868
FXUS65 KBYZ 170848
AFDBYZ

Area Forecast Discussion
National Weather Service Billings MT
248 AM MDT Sat May 17 2025

.KEY MESSAGES...

- Scattered showers and a few thunderstorms over the west half of
  our forecast area today.

- Showers and thunderstorms become more numerous on Sunday.
  Stronger storms could produce localized heavy rain. Southeast
  winds will gust 40-50 mph from Baker to Ekalaka Sunday.

- Light to moderate precipitation will become more widespread
  Sunday night into Monday. Precipitation will come to an end from
  west to east later Monday into Tuesday.

- Significant rain totals Sunday into Monday may cause localized
  flooding over recent burn scars and areas with saturated ground.
  Increased flows on area waterways can also be expected.

- Unsettled weather continues through the work-week.

&&

.DISCUSSION...

Today through Sunday...

Weak zonal shortwave moving thru eastern MT continues to produce
isolated showers over western and central zones. We are finally
seeing some drying from the west, and this is notable on the
latest 700mb analysis, so fully expect this activity to diminish
over the next few hours...probably before sunrise. Current dewpt
depressions are low enough for at least a mention of patchy fog
across much of the area early this morning. In addition, Baker &
Alzada stand at 36F as of 130am, so expect localized frost in our
eastern valleys early today. Satellite imagery shows a deepening
trof along the Pacific coast and flow across the northern great
basin starting to back to the southwest. The departing wave to our
east and falling pressures to our west will bring a shift to
easterly winds today. Temps will jump a few more degrees and highs
today will be widespread in the 60s to near 70F.

The easterly winds and warming mid levels may result in a bit of
mid level capping today. High res models are in some disagreement
here but by late afternoon surface temps should be warm enough to
achieve 250-500 j/kg of SBCAPE with little CIN. Scattered (west)
to isolated (central) pops should suffice for today. Greater
shower potential emerges by this evening in our west per ascent
from the upstream trof. If there is enough capping to slow down
weak convection today, the trofs arrival should overcome that by
evening. None of today`s activity will be severe, due to very
modest instability and shear.

Shower potential continues through tonight in our west. This will
be followed by a high (40% east to 90% west) chance of showers
and t-storms across the region on Sunday. Secondary trof from the
north Pacific drops thru the PacNW and forms a low over ID, while
the initial trof lifts into WY/CO. There are some complex
interactions here and thus there is still plenty of ensemble
spread in magnitude and locations of heaviest rainfall beginning
Sunday. See details in the extended discussion below.

Sunday`s convection has potential to be a bit stronger (CAPE of
500-1000 j/kg and bulk shear near 30kts) and it will definitely be
wetter as the SE low level flow taps into plains moisture
(pushing pwats to near 0.80" or 1-2 standard deviations above
normal). Greatest instability seems to set up in our central
zones, from roughly Billings to Hardin and Sheridan, along
inverted trof axis. It is in this area that the 00z HREF shows a
couple of weak updraft helicity tracks. Cannot rule out some
stronger storms in this area tomorrow afternoon, but it is also
possible that cloud cover and shower development early in the day
will prohibit this from happening. Something to watch. Convection
has the potential to produce heavy rain and this includes the
Flood Watch area (southern Big Horn and Sheridan Counties).

One last item to consider. Pressure falls in our central zones on
Sunday will allow for increasing SE winds over our east, strongest
near the Dakotas line. Current indications are for 40-50 mph gusts
from Baker to Ekalaka. There is a 50-60% chance of gusts reaching
50 mph in this area.

Temps on Sunday will max out in the mid 50s to lower 70s, warmest
in an area from Billings-Sheridan to Miles City-Broadus, as the SE
winds keep things cooler in our far east.

JKL

Sunday Night through Saturday...

