420
AWUS01 KWNH 071519
FFGMPD
MTZ000-072115-

Mesoscale Precipitation Discussion 1220
NWS Weather Prediction Center College Park MD
1118 AM EDT Mon Sep 07 2026

Areas affected...northwestern MT

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

Valid 071518Z - 072115Z

SUMMARY...Localized flash flooding will be possible across
portions of northwestern MT over the next 6 hours as an additional
1-2 inches of rain fall atop areas that have received heavy rain
over the past few days. Hourly rainfall totals are expected to
peak over 0.5 inches through 21Z.

DISCUSSION...Water vapor imagery from 15Z showed a strong mid to
upper level low/trough advancing from north-central ID into
western MT. Out ahead of this feature was a west-east oriented
axis of heavy rain, noted on area radar imagery, with embedded
thunder, located from ~20 miles south of Cut Bank to ~40 miles
south of Havre. Gauge reports from within this axis of heavy rain
have shown 0.5 to 1.0 inches of rain in and around Great Falls but
with much of that falling in about 15-30 minutes. The 12Z sounding
from TFX showed a precipitable water value of 1.01 inches, which
was near the climatological max for early September, but limited
MUCAPE of 235 J/kg. The strong dynamics corresponding to the upper
trough were making up for the lacking instability with left-exit
region upper level jet divergence and diffluence advancing through
west-central MT.

As the mid to upper-level low advances northeastward into
north-central MT through 21Z, weak elevated instability, locally
exceeding 500 J/kg, is forecast by the RAP to affect locations
from the northern Rockies into the upper Missouri Valley and the
U.S./Canadian border. A deformation axis is likely to become
better defined later this morning into the afternoon over
northwestern MT, containing stratiform moderate rain with embedded
cores of heavy rain and possible thunder.

Hourly rainfall of 0.5 to 1.0 inches associated with the ongoing
northward moving axis of heavy rain over west-central MT will
continue to translate north over the next 2-3 hours while steady
moderate to heavy rain sets up to the west. Through the early
afternoon, additional pockets of heavy rain are expected into
parts of northwestern to north-central MT with hourly rainfall up
to ~0.5 inches and 6 hour totals in the 1 to 2 inch range.

Due to 1-3 inches of rainfall that has fallen over the past 2-3
days across portions of western MT, additional localized heavy
rainfall of 1 to 2 inches may result in localized areas of rapid
rises/inundation within areas of sensitive terrain.

Otto

...Please see www.wpc.ncep.noaa.gov for graphic product...

ATTN...WFO...MSO...TFX...

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

LAT...LON   49131247 49111025 49081007 48411002 48081041
            47671219 47101316 47011386 47461491 48161548
            48811547 49081533 49091529