Mesoscale Discussion 2304
NWS Storm Prediction Center Norman OK
0246 PM CDT Thu Sep 17 2026

Areas affected...Parts of southern and central Idaho

Concerning...Severe potential...Watch unlikely 

Valid 171946Z - 172215Z

Probability of Watch Issuance...20 percent

SUMMARY...The risk of a few severe storms capable of producing large
hail and severe wind gusts will increase through the remainder of
the afternoon. A watch is not currently expected, though trends are
being monitored.

DISCUSSION...Thunderstorms are gradually increasing in coverage
across parts of southern and central ID. These storms are being
aided by increasing large-scale forcing for ascent preceding a
closed midlevel low over northern CA and localized terrain
circulations. These storms will continue developing and spreading
northward across southern into central ID this afternoon, given deep
southerly steering flow. The 18Z BOI sounding sampled steep
deep-layer lapse rates and an elongating/mostly straight hodograph
(around 30-40 kt of cloud-layer shear per VWP and mesoanalysis
data). Initially, cellular storms will pose a risk of large hail and
locally severe wind gusts. With time, storms may organize into small
clusters and pose an increasing risk of severe wind gusts.

Currently, the risk appears too localized for a watch, though
convective trends are being monitored.

..Weinman/Hart.. 09/17/2026

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

ATTN...WFO...PIH...MSO...BOI...

LAT...LON   42241289 42401350 42681421 43511524 43861557 44261571
            44561562 44741525 44731470 44431345 44131257 43801213
            43221198 42691207 42271250 42241289 

MOST PROBABLE PEAK WIND GUST...55-70 MPH
MOST PROBABLE PEAK HAIL SIZE...1.00-1.75 IN

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0247 PM CDT Thu Sep 17 2026

Valid 172000Z - 181200Z

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS THIS AFTERNOON
AND EVENING OVER PARTS OF IDAHO/UTAH AND A SMALL PART OF 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.

...20z Update...
No changes were made to the existing outlook with this update as the
forecast remains generally on track. See MCD #2304 and the
discussion below for additional information.

..Chalmers/Weinman.. 09/17/2026

.PREV DISCUSSION... /ISSUED 1110 AM CDT Thu Sep 17 2026/

...ID/UT...
Morning water vapor imagery shows an upper low over northern CA,
with a band of strong flow aloft with associated embedded shortwave
troughs extending across NV into UT/ID.  Strong heating across this
area will aid in the development of scattered thunderstorms from
southern ID into northern UT by mid-afternoon.  Sufficient vertical
shear is present for rotating updrafts capable of large hail and
gusty/damaging winds.  While coverage of storms will likely be
limited, one or two severe cells are expected later today - mainly
over the eastern Snake River Plain.

...CO/KS/NE...
The remnants of a weak tropical low are tracking northeastward
across the four-corners region this morning.  Ample moisture and
weak ascent associated with this feature will lead to considerable
clouds and scattered thunderstorms today over the mountains of CO
and southern WY.  This activity will spread eastward into eastern CO
and western NE/KS with high-based convection posing a risk of
locally damaging wind gusts.  The extent of this severe threat will
be modulated by daytime heating, which is uncertain at this time due
to lingering cloud cover.

Day 2 Convective Outlook  
NWS Storm Prediction Center Norman OK
1210 PM CDT Thu Sep 17 2026

Valid 181200Z - 191200Z

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ...

...SUMMARY...
Isolated severe storms will be possible across parts of Idaho/Utah,
Nebraska/Iowa, and the Piedmont of the Carolinas into north Georgia.

...Great Basin...
A closed midlevel low now over northern CA will move slowly eastward
over northern NV/southern ID tomorrow.  Ascent immediately
east-through-north of the midlevel low, in combination with daytime
heating in cloud breaks and low-level moisture on the northwest edge
of the monsoonal plume, will support scattered thunderstorms Friday
afternoon.  While MLCAPE will likely remain near or below 1000 J/kg,
midlevel lapse rates near 8 C/km and deep-layer southerly shear will
favor some organized/supercell structures capable of producing
isolated large hail and strong outflow gusts.

...IA/NE area...
Low-level warm advection is expected to support clusters of elevated
thunderstorms into Friday morning from northeast NE into northern
IA/southern MN along and north of a slow-moving baroclinic zone. 
The potential remains for an MCV to emerge from the overnight storm
clusters, though its phasing with the surface warm sector and the
formation of additional storms along the surface wind shift Friday
afternoon are in question.  If storms form along this boundary,
there will be a narrow zone where curved low-level hodographs and
moderate-strong buoyancy could favor isolated severe storms. 
Farther southwest, storm development is uncertain given (at best)
very weak forcing for ascent.  While an isolated downburst could
occur with a high-based storm, the overall threat for severe gusts
appears too low to maintain a MRGL into KS.

...Piedmont...
Lingering overnight convection in the vicinity of the Appalachians,
around the east/northeast periphery of the persistent midlevel high
over the south central U.S., could combine with terrain circulations
to provide weak convective outflow and/or differential heating zones
to help focus thunderstorm development Friday afternoon.  Storms are
expected to move generally southward into an environment with strong
surface heating/steep low-level lapse rates/moderate buoyancy with
the potential for precipitation-loaded downbursts with isolated wind
damage.

..Thompson.. 09/17/2026

Day 3 Convective Outlook  
NWS Storm Prediction Center Norman OK
0225 PM CDT Thu Sep 17 2026

Valid 191200Z - 201200Z

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM THE
SOUTHERN HIGH PLAINS TO THE CORN BELT AND OHIO VALLEY...AND FROM
NORTHEAST COLORADO INTO SOUTHEAST WYOMING...

...SUMMARY...
Occasional wind damage and/or large hail will be possible across
parts of the central High Plains and from the central Plains to the
Ohio Valley.

...Synopsis...
The persistent midlevel ridge over the south central CONUS will
begin to weaken as a midlevel trough moves from ID/UT to WY/CO. 
There will be a belt of modestly enhanced midlevel flow from the
central Plains to the Corn Belt/OH Valley, around the northern
periphery of the ridge.  This belt of somewhat stronger flow will
coincide with a slow-moving baroclinic zone with richer low-level
moisture into the OH Valley and deeper mixing with hot surface
temperatures along/south of the front across KS/OK/TX.  Scattered
thunderstorms are probable along the front, with the strongest
storms expected during the afternoon/evening.  Water loaded
downdrafts and up to 30 kt midlevel flow suggests the potential for
occasional downbursts/wind damage across the Corn Belt/OH Valley,
while deeper mixing/inverted-V profiles will favor higher based
storms with occasional strong-severe outflow gusts across the
Plains.  Vertical shear within the warm sector will be on the lower
margins for organized severe storms, so this corridor will be
maintained as a MRGL risk.

Wind profiles and forcing for ascent will be a bit stronger across
the central High Plains, downstream from the midlevel trough
approaching the central Rockies.  There will be a conditional threat
for a couple of supercells, though uncertainty remains regarding the
magnitude of buoyancy and how much clouds will inhibit surface
heating.  Isolated large hail and strong outflow gusts will be
possible, mainly during the afternoon/evening.

..Thompson.. 09/17/2026