low-level hodograph will
   support the potential of at least a few supercells from
   south-central/southeast PA into far northern VA. Initially discrete
   storms will have the greatest opportunity to produce tornadoes.
   Otherwise, storm-scale clustering and the degree of shear may yield
   a couple bowing segments with 45-65 kt gusts capable of producing
   damaging wind swaths to the coast.

   ...Northern Lower MI...
   A vigorous shortwave trough will translate east across northern
   Ontario. Convection within the warm conveyor over northwest Lower MI
   ahead of a sharp cold front may attempt to intensify as it shifts
   east before spreading into southern Ontario. Scant buoyancy will
   remain the primary limiting factor and lightning may be sporadic.
   But mixing of strong 700-mb winds to the surface may produce
   isolated damaging winds. For additional near-term discussion, please
   see MCD 1672.

   ...South-Central TX...
   A mid-level impulse should remain quasi-stationary over central TX
   through the period, with a slow-moving ongoing convective cluster
   drifting south. Robust boundary-layer heating to the southwest may
   encourage late afternoon storm development along the trailing
   outflow over the Hill Country. Weak deep-layer shear will be a
   mitigating factor, but moderately large buoyancy should encourage
   potential for wet microbursts capable of locally damaging winds.

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