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000
FXUS65 KMSO 222047
AFDMSO

Area Forecast Discussion
National Weather Service Missoula MT
247 PM MDT Wed May 22 2013

.DISCUSSION...Several impacts are expected this evening. First, a
very unstable atmosphere will linger over southwest Montana into early
this evening. The low level wind environment is such that a few
storms could be strong into early this evening from Butte to
Anaconda and northward to Seeley Lake and Ovando.

Second, a cold front is moving across western Montana this
afternoon and evening with a quite cold air mass for this time of
year wedging into the area. Snow levels are lowering to near
valley levels from Hamilton to Missoula northward across northwest
Montana. However, we are currently forecasting the precipitation
to remain mainly in the form of rain except above about 4000 feet
and in the lower valleys of far northwest Montana from Thompson
Falls to Hot Springs and near Libby. This is one part of the
forecast that our confidence is not high...meaning there is a
small possibility of the rain temporarily changing to snow in some
of the other valleys, such as Mission, Missoula, Bitterroot, Flathead,
and Seeley/Swan.

The third overnight concern is the moderate to heavy
precipitation. The cold front is expected to stall near the
continental divide resulting a prolonged period of moderate to
heavy showers especially from about Drummond northward across
Seeley Lake, the Mission Valley, Flathead Valley, Eureka, Swan
Valley, and Glacier Park. Some areas should see more than one inch
of precipitation tonight causing local ponding of water and stream
rises. However, with the precipitation changing to snow in the
mountains with lowering snow levels this evening, the bigger
streams and rivers should not see a major reaction. In other
words, the precipitation goes to snow pack rather than runoff.

Fourth, but also very important is overnight temperatures in west
central Idaho. A cold air mass has shifted into Idaho. Partial
clearing is expected tonight from Grangeville to Orofino. As such,
temperatures should fall to near or a little below the freezing
point in many valley locations. Those that have started their
gardens and sensitive outside plants may want to protect tonight.

Another weather disturbance should brush western Montana late
Thursday and Thursday night. However, some of the more recent
forecast guidance is trending towards a more eastward shift of the
main precipitation core east of the divide. At this point we have
only trended down the precipitation forecast some for Thursday
night.

A series of weather disturbances are expected to shift across
northern Idaho and western Montana Saturday, Sunday and Monday.
This should translate into occasional showers. Despite the
occasional precipitation, temperatures should moderate to near
normal values over the weekend into Memorial Day.

&&

.AVIATION...A low pressure system will maintain through the
Pacific Northwest for the next couple of days. Widespread rain
and snow will keep lowered ceilings and visibility. Obscured
mountains and passes due to snow and cloud cover will be common
through tonight. Snow levels have lowered to many of the valley
locations of northwest Montana and western Idaho this afternoon
and snow levels continue to lower throughout the region. Moderate
to heavy rain and snow showers with embedded thunderstorms should
be expected through this evening. Locations between KSMN, KBTM and
KMSO have the most instability and thunderstorms will be in the
vicinity through this evening. Drier air will move into north
central Idaho overnight and precipitation will decrease. Rain and
snow will continue through much of western Montana and along the
divide through much of Thursday. Saturated surface conditions may
also lead to early morning fog development Thursday morning.

&&

.MSO WATCHES/WARNINGS/ADVISORIES...
MT...WINTER WEATHER ADVISORY until 6 AM MDT Thursday Lower Clark Fork
     Region.

     WINTER WEATHER ADVISORY until 6 AM MDT Thursday Kootenai/Cabinet
     Region.

ID...None.
&&

$$

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