SPC AC 071651

   Day 2 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1151 AM CDT Mon Sep 07 2020

   Valid 081200Z - 091200Z

   ...NO SEVERE THUNDERSTORM AREAS FORECAST...

   ...SUMMARY...
   The risk for severe thunderstorms appears negligible across the U.S.
   for Tuesday through Tuesday night.

   ...Discussion...
   Downstream of a prominent and blocking mid-level ridge centered to
   the west of the British Columbia/Pacific Northwest coast, a
   significant short wave trough near the northern U.S. Rockies is
   beginning to split off the main branch of mid-latitude westerlies. 
   Models generally indicate that this impulse will dig into the
   eastern Great Basin by 12Z Tuesday, where it will continue evolving
   into a closed mid-level low and become quasi-stationary Tuesday
   through Tuesday night.  As this occurs, cold surface ridging,
   already nosing southward to the lee of the Rockies, is forecast to
   surge from the Texas/Oklahoma Panhandle vicinity through the Edwards
   Plateau and adjacent Rio Grande Valley vicinity of Texas by 12Z
   Wednesday.

   Farther east, in the wake of a vigorous short wave trough
   accelerating across and east-northeast of Newfoundland and Labrador,
   the frontal zone is expected to stall across the St. Lawrence Valley
   through the lower Great Lakes region.  There may also be little
   movement farther to the southwest, into the lower Missouri Valley
   and adjacent portions of the Great Plains.  However, the shallow
   leading edge of the cooler air may begin to gradually erode, as
   mid-level heights rise on the western periphery of prominent
   subtropical ridging centered over the western Atlantic.

   In association with the subtropical ridging, seasonably high
   moisture content air may begin to return to portions of the south
   Atlantic and Gulf coastal plain.  Otherwise, a plume of more modest
   lower/mid tropospheric moisture will become generally focused along
   and above the surface frontal zone, from the southern Great Plains
   into the lower Great Lakes.

   There may be a corridor of modest diurnal boundary-layer
   destabilization (including CAPE of 1000-2000 J/kg) along the frontal
   zone, which could provide support for scattered vigorous
   thunderstorm development Tuesday afternoon and evening.  However,
   this activity seems likely to either be quickly undercut by the
   southward surging portion of the front across the Great Plains, or
   form in response to warm advection above the front elsewhere.  And,
   with stronger mid/upper flow (and associated convective-layer shear)
   likely to remain displaced to the north and west of this zone,
   severe weather still appears negligible at this time.

   ..Kerr.. 09/07/2020

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