SPC AC 042000

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0300 PM CDT Wed Aug 04 2021

   Valid 042000Z - 051200Z

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER WESTERN
   THROUGH CENTRAL MONTANA...

   ...SUMMARY...
   Isolated severe thunderstorms are possible across parts of Montana
   from mid-afternoon through this evening.

   ...Southern New England...

   Precipitation along a warm conveyor belt continues to spread
   northward into southern New England. Composite radar imagery loop
   and RAP analysis indicates a mesoscale circulation/vort max off the
   Middle Atlantic coast lifting northward. A surface low associated
   with this feature along with an accompanying warm front will move
   just inland of the southern New England coast late tonight. The warm
   sector will likely be accompanied by dewpoints in the upper 60s F
   which will contribute to some destabilization, but instability will
   remain very marginal with MLCAPE less than 400 J/kg due to very weak
   lapse rates. While 0-1 km hodograph size will increase over southern
   New England as the low-level jet strengthens, confidence remains low
   that convective updrafts will become sufficient for an isolated
   tornado threat given the anticipated very marginal thermodynamic
   environment.

   ...Montana...

   The previous forecast appears on track in this region with no
   significant changes needed in the categorical risk area for this
   update.

   ..Dial.. 08/04/2021

   .PREV DISCUSSION... /ISSUED 1130 AM CDT Wed Aug 04 2021/

   ...MT...
   A shortwave impulse near the WA/ID/BC border area will dampen as it
   gradually slides east then southeast into MT. Isolated to possibly
   scattered thunderstorms are expected downstream of this feature
   later this afternoon and persisting into this evening. This
   convection should be rather high-based as deeply mixed thermodynamic
   profiles ensue at peak heating, with only weak buoyancy as MLCAPE
   holds below 1000 J/kg. The somewhat more favorable overlap of
   buoyancy with effective shear should occur across northwest MT just
   ahead of the mid-level impulse. Here, a threat exists for marginally
   severe hail in addition to isolated severe wind gusts. Farther east
   in central MT, the wind hazard should dominate with small hail also
   possible.

   ...Coastal southern New England...
   Guidance differs with the potential inland penetration of a
   surface-based warm sector tonight, ahead of a broad upper trough
   over the eastern U.S. and embedded shortwave impulses along the
   Eastern Seaboard. The 14Z RAP is more aggressive than the 12Z NAM
   with this scenario, advecting low 70s surface dew points into
   coastal southern New England overnight. Extensive stratiform and
   embedded convective elements off the Mid-Atlantic and Northeast
   coast at present suggests that this degree of destabilization
   appears unlikely. Thus, have refrained from an upgrade to cat 1/MRGL
   risk at this juncture.

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