SPC AC 090619

   Day 2 Convective Outlook CORR 1
   NWS Storm Prediction Center Norman OK
   0119 AM CDT Thu Jul 09 2020

   Valid 101200Z - 111200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS CENTRAL
   SD AND NORTH-CENTRAL NE...

   CORRECTED FOR BACKWARDS THUNDER LINE IN AZ/NM

   ...SUMMARY...
   Severe thunderstorms are expected across portions of the northern
   and central Plains Friday. Greatest severe weather risk is currently
   forecast from central South Dakota into central Nebraska Friday
   evening/night. An isolated strong storm or two is also possible
   across coastal portions of the northern Mid-Atlantic States and
   southern New England.

   ...Synopsis...
   Upper ridging is expected to build across the Southwest while a belt
   of enhanced flow aloft extends along its northern periphery from
   northern CA/Pacific Northwest eastward in the Mid/Upper MS Valley. A
   shortwave trough is expected to progress through this belt of
   enhanced flow, reaching the northern High Plains by Friday afternoon
   and the Upper Midwest by early Saturday morning. 

   Weak cyclonic flow aloft is anticipated over most of the eastern
   CONUS. The only exception is in the vicinity of a tropical low,
   which is forecast to begin the period just off the Delmarva
   Peninsula. This low is expected to move northward along the
   Mid-Atlantic coast throughout the day, reaching the Hudson Valley by
   early Saturday morning.

   ...Great Plains...
   A decaying convective line may be moving into the Ozark Plateau and
   adjacent portions of eastern OK early Friday morning. Some severe
   risk is possible downstream as this line continues southeastward.
   However, significant variability exists within the guidance
   regarding the presence and/or location of this line, resulting in
   low predictability and preclude any severe probabilities in this
   area with this outlook.

   Farther north, surface cyclogenesis is expected over the northern
   High Plains ahead of the approaching shortwave trough. The surface
   pressure gradient will tighten as a result of this cyclogenesis,
   contributing to northwestward low-level moisture advection. By
   Friday afternoon, 60 deg F surface dewpoints will likely reach into
   far eastern MT, with mid 60s dewpoints into southern NE.
   Consequently, moderate to strong instability will develop east of
   the surface trough, which will combine with low-level convergence
   and increasing large-scale ascent to support convective initiation.

   Initially, storms will likely be discrete, with long, straight
   hodographs suggest splitting supercells capable of large hail
   (isolated very large) and strong wind gusts. Eventual upscale growth
   into an organized convective line appears probable, 
   with a risk for strong wind gusts then continuing downstream across
   portions of SD and NE. 

   ...Northern Mid-Atlantic into Southern New England...
   Increased low to mid level flow throughout the eastern and northern
   periphery of the tropical low mentioned in the synopsis will result
   in the potential for a few stronger storms within the convective
   band surrounding the system. Damaging wind gusts and/or a brief
   tornado or two are possible with any more persistent updrafts. Poor
   lapse rates and warm temperature profiles will limit instability,
   reducing the overall severe potential.

   ..Mosier.. 07/09/2020

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