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000
FXUS65 KMSO 202051
AFDMSO

Area Forecast Discussion
National Weather Service Missoula MT
251 PM MDT Mon May 20 2013

.DISCUSSION...Lots to talk about for next few days. First, a much
warmer air mass will sweep northward in response to a ridge of
high pressure temporarily building over the northern Rockies
Tuesday.

Second...as the air mass warms up, it will also become quite
unstable by Tuesday evening and support isolated thunderstorm
development. This instability may then combine with a favorable low
level wind field Tuesday night to support scattered convection
and a few strong thunderstorms from the Clearwaters northward.
Storms should generally develop from Grangeville to Orofino and
quickly shift northward across far northwest Montana near Libby
and Troy. Storms have the potential to produce small hail and
strong gusty winds.

As the main upper low pressure develops over Oregon and Washington
Wednesday, additional showers should develop across west central
Montana and further grow in terms of precipitation as it shifts
northward across northwest Montana. This brings the next possible
weather concern. As this precipitation shifts into northwest
Montana, some of the cold low level air from the parent weather
system over Oregon and Washington should slide into northwest
Montana...especially Troy to Libby. This colder low level air
combined with possible moderate to heavy precipitation may drive
the snow level to the valley floors, especially in the Libby area
Wednesday. Heavy wet snow could be a problem given many of the
trees have gone through spring green up and would be more
vulnerable to any snow weight. There is still some uncertainty in
the amount of cold air that will undercut the heavy precipitation,
although. The cooler air should shift eastward to Kalispell and
Glacier Park Wednesday evening and night. However, some moderation
is expected, so snow levels should remain above the valley floors
in the Kalispell area and possibly some wet slushy snow in the
Glacier Park valleys.

As the main cold front shifts across Missoula and then into
southwest Montana...Anaconda and Butte...scattered thunderstorms
may also occur Wednesday.

The main upper low pressure area over Washington and Oregon is
expected to direct additional surges of moisture and energy
northward across mainly western Montana Thursday into Friday. Snow
should continue to accumulate in the higher terrain. Enough
precipitation may occur by Thursday and Friday to support
significant stream and river rises. However, no significant
flooding is expected at this time.

&&

.AVIATION...The atmosphere will quickly stabilize this evening
with partial clearing most areas from Kalispell to Salmon and Butte.
These general VFR conditions should continue into Tuesday.

&&

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

$$

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