SPC AC 290526

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1226 AM CDT Thu Jul 29 2021

   Valid 291200Z - 301200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS OVER A PORTION
   OF THE MIDDLE ATLANTIC REGION...

   ...SUMMARY...
   Thunderstorms associated with wind damage and a risk for a tornado,
   will be possible from the Ohio Valley, central Appalachians and
   Mid-Atlantic states on Thursday.

   ...Northeast through Middle Atlantic area...

   Upper pattern in this region will be characterized by a broad trough
   today. MCV/shortwave trough embedded within the synoptic trough will
   move from the lower Great Lakes to the Middle Atlantic. At the
   surface a warm front will move northward through the Middle
   Atlantic, while a cold front advances southeast, reaching PA and the
   central Appalachians by evening. Thunderstorms should be in progress
   at the start of the period in association with the MCV over the
   southern portion of the Great Lakes. Other storms may be ongoing in
   the warm-advection regime north of the warm front over a portion of
   the Middle Atlantic. Widespread multi-layer clouds may precede the
   warm front. However, the warm sector should become moderately
   unstable during the afternoon with 1000-2000 J/kg MLCAPE supported
   by the advection of richer low-level moisture and pockets of
   diabatic heating. Thunderstorms are expected to develop ahead of the
   cold front in association with ascent attending the southeast-moving
   MCV as well as along the southeast-advancing cold front. This area
   will reside within belt of stronger mid-upper flow associated with
   the synoptic trough supporting 40-45 kt effective bulk shear.
   Therefore, some supercells may evolve along with line segments with
   bowing structures as activity develops southeast during the
   afternoon into the evening. Primary threat appears to be damaging
   wind and possibly some hail, though low-level hodographs will be
   sufficient for a few tornadoes, especially if sufficient instability
   can develop in vicinity of the warm front.  

   ...Ohio Valley through the Midwest region...

   Trailing portion of the cold front will extend westward through the
   Ohio Valley and into the Midwest during the afternoon. The
   pre-frontal warm sector should become moderately to strongly
   unstable with 2000-3500 J/kg MLCAPE possible as the moist boundary
   layer warms. Storms will likely develop along the cold front and
   possibly along any remnant pre-frontal outflow boundaries and spread
   southward. Winds aloft will decrease with western extent toward the
   Midwest, but 25-40 kt effective bulk shear will be sufficient for
   some organized structures, especially given expected degree of
   instability. Both multicell lines and a few supercell structures
   will be possible in this regime with damaging wind and hail the main
   threats.  

   ...Central High Plains region...

   Easterly upslope flow will become established in the post-frontal
   region of the central High Plains during the afternoon resulting in
   corridor of moderate instability from eastern WY into NE. The
   proximity to the upper ridge suggests forcing aloft will remain
   weak. However, storms will likely develop over the higher terrain
   and spread east during the late afternoon into the evening, possibly
   evolving into a small MCS. Other more isolated storms might develop
   farther east along the stalled front. Up to 35-40 kt effective bulk
   shear will support some organized structures with isolated damaging
   wind and hail the main threats from late afternoon into the evening.

   ..Dial/Bentley.. 07/29/2021

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