472
FXUS65 KMSO 140813
AFDMSO

Area Forecast Discussion
National Weather Service Missoula MT
213 AM MDT Wed May 14 2025

.DISCUSSION...

KEY MESSAGES:

- Cold and wet in the backcountry with snowfall accumulation
  above 6,000 feet in southwest Montana and Lemhi County today.

- Lightning is the main impact with thunderstorms across western
  Montana today.

- Thursday morning rain for northwest Montana, with light
  mountain snow above 6,000 feet.

- Unsettled pattern persists, with possible brief warm/dry window
  Saturday.

The Northern Rockies have received a decent amount of moisture over the
past several days. Some locations have clipped an inch or more of
precipitation or more in the last 72 hours...1.02 inches at the
West Fork RAWS near Painted Rocks, 1.00 inches at Joseph Plains
RAWS southwest of Grangeville, Idaho, and 0.74 inches at our
Hungry Horse Dam COOP. In fact, the May 12-13 total at Hungry
Horse ranks as the 3rd highest for this two-day period since 1949.

The models were challenging when it came to determining whether the
Bitterroot Valley and parts of the Blackfoot region would receive
a wetting rain (defined as 0.10" or greater). This matters
greatly to fire partners, as wetting rain penetrates light or
medium surface fuels, reducing fire intensity and spread.
Interestingly, the area south of Hamilton received more rainfall
than expected in the median(50th percentile) of model
guidance...0.68 inches at Little Rock Creek RAWS and 0.50 inches
at the Darby Ranger Station. One reason for the higher totals is
that the deformation band ended up further north and west than
originally forecast. Still, despite the widespread rain, some
areas received hardly anything:
just 0.01" in Plains, 0.02" in Salmon Idaho and at Hot Springs
RAWS, 0.03" at Selway Lodge, and 0.04" in the Lubrecht Forest(per
University of Montana sensors).

Let`s talk about today: The slow-moving upper-level trough that
brought this moisture will continue to shift eastwards towards
Wyoming. Before it does, a mid-level circulation embedded within
the trough over southeastern Idaho will continue to wrap moisture
into Lemhi County and southwest Montana. Webcams this morning
depicted snow falling at Gilmore Summit and Homestake Pass, though
it wasn`t sticking to the roads. With continued instability and
residual moisture, showers and thunderstorms could develop today.
Numerous storms formed yesterday, mostly terrain-induced. A few
cloud-to-ground lightning strikes reached the valleys, for
instance, storms developed over the Cabinet Range and successive
outflows triggered storms in Thompson Falls. Remember: "If
thunder roars, go indoors". Today`s flow has turned more
northwesterly, shifting the higher probability for lightning to
Lincoln County eastward and southward, including the Flathead
Valley, Mission Valley, Glacier Park, and south to Arlee (40-60%
chance).

Looking ahead to Thursday: Models show a shortwave trough diving south out
of British Columbia, bringing steady morning rain to northwest and Glacier
Park, with totals between 0.20" and 0.45". Snow accumulation of 2
to 4 inches is possible above 6,000 feet. Precipitation should
become more showery by afternoon, with lower rain chances across
north-central Idaho, the Bitterroot Valley, and Lemhi County.

Friday: Somewhat of a break? Maybe. It depends on the timing of the next
west-to-east shortwave trough. Idaho could see morning showers (50-60%
chance), then the focus shifts to western Montana in the afternoon.
Downslope-prone areas-Plains, Missoula, the Bitterroot, Salmon, and some
valleys in southwest Montana, have lower chances (15-40%).

Saturday: A brief pause, or...not? There`s potential for a short-
lived break, but it may not last. Another large trough, breaking
off from the Aleutian Low, is forecast to arrive on the U.S. West
Coast Saturday. If it digs well into California/Nevada, we may get
brief warm, dry high pressure. The NBM currently gives a 60-80%
chance of valley highs reaching the upper 60s(around elevations of
3,000 feet), possibly the warmest day through early next week.
But if the trough digs less, expect quicker deterioration.

Sunday and Beyond: Wet Pattern Resumes
Ensemble and NBM guidance suggest Sunday`s system will impact
north-central Idaho, west-central and southwest Montana, and
Lemhi County, bringing another round of widespread precipitation
(70-80% probability).

Another short-lived break may come Monday, but the progressive wave train
doesn`t stop. Models continue to show upstream blocking, with
persistent upper ridging over the central Aleutians. This could
anchor the Aleutian Low over the Gulf of Alaska between May
21-25, promoting ridging over the Northern Rockies, a possible
pattern shift toward drier and warmer weather.

&&

.AVIATION...Lower ceilings and mountain obscuration are expected
today from Lemhi County to southwest Montana due to wrap- around
rain and snow. Afternoon showers and thunderstorms are expected
across northwest and west-central Montana, with potential impacts
including: lightning, gusty winds to 20 knots, brief heavy rain,
pea-sized hail or graupel. Steady rainfall and snow above 6,000
feet MSL is expected to develop after midnight tonight, 15/0600Z
across northwest Montana and continue through 15/1800Z(12 pm MDT).
During this period, lower ceilings and mountain obscuration are
likely. Periodic showers could impact the Butte (BTM) area
Thursday afternoon.

&&

.MSO WATCHES/WARNINGS/ADVISORIES...
MT...None.
ID...None.
&&

$$