Area Forecast Discussion
Issued by NWS Missoula, MT

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353
FXUS65 KMSO 182009
AFDMSO

Area Forecast Discussion
National Weather Service Missoula MT
209 PM MDT Sun Apr 18 2021

.DISCUSSION...

A vigorous cold front will move south across the Northern Rockies
this afternoon and evening, bringing a blast of cold air and high-
elevation snow across the Continental Divide. Boaters on Flathead
Lake this afternoon and evening will find conditions a bit
inhospitable due to a sudden shift to northerly winds and increased
wave activity. Area travel will be impacted by snow, blowing snow,
and low visibility across southwest Montana and over mountain passes
(particularly passes along the Continental Divide, such as Marias,
MacDonald, and Homestake, where 1-inch per hour snow could develop
for a time). For high-elevation southwest Montana valleys above 4500
feet, there will be a mix of snow accumulating, melting, and icing
up on roadways. Travel conditions may be drastically different among
locales. The snow will quickly subside by mid morning on Monday,
with a relatively cold and brisk remainder of the day throughout
much of the region.

Weak high pressure will develop across the Northern Rockies Monday
through Wednesday, with temperatures gradually moderating to near
the seasonal average.

Thursday through Friday, another storm system is anticipated to
sweep south across the Northern Rockies, bringing another
opportunity for at least brief snow impacts across the Continental
Divide Thursday overnight. Models and their ensembles have trended
toward depicting an increasingly dynamic storm system during this
time, however clarity in specific timing is lacking at the moment.

High pressure is expected to return to the Northern Rockies Friday
to Saturday, rapidly warming and drying the region. The larger scale
pattern appears to transition from northerly flow to westerly flow
over the weekend, with a series of Pacific storm systems set to move
across the region during much of the following week. Thunderstorms
may become a bigger part of the forecast picture by Sunday
afternoon, as southwest flow aloft intensifies and the atmosphere
destabilizes.

&&

.AVIATION...A Canadian cold front has already increased winds out
of the N-NE at KGPI as of 19/20Z, with gusts up to 33 knots
observed. These kinds of winds will occur at all area terminals
throughout the afternoon and evening as the front makes its way
across the region. Precipitation will begin to increase at KGPI by
19/00Z or around there, beginning as a rain but transitioning to
a rain/snow mix through Monday, 19/15Z. KMSO, KBTM, and KSMN will
also experience similar precipitation trends shortly thereafter,
and will continue through roughly 19/18Z. Winds and precip will
diminish after 19/18Z.

&&

.MSO WATCHES/WARNINGS/ADVISORIES...
MT...Winter Weather Advisory from 6 PM this evening to noon MDT
     Monday Butte/Blackfoot Region.

     Wind Advisory until midnight MDT tonight for Flathead Lake in
     the Flathead/Mission Valleys.

ID...None.
&&

$$



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