SPC AC 041259

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0759 AM CDT Thu Jun 04 2020

   Valid 041300Z - 051200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS THE
   NORTHERN PLAINS/UPPER MIDWEST AS WELL AS THE SOUTHERN PLAINS AND
   MID-ATLANTIC STATES...

   ...SUMMARY...
   Severe thunderstorms are expected across parts of the northern and
   Central Plains to Middle Missouri and Upper Midwest today and
   tonight. Other severe thunderstorms are expected across the southern
   Plains as well as the Northeast/Mid-Atlantic States.

   ...Northern Plains through Mid Missouri Valley...
   Low-amplitude westerlies with embedded mid-level perturbations and
   strengthening late-day winds aloft (reaching 55-70kt at 500 mb over
   the Dakotas) will influence the region. In the wake of a weak
   shortwave trough over South Dakota this morning, an additional
   mid-level impulse/speed max emanating from the International
   Border-vicinity Rockies will reach the Dakotas/middle Missouri
   Valley by late afternoon/early evening.

   A corridor of modest low-level moisture will be maximized near a
   front across South Dakota where low/mid 60s F surface dewpoints are
   expected, while 50s/some low 60s F dewpoints will exist across 
   Nebraska near/east of a surface trough across other northern High
   Plains areas within a post-frontal upslope regime. This moisture
   beneath 7.5-8.5 C/km mid-level lapse rates will generally result in
   1500-2000 J/kg MLCAPE by peak heating. 

   As air mass destabilization occurs and forcing for ascent increases
   into the region, widely scattered storms are expected to initially
   develop/intensify this afternoon across the western Dakotas and
   possibly far southeast Montana/northeast Wyoming, with a further
   upswing in coverage/intensity as storms increase in vicinity of the
   frontal zone across South Dakota. The aforementioned westerlies will
   yield 40-50 kt effective bulk shear, plentiful for initial
   supercells capable of very large hail late this afternoon/early
   evening. Storms could congeal into one or more clusters as they
   spread southeastward this evening with damaging wind/hail potential
   continuing. 

   A categorical upgrade to Enhanced Risk may be warranted in future
   outlooks, should confidence increase in terms of a greater
   coverage/likelihood of severe.

   ...Southern Plains...
   Ample heating (near or above 100F in some locations) will occur with
   the boundary layer becoming moderately unstable (1500-2500 J/kg
   MLCAPE) to the east of a lee trough/dryline. At least widely
   scattered thunderstorms are likely to develop by mid/late afternoon,
   particularly across the Texas Panhandle and western Oklahoma. Such
   development will likely be aided by a subtle ridge-riding mid-level
   impulse, which should be favorably timed with peak heating/maximum
   destabilization. Storms will be high based with modest (25-35 kt)
   vertical shear supportive of mostly multicells, but some marginal
   supercell structures are also possible. Large hail and
   severe-caliber wind gusts will be the primary risks. Activity may
   congeal into a southeastward-moving MCS with a continuing wind risk
   during the evening, potentially reaching the Texas Low Rolling
   Plains, western North Texas, and/or south-central Oklahoma.

   ...Northeast/Mid-Atlantic States...
   A moist air mass (generally low to mid 60s F surface dewpoints) will
   remain in areas along/south of a roughly west/east-oriented front
   across the region. While the storms are not expected to be as
   intense/fast-moving as yesterday across the Mid-Atlantic, moderate
   warm sector destabilization should allow for an increase and
   intensification of storms into the afternoon. This will also be
   influenced by weak mid-level ascent paralleling the middle/upper
   Ohio River Valley, at least partially influenced by earlier MCV
   development. Such anticipated storm development should initially
   occur near the mountains/higher terrain and along the lee trough
   this afternoon. A modestly strong belt of westerlies aloft will
   contribute to around 25-35 kt of effective shear, which will be
   sufficient for organized/sustained multicells. Wind damage will be
   the most probable severe hazard, but some hail is possible as well.

   ..Guyer/Gleason.. 06/04/2020

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