SPC AC 210552

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1252 AM CDT Wed Aug 21 2024

   Valid 211200Z - 221200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS OF
   EASTERN MONTANA AND WESTERN NORTH DAKOTA....

   ...SUMMARY...
   Scattered thunderstorms, some with severe hail and damaging gusts, 
   are expected to develop today over parts of far eastern Montana and
   western North Dakota.

   ...Synopsis...
   A mid-level shortwave trough over north-central California early
   this morning will move across the Great Basin and emerge across
   eastern Montana this afternoon. This shortwave trough will be the
   focus for a severe weather threat this afternoon/evening across
   eastern Montana and western North Dakota. Farther east, a mid-level
   trough centered over the Plains will start to break down during the
   day today. As this occurs, surface troughing is forecast to develop
   from the northern Plains to the central High Plains. Scattered
   strong to potentially severe storms are expected along the extent of
   this surface trough from western South Dakota to eastern Colorado. 

   ...Eastern Montana and western North Dakota...
   Significant heating is forecast across eastern Montana this
   afternoon with temperatures forecast to warm into the mid to upper
   90s (depending on the degree of mixing). This will result in
   moderate instability and a mostly uncapped atmosphere by mid to late
   afternoon. DCVA ahead of the approaching mid-level trough will
   result in sufficient deep-layer ascent for thunderstorm development
   by mid to late afternoon. These storms will likely be supercellular
   in nature as mid-level flow strengthens to near 50 knots while the
   associated mid-level jet streak overspreads the region. The
   long-straight hodograph may also support some splitting storms.
   Large hail (up to golf ball size) and severe wind gusts of 60 to 80
   mph will be the primary threats associated with this activity. 

   ...Western South Dakota to eastern Colorado...
   Scattered thunderstorm activity is expected within a broad surface
   trough from western South Dakota to eastern Colorado where moderate
   to strong instability will be in place by mid-afternoon. Mid-level
   flow will be mostly weak across this region (15 to 20 knots) which
   will limit the overall shear magnitude. However, moderate
   southeasterly flow ahead of this surface trough, coupled with the
   westerly mid-level flow may result in adequate shear for storm
   organization and perhaps a few slow-moving supercells. The deeply
   mixed boundary layer across this zone will support a threat for
   severe wind gusts with isolated hail as a lesser threat
   (predominantly associated with any more robust supercell development
   which may occur). A few regions within the marginal risk may have a
   greater threat (southwest South Dakota/northern Nebraska and
   northeast Colorado), but it is not clear at this time whether
   coverage/intensity will be sufficient for a slight risk.

   ...Southeast Arizona...
   A surge of monsoon moisture across southern Arizona will result in
   moderate (1500-2000 J/kg MLCAPE) instability and scattered to
   widespread thunderstorm development this afternoon. Storm
   organization will likely be somewhat weak given the weak shear (~15
   knots). However, the moderate instability in a deeply mixed boundary
   layer will support an isolated severe-wind threat. In addition, the
   widespread nature of the convection will likely result in numerous
   outflow boundary collisions which may locally enhance updraft
   intensity and support a few stronger cells.

   ..Bentley.. 08/21/2024

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