SPC AC 040553

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1253 AM CDT Tue Aug 04 2020

   Valid 041200Z - 051200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS WESTERN
   PORTIONS OF THE PLAINS...AND FROM THE CHEASPEAKE BAY AREA TO NEW
   ENGLAND...

   ...SUMMARY...
   A few tornadoes will be possible from the Virginia Tidewater into
   New England today and into tonight, in association with Isaias.
   Scattered severe thunderstorms capable of producing large hail and
   damaging winds are also expected across portions of the northern and
   central Plains.

   ...Synopsis...
   Tropical Storm Isaias is forecast to shift quickly
   north-northeastward today, from near the Chesapeake Bay early,
   across the Mid Atlantic region and then into/across New England,
   gradually weakening with time.  This north-northeastward
   acceleration will occur as Isaias begins being influenced by
   deep-layer south-southeasterly winds on the eastern side of a large
   eastern U.S. trough.  Meanwhile, ridging will persist over the
   western U.S., but will be suppressed later in the period, as eastern
   Pacific troughing shifts slowly inland.  Between the western ridge
   and the eastern trough, a belt of enhanced northwesterly flow aloft
   will remain over the Plains, with several short-wave perturbations
   within this flow field moving eastward out of the Intermountain West
   and into the Great Plains.  

   ...Delmarva/Chesapeake Bay region to New England...
   As Isaias shifts northeastward across the Mid-Atlantic Region into
   New England, an accompanying/very moist low-level environment will
   permit maintenance of modest CAPE along and east of the track of the
   storm.  With a very strong wind field -- veering strongly with
   height -- also spreading northward in tandem with Isaias, a
   favorable environment for rotating cells -- and a few tornadoes --
   is apparent.  Overnight, as the storm eventually approaches the St.
   Lawrence Valley, limited tornado risk may spread as far north as
   Maine.

   ...Western portions of the Plains...
   Lee troughing is forecast to prevail across the High Plains, as
   northwesterly flow aloft persists.  As daytime heating commences
   over the High Plains, modest low-level moisture (50s dewpoints) will
   contribute to moderate destabilization (1000 to 2000 J/kg
   mixed-layer CAPE during the afternoon).

   As perturbations aloft move atop the lee trough, scattered storms
   are forecast to develop from the western Dakotas southward to the
   central and southern High Plains.  With moderate mid-level
   northwesterlies atop low-level southeast flow, shear will support
   both organized/rotating storms, and southeastward advance of
   convection across the High Plains through the afternoon/early
   evening.  Large hail will be possible during the early-stage
   convection, along with locally damaging winds.

   With time, several clusters of storms are expected to evolve/grow
   upscale, as a south-southeasterly low-level jet develops from the
   Texas Panhandle/South Plains into western Kansas.  Risk for damaging
   winds will therefore likely linger well into the evening into parts
   of western Kansas, and possibly into/across the Texas and Oklahoma
   Panhandle region depending upon degree of linear organization/cold
   pool expansion.

   ..Goss/Wendt.. 08/04/2020

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