SPC AC 211709

   Day 2 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1209 PM CDT Thu Aug 21 2025

   Valid 221200Z - 231200Z

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS
   OF THE NORTHERN/CENTRAL HIGH PLAINS TO THE UPPER MIDWEST...AND OVER
   SOUTHWEST AZ AND FAR SOUTHEAST CA...

   ...SUMMARY...
   Strong to locally severe thunderstorms are possible Friday from
   parts of the central and northern High Plains into the Upper
   Midwest, and across southwest Arizona into far southeast California.

   ...Northern/Central High Plains to Upper Midwest...

   An upper cyclone centered over Manitoba Friday morning and its
   attendant shortwave trough extending into the northern Plains will
   develop southeast into Ontario and Upper Midwest through early
   Saturday. At the surface, a trailing cold front will spread
   east/southeast across the region, extending from northeast MN into
   southwest NE/northeast CO by 00z, before continuing on to portions
   of the U.P. of MI to southwest KS/southeast CO by Saturday morning.

   Ahead of the front, seasonal boundary layer moisture will be in
   place, and pockets of weak to moderate destabilization are forecast.
   However, lapse rates are generally expected to remain modest across
   the Upper Midwest, which may limit updraft intensity somewhat.
   Additionally, remnant convection from the Day 1/Thu period may be
   ongoing across parts of MN at the beginning of the period. The
   extent of this activity and associated cloud cover may temper
   stronger heating/destabilization as well. Nevertheless, as
   large-scale ascent increasing with the approach of the upper trough
   and along the surface cold front, isolated strong to severe storms
   may develop during the afternoon and evening, posing a risk for
   strong gusts and marginal hail.

   Further southwest into the High Plains, the post-frontal upslope
   flow regime will likely result in isolated thunderstorm development
   across parts of western WY/CO, with convection spreading into parts
   of SD/NE. Steeper lapse rates and a deeper, mixed boundary layer may
   support strong/severe gusts and isolated hail.

   ...Southwest AZ into far southeast CA...

   A surge of monsoonal moisture is expected over the region on Friday
   amid steep lapse rates and hot boundary layer conditions. This
   should support modest destabilization, with strong DCAPE near 1500
   J/kg noted in forecast soundings. Around 20-35 kt east/northeast
   mid/upper flow atop weak westerly low-level winds will foster around
   30-40 kt effective shear magnitudes. Initial thunderstorm activity
   is expected to develop over higher terrain. Most CAMs guidance and
   HREF calibrated/ML guidance suggest sufficient clustering and
   surging outflow will move east/southeast across southwest AZ/far
   southeast CA. Given the deeply mixed boundary layer/steep lapse
   rates, have introduced a Marginal risk (level 1 of 5) for severe
   wind gust potential across the lower deserts/Lower CO Valley
   vicinity.

   ..Leitman.. 08/21/2025

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