SPC AC 311612

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1112 AM CDT Thu Oct 31 2019

   Valid 311630Z - 011200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS PARTS OF
   NC...VA...WV...MD...DC...PA...

   ...SUMMARY...
   An extensive squall line of strong to severe thunderstorms is
   expected to develop during the late afternoon into this evening,
   near and east of the Appalachians from parts of New York to the
   Carolinas. At least a few tornadoes and scattered to widespread
   damaging winds are possible.

   ...Northeast to the Carolinas...
   Have upgraded peak probabilities for both wind and tornado hazards
   with an active period of severe weather anticipated later this
   afternoon into the evening.

   An amplifying shortwave trough now over IL will accelerate
   east-northeast toward the Saint Lawrence Valley through tonight
   toward PA/NY, while taking on a more neutral-to-negative tilt with
   peak 500 mb wind speeds exceeding 120 kt. Strong surface
   cyclogenesis is expected in advance of the amplifying trough, with a
   cyclone moving from OH to far southern QC by early Friday. South of
   the deepening cyclone, a strong cold front will sweep eastward
   across the Appalachians this afternoon, reaching the Atlantic coast
   overnight. The cold front will be the primary focus for severe
   thunderstorms through this evening.

   Northward advection of low-level moisture will continue with mid 60s
   boundary-layer dew points up the OH Valley west of the Appalachians
   and across southeast PA to coastal southern New England. Poor low to
   mid-level lapse rates amid widespread clouds will hinder
   destabilization with northern extent in the Northeast. Richer
   low-level moisture (upper 60s to low 70s boundary-layer dew points)
   and some surface heating in cloud breaks will support a confined
   plume of MLCAPE between 750-1500 J/kg across parts of VA into the
   Carolinas at peak heating. 

   Within the warm sector, vertical shear will increase in response to
   the approach and deepening of the mid-level trough/surface cyclone.
   Long hodographs with effective bulk shear of 50-60 kt are expected,
   and there will also be enlarged low-level hodograph curvature for
   numerous rotating storms amid effective SRH of 300-500 m2/s2.

   Strong linear forcing for ascent along the cold front will be the
   primary focus for storm development, mainly east of the Appalachians
   later this afternoon/evening. The strong linear ascent will favor
   upscale growth into a squall line, though strong deep-layer shear
   and enough east component to storm motions should allow embedded
   supercell structures to persist. The modest buoyancy and very strong
   850-700 mb flow will favor damaging winds as the primary threat.
   These could be potentially widespread within longer-tracked bowing
   segments. At least a few tornadoes are also anticipated with
   embedded supercell and mesovortex structures, particularly from
   central NC to central MD. 

   The severe threat will diminish during the late evening to overnight
   given an increasing dearth of surface-based instability with
   east/northeast extent.

   ..Grams/Wendt.. 10/31/2019

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