AWUS01 KWNH 181447
FFGMPD
WYZ000-UTZ000-IDZ000-182040-

Mesoscale Precipitation Discussion 1321
NWS Weather Prediction Center College Park MD
1045 AM EDT Fri Sep 18 2026

Areas affected...northern UT into eastern ID/western WY

Concerning...Heavy rainfall...Flash flooding possible

Valid 181445Z - 182040Z

SUMMARY...Localized flash flooding will be possible across
portions of northern UT into eastern ID/western WY through 21Z.
Repeating/training of heavy rain will have the potential to
produce hourly rainfall between 1 and 2 inches and 15-minute
rainfall over 0.5 inches.

DISCUSSION...Through 1420Z, area radar imagery across northern UT
into southeastern ID showed two axes of thunderstorms with recent
expansion just northeast of the Great Salt Lake. This region of
the U.S. was located within favorable southwesterly diffluent flow
aloft, just ahead of the southern lobe of an eastward advancing
closed low in northern NV. The 12Z sounding from SLC showed MLCAPE
just shy of 1100 J/kg with weak inhibition and 1.0 inches of
precipitable water (PW), which is near the climatological max
regarding PW for mid-September.

As the upper low continues to advance east over the next 6 hours,
low level moisture transport and daytime heating will continue to
increase instability across the region with an expanding coverage
of 500-1500 J/kg expected by 20Z from UT into eastern ID and far
western WY. The combination of increasing CAPE and sufficient
shear could allow for a few organized cells embedded within the
region of expanding thunderstorms which are forecast to move into
portions of eastern ID and western WY later this morning into the
afternoon. Visible satellite imagery showed mostly clear skies to
the immediate south of the ongoing storms in northern UT which
should allow for increasing instability/convective development in
the 18-21Z time frame with the diurnal cycle. Mean southwesterly
winds will promote repeating cells and training at times, with
hourly rainfall over 1 inch at times (locally near 1.5 inches
possible) along with 15-minute rainfall over 0.5 inches at times,
resulting in concerns for flash flooding. Any areas of flash
flooding will carry the greatest threat across sensitive burn
scars and any impervious/urban areas.

Otto

ATTN...WFO...GJT...PIH...RIW...SLC...

ATTN...RFC...KRF...PTR...STR...NWC...

LAT...LON   44311113 43621002 42060925 39781040 39151209 
            39281329 39801380 40851398 41751366 42901278 
            44101222 

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