929
FXUS65 KMSO 151946
AFDMSO

Area Forecast Discussion
National Weather Service Missoula MT
146 PM MDT Sat Jun 15 2024

...Late Spring Snow Storm to hit the Mountains Sunday night into
Monday...

.DISCUSSION...Our late spring winter weather event is coming into
focus. The cold trough has slid into the Northern Rockies as of
noon Saturday the 15th and snow levels dropped to around 5000 feet
along the Idaho and Montana border, moderate to heavy snow was
observed on the peak camera at Lookout Pass Ski area, and
temperatures are between 15 and 20 degrees colder than what
occurred Friday. The cold front has shifted eastwards and is
currently making its way into southwest Montana and Lemhi County
early this afternoon. This feature brought a plethora of cloud-
cover and showers to the region. Along the front between 11am and
1 pm MDT, scattered thunderstorms developed and brought gusty
northwest winds to 45 mph and was accompanied by a temperature
drop. Temperatures in Deer Lodge fell from 66 degrees at 1045am to
48 degrees by 1215pm MDT.

Scattered showers will persist through this evening as the trough
continues to influence our weather. Patchy heating from the sun
could induce a few weak thunderstorms(20% chance) across northwest
Montana and western Clearwater County. It will be a breezy
afternoon across western Montana. There is a 40% chance that wind
gusts could exceed 35 mph in the Flathead to Mission valleys,
along the Divide, and in Lemhi County. Some clearing tonight is
expected and will allow low temperatures to fall into the 30s in
quite a few locations. You might want to cover your sensitive
plants tonight! Here are the current probabilities for a frost:
40% in Pierce, Idaho, 40 to 60% north and west of Cottonwood,
Idaho, 60% between Kila and Happys Inn, and 70 to 100% across
southwest Montana, and southern Lemhi County.

We expect the coldest part of the trough to rotate south over our
region late Sunday and eventually a piece of energy in the middle
atmosphere is expected to close off and rotate over Lemhi County,
further lowering snow levels to the north and west of its track
(deformation zone).

Sunday continues cool, overcast, and breezy. Shower activity
initially limited to the Canadian border will expand late
afternoon south of I-90 in response to a low pressure system
strengthening in eastern Washington.

Overnight conditions get progressively cooler and snow levels in
general lower across the region, an average 5000 feet or above.
There is increasing confidence of a mid-level low pressure system
closing off and developing a deformation zone after midnight
Sunday. There is model consensus that the deformation will develop
over far southeastern Clearwater County, into the southern
Bitterroot Valley, and eventually moving east of Missoula along
I-90. Under the deformation zone snow levels will likely lower an
additional 1000 to 1500 feet. We have added Clearwater County
(southeastern portion) and the Bitterroot Valley to the winter
storm watches for concerns of back country users in Idaho and the
potential for power outages due to downed tree limbs in the
Bitterroot Valley. As little as 1 to 2 inches of heavy wet snow
will be sufficient on leafed out tree branches to cause breaks and
potentially power outages. Closed lows are notorious for shifting
track so our concerns will shift with the exact track of the
system.

In general, Monday will be a wet and raw day. Plan accordingly for
any outdoor plans, hypothermia can happen even in June.

Tuesday a secondary wave drops out of Canada and scattered showers
redevelop. Snow levels rise above 6000 feet but snow showers will
be possible in Glacier National Park.




&&

.AVIATION...Westerly winds will gust to about 25-30 knots this
afternoon across all of western Montana and north central Idaho.
A few isolated thunderstorms are possible, primarily across
northwest Montana and over Clearwater County, Idaho (20%). Some
clearing is expected tonight. VFR conditions are expected on
Sunday before the winter storm arrives Sunday night.


&&

.MSO WATCHES/WARNINGS/ADVISORIES...
MT...Winter Storm Watch from late Sunday night through Monday
     afternoon for Bitterroot/Sapphire Mountains...
     Butte/Blackfoot Region...Potomac/Seeley Lake Region.

     Winter Storm Watch from Monday afternoon through Tuesday evening
     for West Glacier Region.

     Lake Wind Advisory until 10 PM MDT this evening for
     Flathead/Mission Valleys.

     Winter Storm Watch from late Sunday night through Monday morning
     for Missoula/Bitterroot Valleys.

ID...Winter Storm Watch from Sunday evening through Monday afternoon
     for Southern Clearwater Mountains.

     Wind Advisory until 8 PM MDT this evening for Eastern Lemhi
     County...Western Lemhi County.

&&

$$