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000
FXUS65 KMSO 210947
AFDMSO

Area Forecast Discussion
National Weather Service Missoula MT
347 AM MDT Tue May 21 2013

...Thunderstorms possible this afternoon/evening...

...Persistent rainfall, in addition to high terrain snowfall, expected
Wednesday and Thursday...

.DISCUSSION...The first half of today should begin fairly
pleasant, which will be rather deceiving since the evening and
overnight hours will be quite a departure from this morning`s
weather. Cloud cover will steadily increase, with winds kicking up
across all of western Montana and central Idaho. As the winds help
to mix out the atmosphere, temperatures will warm to near 80
degrees in many valley locations this afternoon. The atmosphere
will then destabilize and allow a scenario ripe for thunderstorms,
some potentially strong and damaging, especially across Lincoln
County in northwest Montana. This will be the favored region for
thunderstorm development, however most of the Northern Rockies
could see a storm or two through this evening.

Tonight, the cut off low pressure system advertised in previous
forecasts to bring heavy rainfall to the region will begin more
fully impact central Idaho. Rain will spread across west central
Montana Wednesday morning, and very literally remain stationary
through Thursday as well. By later in the day Thursday, rain will
finally shift north into Alberta.

During the Wednesday-Thursday time frame, over one inch of rainfall
has the potential to fall across Glacier Park and surrounding
terrain, while the rest of Flathead County and Lincoln County
could see between 0.75 and 1.00 inch of rainfall. In addition, 2 to
4 inches of snowfall could coat the higher terrain across
northwest Montana, as temperatures and snow levels take a
significant dive.

Forecast models are struggling with how to handle each push of
energy ejecting off the parent coastal cut off low pressure system
beginning Thursday night and continuing through the weekend. Thus
have made only minimal changes to the overall forecast which had
above normal chances for precipitation continuing through the
weekend. In addition to periodic chances for showers, there will
also be at least a slight chance for thunderstorms each afternoon.
But the flow remains modest at best and with abundant moisture
around expect the key impact (aside from lightning) to be
occasional moderate rainfall, which could hamper weekend plans.
Temperatures will show a gradual warming trend with values working
their way back up to around normal for late May.

Some model consistency remains regarding the dissipation of that
stubborn cut off low with models each showing it expanding into a
broad, coastal trough by the start of next week. This should
promote a continued gradual warming trend but also keep a steady
stream of Pacific moisture moving across the Northern Rockies.

&&

.AVIATION...Clear skies and calm winds will be present this
morning, but the Northern Rockies will see a change as today
progresses. Cloud cover will thicken and lower, with south to
southeast winds increase, some gusts to 30 to 35 mph through this
evening. Thunderstorm potential will also increase this afternoon
and evening, especially for KGPI and northwest Montana airfields.
Thunderstorms that do develop could be locally strong, causing
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. Low cloud decks and
visibility will be common during the day Wednesday.

&&

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

$$

Dickerson/Allegretto

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