032
FXUS65 KBYZ 181138
AFDBYZ

Area Forecast Discussion
National Weather Service Billings MT
538 AM MDT Fri Sep 18 2026

.KEY MESSAGES...

- A few strong thunderstorms are possible, mainly around and east
  of Billings this afternoon and evening.

- Flood Watch tonight through Sunday for much of the area.
  Widespread precipitation this afternoon through the weekend,
  with the potential for locally heavy rainfall widespread through
  Saturday night.

- Chance for at least an inch of rainfall is greater than 80
  percent across much of southern Montana and northern Wyoming.
  The chance for at least 2 inches of rainfall is 50 to 80
  percent, greatest across south-central Montana.

&&

.UPDATE...
Light showers have exited far southeast MT, and in our west an
area of showers has reached West Yellowstone and is approaching
Gardiner. The latter area is in response to diffluent flow and
instability associated with a cut off low forming over WA/OR/ID.
Showers should spread into the Beartooth-Absarokas over the next
few hours. Main issue over the next 6 hours is with regard to the
low stratus and fog that has spread into southeast MT (east of
Miles City-Broadus) and to a bit lesser extent southern Big Horn
into eastern Sheridan Counties as of 09z. Local cameras show most
of this is stratus but it is still evolving. Expect areas of
fog/stratus through sunrise this morning from Rosebud-Treasure-
Big Horn-Sheridan Counties eastward. Have made some forecast
adjustments to account for this. JKL

&&

.DISCUSSION...
Today through Thursday...

Patchy fog may develop early this morning amongst the river
valleys and lower elevations east of Billings. The latest HiRes
models depict the greatest visibility reductions to far southeast
Montana and northeastern Sheridan County. Will continue to
monitor this through the morning hours.

A very active weekend of weather remains on track across the
region. Flood Watches remain in effect for all counties except for
Fallon, Custer, and Carter Counties, beginning this evening and
lasting through the day Sunday.

A deepening upper low is slowly moving into the region over the
next few days. This will bring very high precipitable water values
(200 to 250% of normal) through the weekend. At the surface,
southeasterly flow will contribute to the high PWAT values, as
Gulf and Plains moisture is advected into the region. Showers and
thunderstorms will begin to develop this afternoon, becoming
widespread overnight and lasting into Sunday.

The risk for severe thunderstorms today is currently low, with
the latest SPC outlook depicting a general risk of thunderstorms
across much of Montana and Wyoming. There are two areas of
interest for a few stronger storms this afternoon. The first area
is southwestern MT where the greatest upper diffluence is shown by
models. Models do show MUCAPE values climbing towards 1000 J/kg
as well. Something to note however, is that the latest HREF
depicts stronger storms further west than into Park County. The
second area of stronger storm potential exists mainly east of
Billings where MUCAPE values are shown approaching 2000 J/kg. Low-
level shear values are going to be fairly weak (generally less
than 15 knots) across much of the area while mid- layer shear will
be modest (20 to 40 knots). For both areas, the main threats are
gusty winds, heavy to very heavy rainfall, and hail. Those with
outdoor and sports viewing plans should pay attention to the
forecast and be prepared in case a strong storm develops in your
vicinity tomorrow.

The probability for at least an inch of rainfall tonight through
Sunday is 75 to 100% for most of south-central Montana and
northern Wyoming. Meanwhile, while the chance over far eastern
Montana is currently 30 to 60%. The probability for at least 2
inches of rainfall is 40 to 80% from Rosebud County and west,
greatest around and west of Billings. The chance for stronger
thunderstorms exists south and east of Billings on Saturday. For
the rest of the area, the risk of stronger storms will be low with
widespread showers and embedded thunderstorms expected. The risk
for locally very heavy rainfall exists as well, with the latest
run of the REFS 3-hour QPF Max showing pockets of 2-4+ inches of
rain in 3 hour periods on Saturday.

While much of the rain will fall over the course of the weekend,
the risk for locally higher amounts warranted the aforementioned
flood watches. Recent burn scars will be highly susceptible for
flash flooding and debris flows in the current long duration
precipitation forecast, especially if thunderstorm precipitation
impacts them. Those living near recent burn scars should have a
plan of action to seek safety should the heavy rainfall develop in
those locations. Pay attention to local officials directions and
for any Flash Flood Warnings that may be issued.

Precipitation should taper off Sunday night into Monday from west
to east as the upper low moves into the Northern Plains states.
While much drier weather is forecast for next week, depending on
how much rain falls this weekend, there will probably be a good
chance for at least afternoon and evening showers and
thunderstorms each day into mid-week just from the residual
surface moisture across the region.

Snowfall...With the increased precipitation forecast, snow
amounts in the highest peaks of area mountains have climbed into
the 3 to 6 inch range. Snow levels start out around 12kft tonight,
dropping to 11kft Saturday afternoon and possibly falling to
around 10kft Saturday night. Currently, it appears unlikely that a
Winter Weather Advisory will be needed for the Beartooth Pass.
The most likely timeframe where one could be needed though would
be Saturday night into Sunday morning. That said, anyone planning
travel over the pass this weekend should be prepared for winter
conditions.

High temperatures are forecast in the 70s to mid 80s today,
warmest in southeast Montana. Over the weekend, temperatures look
to be in the 60s before climbing back into the 70s by Monday.

Matos/Chambers

.AVIATION...

12z Discussion.

Low stratus and fog will impact areas south/east of K00U-KMLS
(this includes KSHR) with IFR or lower early this morning. Expect
the fog and stratus to dissipate gradually between 14-18z. VFR
will prevail otherwise today.

The chance of showers and thunderstorms will increase this
afternoon and evening, initially over the mountains and foothills
then spreading over the lower elevations beginning around 00z.
Storms will be capable of producing erratic wind gusts and local
MVFR/IFR in moderate to heavy rain. Late tonight as the risk of TS
decreases, widespread showers will cause flight conditions to
deteriorate with MVFR/IFR becoming common. Mountains will be
occasionally obscured today, becoming mostly obscured tonight.

JKL

&&

.PRELIMINARY POINT TEMP/POPS...

    Tdy Sat     Sun     Mon     Tue     Wed     Thu
-------------------------------------------------------
BIL 080 055/061 053/063 050/072 051/077 052/077 052/079
    2/T 9+/T    98/T    20/B    00/B    11/U    00/U
LVM 075 049/060 046/066 044/074 046/077 047/077 047/079
    6/T +9/T    85/T    10/U    01/B    10/U    10/U
HDN 081 054/065 050/064 047/075 046/079 047/080 046/082
    1/U 9+/T    99/T    30/B    00/B    10/U    00/U
MLS 082 057/065 053/064 050/071 051/078 052/079 052/081
    0/U 69/T    +9/W    31/B    00/U    00/U    10/U
4BQ 082 057/066 054/063 050/069 051/077 053/079 052/081
    0/B 78/T    +9/T    51/B    00/U    12/W    10/U
BHK 080 050/066 047/062 045/069 046/075 048/078 048/080
    0/B 37/W    99/W    31/B    00/B    11/U    20/U
SHR 080 051/066 047/063 044/071 044/077 045/078 046/081
    4/T 89/T    +9/T    61/B    01/U    12/W    10/U

&&

.BYZ WATCHES/WARNINGS/ADVISORIES...
MT...Flood Watch in effect from this evening through Sunday
      afternoon FOR ZONES
      29>31-34-36-40-42-56>58-63>68-138-139-141-169>173-228-235.
WY...Flood Watch in effect from this evening through Sunday
      afternoon FOR ZONES 198-199.

&&

$$
weather.gov/billings