Sep 21, 2017 0730 UTC Day 3 Severe Thunderstorm Outlook
Updated: Thu Sep 21 06:46:53 UTC 2017 (20170921 0730Z Day 3 shapefile | 20170921 0730Z Day 3 KML)
Probabilistic to Categorical Outlook Conversion Table
Categorical Graphic
0730 UTC Day 3 Outlook Graphic
Day 3 Risk Area (sq. mi.) Area Pop. Some Larger Population Centers in Risk Area
No Risk Areas Forecast
Probabilistic Graphic
20170921 0730 UTC Day Probabilitic Graphic
Probability of severe weather within 25 miles of a point.
Hatched Area: 10% or greater probability of significant severe within 25 miles of a point.
Day 3 Prob. Risk Area (sq. mi.) Area Pop. Some Larger Population Centers in Risk Area
No Risk Areas Forecast
   SPC AC 210646

   Day 3 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0146 AM CDT Thu Sep 21 2017

   Valid 231200Z - 241200Z


   A very low severe risk will exist across portions of the southern
   and central High Plains northeastward to central Minnesota.

   A longwave trough will continue to persist across the West while
   only making slow eastward progress.  Meanwhile, ridging will remain
   in place across the Ohio Valley and Great Lakes region, and strong,
   meridionally oriented flow will remain in place across the High
   Plains and Upper Mississippi Valley.  

   At the surface, a nearly stationary boundary will extend from
   northwestern Minnesota south-southwestward to a surface low over
   eastern Colorado.  A surface trough will extend southward from the
   low across eastern New Mexico.  To the east of these features,
   southerly low-level flow will maintain a modified maritime tropical
   airmass across much of the Plains and Upper Mississippi River
   Valley.  An anticyclone will remain nearly stationary across the
   East, while surface ridging slowly builds into portions of the
   northern Plains.

   Sustained convergence along the surface boundary and lee trough
   should foster ongoing deep convection at the beginning of the
   period, and these storms should increase in coverage and intensity
   throughout the day in response to surface heating and attendant
   destabilization.  Although shear profiles in much of the Plains
   would support updraft organization (and perhaps a few instances of
   rotation), marginal mid-level lapse rates will act to limit the
   overall magnitude of the severe risk in most areas.  Low (5%) severe
   risk areas may be needed in later outlooks where late-day
   destabilization can be maximized.

   ..Cook.. 09/21/2017