591
FXUS65 KBYZ 120940
AFDBYZ

Area Forecast Discussion
National Weather Service Billings MT
340 AM MDT Mon May 12 2025

.KEY MESSAGES...

- Warm today with high temperatures in the 70s to 90s.

- Expect additional rises on rivers and streams due to mountain
  snowmelt early this week.

- Breezy tonight with gusts to 30-50 mph.

- Confidence increasing for a cool and wet weather system during
  the middle of this week. There is a 50-90% chance of at least 1
  inch of precipitation (greatest over southern mountains &
  foothills) and an 80% chance of at least a foot of snow in the
  mountains above 7000 feet.

- Midweek precipitation may add to elevated water levels on rivers
  and streams along the foothills.

&&

.DISCUSSION...

Today through Monday night...

Through Monday night, a 500 mb low will move eastward from the
West Coast to the interior Pacific Northwest. This evolution will
place the Northern Rockies under south/southwesterly flow aloft.
The stronger flow at 700 mb will be over the western foothills,
where winds of 30-40 kt will mix down to the surface and produce
wind gusts of 35-45 mph during the day today for Livingston, Big
Timber and Harlowton. There will also be a pocket of dry air
aloft moving through the flow that will give a dry day for much of
the area.

Partly cloudy skies combined with more downsloping flow in the
lower levels should produce high temperatures ranging from the 70s
in the western foothills to 80s in central areas to lower 90s in
southeast MT. The warm weather will cause additional snow melt in
the mountains today, so streams and rivers will be running faster
and higher. Additional information can be found on our website at
weather.gov/byz.

Late this afternoon, moisture aloft will begin to increase from
the west, and high-resolution models suggest isolated showers and
thunderstorms can develop over the western mountains and move
northeastward over the lower elevations. In accord with this
thinking we have a 15-20% chance of showers and thunderstorms for
areas near and west of Roundup-Reed Point-Red Lodge. CAPE looks
low, so not expecting severe thunderstorms at this time. A wind
gust to 50 mph cannot be ruled out in these areas.

This evening, a cold front will cross the area, with cooler air
moving in behind it. 00z HREF suggests that winds will gust to
30 mph for a few hours behind the front. A few locations can gust
to 40-50 mph. Scattered showers will then occur over more of
south central MT tonight, with the probability of precipitation
ranging from about 15% just west of Miles City-Wyola to near 70%
over northern Park to northern Wheatland Counties. Precipitation
amounts will be light at generally less than 0.10 inch for
tonight. Low temperatures will range from the 40s in the west to
the 50s in southeast MT. RMS

Tuesday through Monday...

Uncertainty still remains in the exact track of the next system,
which will highly influence precipitation amounts Tuesday through
Thursday. As an example, along a line from Billings to Miles City
to Baker, there is a 55-65% chance of more than 1 inch of
precipitation, however, along that same line, there is also a 20%
chance of less than 0.5 inch (which is still an 80% chance of 0.5
inch or more). Looking at ensemble spreads (25th to 75th
percentile), the difference between the low end and high end
precip amounts is around 0.75 inch to 1.5 inches. So, this system
has the potential to bring high precip amounts across the region.
Currently, the area expected to receive the most precip is from
Big Timber to Miles City and south, with a 50-90% chance (highest
over and near the Bighorn mountains) of at least 1 inch of
precip.

Snow levels are expected to quickly fall to 7000 feet. Currently,
there is an 80% chance of at least a foot of snow over the highest
mountain peaks. Over the Absaroka/Beartooth mountains, there is
also a 20% chance of more than 2 feet of snow. Winter Storm
Watches are in effect over the mountains beginning Tuesday.

After the system exits on Thursday, shortwaves in the flow will
continue to bring light precip into the weekend. By Sunday, a
deeper trough or evolving low is expected to move in, bringing
another chance (50-70%) of heavier precip into early next week.

Highs will range from upper 50s over the west to mid 70s over the
east Tuesday. Then, highs will decrease to mid 50s to low 60s for
Wednesday and Thursday. Friday and Saturday will be closer to
seasonal normals, with highs generally in the 60s. Then, with the
next system, highs will become a little cooler again, generally
in the 50s, for Sunday and Monday. Archer
&&

.AVIATION...

Breezy conditions will pick up across the western foothills
this morning, including near KLVM, with SW gusts up to 40 kts
through the afternoon. A frontal passage late this evening into
Tuesday morning will bring surfaces gusts around 15-25 kts near
all TAF sites and scattered showers and isolated thunderstorms for
locations west of KMLS and KSHR. Archer

&&

.PRELIMINARY POINT TEMP/POPS...

    Tdy Tue     Wed     Thu     Fri     Sat     Sun
-------------------------------------------------------
BIL 086 049/067 047/057 047/061 042/064 046/063 043/056
    1/B 35/W    99/W    67/T    23/W    56/T    78/W
LVM 077 043/058 041/055 040/058 038/060 041/060 038/052
    2/T 69/T    88/T    67/T    24/T    68/T    78/W
HDN 089 049/070 046/056 045/061 041/064 044/066 043/058
    1/B 22/W    99/W    78/T    32/W    46/T    78/W
MLS 090 055/074 049/059 047/061 041/063 041/065 041/056
    1/B 10/U    99/W    77/W    21/B    24/W    46/W
4BQ 090 054/073 049/056 046/058 041/061 042/063 041/054
    1/U 00/U    99/T    87/T    31/B    24/T    57/W
BHK 092 052/076 047/060 042/058 038/061 035/063 037/055
    1/U 00/U    88/W    87/W    41/B    12/W    35/W
SHR 086 047/068 041/053 040/058 037/060 039/063 038/055
    1/B 03/W    ++/T    89/T    33/T    36/T    68/T

&&

.BYZ WATCHES/WARNINGS/ADVISORIES...
MT...Winter Storm Watch in effect from Tuesday morning through
      late Thursday night FOR ZONE 67.
     Winter Storm Watch in effect from Tuesday evening through
      late Thursday night FOR ZONE 171.
WY...Winter Storm Watch in effect from Tuesday evening through
      late Thursday night FOR ZONE 198.

&&

$$
weather.gov/billings