SPC AC 171206

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0706 AM CDT Tue Sep 17 2024

   Valid 171300Z - 181200Z

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

   ...SUMMARY...
   Thunderstorms capable of severe wind gusts and isolated large hail
   will be possible this afternoon and evening across parts of the
   central and northern High Plains.

   ...High Plains/Central and Southern Rockies...
   Water-vapor imagery early this morning shows a potent mid-level low
   and associated trough centered over northeast NV.  This feature will
   become negatively tilted and move into the High Plains late today
   while the low moves east-northeast reaching southeast MT Wednesday
   morning.  Strong attendant forcing for ascent will overspread the
   High Plains this afternoon into the early evening with an
   accompanying 50-60 kt 500-mb speed max moving into CO and eventually
   western NE/SD.  In the low levels, a lee trough will sharpen with
   southerly flow maintaining a fetch of seasonably adequate moisture
   (50s deg F surface dewpoints) into the High Plains.

   Ahead of the approaching trough, partial clearing will result in
   strong surface heating this afternoon over the central High Plains.
   MLCAPE is expected to peak near 1500 J/kg from eastern Colorado
   northward into western Nebraska.  Initial storm activity late this
   morning through the early afternoon will develop over the high
   terrain of CO/northern NM and move/develop east into richer moisture
   by mid-late afternoon.  The increasing large-scale ascent coupled
   with heating will eventually result in numerous storms mainly in the
   form of linear clusters evolving into linear bands of convection by
   early evening.  The greatest combination of instability, lift
   and deep-layer shear is forecast across western Nebraska and
   northeast Colorado, suggesting an enhanced threat for severe wind
   gusts.  Based on some of the latest CAM data, have expanded the
   Slight-Risk equivalent wind probabilities farther south into
   northeast NM/northwest TX Panhandle.  Some consideration was given
   to expand the ENH Risk into southwestern SD, but the later timing of
   storms lends enough uncertainty to preclude this change this outlook
   update.

   ...Northern High Plains...
   Ahead of the approaching mid-level low, divergent south to
   south-southeasterly flow will be in place across the northern
   Rockies and northern Plains.  During the afternoon, thunderstorms
   will develop in the north-central Rockies and move north to
   northeastward across the northern High Plains.  An axis of moderate
   instability is forecast by afternoon from the Dakotas northwestward
   into northeast Montana.  As large-scale ascent and moderate
   deep-layer shear overspread the northern High Plains late this
   afternoon and evening, organized storms will be possible.  Severe
   wind gusts and isolated large hail will be the primary threats.

   ...Eastern Florida...
   A mid-level trough will move across the eastern Gulf of Mexico
   today, as an associated surface trough moves across the Florida
   Peninsula. Surface dewpoints in the lower to mid 70s F, along with
   surface heating will contribute to moderate instability by
   afternoon. As surface temperatures warm today, scattered
   thunderstorms are expected to develop along and ahead of the surface
   trough.  RAP forecast soundings at 21Z in eastern Florida have
   MLCAPE in the 2000 to 3000 J/kg range, with 0-6 km shear around 20
   knots.  This, combined with steep low-level lapse rates could be
   enough for a marginal threat for locally severe wind gusts.

   ..Smith/Mosier.. 09/17/2024

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