SPC AC 181938

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0238 PM CDT Wed Aug 18 2021

   Valid 182000Z - 191200Z

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

   ...SUMMARY...
   Tornado potential will continue through tonight across parts of the
   Mid-Atlantic region in association with post-Tropical Cyclone Fred.
   Storms may produce isolated damaging wind and large hail across a
   portion of the Great Basin including much of Utah.

   ...20z Update - Middle Atlantic Region...

   Minor adjustments have been made to the Marginal and Slight risk
   areas based on the progression of post-Tropical Cyclone Fred and
   latest observational trends. Tornado and wind probabilities have
   been trimmed on the southwestern edges of the previous outlook area
   across parts of VA/WV/PA as the system continues to lift
   north/northeast. A small northward expansion has been made to the
   Slight risk area across PA toward the NY border where some breaks in
   cloudiness may allow for additional destabilization. Some
   consideration was given to extending the Slight risk into parts of
   southern/southeast NY. However, cloud cover has limited greater
   destabilization, and if the surface warm front lifts northward it
   may be ill-timed with the diurnal heating cycle, instead coinciding
   with boundary-layer stabilization during the evening hours beyond
   peak heating. As a result, confidence is too low to expand 5%
   tornado probabilities further north into NY. Reference MCDs 1554 and
   1555 for more short-term details on the severe threat across parts
   of the Mid-Atlantic vicinity.

   Otherwise, no other changes have been made to the previous outlook.

   ..Leitman.. 08/18/2021

   .PREV DISCUSSION... /ISSUED 1208 PM CDT Wed Aug 18 2021/

   ...Middle Atlantic Region...

   The center of post Tropical Cyclone Fred is located near the
   northeast WV / southwest PA border and should move north northeast
   to near the central PA / NY border by early evening. A warm front
   extends east from this low through southern PA into central NJ, and
   this boundary will continue to advance northward during the day.
   Visible imagery show numerous breaks between convergence bands,
   especially across northern VA into MD and southern PA. This zone of
   partial clearing and diabatic warming of the moist surface layer
   should spread northward with time contributing to 1000-1500 J/kg
   MLCAPE, especially near and south of the warm front. The greatest
   potential for a few tornadoes will probably be across central PA.
   Here, convection developing within the convergence bands will move
   north and interact with the warm front where low-level hodograph
   size should be maximized (250-350 m2/s2 SRH) during afternoon peak
   heating.  

   ...Great Basin region...

   A convective outflow boundary is situated across northern UT from
   near Salt lake City southwestward to the NV border. South of this
   boundary, anomalously rich low-level moisture is in place with
   dewpoints in the upper 50s F. Areas of partial clearing with strong
   diabatic heating should support up to 1500 J/kg MLCAPE this
   afternoon, and thunderstorms will likely increase in coverage as the
   boundary layer continues to destabilize. Increasing mid-upper flow
   associated with an amplifying upper trough will support 30-45 kt
   effective bulk shear supportive of some supercell structures.
   Isolated damaging wind and large hail are the primary threats into
   the early evening.

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