SPC AC 211250

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0750 AM CDT Sun Jun 21 2020

   Valid 211300Z - 221200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS EASTERN NE TO
   NORTHERN OK...

   ...SUMMARY...
   Numerous severe storms should affect portions of the Great Plains,
   centered on eastern Nebraska to northern Oklahoma. Very large hail,
   intense damaging wind gusts, and a few tornadoes will be possible.

   ...Central States...
   An active severe weather episode is expected with the greatest
   coverage of severe storms anticipated from the Mid-MO Valley into
   Oklahoma during the late afternoon and evening. 

   A slow-moving MCS is ongoing across parts of south-central OK into
   north-central TX. This activity should struggle to intensify as
   low-level warm advection subsides, but isolated severe hail and wind
   will remain possible this morning. For additional short-term
   guidance, please see MCD 951. The remnant MCV should be the focus
   for renewed thunderstorm development across the Ark-La-Tex towards
   the Lower MS Valley. Isolated damaging winds should be the primary
   hazard this afternoon.

   A broadly cyclonic mid-level flow regime will exist to the south of
   a low that slowly shifts east from southern Manitoba into northwest
   Ontario. Multiple perturbations embedded within this flow regime
   should aid in scattered mid to late afternoon thunderstorm
   development along the dryline/lee trough from south-central NE south
   into west TX and a slow-moving front extending northeast into MN.
   The strongest boundary-layer heating is expected from NE southward
   beneath a pronounced EML (well sampled by 12Z LBF/DDC/AMA
   soundings). This should yield large buoyancy with a broad swath of
   MLCAPE from 2500-4000 J/kg. While mid-level flow will not be overly
   strong, 30-40 kt effective bulk shear will yield robust storm
   organization.

   Rapid storm development is likely around 20-21Z across western KS to
   eastern NE. These storms should quickly become severe, with an
   initial threat for scattered large to very large hail with a mix of
   supercells and multicell clusters. With time, storms will congeal
   and grow upscale into multiple MCSs or perhaps very large MCS as a
   southerly low-level jet strengthens this evening from the southern
   High Plains to the Mid-MO Valley. At least scattered severe wind
   gusts appear probable. The corridor for a forward-propagating MCS
   with embedded bows capable of potentially widespread severe wind may
   evolve across parts of southwest to south-central KS and northwest
   to north-central OK along the nose of the stronger low-level jet
   impinging on the northwest extent of upper 60s to low 70s surface
   dew points. However, guidance has been underdone with the MCS across
   the Red River Valley this morning, and this renders uncertainty to
   how pervasive high-quality boundary-layer moisture and resultant
   extreme instability may actually be later today. Thus will defer to
   later outlooks regarding a potential upgrade to category 4/Moderate
   Risk.

   Relatively large surface temperature/dew point spreads should be a
   limiting factor to the overall tornado risk in western KS, but a
   couple tornadoes are possible here within the initial discrete
   supercell phase. Lower spreads into NE may support a risk for a
   couple tornadoes as well, and a couple QLCS tornadoes will be
   possible later in the evening at MCS maturation farther south in
   KS/OK. 

   ...OH and TN Valleys...
   An MCV over far western KY will drift across the OH Valley today.
   Diabatic heating will be slowed near the MCV by relatively
   widespread early-day stratiform. Nevertheless, boundary-layer
   heating farther east and south of the morning precip may overlap
   with modest enhancement to mid-level flow to support multicell
   clustering. Isolated damaging winds should be the primary threat.

   ..Grams/Kerr.. 06/21/2020

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