SPC AC 170712

   Day 3 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0212 AM CDT Fri Sep 17 2021

   Valid 191200Z - 201200Z

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS LATE SUNDAY
   AFTERNOON AND EVENING ACROSS PARTS OF THE EASTERN DAKOTAS AND
   WESTERN MINNESOTA...

   ...SUMMARY...
   Scattered strong storms may impact parts of the eastern Dakotas and
   western Minnesota late Sunday afternoon and evening, accompanied by
   at least some risk for severe weather.

   ...Synopsis...
   A belt of westerlies across the northern mid-latitudes of the
   Pacific through North America will remain modestly amplified through
   this period and beyond.  Within this regime, larger-scale mid-level
   troughing is forecast to continue to evolve while shifting eastward,
   from the Pacific Northwest through the northern Rockies and adjacent
   Great Plains.  In response to the approach of this trough, weak
   mid-level troughing, with perhaps a lingering embedded
   lower/mid-level cyclonic circulation, may accelerate
   north-northeastward toward the lower Ohio Valley

   Seasonably high moisture content will remain confined to a broad
   plume east/southeast of this lower latitude troughing, across much
   of the Southeast into southern western Atlantic, to the south and
   west of a wind shift associated with a possible developing tropical
   cyclone progressing away from the Atlantic Seaboard.  However,
   models do indicate a gradual further moistening of southerly return
   flow to the east of surface troughing across the Great Plains.  It
   appears that this will contribute to increasing potential
   instability beneath  a narrow plume of warm elevated mixed-layer air
   initially over the northern Great Plains at 12Z Sunday.

   ...Parts of eastern Dakotas into western Minnesota...
   The most substantive boundary-layer destabilization Sunday appears
   likely to become focused along and just east of a corridor of
   stronger daytime heating, beneath the warmest elevated mixed-layer
   air.  At 12Z Sunday, it appears that this elevated mixed-layer may
   extend in a narrow plume across central South Dakota through eastern
   North Dakota, before slowly shifting eastward during the day, and
   eroding in response to lift associated with a weak short wave
   perturbation preceding the primary approaching mid-level troughing. 

   It appears possible that thunderstorm initiation may coincide with
   the peak late afternoon or early evening boundary-layer instability,
   when CAPE may reach 2000+ J/kg, in the presence of modestly sheared,
   30-50 kt southwesterly flow in the 850-500 mb layer.  If this
   occurs, organized convective development is possible, initially
   including a few supercells, then one or two upscale growing
   clusters, with potential to produce severe hail and wind.

   Thunderstorm development may continue and perhaps increase southward
   toward the central Great Plains overnight, as a cold front
   progresses east of the Rockies and merges into the lee surface
   troughing.  However, in the presence of weak and/or waning
   boundary-layer instability, the risk for severe weather is expected
   to become increasingly negligible.

   ..Kerr.. 09/17/2021

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