SPC AC 181623

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1123 AM CDT Tue Mar 18 2025

   Valid 181630Z - 191200Z

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS TONIGHT ACROSS
   PARTS OF SOUTHEASTERN NEBRASKA THROUGH SOUTHWESTERN AND CENTRAL
   IOWA...

   ...SUMMARY...
   Isolated, marginally severe hail is possible by late tonight in a
   corridor from north central Kansas through central Iowa.

   ...Discussion...
   Much of the nation remains under the influence of one prominent belt
   of westerlies emanating from the northern mid-latitude Pacific. 
   Within this regime, mid-level troughing is in the process of digging
   across the Intermountain West and Rockies.  This includes one
   significant embedded smaller-scale perturbation approaching the
   southern Rockies, which is forecast to accelerate east-northeastward
   across the south central Great Plains by late tonight.

   Models indicate that this will be accompanied by a strong mid/upper
   jet streak, including a core intensifying in excess of 100 kt around
   500 mb, nosing across the Texas Panhandle vicinity through the lower
   Missouri Valley from early this evening through 12Z Wednesday.  It
   appears that this may be accompanied by substantive further
   deepening of a developing low across central through northeastern
   Kansas, within already deep surface troughing now present across the
   central/southern Great Plains into lower/middle Missouri Valley.

   In the wake of the prior cyclone which impacted much of the central
   and eastern U.S. and Gulf Basin late last week into the weekend, 
   boundary-layer moistening across the western Gulf is underway, but
   still only supportive of rather modest low-level moistening across
   the Texas Gulf coast into southeastern Great Plains (including low
   to mid 50s F surface dew points).  It appears that this will reach
   portions of the lower central Great Plains into Upper Midwest late
   this evening through early Wednesday.  However, relatively warm and
   dry air in the lower/mid-troposphere, associated with a broad plume
   of elevated mixed-layer air overspreading the central/southern Great
   Plains and middle/lower Mississippi Valley, will tend to impede an
   appreciable risk for thunderstorm development.

   ...Central Great Plains toward the Upper Midwest...
   Perhaps as early as late this evening, into the overnight hours,
   model output continues to indicate that strengthening mid/upper
   forcing for ascent, coupled with better low-level moisture return,
   will overcome the inhibition and support a corridor of thunderstorm
   development.  This likely will be focused within strengthening
   lower/mid-tropospheric warm advection, along a frontal zone
   extending to the east-northeast of the surface cyclone, across
   southeastern Nebraska through central Iowa by late tonight.  

   Based at least on the latest NAM, it appears that the more sustained
   and stronger thunderstorm development may initiate as early as late
   evening near/east-northeast of the Concordia KS vicinity, before
   spreading northeastward.  This seems likely to occur as convection
   begins to become increasingly rooted below the 700 mb level, closer
   to 850 mb, with soundings perhaps becoming characterized by "loaded
   gun" thermodynamic profiles above the near-surface stable layer.

   Although most unstable CAPE may mostly peak in the 500-750 J/kg
   range, stronger cells may become conducive to marginally severe
   hail, aided by strong shear in the convective layer.  

   The potential for strong surface gusts is somewhat more uncertain. 
   Although the risk for strong downdrafts to reach the surface appears
   negligible, gravity wave induced surface pressure perturbations
   supportive of gusty winds may not be entirely out of the question.

   ..Kerr/Bunting.. 03/18/2025

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