SPC AC 200559

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1259 AM CDT Sat Jun 20 2020

   Valid 201200Z - 211200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FOR PORTIONS OF
   THE DAKOTAS INTO NEBRASKA...

   ...SUMMARY...
   A few strong to severe storms will be possible over the Dakotas into
   Nebraska this afternoon and evening, with a more isolated threat
   over a large area from the southern Plains into the Mississippi
   Valley and also portions of the Carolinas. Isolated large hail and
   locally damaging gusts will be the main hazards with these storms.

   ...Synopsis...
   A mid/upper-level trough will move from the central/southern Plains
   into the mid-MS Valley and upper Midwest today, with multiple MCVs
   likely to move in conjunction with the larger-scale trough as they
   become embedded within modest west/southwest flow aloft. Another
   shortwave trough is forecast to move into the Dakotas on the
   southern periphery of a closed mid/upper-level low over the Canadian
   Prairies. The persistent upper-level trough/low over the East is
   forecast to shift slowly eastward through the period. 

   ...Dakotas/Nebraska...
   Scattered thunderstorm development is expected this afternoon from
   eastern ND into central SD and north-central NE, in conjunction with
   the approaching shortwave trough and a weak surface trough. While
   low-level moisture will be relatively limited (dewpoints largely in
   the 50s F), cool midlevel temperatures will allow MLCAPE of 500-1500
   J/kg to develop as the boundary layer warms. Effective shear of
   25-35 kt will support organized multicells or perhaps a supercell or
   two, with an initial threat of hail. Steep low-level lapse rates
   will also support an isolated severe wind risk, especially with any
   upscale-growing clusters that may develop during the late afternoon
   or early evening.

   ...TX/OK...
   Convection will likely be ongoing at the start of the period this
   morning across portions of TX/OK. This activity will likely weaken
   with time as the low-level jet weakens by midday, with a composite
   outflow boundary expected to move slowly northward somewhere across
   north TX/southern OK from afternoon into early evening. The best
   chance of surface-based thunderstorm development during the
   afternoon is expected over the higher terrain of southwest TX, where
   strong instability and modestly favorable vertical shear will again
   favor high-based thunderstorms capable of localized severe
   hail/wind. 

   Further north, surface-based development along the remnant outflow
   boundary cannot be ruled out through early evening, though the more
   likely scenario is for slightly elevated convection to develop late
   evening/overnight near/north of the boundary, in response to an
   increasing low-level jet. This activity would pose an initial threat
   of isolated large hail, with some upscale growth potentially
   resulting in a damaging wind risk as well. Considerable uncertainty
   remains regarding the location of the boundary and timing of
   initiation, so a broad Marginal Risk has been maintained from north
   TX into OK. 

   ...Mid-MS Valley into the upper Midwest...
   A broad region from the mid-MS Valley into the Upper Midwest will
   likely become moderately unstable this afternoon, in advance of a
   weak front. Scattered thunderstorm development and/or
   intensification of morning convection is possible after midday near
   the frontal boundary, and also possible in association with any MCVs
   moving across the region. Effective shear of 25-35 kt will be
   marginally supportive of organized convection, with an attendant
   risk of locally damaging wind and marginally severe hail. 

   ...Carolinas...
   As the upper trough/low shifts slowly eastward, midlevel flow will
   become northwesterly across the Carolinas. Scattered thunderstorm
   development is expected by early afternoon, most likely focused
   along the sea breeze. Elongated hodographs may support at least
   weakly organized storms capable of locally damaging wind and
   marginally severe hail.

   ..Dean/Squitieri.. 06/20/2020

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