SPC AC 221727

   Day 2 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1227 PM CDT Sat May 22 2021

   Valid 231200Z - 241200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
   THE CENTRAL AND NORTHERN HIGH PLAINS INTO THE NORTHERN PLAINS...

   ...SUMMARY...
   Severe thunderstorms capable of severe gusts and large hail are
   possible across portions of the northern Great Plains tomorrow
   afternoon and evening.

   ...Synopsis...
   A mid-level trough will eject into the northern High Plains, with
   surface lee troughing encouraging the northward advection of a warm,
   relatively moist airmass across the northern and central CONUS
   tomorrow. Deep-layer ascent associated with the approaching trough,
   and strong surface heating with upslope flow will support scattered
   strong to severe thunderstorm development along the lee of the
   Rockies, from the U.S./Canada to Mexico borders. Upper ridging and
   surface high pressure will dominate much of the Ohio Valley into the
   Southeast, with deep-layer northwesterly flow prevailing across the
   Northeast, where scattered storms (some strong to locally severe)
   are likely tomorrow afternoon ahead of a cold front.

   ...Northern High Plains into the Dakotas...
   Weak surface low development is expected along a warm front across
   the northern High Plains during the morning hours, with the low
   expected to be positioned across western SD by afternoon peak
   heating. Ahead of the low, low 60s F dewpoints will reach northern
   SD during the afternoon, overspread by 7+ C/km mid-level lapse
   rates, supporting widespread 1500 J/kg MLCAPE. 30-50 kts of
   effective bulk shear is expected ahead of an eastward moving cold
   front given ample speed shear. However, troposheric flow will be
   meridional, with unidirectional wind shear above 700 mb expected
   across the central/northern Plain states tomorrow. As such, storms
   should rapidly grow upscale into linear segments, with damaging
   winds and large hail expected as the primary severe hazards. Ahead
   of the surface low and along the warm front across portions of
   western SD, relatively greater backing of the surface winds will
   contribute to locally better low-level veering vertical wind
   profiles. Modestly enlarged hodographs/200+ m2/s2 effective SRH will
   promote some tornado threat with circulations embedded in line
   segments, along with any storms that can manage to stay discrete
   ahead of the line.

   ...Central into the southern High Plains...
   Low-level convergence due to upslope flow and afternoon peak heating
   will encourage scattered thunderstorm development along the lee of
   the higher terrain. Ahead of the storms, a moist low-level airmass
   (characterized by low 60s F dewpoints) will contribute to 1000-1500
   J/kg MLCAPE. Relatively weak deep-layer and low-level shear will
   foster cold pool mergers with multicellular clusters/transient
   supercells upscale-growing into line segments an hour or so after
   maturity. Large hail may occur with the initial, more discrete storm
   modes, with damaging gusts becoming more prevalent after upscale
   growth.

   ...Northeast...
   Mid 60s F dewpoints beneath 5.5-6 C/km mid-level lapse rates will
   foster 500-1000 J/kg MLCAPE ahead of the cold front, where at least
   isolated thunderstorm development is expected tomorrow afternoon.
   While tropospheric flow will be unidirectional (northwesterly),
   40-50 kts of effective bulk shear suggests that some of the storms
   may become organized/strong. As temperatures warm into the 80s F,
   modest boundary-layer mixing may encourage downward momentum
   transport of the stronger flow aloft, with a couple of damaging
   gusts possible with the stronger storms.

   ..Squitieri/Gleason.. 05/22/2021

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