535
FXUS65 KBYZ 200038
AFDBYZ

Area Forecast Discussion...Updated
National Weather Service Billings MT
638 PM MDT Sat Sep 19 2026

.KEY MESSAGES...

- A few strong thunderstorms are expected over southeast Montana
  through this evening. Main threat is heavy rain and strong
  winds.

- Flood Watch in effect through Sunday for much of the area.
  Widespread precipitation through the weekend.

- Widespread 50-95% chance of an inch of rainfall. 50-80% chance
  of two inches of rainfall for portions of south central Montana.

&&

.DISCUSSION...

Rest of Today through Friday...

Widespread rain will continue over the next 24 hours. An estimated
0.75-1.25" of rain has fallen in a swath extending from Sweet
Grass County and northeast through northern Custer County.
Localized heavier amounts up to 3 inches has occurred within this
watch as well. Expect more rain to occur over the next 24 to 30
hours.

The root cause of all this rain is an upper level low that is
positions over southwest Idaho as of noon. Moisture brought in
with it the Pacific, as well as a sharp easterly surge of low-
level moisture has PWATs approaching 1.3" at and east of
Yellowstone County. While widespread rain will persist, the larger
issue is keeping an eye out for pockets of localized, heavier
rainfall.

Analysis shows CAPE up to 500 J/kg across southeast MT this
afternoon, with modest shear (especially for mid September). These
ingredients combined with the ample ascent provided by the upper
low will bring about storm potential through the evening.
Soundings show long, skinny CAPEs alongside a deep, saturated
layer, further indicating the heavy rain potential of these
storms. Due to all of this, a rare Moderate risk of Excessive
Rainfall exists from Yellowstone County through Custer and Powder
River counties, meaning that any storm has at least a 40% chance
of inducing flash flooding. These rainfall rates could be as high
as 3" of rain per hour, and the most likely timeframe of these
storms occurring is now through 9pm. Do not wait for a storm to
occur, have a plan ready now in case of possible flash flooding.
Flood Watch for all of southeastern MT and north-central WY continues
to remain in effect.

Relatively low amount of cooler air is brought in with this
system, only dropping snow levels at around 10,000 ft. So
elevations above this will see 1-4 inches of snow.

As the upper low shifts through Wyoming on Sunday, we will still
receive additional rain from the wrap-around. However, little
instability exists during this period, so the chance for localized
areas of heavy rain with rain rates of 2-3" per hour is much
lower. The only exception (while still being a lower chance than
today) is areas at and south of US-212 from Crow Agency through
Alzada, where CAPE up to 200 J/kg is forecasted to occur. All rain
is expected to shift east of Montana by midnight Sunday night.

As we enter the work week, expect a quick warm-up to slightly
above normal temperatures and mainly dry conditions throughout the
week. A week shortwave is currently progged to cross southern MT
on Wednesday, bringing with it a small (less than 20%) chance of
showers and storms to the area. Vertz

.AVIATION...

00z update:

Numerous showers continue across portions of south central MT
with more scattered showers over southeast MT and north central WY
and isolated thunderstorms over far southeast MT. MVFR-IFR
conditions will persist tonight into Sunday over much of the area
due to low clouds and rain showers. Patchy fog is likely tonight-
Sunday morning mainly in the area bounded by KRPX, KBIL, KSHR,
and KMLS. In the KLVM area, the chance of rain is low, but low
clouds could still develop from recent rainfall contributing to
surface moisture under periods of clear skies. The chance of rain
should diminish over south central MT from west to east Sunday
afternoon. Mountains will be obscured through Sunday. RMS

&&

.PRELIMINARY POINT TEMP/POPS...

    Sun     Mon     Tue     Wed     Thu     Fri     Sat
-----------------------------------------------------------
BIL 051/062 050/070 049/077 054/079 054/081 054/080 053/072
    +8/W    20/B    00/U    01/B    00/U    00/B    11/B
LVM 043/066 042/074 045/078 049/078 048/081 049/081 048/072
    62/W    00/U    00/U    10/B    10/U    00/B    13/W
HDN 049/063 047/072 045/079 049/080 048/083 049/083 049/075
    +8/W    30/B    00/U    11/B    00/U    00/U    11/B
MLS 053/061 049/069 050/077 053/078 053/082 056/083 054/075
    ++/W    20/B    00/U    01/B    00/U    00/U    01/B
4BQ 053/061 049/067 049/076 054/079 053/082 057/083 055/076
    99/T    61/E    00/U    12/T    20/U    00/U    02/W
BHK 048/061 044/067 044/073 048/079 050/083 052/084 052/078
    99/W    40/E    00/N    03/T    30/U    00/N    11/B
SHR 046/062 044/070 042/077 048/078 046/082 048/083 049/077
    88/T    61/B    00/U    12/W    00/U    00/U    01/B

&&

.BYZ WATCHES/WARNINGS/ADVISORIES...
MT...Flood Watch in effect through Sunday afternoon FOR ZONES
      29>34-36-37-40-42-56>58-63>68-138-139-141-169>173-228-235.
WY...Flood Watch in effect through Sunday afternoon FOR ZONES
      198-199.

&&

$$
weather.gov/billings