SPC AC 231953

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0253 PM CDT Sun May 23 2021

   Valid 232000Z - 241200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
   THE CENTRAL INTO NORTHERN HIGH PLAINS...

   ...SUMMARY...
   Scattered to numerous severe thunderstorms are still expected across
   a large portion of the High Plains this afternoon into evening. The
   most likely corridor for a few tornadoes, large hail, and
   significant severe wind is across northeast Colorado and western
   portions of South Dakota/Nebraska.

   ...20Z Update...
   No major changes have been made to the current Day 1 Outlook. Strong
   to occasionally severe storms have already developed along the CO
   Front Range, and additional thunderstorm development and
   intensification is still expected across much of the High Plains
   into the early evening hours. Storms will start off as initially
   discrete, with the potential to produce large hail (some
   significant) and perhaps a few tornadoes, especially from northeast
   CO northeastward, amidst backed, strengthening low-level flow.
   However, a relatively quick transition to linear convective modes
   will transpire, with damaging gusts becoming the primary concern
   from ND to the Trans Pecos region in southwest TX.

   While convection has struggled to organize thus far across portions
   of southern New England into the Mid Atlantic, surface temperatures
   have warmed into the 80s to near 90F, with 18Z RAP forecast
   soundings depicting a dry sub-cloud layer. As such, potential
   remains for a damaging gust into the early evening hours.

   ..Squitieri/Gleason.. 05/23/2021

   .PREV DISCUSSION... /ISSUED 1136 AM CDT Sun May 23 2021/

   ...Near Colorado Front Range/North-Central High Plains...
   On the southeast periphery of the prominent upper trough centered
   over the Great Basin/Intermountain West, a shortwave trough/speed
   max is readily apparent in water vapor imagery over the Four Corners
   area late this morning, and are likely to reach the north-central
   High Plains by around peak heating. Surface cyclogenesis will
   continue across the Nebraska Panhandle toward the Black Hills in
   vicinity of a northward-shifting warm front, while secondary surface
    cyclogenesis will also occur across southeast/east-central
   Colorado. A relatively moist air mass extends west-northwestward
   toward the Front Range/Black Hills with upper 50s/lower 60s F
   surface dewpoints. Moderately steep mid-level lapse rates of 7-7.5
   C/km should support an increasingly expansive plume of moderate
   buoyancy with MLCAPE of 1000-2000 J/kg by peak heating. 

   As pronounced mid-level height falls overspread the region this
   afternoon into evening and lingering cloud cover/inhibition quickly
   erode, thunderstorms will rapidly develop by early to mid-afternoon
   within a strengthening deep-layer shear regime. Supercells will be
   favored within the first couple hours of initiation, but a
   transition into one or more northeastward-moving QLCS are expected
   especially across westerns portions of South Dakota/Nebraska. Large
   hail will be likely mainly early, with severe wind gusts becoming
   the primary hazard as the aforementioned upscale growth occurs. A
   somewhat more focused tornado potential via a somewhat longer
   duration of discrete supercellular mode may exist across
   northeast/east-central Colorado into adjacent southwest Nebraska,
   although tornado potential will exist farther north in vicinity of
   the warm front even with a more mixed/transitional convective mode.

   ...Southern High Plains...
   Isolated to scattered thunderstorms are expected to develop this
   afternoon near the dryline, including near the Raton Mesa southward
   to the Trans-Pecos of far west Texas. A mix of supercells and
   multicell structures will be most favored in the Raton Mesa vicinity
   along the glancing periphery of the ejecting trough. Large hail,
   severe wind gusts, and a tornado or two are possible.

   ...Northeast States...
   Convective coverage will likely remain sparse owing to modest
   convergence along a predominately west/east-oriented cold front,
   which will be pushing south across the region this afternoon.
   Despite weak mid-level lapse rates, surface temperatures warming
   into the 80s amid upper 50s to low 60s dew points should support
   weak buoyancy with MLCAPE of 250-750 J/kg. Along the periphery of a
   mid-level speed max ejecting across northern Maine, adequate and
   nearly unidirectional deep-layer shear will exist for a few cells
   capable of producing locally damaging winds.

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