SPC AC 041630

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1130 AM CDT Thu Jun 04 2020

   Valid 041630Z - 051200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS WESTERN TO
   SOUTH-CENTRAL SD...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS SOUTHERN GREAT
   PLAINS AND UPPER OHIO VALLEY/MID-ATLANTIC STATES...

   ...SUMMARY...
   The most likely corridor for severe thunderstorms is across parts of
   western to south-central South Dakota where very large hail, severe
   wind gusts, and a couple tornadoes are possible. Elsewhere,
   scattered severe thunderstorms are possible in the Upper Midwest,
   southern Great Plains, and Upper Ohio Valley to Mid-Atlantic States.

   ...Northern Great Plains through Mid Missouri Valley...
   Primary change is to upgrade to a cat 3/Enhanced Risk across parts
   of western to south-central SD for large hail given the potential of
   a couple long-track supercells.

   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
   mid-level impulse over eastern SD, an additional impulse/speed max
   emanating from the northern Rockies will reach the Dakotas/Mid-MO
   Valley by late afternoon/early evening.

   A corridor of modest low-level moisture will be maximized near a
   front across SD where low/mid 60s F surface dewpoints are expected,
   while 50s/some low 60s F dewpoints will exist across NE 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, isolated to scattered storms are expected to
   initially develop off the Big Horns and spread into western SD
   during the late afternoon The aforementioned westerlies will yield
   around 50 kt effective bulk shear, plentiful for at least a few
   supercells capable of very large hail late this afternoon/early
   evening. A couple of these may become longer-tracked as signaled by
   majority of morning CAM guidance. These supercells may congeal into
   a cluster/small MCS as they spread southeast across the Mid-MO
   Valley tonight. With activity impinging on the northern corridor of
   strong buoyancy emanating north from the eastern KS vicinity, the
   setup could result in sustaining severe wind potential overnight
   across the valley into IA. 

   ...Southern Great Plains...
   Hot temperatures from the upper 90s to slightly above 100 will occur
   with the boundary layer becoming moderately unstable (1500-2000 J/kg
   MLCAPE) to the east of a lee trough/dryline. Scattered thunderstorms
   are likely to develop by late afternoon across the Panhandles and
   spread into western OK and western north TX in the early evening.
   This 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
   moderate (25-35 kt) vertical shear supportive of mostly multicells,
   but some marginal supercell structures are also possible. Severe
   wind gusts and some hail will be the primary risks before waning
   after dusk when MLCIN substantially increases.

   ...Upper OH Valley to Mid-Atlantic States...
   A moist air mass with 60s surface dew points will remain across the
   region. While the storms are not expected to be as intense as
   yesterday across the Mid-Atlantic, moderate warm sector
   destabilization should allow for an intensification of storms this
   afternoon. Ascent tied to an MCV near the OH/IN border and along a
   developing lee trough east of the central Appalachians should
   support scattered thunderstorms during the late afternoon to early
   evening. 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 marginally severe hail is possible.

   ...Lower OH Valley to Mid-South...
   An MCV is drifting east across southeast MO. Thinning of cloud
   coverage in vicinity of this feature should support some
   boundary-layer heating beneath weak mid-level lapse rates. While
   CAMs suggest a late day increase in convection amid sufficient
   mid-level flow to support multicell clustering, uncertainty exists
   in whether buoyancy will be large enough to warrant a greater risk
   beyond locally damaging winds.

   ..Grams/Moore.. 06/04/2020

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