SPC AC 031625

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1125 AM CDT Mon Aug 03 2020

   Valid 031630Z - 041200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS THIS EVENING
   THROUGH THE OVERNIGHT HOURS ACROSS PARTS OF FAR EASTERN SOUTH
   CAROLINA...EASTERN NORTH CAROLINA AND PARTS OF SOUTHEASTERN
   VIRGINIA...

   ...SUMMARY...
   A few tornadoes are possible across parts of the southern Mid
   Atlantic Coast region in association with the landfall and inland
   migration of Isaias this evening through tonight.  Otherwise,
   isolated severe storms are possible across parts of the Rockies and
   adjacent portions of the Great Plains late this afternoon and
   evening.

   ...Synopsis...
   Mid/upper flow remains highly amplified across much of Canada and
   the U.S., but significant troughing now inland of the British
   Columbia and Pacific Northwest coast is forecast to continue
   eastward and contribute to the suppression of ridging across the
   Canadian Prairies and northern U.S. Rockies.  Models suggest that
   the mid-level troughing may split east of the Canadian Rockies, with
   the northern portion pivoting through northern Alberta and the
   southern portion gradually turning east-southeastward along the
   Montana international border area tonight.

   Farther east, positively tilted large-scale mid-level troughing
   likely will be maintained along an axis across Quebec and Ontario
   into the central/eastern Gulf Coast states.  A fairly significant
   digging short wave perturbation across the Upper Midwest and
   adjacent Great Lakes region may contribute to reinforcement of this
   troughing, and low-level cooling and drying southward through much
   of the Mississippi Valley.

   Seasonably high moisture content will remain confined to portions of
   the southern through middle Atlantic Seaboard, in association with
   northeastward accelerating Tropical Storm Isaias.

   ...Southern Mid Atlantic Coast...
   Primary severe weather potential associated with Isaias appears
   likely in association with the north-northeastward progression of
   its right front quadrant with respect to storm motion, mainly
   northeast through east of the low-level circulation center.  This
   appears likely to be where low-level convergence and inland
   boundary-layer destabilization will become maximized, coincident
   with most favorable enlargement of clockwise-curved low-level
   hodographs.  Beneath south-southeasterly 850 mb flow strengthening
   to 50-70+ kt, strong to extreme low-level shear will support
   potential for low-topped supercells with strong low-level
   mesocyclones.  And a moist boundary layer with mid 70s F+ surface
   dew points probably will remain sufficiently unstable this evening
   and overnight to support a risk for a few tornadoes.

   ...Rockies and adjacent Great Plains...
   Steep lower/mid tropspheric lapse rates and moderate to strong shear
   beneath modest westerly to northwesterly mid-level flow will
   contribute to a conditional potential for severe storms today,
   particularly across parts of central and eastern Montana, aided by
   forcing for ascent ahead of the short wave trough.  However,
   low-level moisture appears sufficient to only support seasonably
   modest boundary-layer instability, which may result in fairly sparse
   coverage of vigorous thunderstorm development late this afternoon
   and evening.

   ...Appalachians vicinity...
   A belt of seasonably moderate to strong southwesterly mid/high-level
   flow ahead of the troughing may be contributing to favorable
   deep-layer shear across the region.  However, with general surface
   ridging present across much of the region, along with weak lapse
   rates, and generally modest (at or below 20 kt) lower/mid
   tropospheric flow, severe weather probabilities appear generally
   less than 5 percent.

   ..Kerr/Cook.. 08/03/2020

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