552
FXUS65 KBYZ 200638
AFDBYZ

Area Forecast Discussion
National Weather Service Billings MT
1238 AM MDT Sun Sep 20 2026

.KEY MESSAGES...

- Flood Watch in effect through Sunday for all of the area.
  Heaviest rain shifts over SE MT, NE WY Today.

- Patchy fog this morning and again overnight tonight into Monday
  AM.

- Warmer and drier for the work week. Isolated showers linger.

&&

.DISCUSSION...
Next 7 Days...

The upper level low is slowly progressing through Central WY
tonight with an ill-defined surface low roughly vertically
stacked. The flow around this feature continues to advect plains
moisture into the region which is further aided by the gradual
upslope. The Billings airport has already picked up a storm total
rainfal around 3" with rain continuing to fall. These totals are
due to the very saturated atmosphere through the entire column.
Even the stratiform rainfall is producing rates up to 0.5" per
hour in some locations. Areas of SE MT that were under convection
for much of last evening were seeing 1"/hr rates for almost 2
hours which produced some localized flooding.

Today we will start to see the trough make its exit to the east.
This will be beneficial to Sheridan, WY as they will finally see
surface winds become more upslope. They have largely missed out on
most of the precipitation so far due to the persistent westerly
component of the winds resulting in continuous downslope. Across
the rest of the CWA, a TROWAL will set up on the backside of the
low which will, in part, be the reason that showers stick around
through most of the day today. This feature in combination with
the very efficient warm conveyer belt of the low will keep much of
the eastern 2/3 of the CWA with scattered areas of rain through
the daytime hours. Due to the lack of instability, this rain will
be all stratiform which will keep rainfal rates, and in turn
totals, on the lower end compared to what we have seen already.
That being said, places in NE WY and SE MT could still pick up
over another inch before all is said and done. Some CAMs are
depicting a banding feature of rain through the central part of
the CWA this evening in association with the exiting TROWAL and,
therefore enhanced area of CSI. This type of feature is a lot more
common in wintertime systems, however is still possible in
transition season events such as this.

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

The other thing to watch out for this morning is patchy fog across
the entire forecast area. With much of the CWA already completely
saturated, with rain on the gounod, fog should easily develop in
the light winds. Some locations will be more prone to fog
development than others meaning that visibility could change
significantly over short distances. In some areas, this saturated
environment will result in low stratus clouds and minimal fog.

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...

06z update:

Slow-moving weather system will continue to produce numerous
showers today, mainly east of KHWQ-KLVM, with heaviest precip
shifting from west to east and finally tapering off tonight. The
showers and very moist surface will result in areas of MVFR/IFR
with low clouds and patchy fog, and local LIFR east of KBIL.
Mountains will be frequently obscured. Any thunderstorms today
will be east of KBIL and weak. Expect improvement to VFR in the
west after daybreak. Tonight, after shower activity diminishes,
any clearing skies will likely be followed by fog development. Fog
late tonight into early Monday could be locally dense and will
need to be monitored, and all TAF sites may be impacted. JKL

&&

.PRELIMINARY POINT TEMP/POPS...

    Tdy Mon     Tue     Wed     Thu     Fri     Sat
-------------------------------------------------------
BIL 064 049/070 048/078 053/079 053/081 053/083 054/075
    6/W 21/B    00/U    21/B    00/U    00/U    01/B
LVM 067 040/074 042/078 047/078 046/081 048/082 048/074
    2/W 20/U    00/U    10/U    00/U    00/U    11/B
HDN 063 046/072 045/079 049/080 048/083 050/085 050/077
    9/T 30/B    00/U    22/W    00/U    00/U    01/B
MLS 062 048/069 049/077 052/079 052/082 055/085 056/077
    9/W 21/B    00/U    13/W    00/U    00/U    00/U
4BQ 061 049/067 049/076 053/078 053/082 056/084 056/078
    9/T 52/E    00/U    22/W    10/U    00/U    00/U
BHK 061 043/067 043/074 047/079 048/084 051/085 053/080
    9/W 41/B    00/N    23/T    10/U    00/N    00/U
SHR 061 042/070 040/077 046/078 045/082 048/083 049/079
    9/T 62/W    00/U    22/T    00/U    00/U    01/U

&&

.BYZ WATCHES/WARNINGS/ADVISORIES...
MT...Flood Watch in effect through this 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 this afternoon FOR ZONES
      198-199.

&&

$$
weather.gov/billings