073
FXUS65 KMSO 231115
AFDMSO

Area Forecast Discussion
National Weather Service Missoula MT
415 AM MST Sun Feb 23 2025

.DISCUSSION...

KEY MESSAGES:

-  Increased snowmelt and runoff into Monday, bringing the risk
   of minor flooding to low-lying and poor drainage areas and to
   small streams and creeks from potential ice jam releases.

-  Cold ground conditions may cause rain to refreeze during
   nighttime hours.

-  Trough to bring mountain snow and mixed precipitation Monday
   evening through Tuesday night.

-  Anomalously warm and drier end to the week. Temperatures could
   reach the 50s.

An atmospheric river is moving in this morning, delivering a huge amount
of moisture. Satellite images show a continuous stream of clouds
from the Pacific Northwest that started near Hawaii on Friday.
This high moisture (nearly 5 standard deviations above normal)
means heavy snowfall above 6,000, and moderate rain (which is
defined as 0.11 to 0.30 inches per hour) at lower elevations near
the Idaho/Montana border.

Rain is falling on very cold ground, which may refreeze and create icy
conditions, especially on side roads. Winter weather advisories are
in effect, so please travel carefully. Flood watches are also active,
as there could be flooding due to poor drainage, and there
continues to be a possibility for ice jam breakage. Clear snow
from around your house and make sure water can drain away from
your property.

The system is bringing breezy southerly winds. Normally calm
valleys (like in the Bitterroot Valley) are experiencing gusts up
to 30 mph with temperatures in the 40s. Higher valleys are
gusting near 40 mph, with the Camas Prairie/Grangeville Idaho
potentially reaching 50 mph this morning. These winds will speed
up the sublimation(the snow skips the liquid stage and directly
changes into water vapor) of the snow cover.

There will be a brief lull in the rain this afternoon. Later,
another surge of moisture will hit north-central Idaho by late
afternoon and then western Montana by early evening, bringing
heavy mountain snow above 6,500 feet and moderate rain in the
valleys. A weak cold front will pass by midnight, ending the
widespread precipitation, though showers will continue into Monday
morning. Snow levels will fall to around 3,500 ft in far
northwest Montana and 4,600 to 5,600 ft in other areas, with
heavy slush possible at high elevations like Lost Trail Pass and
Georgetown Lake by Monday morning.

Outlook for Monday Through Friday:
Monday morning may see a brief break, followed by warming
temperatures into the 40s and 50s. A warm front will bring light
precipitation by late Monday afternoon, and some instability could
trigger a squall line Monday evening, potentially leading to a
short period of heavy snow with gusty winds over 30 mph at
mountain passes. The unsettled pattern continues into Tuesday,
with mountain areas (above 3,500 ft) near the Idaho/Montana border
possibly receiving 6 to 12 inches of snow, while valleys see
lighter amounts. By Thursday through Friday, upper-level ridging
will reduce precipitation and increase sunshine (with morning
fog), and temperatures are expected to be above average, possibly
reaching the 50s or higher in some locations. This ridge pattern
seems a bit similar to what occurred on March 1, 2022.


&&

.AVIATION...Expect lowering ceilings and visibility due to the
incoming atmospheric river moisture through 18z (11am MST) today.
Strong westerly flow between 50 and 60 knots can be expected in
the saturated icing layer from 6000 feet MSL up to 15,000 ft MSL
through 18z/11am MST this morning across northwest Montana.
South to southwest winds will predominantly affect KHRF, KGPI,
and KBTM. Expect widespread rain and areas of fog throughout the
day Sunday across all of the Northern Rockies.

The presence of low level wind shear is possible this
evening between 00z Monday(5pm MST Sunday) and 6z Monday(11pm MST
Sunday). Stronger westerly flow (60+ kts) with mountain wave
activity is expected towards the Divide. Another round of heavy
precipitation, and icing threat is expected later this afternoon
(22z/3pm MST) through (07z Monday/12am MST). The westerly mid-
level jet streak associated with this wave will generally be 50 to
70 knots for much of Sunday night, so mountain wave turbulence
will again be another issue.

&&

.MSO WATCHES/WARNINGS/ADVISORIES...
MT...Flood Watch through Monday afternoon for Bitterroot/Sapphire
     Mountains...Butte/Blackfoot Region...Flathead/Mission
     Valleys...Kootenai/Cabinet Region...Lower Clark Fork
     Region...Missoula/Bitterroot Valleys...Potomac/Seeley Lake
     Region...West Glacier Region.

     Winter Weather Advisory until 8 AM MST this morning for
     Flathead/Mission Valleys...Kootenai/Cabinet Region...Lower
     Clark Fork Region...Potomac/Seeley Lake Region...West
     Glacier Region.

ID...Flood Watch through Monday afternoon for Lower Hells
     Canyon/Salmon River Region...Northern Clearwater
     Mountains...Orofino/Grangeville Region...Southern
     Clearwater Mountains.

&&

$$