SPC AC 121959

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0259 PM CDT Thu May 12 2022

   Valid 122000Z - 131200Z

   ...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS FOR PORTIONS OF
   NORTHEAST NEBRASKA...EASTERN SOUTH DAKOTA...SOUTHEAST NORTH
   DAKOTA...AND WESTERN MINNESOTA...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS THE LOWER
   MISSISSIPPI VALLEY...

   ...SUMMARY...
   Scattered to numerous severe thunderstorm gusts (some 75+ mph),
   large hail and a few tornadoes are expected over parts of the
   eastern Dakotas, eastern Nebraska, western Iowa, and
   central/southern Minnesota this afternoon into tonight. Other severe
   storms are expected across the southern Plains and Lower Mississippi
   Valley.

   ...Discussion...
   Only two notable changes were made to the previous convective
   outlook and are listed below.
   1) Upgraded severe wind probabilities from 30% to 45% for northern
   portions of NE and southeast SD immediately ahead of an evolving bow
   that is forecast to quickly move north-northeast at 55 kt.  Severe
   gusts ranging from 60-100 mph are possible in the short-term over NE
   into southeast SD over the next several hours.  See MCD #737 for
   short-term details.
   2) Reduced hail probabilities from 30% to 15% for NE and focused the
   significant hail area to be over eastern SD where supercell
   convective mode is most likely.

   ..Smith.. 05/12/2022

   .PREV DISCUSSION... /ISSUED 1156 AM CDT Thu May 12 2022/

   ...Northern/central Plains and Upper Midwest...
   Have made spatial (westward) adjustments to the categorical Moderate
   Risk along with some increased potential for initial severe-discrete
   storms, which includes the potential for a strong tornado, across
   South Dakota. In general, scattered to numerous thunderstorms are
   expected to develop in an arc near the surface low and cold front by
   mid/late afternoon, from central/eastern South Dakota southward
   across central/eastern Nebraska and into at least northern Kansas. A
   brief interval, early in the convective cycle, may support discrete
   to semi-discrete supercells before the convection becomes
   quasi-linear. The most common severe type should evolve quickly to
   thunderstorm gusts through early evening -- some of which may be
   significant (65+ kt) strength, especially from parts of eastern
   South Dakota and southeast North Dakota into western Minnesota.

   The warm sector is forecast to destabilize throughout the day, with
   a combination of at least weak large-scale ascent (increasing
   northward), diurnal heating and related lift from below, and
   ultimately frontal convergence, eliminating a basal EML inversion
   and supporting convection initiation. Activity is expected to
   intensify quickly as it impinges on a narrow but very favorable
   prefrontal corridor where 60s to near 70 F surface dewpoints
   contribute to MLCAPE in the 3500-4500 J/kg range in and near the
   "moderate" area, decreasing gradually with southward extent as more
   low-level moisture is mixed out. Low-level and deep shear will be
   greatest near the triple point and warm front across South Dakota,
   with 300-500 m2/s2 effective SRH and 45-55-kt effective-shear
   magnitudes. A substantial component of mid/upper winds parallel to
   the axis of convective forcing indicates potential for fairly fast
   merging of early discrete and sporadically supercellular convection.
   Surges of wind from resulting LEWP/bowing segments will pose the
   greatest overall severe hazard, with line-embedded mesovortices and
   perhaps a few associated tornadoes also possible. With the warm
   sector's not being very broad, the convective event should diminish
   late this evening into early overnight hours as it outruns the most
   favorable instability.

   ...Eastern Minnesota/northern Wisconsin/northern Michigan today...
   An ongoing linear complex of storms may persist eastward today in
   vicinity of the warm front and nearby moist/unstable air mass. For
   additional details, see Mesoscale Discussion 733.

   ...Portions of Mississippi/Louisiana and western Alabama...
   Have upgraded portions of the region to a categorical Slight Risk.
   Morning upper-air data reflects moderately strong
   north-northeasterly mid/high-level winds across the region to the
   west of the low centered in the Atlantic off the coastal Southeast.
   Ample insolation/heating is occurring in conjunction with a moist
   air mass across the region. This will contribute to scattered
   thunderstorm development this afternoon with gradual clustering and
   some potential for upscale evolution into multiple
   southwestward-moving semi-organized clusters. Isolated wind damage
   is expected to the primary severe hazard.

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