SPC AC 021952

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0252 PM CDT Wed Sep 02 2020

   Valid 022000Z - 031200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
   THE MID-ATLANTIC...

   ...SUMMARY...
   The most likely corridor for at least a couple tornadoes and locally
   damaging winds will continue from northern Virginia to southeast
   Pennsylvania through 10 PM EDT.

   ...20Z Update...
   No substantial changes have been made to the ongoing forecast.
   Storms are still expected to increase in both coverage and intensity
   across parts of the I-95 corridor this afternoon and evening, mainly
   from northern VA to DC/MD and into southeastern PA. Around 1000-2000
   J/kg of MLCAPE has already developed across this region, and
   deep-layer shear remains strong. Latest short-term guidance
   continues to suggest that isolated supercells will develop eastward
   off the higher terrain of the central Appalachians/Blue Ridge
   Mountains, posing a threat for both damaging wind gusts and a couple
   of tornadoes. For more information on the near-term severe threat
   across this region, see Mesoscale Discussion 1669.

   Elsewhere, storms have developed beneath an upper low across the far
   eastern TX Panhandle and western OK, where slightly cooler mid-level
   temperatures are present (around -8 degrees C at 500 mb). Even
   though a pocket of moderate instability has developed across this
   area with diurnal heating, deep-layer shear remains negligible.
   Current expectations are for ongoing convection to remain rather
   disorganized.

   ..Gleason.. 09/02/2020

   .PREV DISCUSSION... /ISSUED 1130 AM CDT Wed Sep 02 2020/

   ...Mid-Atlantic and Northeast...
   Primary feature of concern for late afternoon to early evening
   severe potential is an MCV tracking east from the Upper OH Valley. 
   This feature in conjunction with low-level convergence along the lee
   trough should aid in at least isolated thunderstorms off the Blue
   Ridge from central VA to southern PA. Additional storms will likely
   develop in the lee of the Lower Great Lakes and spread east. Warm
   mid-level temperatures from -4 to -6 C at 500 mb and similarly poor
   lapse rates aloft will limit updraft accelerations. Where cloud
   breaks are present east of the Blue Ridge, boundary-layer warming
   should support MLCAPE in excess of 1000 J/kg from MD southward given
   the presence of mid 70s surface dew points. Sufficient low and
   deep-layer shear will be present for a few supercells to form. These
   will be capable of producing a risk for at least a couple tornadoes
   across parts of the Mid-Atlantic region, along with locally damaging
   winds. A cat 2/SLGT risk has been introduced for this possibility. 

   ...Mid-South...
   A convectively augmented shortwave trough will drift east across
   northern TX through the period, with a series of downstream MCVs
   gradually progressing east-northeast towards the OH Valley. A belt
   of enhanced 700-mb southwesterlies should be centered on the
   ArkLaTex to TN Valley at peak heating. This should aid in 
   relatively greater potential for downbursts to occur as convection
   likely redevelops ahead of MCVs in central AR and the ArkLaTex. Poor
   mid-level lapse rates and flow weakening with height above 700 mb
   suggest a cat 1/MRGL risk remains warranted. 

   ...South-central TX...
   A conditional large hail/severe wind/brief tornado risk remains
   during the late afternoon to early evening, focused with potential
   peak heating convective development in association with an MCV near
   Del Rio that should drift east. Some uncertainty exists with the
   degree of midday non-severe convection ongoing as outflow has
   already reached Austin. Still, with ample boundary-layer heating
   occurring to the west of this outflow, large buoyancy should develop
   within a rather confined corridor in parts of the Brush Country.
   Deep-layer shear may be adequate for a couple transient
   supercells/small clusters.

   ...ND...
   A pronounced shortwave trough will dig as it progresses from the
   Canadian Prairies into northern MN by tonight. A sharp cold front
   will sweep southeast across ND during the late afternoon to
   mid-evening. Higher-based showers that may deepen into a few
   thunderstorms are expected along the front amid scant buoyancy.
   Inverted-v thermodynamic profiles should support mixing of
   strengthening mid-level flow such that isolated severe wind gusts
   are possible.

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