774
FXUS65 KBYZ 191043
AFDBYZ

Area Forecast Discussion
National Weather Service Billings MT
443 AM MDT Sat Sep 19 2026

.KEY MESSAGES...

- A few strong thunderstorms are possible over southeast Montana
  today.

- Flood Watch in effect through Sunday for much of the area.
  Widespread precipitation through the weekend, with potential for
  locally heavy rainfall through tonight.

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

&&

.UPDATE...
Upper low continues to evolve very slowly over southern ID,
leaving our forecast area under a very diffluent region aloft.
Easterly 700mb winds have veered to southerly over the south, but
remain easterly over the north...and as a result the heaviest
precip as of 1030z is across our north. This includes Hysham to
Miles City where t-storms recently developed along what amounts to
an elevated warm front (w/ instability). As the low continues to
move slowly eastward into western WY today, precip w/ the trowal
should wrap back southward, with upslope enhancement from Red
Lodge to Sheridan tonight & Sunday. Our southern upslope areas
seem to be where the forecast is most uncertain at this point, as
they depend on mid level flow turning to the N-NE. Otherwise,
showers and some embedded t-storms continue. If there is a risk of
a few strong storms today it will be in our S/SE from Sheridan
across far southeast MT, where instability will be greatest, but
HREF continues to show helicity tracks most favored further south
in eastern WY. Something to watch.

Billings airport is up to 2.03" of total rainfall since Friday
evening. This already ranks as the 3rd wettest 2-day precip total
in September (dating back to 1934). Another 0.18", which is all
but certain, will put the current event in the top spot for 2-day
precip at Billings in September.

JKL

&&

.DISCUSSION...

Next 7 Days...

An upper level trough is currently centered over southern ID and
is slowly moving eastwards. This feature is our primary weather
maker for the next few days. Showers and thunderstorms already
made their presence known last evening with rainfall rates under
the strongest cores exceeding 2" /hr. This will be the theme over
the next day or so.

The low pressure system is vertically stacked for the most part
due to the same limited upper level winds that are making it move
as slow as it is. This is aiding in the limited shear across the
region with the exception of directly in the low itself. This
limited shear is the primary reason that there is minimal concern
for any organized severe weather over the next 2 days. The one
possible exception to this, is as the low approaches the Bighorns
this afternoon, some CAMs show a short lived complex of stronger
storms. This is believable as the synoptic lifting mechanism
combines with the orographic one, the minimal (500 J/Kg) CAPE
should be sufficient for stronger storms to roll off the Bighorns.
Due to the quite weak low level shear though, these storms should
not last long. With the vertically stacked vorticity and slow
moving cold core low, I would not be shocked to see a few cold air
funnels during the day today though.

The main story with this event will be the large amounts of
rainfall. PWAT values could exceed 1.25" over much of the CWA
today as the easterly surface winds ahead of the low advect in all
of the moisture out of the Great Plains. Completely saturated
soundings depicted by nearly all models are a rare occurrence in
this climate; these are indicative of efficient rainfall
producing storms. As mentioned above, we have already seen very
heavy rainfall rates, and the likelihood of this continuing under
the strongest cores is quite high. Even locations that do not
have any convection move overhead will still see steady stratiform
rainfall through the day today which will easily total at least a
half inch for the entire forecast area. Actually, the NBM 10th
percentile shows over 1.5" for a corridor from Big Timber to
Hysham, including Billings. This would suggest that there is at
least a 90% chance that we see that much rainfall. While the NBM
90th percentile values are a bit high, localized pockets of 4+
inches should be expected in association with the convective
cores. Flash flooding, esepcailly in locations prone to such, is a
large concern over the next 24 hours as some locations could pick
up the equivalent of 25% of their average annual rainfall in just
a few hours.

In terms of mountain snow, confidence is increasing that snow
levels remain at or above 10,0000 ft for the duration of the
event. This could bring a light accumulation to the peak of the
Beartooth Pass Sunday AM, in addition to mixed precip throughout
the event. The highest peaks of the mountains (ie. Granite Peak)
could pick up a total of 3-6" of snowfall through the entire
event.

Sunday we will be behind the low pressure system, however wrap
around precipitation is still possible. Scattered showers across
the forecast area will linger through the day due to a TROWAL like
feature but will have less moisture left to work with. Impacts
with this rain on Sunday should be confined to locations that
already picked up a lot of rainfall Today, meaning that widespread
impacts are not expected.

Looking ahead to the work week, we should see much drier
conditions. Behind the weekend low a dirty ridge builds in across
the area. This will allow for temperatures to slowly rebound
through the week to near average. There will be a stream of
monsoonal moisture that is originating in Tropical Pacific
reaching into western parts of the CWA. This could bring isolated
showers and storms for much of the week, but nowhere close to what
we are seeing this weekend. As of right now the chance for precip
for locations west of Billings is 30% with even lower numbers
elsewhere. WMR

.AVIATION...

10z discussion.

A slow-moving weather system will bring widespread rain showers
and some embedded thunderstorms today and tonight, with patchy
nighttime fog as well. MVFR/IFR flying conditions will be common,
and all TAF sites will be impacted by this system. Mountains will
be mostly obscured through the period. JKL

&&

.PRELIMINARY POINT TEMP/POPS...

    Tdy Sun     Mon     Tue     Wed     Thu     Fri
-------------------------------------------------------
BIL 059 051/063 049/072 050/077 052/079 053/081 053/080
    +/T 98/W    20/B    00/U    00/B    10/U    00/B
LVM 059 043/066 041/075 044/078 046/079 047/081 048/080
    +/T 84/W    00/U    00/U    01/U    00/U    00/B
HDN 063 048/063 046/073 045/079 049/081 048/083 049/083
    +/T 97/W    30/B    00/U    11/U    00/U    00/U
MLS 065 052/062 049/070 050/078 053/080 053/082 055/083
    +/T 99/W    31/B    00/U    12/W    10/U    00/U
4BQ 066 052/061 049/068 049/077 053/079 053/082 055/082
    +/T 99/T    61/B    00/U    12/T    10/U    00/B
BHK 065 047/060 044/067 045/075 049/080 050/083 051/082
    7/T 99/W    41/B    00/U    01/U    10/U    00/B
SHR 063 044/063 042/071 042/077 046/079 046/082 047/083
    9/T 98/T    41/B    00/U    11/U    10/U    00/U

&&

.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