805
FXUS65 KBYZ 200145
AFDBYZ

Area Forecast Discussion
National Weather Service Billings MT
745 PM MDT Sun May 19 2024

.UPDATE...
Satellite imagery shows troffing over the northern Rockies and a
WSW flow over our region, along with a departing jet streak
lifting thru northeast MT. Very little is happening across our
cwa this evening. Central and east parts are dry (too stable w/
subsidence) but there are a few light showers near the
mountains/foothills, mainly in the west where reasonably steep mid
level lapse rates linger. There is a shortwave over OR/ID that
will track eastward and provide synoptic scale ascent for our SW
mountains late tonight into early Monday, coinciding w/ mid level
winds shifting to easterly, so expect increasing chances of
showers over the Beartooth-Absarokas late tonight (though much
better precip chances take shape during the day tomorrow).

Forecast is in good shape. Have tweaked pops down and removed any
mention of thunder for the remainder of the night. By the way, it
will also be chilly tonight with 30s to around 40F for lows.
Increasing clouds from the SW will keep temps from dropping
further, but some locations in our west could see a frost tonight.
JKL

&&

.DISCUSSION...
Through Tuesday night...

The ongoing light to moderate rain showers this afternoon will
decrease through the evening hours today. So far, there has been
no lightning in our area, but we still can`t rule out a rumble of
thunder this afternoon. Overall, the showers will remain benign.

By tonight, dry conditions will persist before another more
robust weather system brings precipitation chances back to the
region Monday through Tuesday. This next system will mainly impact
the western and southern foothills, as well as the
Beartooth/Absaroka and Bighorn Mountains. While rain is expected
over the lower elevations, the previously mentioned mountains
could see heavy snow above 7000 feet. Those working to clear the
Beartooth Highway or with travel plans along US-14 through the
Bighorn Mountains Monday through Tuesday should pay close
attention to the forecast.

Monday will start out dry with temperatures warming into the 50s
to lower 60s. By late morning, chances of precipitation will
increase west and south of Billings as a trough digs south into
Idaho and eventually Wyoming. By Monday afternoon and evening,
chances of precipitation will spread east through Billings (40-90%
chance, highest over the mountains), however, areas from Miles
City to Baker look to remain dry (less than 10% chance of
precipitation). As we get into Tuesday, the chance of
precipitation will remain mainly over the southern mountains and
foothills (50-95% chance, highest over the mountains). By Tuesday
evening and night, the upper trough will move east and
precipitation will come to an end.

As far as precipitation and snowfall amounts go for the Monday
through Tuesday event, some uncertainty remains. There continues
to be a disconnect between the deterministic forecast values and
their respective probabilities. At this time, the probabilities
are much lower than the associated deterministic values. With that
said, the NBM probabilities for 0.25 or more inches of liquid
(rain or melted snow) is high over the Beartooth/Absaroka and
Bighorn Mountains, as well as their respective foothills (70-90%).
These probabilities drop quickly as you move north and east over
the lower elevations with Billings having a 40% chance and Miles
City having a 10% chance. With snow levels hovering around 7000
feet for this event, snow is expected in the higher elevations of
the mountains. Probabilities remain moderate for 10 or more inches
of snow in the Beartooth/Absaroka and Bighorn Mountains, however,
the deterministic values suggests 8-14 inches over the higher
terrain.

With these amounts and the ongoing work to clear the Beartooth
Highway, we have gone ahead and issued a Winter Storm Watch for
the Beartooth/Absaroka Mountains covering the period from 12PM
MDT Monday through 12PM MDT Tuesday. Heavy snow looks likely for
the Bighorn Mountains too, but decided to wait on any products as
the impactful snow does not get started until Monday evening and
night there. Arends

Wednesday through Sunday...

The extended forecast still was looking quite active, especially
for the Wednesday through Friday time frame. The pattern shaping
up supports a very good chance of rainfall, with below seasonal
temperatures.

Wednesday through early Friday: An upper low pulls into the Great
Basin Wednesday afternoon, spreading energy and moisture into the
west for an increasing chance of showers. As this upper low moves
across northern Wyoming, this will spread the increasing chance
of showers across central and southeast zones Wednesday night
through Thursday night. The chance of precipitation will be 30-50%
Wednesday afternoon and night west and central. The chance
increases to 50-80% for the entire area Thursday into Thursday
night as the system swings across northern Wyoming (which is a
really good position for precipitation). Chance of 0.25 inches
through the period was 50-70%, with 35-55% chance of 0.50 inches.
Another aspect to keep our eye on is snow potential for the higher
elevations. Snow amounts were falling in the 6-12 inch category,
with a 40% chance of amounts above 10 inches.

Friday afternoon through Sunday: The mid week system moves east
and small scale ridging pulls in for a brief period of drying
Friday afternoon through early Saturday. The flow opens up to the
southwest as the next upper system drops into the Great Basin and
brings up more energy and lift and another increasing chance of
showers Saturday afternoon through Sunday. At this time, the focus
for highest precipitation chances (30-50%) is over western and
central zones (including the mountains and foothills). The bulk of
the heaviest rainfall (over an inch in some cases) is currently
progged over northern Montana with this system. Will have to watch
this closely, as ensemble trends have been to drag some of the
higher rain amounts farther south with each passing run.

Temperatures: Below seasonal readings in the 60s will be across
the entire area Wednesday through Friday, with the warmest day
(mid to upper 60s) being Wendesday. The weekend looks slightly
warmer, with highs around 70 degrees. TWH

&&

.AVIATION...

Isolated showers and NW winds will dissipate through the evening.
Showers will be present over the mountains and cause obscurations
at times. Scattered showers and isolated thunderstorms will begin
to develop late Monday morning into the afternoon, affecting all
sites. MVFR conditions are possible under any thunderstorm with
heavy rain. TS

&&

.PRELIMINARY POINT TEMP/POPS...

    Mon     Tue     Wed     Thu     Fri     Sat     Sun
-----------------------------------------------------------
BIL 038/060 042/062 042/068 045/057 041/062 043/071 048/069
    15/T    45/T    13/W    58/T    64/T    13/W    35/T
LVM 033/055 036/060 037/062 040/051 036/060 040/068 041/064
    28/T    46/T    16/T    79/T    64/T    14/T    46/T
HDN 036/062 041/062 037/071 043/061 039/063 040/073 047/072
    14/W    35/R    12/W    47/T    74/T    22/W    34/W
MLS 037/063 042/064 042/070 045/063 040/061 041/072 047/070
    11/B    12/R    11/B    26/T    64/W    21/B    23/W
4BQ 038/060 044/061 040/070 045/067 040/060 041/072 047/072
    12/W    24/R    21/U    25/W    64/W    21/B    23/W
BHK 036/063 040/064 038/070 042/065 038/059 038/070 045/071
    11/B    12/R    11/U    25/W    64/W    21/B    23/W
SHR 036/057 039/055 033/068 040/062 036/059 038/070 044/069
    27/T    78/T    32/W    26/T    64/W    22/W    34/T

&&

.BYZ WATCHES/WARNINGS/ADVISORIES...
MT...Winter Storm Watch in effect from Monday afternoon through
      Tuesday morning FOR ZONE 67.
WY...None.

&&

$$
weather.gov/billings