SPC AC 220600

   Day 2 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0100 AM CDT Wed Jul 22 2020

   Valid 231200Z - 241200Z

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
   THE NORTHEAST/MID-ATLANTIC TO THE CENTRAL/SOUTHERN APPALACHIANS
   ...NORTHERN/CENTRAL PLAINS...AND NORTHERN ROCKIES/HIGH PLAINS...

   ...SUMMARY...
   Isolated strong to severe thunderstorms appear possible Thursday
   across parts of the Northeast/Mid-Atlantic to the central/southern
   Appalachians, northern/central Plains, and northern Rockies/High
   Plains.

   ...Northeast/Mid-Atlantic to the Central/Southern Appalachians...
   Another in a series of positively tilted shortwave troughs should
   move across the Northeast and Mid-Atlantic on Thursday. The stronger
   mid-level flow associated with this feature is expected to remain
   generally north of the surface warm sector, with a broad corridor of
   20-30 kt west-southwesterly winds forecast across much of the
   Northeast and Mid-Atlantic. A weak surface low initially over New
   England should develop northeastward and offshore through the day,
   with a trailing cold front moving southeastward through the period.

   Mid-level lapse rates should remain rather poor across these regions
   Thursday, with diurnal heating of a moist low-level airmass likely
   the main support for instability. Storms that form along/ahead of
   the front may be capable of producing damaging winds, as low-level
   lapse rates steepen with diurnal mixing/heating of the boundary
   layer. At this point, the potential for greater destabilization
   appears better along/east of the Appalachians. However, modest
   deep-layer shear and the previously mentioned poor mid-level lapse
   rates may tend to limit updraft strength, with mainly small bowing
   line segments and clusters likely. Also, with fairly widespread
   convective overturning expected in the Day 1 period, it seems too
   early to attempt to pinpoint a corridor of greater potential for
   damaging winds. Have therefore opted to expand the Marginal Risk for
   wind considerably based on latest guidance. Shear does become much
   more modest with southward extent into the central/Southern
   Appalachians vicinity, but loosely organized clusters capable of
   strong/gusty winds and occasional damage still appear possible.

   ...Northern/Central Plains...
   Upper ridging centered over the Plains Thursday will be eroded by a
   northeastward-ejecting shortwave trough over the northern/central
   Plains. A surface low over the northern High Plains should
   consolidate and likewise develop northeastward into southern AB/SK
   by Thursday evening. Low-level moisture is forecast to continue
   streaming northward across the northern/central Plains to the east
   of a surface trough/dryline. Convective initiation may be delayed
   until early Thursday evening owing to a stout low-level temperature
   inversion. Nonetheless, once storms form, they will likely encounter
   moderate to strong instability with steep mid-level lapse rates
   overlying the rich low-level moisture across the warm sector. The
   main limiting factors precluding the introduction of greater severe
   probabilities for large hail and damaging wind gusts remain marginal
   effective bulk shear around 20-30 kt, a late start to initiation,
   and fairly isolated storm coverage overall. Still, potential for an
   upgrade to Slight Risk across far eastern MT into the western
   Dakotas remains possible pending additional guidance. Any storms
   that do form across the warm sector should move northeastward
   Thursday night across ND and vicinity, with a continued isolated
   hail/wind threat.

   ...Northern Rockies/High Plains...
   A more amplified upper trough should move eastward across BC/AB and
   the Pacific Northwest on Thursday. Large-scale lift preceding this
   feature along with terrain-driven circulations should support
   scattered storms over parts of the northern Rockies into the
   adjacent High Plains Thursday afternoon and evening. Low-level
   moisture across this area is expected to be much more limited than
   locations farther east. But, it should still be sufficient in
   combination with steepening mid-level lapse rates and strengthening
   deep-layer flow to support organized storms. Both large hail and
   severe wind gusts appear possible with any storms that initially
   form over the higher terrain of the northern Rockies, and
   subsequently spread eastward into the northern High Plains. Opted to
   expand and combine the previously separate Marginal Risk areas
   across the northern Plains, as there appears to be minimal
   separation between the two potential severe regimes.

   ...Florida Peninsula and the Gulf Coast...
   A tropical wave currently over the eastern Gulf of Mexico may have
   some potential for gradual development over the next few days as it
   moves generally west-northwestward (see latest forecasts from the
   National Hurricane Center for more information). If this slow
   development occurs, then a modestly strengthened low-level flow
   field may support bands of storms capable of producing strong/gusty
   winds across parts of the Florida Peninsula to the central Gulf
   Coast Thursday. Too much uncertainty exists at this time to include
   5% wind probabilities across these regions, but latest observational
   and model trends will continue to be monitored regarding this
   tropical wave.

   ..Gleason.. 07/22/2020

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