849
AWUS01 KWNH 191402
FFGMPD
MTZ000-191900-

Mesoscale Precipitation Discussion 1332
NWS Weather Prediction Center College Park MD
1000 AM EDT Sat Sep 19 2026

Areas affected...south-central and southeastern MT

Concerning...Heavy rainfall...Flash flooding likely

Valid 191358Z - 191900Z

SUMMARY...At least localized pockets of flash flooding will be
likely across south-central and southeastern MT over the next 4-6
hours. Embedded hourly rates of 1.0 to 1.5 inches are expected
with additional totals of 2 to 3+ inches through 19Z.

DISCUSSION...Water vapor imagery at 1330Z showed a potent closed
upper level low slowly advancing toward the southeast over
southern ID with the warm conveyor belt arcing northward toward a
deformation zone across MT. Infrared satellite and radar imagery
along with lightning data showed a uptick in the intensity of
rainfall across eastern MT since 11Z, within a diffluent upper
flow pattern, along a west-east axis located ~25 miles north of
Miles City. This axis of heavy rain was focused within a band of
convergence within the 700-600 mb layer where overlap occurred
with an estimated 100-500 J/kg MUCAPE in place (via 13Z SPC
mesoanalysis data). While ground truth was difficult to determine
due to a lack of observation stations, MRMS-estimated hourly
rainfall was just over 1 inch across localized portions of
Garfield County ending 13Z.

Little change is forecast over the next 1-2 hours with the
latitude of the elevated convergence axis, but beyond that, the
axis is forecast to become disrupted by recent runs of the RAP
with some southward displacement and fragmentation given the
forecast development of embedded circulations within the zone of
convergence. The result will be continued steady rainfall for much
of the region but with embedded pockets of higher hourly rainfall
totals (1.0 to 1.5 inches) where locally higher instability is
encountered. There will also a notable upslope component with
easterly to northeasterly low level flow into the higher terrain
west and northwest of Billings. Additional 6-hour rainfall totals
of 1 to 2 inches are expected but localized totals of 2 to 3+
inches will also be possible through 19Z. Given overlap with heavy
rainfall that has already occurred over south-central MT, and the
localized higher rainfall rates over the next several hours, at
least localized flash flooding is considered likely.

Otto

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

ATTN...WFO...BYZ...GGW...TFX...

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

LAT...LON   47440643 47280471 46960414 45780410 45280484
            45070625 45010753 45030905 45140998 45491074
            45851139 46361170 46831128 47031048 47240888