SPC AC 300722

   Day 3 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0222 AM CDT Thu Jul 30 2020

   Valid 011200Z - 021200Z

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS SATURDAY
   AFTERNOON AND EVENING ACROSS PARTS OF THE LOWER OHIO AND TENNESSEE
   VALLEYS...

   ...SUMMARY...
   Strong thunderstorms may impact parts of the lower Ohio and
   Tennessee Valleys Saturday afternoon and evening, accompanied by at
   least some risk for damaging wind gusts and a couple of tornadoes.

   ...Synopsis...
   Mid/upper flow across much of North America will remain highly
   amplified through this period and beyond.  This is expected to
   continue to include prominent subtropical ridging centered over the
   Southwest, and ridging within the mid-latitude westerlies, across
   the northern U.S. intermountain region and Rockies into the Canadian
   Northwest Territories. 

   East of this ridging, models indicate that large-scale troughing
   within initially split branches of the westerlies may gradually
   begin to consolidate.  As the southern portion of a trough within
   the northern branch digs into the Upper Midwest and adjacent Great
   Lakes, troughing within the southern branch is forecast to begin
   shifting across and northeast of the middle and lower Mississippi
   Valley.  Within lower/mid tropospheric flow trending southwesterly
   to the east of the trough axis,  it appears that this may include a
   convectively generated or enhanced perturbation migrating into and
   through the lower Ohio Valley, accompanied by a developing surface
   low.

   At the same time, recently formed Tropical Storm Isaias may continue
   a north-northwestward track around the western periphery of ridging
   over the western subtropical Atlantic, across/along or just east of
   the Florida Peninsula Saturday through Saturday night.

   ...Lower Ohio and Tennessee Valley vicinity...
   Sizable spread lingers within model output concerning the synoptic
   and sub-synoptic developments Saturday through Saturday night.  In
   general, though, it appears that the lower/mid tropospheric
   perturbation will be accompanied by 30-50 kt southerly to
   southwesterly flow in the 850-500 mb layer, across the warm sector
   of a modest, slowly deepening and northeastward migrating surface
   low.  Although mid-levels may be relatively warm, boundary-layer
   moisture characterized by surface dew points in excess of 70f may
   still contribute to CAPE of 1000-2000+ J/kg by Saturday afternoon. 
   This should be sufficient to support the initiation of vigorous
   thunderstorm development.

   Given this environment, including favorable ambient vertical
   vorticity, and perhaps modest clockwise curved low-level hodographs,
   some supercell structures posing a risk for tornadoes appear
   possible, in additional to upscale growing convection capable of
   producing potential damaging wind gusts.

   ...Florida Peninsula...
   Based on the current forecast track of Tropical Storm Isaias, it is
   not out of the question that at least some increase in potential for
   convection capable of producing tornadoes could occur across
   southeast Florida coastal areas late Friday night into early
   Saturday.  However, given the increasing spread in the forecast
   track as it approaches south Florida, among other factors, severe
   weather probabilities are being maintained at less than marginal
   risk categorization at this time.

   Later Saturday through Saturday night, the current forecast track
   northward along the east coast of the Florida Peninsula would
   suggest generally negligible risk for tornadoes inland of the coast.
   However, with a westward shift of the track, particularly closer to
   (or off) the west coast, this could change considerably.

   ..Kerr.. 07/30/2020

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