Area Forecast Discussion
Issued by NWS Missoula, MT

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FXUS65 KMSO 032050

Area Forecast Discussion
National Weather Service Missoula MT
250 PM MDT THU SEP 3 2015

...Cold & Wet Labor Day Weekend with High Mountain Snow Likely...

.DISCUSSION...A strong and very cold upper level storm system will
continue its slow decent from British Columbia tonight and become
centered over Puget Sound by Friday morning. The ensuing
southwest flow ahead of this system has produced a persistent,
near stationary area of precipitation thats stretching across a
good portion of central Idaho and western Montana today. This
rain should slowly move east this evening with the focus for
overnight rain shifting towards SW Montana & east of the
Continental Divide. Elsewhere only light, occasional showers are
anticipated with perhaps some clearing overnight in NW Montana.

A weak disturbance will move out of Oregon towards the Northern
Rockies on Friday. It will combine with favorable upper level jet
energy and help recharge the persistent area of precipitation.
And the deepening of the upper level low moving down the
Washington/Oregon coastline will actually shift the precipitation
back westward across Montana and into parts of central Idaho.
Thus very similar conditions are expected on Friday as compared to
what has transpired today (cool, wet & cloudy). The upper level
low, now over Oregon, will slowly drift towards Idaho and Montana
Friday night into Saturday with impulses of energy likely
providing enhanced areas of precipitation over central Idaho and
along the Idaho/Montana border.

The center of low pressure will slowly move over western Montana
late Saturday night and begin to open into a slightly more
progressive wave on Sunday. However models continue their
struggles in accurately depicting the timing of this transition
and eventual exit of the weather system. At this time there is
more evidence to support slower movement and progression, more
akin to that of the ECMWF, NAM & Canadian models. Regardless,
precipitation will persist Saturday night with the heaviest
amounts focused across SW and west central Montana.

SNOW: All models support lowering snow levels down to at least
7000ft by late Saturday night. However late afternoon model runs
of the Canadian and ECMWF suggest the slower progression and
overhead path of the cold, upper low could drive the snow level to
somewhere between 6000ft to 6500ft under the heaviest
precipitation. Anyone with camping, backpacking or hunting plans
this weekend, especially in southwest Montana, should be prepared
to encounter at least a mix of very cold rain and some snow.

Models are now supporting additional waves of energy dropping into
the Northern Rockies after the passage of this main weather
system. Thus have trended chances for precipitation upward and
cooled temperatures. However the more showery regime under
northwest flow aloft should produce lighter amounts of rain. Mid
to late next week, forecast confidence is low. However, models
tend to agree that we will be in west to northwest flow aloft,
which will lead to continued cool, autumn- like weather.
Occasional chances for light rain cannot be ruled out, but timing
any weak systems moving through the jet stream is imprudent at
this time due to high run- to-run variability.


.AVIATION...Mid-level cloud currently affecting much of the area
will persist. Rain showers can be expected at KMSO and KBTM this
this afternoon, then more widespread showers around KSMN Friday
morning. A clap of thunder near KBTM is possible, but
thunderstorms look to be isolated in coverage. The most likely
time for showers are listed above, but with a moist airmass in
place, no location can count on totally dry runways over the next
24 hours. Winds will be light, and likely variable during showers.
Although the public forecast calls for some patchy fog in some
valleys of far western Montana early Friday morning, cloud cover
over TAF sites are expected to prevent fog formation near
terminals...a few scattered low clouds is a more likely scenario.



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