ACUS01 KWNS 171232
SWODY1
SPC AC 171230

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0730 AM CDT Thu Sep 17 2026

Valid 171300Z - 181200Z

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
UTAH/IDAHO INTO SOUTHWEST MONTANA...AND THE CENTRAL HIGH PLAINS...

...SUMMARY...
Thunderstorms with isolated severe wind gusts and hail will be
possible over parts of the Intermountain West this afternoon and
evening. A few strong to severe gusts may also occur across portions
of the central High Plains.

...Utah/Idaho into Southwest Montana...
A closed mid/upper-level low over northern CA this morning will
slowly develop eastward today across the northern Intermountain
region. A 40-50 kt southwesterly mid-level jet and difluence aloft
will overspread parts of the northern Great Basin through this
afternoon. Large-scale ascent will likewise further increase through
the day across this region, with additional thunderstorm development
anticipated. The primary uncertainty regarding the severe risk
remains how much instability will develop given the prevalence of
clouds and ongoing precipitation this morning. With upper 40s to low
50s surface dewpoints already in place, some guidance suggests
sufficient diurnal heating in tandem with cool temperatures aloft
will support up to 1000-1250 J/kg of MLCAPE over southeast
ID/northern UT and vicinity this afternoon. If this occurs, then a
focused corridor of greater severe hail/wind potential may exist
across these areas, as deep-layer shear will be strong enough to
support supercells. Otherwise, a broader risk for isolated
hail/gusty winds will exist across the northern Great Basin with any
cells/small clusters that can develop.

...Central High Plains...
Between a mid/upper-level low over the West Coast and upper ridging
over the Southeast/southern Plains, multiple minor mid-level
perturbations should move northeastward from the Southwest/southern
Rockies towards the central High Plains by this evening. One leading
mid-level vorticity maximum aiding showers and isolated
thunderstorms across NM this morning should support additional
convective development across northeast CO into western NE and
vicinity by late afternoon. While low-level moisture and related
instability should gradually increase across this region through the
day, mid-level lapse rates should remain modest. This will probably
tend to limit updraft intensity, but sufficient deep-layer shear
should be present for some convective organization. A small cluster
or two could eventually evolve and pose some risk for isolated
severe gusts as they spread east-northeastward across the central
High Plains through the evening.

...Midwest/Ohio Valley into the Mid-Atlantic...
Ongoing convection across the Midwest has remained slightly elevated
to the north of a surface front through the early morning. Current
expectations are for this activity to move east-southeastward
through the rest of the morning in a weakly unstable environment.
Various observed 00/12Z soundings downstream across the OH Valley
and southern Mid-Atlantic show very poor lapse rates aloft (i.e.,
around 4.7-5 C/km in the 700-500 mb layer), which will likely hinder
robust updraft intensities. Furthermore, visible satellite imagery
shows extensive cloudiness present across these regions. This will
slow/dampen diurnal heating and related steepening of low-level
lapse rates. While strong/gusty winds may still occur with
convection that develops/spreads southeastward through the day, the
overall risk for severe/damaging winds appears too limited for 5%
probabilities at this time.

..Gleason/Mead.. 09/17/2026

$$
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