As cooler air moves in and atmospheric instability decreases, the
threat of thunderstorm induced heavy rain will come to an end
Sunday evening and night. With that said, scattered to widespread
precipitation is likely to continue Sunday night into Monday over
much of the area with support from a developing closed low near
western South Dakota and Nebraska (70 to 95 percent chance). While
precipitation associated with this low will generally be more
light to moderate, deep atmospheric moisture profiles look to
persist through Monday over eastern Montana, so pockets of
persistent moderate rain could still pose a risk of flooding over
recent burn scars and areas with saturated soils. Because Monday`s
precipitation will be associated with an area of low pressure
south and east of our area, the westward extent of precipitation
looks to set up somewhere near Billings. Therefore, areas west of
Billings only have a moderate chance of seeing precipitation
continue into the day Monday (20 to 60 percent chance), but areas
east of Billings continue to have a high chance, 70 to 95 percent.
By late Monday into Tuesday morning, the low will move farther
east and precipitation will come to an end from west to east
across our area.

When all is said and done, precipitation amounts between Sunday
and Monday across the region look to be healthy once again. During
this time, the National Blend of Models (NBM) continues to
advertise high chances of seeing at least a half inch of
precipitation across our whole area (50 to 90 percent chance), and
moderate to high chances of seeing an inch or more of
precipitation mainly east of Livingston (50 to 70 percent chance).
With cooler air moving in for Monday, not all this precipitation
will fall as rain. Look for snow levels to drop to around 6000 to
7500 feet Monday across the region. With this, a couple inches of
snow is forecast over the mountains with this event once again,
but the chance of seeing significant snow is generally low (10 to
40 percent chance of 8 or more inches of snow, highest in the
Bighorn Mountains).

Because of the previously mentioned threat of heavy rain and
flooding over recent burn scars and areas with saturated soils,
the Flood Watch for portions of south-central Montana and north-
central Wyoming around the Bighorn and Pryor Mountains from Sunday
afternoon through Monday afternoon remains in good shape, and no
changes or additions will be made on this shift.

Active weather looks to continue through the rest of the work
week behind this early week system with progressive zonal flow in
place. During this time, scattered showers and thunderstorms will
be possible most days, but the lack of any significant system or
moisture will inhibit any continued threat of heavy rain. Arends

&&

.AVIATION...

Isolated light rain showers continue to affect western and central
zones as of 07z. Local MVFR is possible, and the precip should
finally taper off before sunrise. There is a low risk (<20%) of
fog at all terminals early this morning. Any fog that develops
will dissipate by 15z. Expect scattered showers and isolated
thunderstorms this afternoon and evening, beginning around 19z in
the west, and affecting western and central parts of the forecast
area with local MVFR. Mountain tops will be occasionally obscured.
Winds will shift to easterly today and gusts could approach 30
knots in the east tonight. JKL

&&

.PRELIMINARY POINT TEMP/POPS...

    Tdy Sun     Mon     Tue     Wed     Thu     Fri
-------------------------------------------------------
BIL 068 049/065 045/053 043/066 044/068 047/068 045/067
    2/T 29/T    97/W    43/W    53/T    35/T    43/T
LVM 065 044/055 036/057 038/063 038/066 041/065 039/067
    4/T 69/T    75/W    24/T    52/T    35/T    33/T
HDN 070 049/069 045/051 042/066 042/068 044/070 045/068
    2/T 29/T    98/W    54/W    54/T    35/T    43/W
MLS 068 047/069 045/051 042/061 041/066 045/069 046/066
    1/B 14/T    99/W    74/W    34/W    24/W    43/W
4BQ 067 047/070 043/047 041/058 040/064 045/068 046/066
    1/B 26/T    99/W    74/W    34/W    13/W    43/W
BHK 063 040/058 040/050 037/053 036/062 039/067 041/064
    0/B 24/T    89/W    85/W    33/W    23/W    43/W
SHR 068 044/070 039/048 038/063 037/066 040/070 040/066
    3/T 29/T    99/W    64/W    44/T    14/T    33/T

&&

.BYZ WATCHES/WARNINGS/ADVISORIES...
MT...Flood Watch in effect from Sunday afternoon through Monday
      afternoon FOR ZONES 138-169-171.
WY...Flood Watch in effect from Sunday afternoon through Monday
      afternoon FOR ZONES 198-199.

&&

$$
weather.gov/billings