SPC AC 241242

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0742 AM CDT Mon Aug 24 2020

   Valid 241300Z - 251200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS LATER TODAY INTO
   TONIGHT FOR PARTS OF MN/WI...

   ...SUMMARY...
   One or more clusters of storms will be capable of producing large
   hail, damaging winds, and an isolated tornado across parts of the
   upper Mississippi Valley later today into tonight.  Other more
   isolated severe storms may occur this afternoon from the
   Mid-Atlantic into New England, and along the northeast Gulf coast
   with Tropical Storm Marco.

   ...Upper MS Valley today into tonight...
   Overnight convection has been rather persistent - an initial cluster
   weakened earlier this morning, but a second small cluster is now
   moving into central/south central MN.  The convection has been
   supported by low-level warm advection on the nose of a weak
   low-level jet, as well as frontogenesis near 850 mb, all in advance
   of a speed max approaching MN from ND.  Substantial uncertainty
   still remains in this forecast regarding thunderstorm timing through
   tonight.

   There is some potential for the morning convection to persist into
   the day, though forcing for ascent will reach a diurnal minimum as
   the low-level jet weakens this morning.  Thereafter, there will be
   some potential for surface-based thunderstorm development later this
   afternoon as surface temperatures warm into the 90s along and south
   of the outflow-reinforced front.  If storms form on this boundary
   this afternoon, the environment will conditionally favor supercells
   capable of producing large hail/damaging winds, and perhaps a
   tornado.  Whether or not afternoon storms form on the boundary,
   additional development is expected late this evening into tonight as
   warm advection again increases, and storms will subsequently move MN
   to WI with an accompanying threat for large hail/damaging winds. 

   ...Northern Gulf coast to southern GA today...
   Tropical Storm Marco is suffering the effects of southwesterly
   vertical shear aloft, with the low-level circulation displaced to
   the southwest of the persistent deep convection.  This sheared
   structure of the cyclone suggests weakening is likely during the day
   as it approaches the coast (see latest NHC advisories for details). 
   Deeper convection will spread onshore as upper 70s boundary-layer
   dewpoints and associated buoyancy spread inland with the cyclone and
   as a diffuse baroclinic zone moves slowly northwestward across the
   FL Panhandle.  Prior to weakening of Marco's wind field, a zone of
   sufficiently strong low-level shear will persist along the north
   central/northeast Gulf coast to support a low-end supercell/tornado
   threat today.  Farther east, residual 30-40 kt low-midlevel flow and
   daytime heating/destabilization on the east edge of the thicker
   cloud shield could support isolated strong outflow gusts today into
   south GA.  

   ...FL Keys later today into tonight...
   Tropical Storm Laura will skirt the south coast of Cuba and emerge
   over the southeast Gulf of Mexico early Tuesday (please see latest
   NHC advisories for details).  Strengthening of the gradient flow on
   the north side of the tropical cyclone will result in sufficient
   vertical shear for isolated rotating storms within the outer
   northern convective bands later today into tonight across the Keys,
   where an isolated tornado will be possible.

   ...New England into parts of the Mid-Atlantic this afternoon...
   In advance of a deepening midlevel trough over ON, a series of
   subtle perturbations will move eastward from the OH Valley/lower
   Great Lakes toward New England and the Mid-Atlantic through tonight.
    Low-midlevel flow/vertical shear will remain rather modest across
   much of this area (effective bulk shear of 20-30 kt), but daytime
   heating and terrain circulations will again initiate at least widely
   scattered thunderstorms this afternoon from VA northward into New
   England.  MLCAPE ranging from 1000 J/kg north to 2000 J/kg south, as
   well as DCAPE in the 750-1000 J/kg range, will support multicell
   clusters capable of producing isolated damaging gusts this
   afternoon.

   ...Northwestern NV and vicinity today...
   Midlevel convection is spreading north-northeastward from CA to
   northwestern NV this morning in association with a midlevel trough. 
   Typical inverted-v profiles can be expected today, and there will be
   a belt of stronger midlevel flow with the midlevel trough.  However,
   buoyancy will likely remain weak, and a combination of clouds/smoke
   may also slow surface heating some.  Gusty outflow winds will be
   possible with both the morning storms and additional convection this
   afternoon rooted closer to the surface, but the overall threat for
   damaging winds appears too low to warrant any severe probabilities.

   ..Thompson/Leitman.. 08/24/2020

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