Special Weather Statement, NWS Missoula, MT

Area Forecast Discussion
National Weather Service Missoula MT
331 PM MDT Thu Aug 21 2014

.DISCUSSION...Widespread showers and thunderstorms have already
developed across much of the Northern Rockies as a very moist
atmosphere responds to sufficient daytime heating. We do not
anticipate these storms reaching severe strength, but they will at
times be strong with key threats of ample small hail and very
heavy rainfall. Another weakness in the upper flow ahead of a large
upper level storm system is forecast by the models to traverse the
region overnight. So just as storms will seem to be winding down
later this evening, a secondary resurgence in activity is expected
in the middle of the night.

The main upper level system will be entering the Pacific Northwest
on Friday and Northern Rockies through the weekend. Locations
across southwest Montana and Lemhi County will fall under a
particularly favorable area for convective development on Friday,
much earlier in the day than normal (likely between 9am and
11am). These storms will quickly become strong and likely poses
very heavy rainfall, which could become particularly problematic
for flood prone areas. A Flash Flood Watch could be warranted for
these areas, however we need to gather additional data over the
next 12 hours in order to make a final call. Elsewhere widespread
showers and thunderstorms will continue to develop throughout the
day with again, heavy rain and small hail the primary threats. In
fact, many models are resolving an area of focused rotation and
area of heavy precipitation developing across Ravalli, Missoula &
Granite Counties by Friday evening and persisting well into early
Saturday morning. Although the intensity for flash flooding is not
anticipated with this precipitation, the resultant amount may
produce some urban flooding concerns (i.e. water on roadways,
standing water in poor drainage areas, etc.).

Farther north along the Continental Divide and in Glacier
National Park heavy wrap around precipitation will be further
enhanced by up slope flow along the leeward side of the Divide.
This will ultimately spill over into the windward side, affecting
Glacier National Park, the Bob Marshall Wilderness and all other
areas, both valley and mountain near the Divide. All forecast
models place the heaviest amounts in these areas, on the order of
1 to 3 inches of liquid between Friday afternoon and Sunday
morning.

During this same time frame unseasonably cold Canadian air will be
descending upon Montana, lowering snow levels across basically the
same area where the heaviest precipitation is expected. This
collusion of factors will force snow levels to between 6500ft and
7000ft for a time and result in generally 2 to 5 inches of snow.
However some of the highest peaks could see more than that if
these factors work out just right. An early season Winter Weather
Advisory may be necessary, but will again wait for additional data
given the complexity and poor confidence with this enter system.
Precipitation will continue throughout the day on Saturday into
Sunday morning, but will gradually taper off and reveal a pleasant,
warmer Sunday.

Monday should see the last upper level wave in the northern tier trough
produce another round of light precipitation before the transition
to a high pressure pattern through midweek. Main concern for building
high pressure and clearing skies is cold mornings and patchy frost on
Tuesday and Wednesday. Otherwise expect nice weather in the afternoons
next week. Focus could then shift to another developing fall-like
weather system toward the end of the week. /ss

&&

.AVIATION...Showers and thunderstorms continue to develop across
the entire region and could impact aviation operations at all TAF
sites. This activity will not only continue through the evening,
but also overnight. Briefly lowered visibility and clings along
with frequent lightning will be the chief threats. Additional
development of showers and thunderstorms are also forecast for
Friday, likely starting as early as 22/1500z near KSMN and KBTM.

&&

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

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