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000
FXUS65 KMSO 212043
AFDMSO

Area Forecast Discussion
National Weather Service Missoula MT
243 PM MDT Tue May 21 2013

...Gusty winds with thunderstorms this afternoon and evening...
...Much colder air to produce snow over Lookout Pass and even
Libby and Yaak...
...Rain and high elevation snow through Friday...

.DISCUSSION...The first in a series of cold fronts is passing
through eastern Oregon and Washington this afternoon. Warm
afternoon temperatures and the cold front passage will bring
showers and thunderstorms through north central Idaho and western
Montana beginning this afternoon. Gusty winds will initially be
the main threat with thunderstorms through this evening.

Much colder air will follow the cold front especially into north
central Idaho and northwest Montana. There is a good chance that rain
will change to snow in northwest Montana Wednesday morning.
Locations of main concern for snow are in the vicinity of Libby
and Yaak and over Lookout Pass. Accumulations are not expected
initially but may begin to accumulate through Wednesday morning.
Precipitation will enhance in the afternoon on Wednesday and there
is the potential for slushy accumulations especially over Lookout
Pass. At this time snow is not forecast for Kalispell, and
locations to the south, but if the low track changes snow may
become possible.

A low pressure system aloft should direct surges of moisture
northward across mainly western Montana Thursday into Friday.
There are at least two significant waves of precipitation. The
first arrives Wednesday night and then a stronger wave hitting
Thursday night and Friday. There is some uncertainty regarding the
location of the heaviest precipitation with the GFS further west
over the Bitterroots northward across the Flathead and the NAM and
ECMWF a bit further east over the continental divide from Wisdom
northward across Bob Marshall and Glacier Park. One way or the
other, Glacier Park southward to McDonald Pass should see
significant precipitation. Snow levels should generally be around
5000 to 6000 feet. However, at night or within heavier shower
cores, the snow level may briefly go down to valley floors, such
as Seeley Lake, Swan Lake and in the vicinity of the Park. In the
higher terrain, heavy snow is expected with 1 to 2 feet possible
in the terrain of Glacier Park by Friday.

Besides the moisture from the weather system over Montana, the
other concern is the very cool air mass that will settle over
northern Idaho and western Montana. Partial clearing may occur
Wednesday night, Thursday night and Friday night allowing areas of
frost. Locations that may be most susceptible to frost include
Grangeville, Pierce, Libby, and Thompson Falls.

The main low pressure system should shift northward into Canada
Friday. However, another trough of low pressure and relatively
cool and unsettled air mass should settle over the region for the
Memorial Weekend.


&&

.AVIATION...High resolution model data show isolated showers and
thunderstorms this afternoon, but by this evening a more defined
line of thunderstorms is forecast to form. Showers with isolated
thunderstorms are already developing ahead of the cold front that
is passing through Washington and Oregon. The main threats with
thunderstorms will be very gusty winds and small hail. As the sun
sets this evening and thunderstorms die off, rainfall will spread
across central Idaho and then western Montana overnight tonight.
Snow levels will lower behind the cold front tonight with snow
possible to the valley floors of northwest Montana. Low cloud
decks and visibility will be common during the day Wednesday.


&&

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

$$

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