Storm Prediction Center Mesoscale Discussion 2320

   Mesoscale Discussion 2320
   NWS Storm Prediction Center Norman OK
   0901 PM CDT Sun Sep 20 2026

   Areas affected...portions of eastern Colorado and western Kansas

   Concerning...Severe Thunderstorm Watch 673...

   Valid 210201Z - 210400Z

   The severe weather threat for Severe Thunderstorm Watch 673
   continues.

   SUMMARY...The initial severe threat has come to an end across the
   area. However, the region is being monitored for additional
   thunderstorm development, such as that suggested by latest numerical
   models. A new watch or the extension in space or time of the
   existing watch may become necessary before Severe Thunderstorm Watch
   #673 expires at 9:00 PM MDT.

   DISCUSSION...The long-track supercell that moved southeast across
   the Plains of Colorado this evening has dissipated bringing an end
   to the initial severe threat. This evolution was handled very well
   by high-resolution guidance. 

   Attention is now focused on areas to the north where high-resolution
   guidance is indicating additional thunderstorm development is likely
   in the next 60-90 minutes. This scenario seems plausible given the
   persistent moist, easterly flow into the foothills expected this
   evening. Initial development may already be underway across portions
   of the Denver and Fort Collins metro areas based on MRMS CREF
   imagery. 

   The overall environment will remain conducive for supercells with
   MUCAPE values in excess of 1000 J/kg, especially south and east of
   the Denver metro. Effective-layer shear remains more than sufficient
   for updraft rotation, with shear values between 40-55 knots across
   the Colorado Plains. Large hail and strong thunderstorm outflows
   will be possible with any sustained supercell this evening and
   overnight. 

   Severe Thunderstorm Watch #673 is scheduled to expire at 9 PM MDT.
   The area will continue to be monitored for additional thunderstorm
   initiation and intensification. If it becomes apparent that
   additional severe thunderstorms are probable this evening, the
   region will either need a new Severe Thunderstorm Watch or an
   extension of Severe Thunderstorm Watch #673. A decision will be made
   regarding the need for a new watch, an extension of the existing
   watch, or cancellation of the existing watch within the hour.

   ..Marsh.. 09/21/2026

   ...Please see www.spc.noaa.gov for graphic product...

   ATTN...WFO...DDC...GLD...PUB...BOU...

   LAT...LON   37620267 38560409 39030494 39620532 40110518 40290471
               40250355 40110245 39960193 39750156 39360106 38750079
               37890098 37630143 37620267 

   MOST PROBABLE PEAK TORNADO INTENSITY...UP TO 90 MPH
   MOST PROBABLE PEAK WIND GUST...55-70 MPH
   MOST PROBABLE PEAK HAIL SIZE...1.50-2.50 IN