Area Forecast Discussion
Issued by NWS Missoula, MT

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FXUS65 KMSO 250921

Area Forecast Discussion
National Weather Service Missoula MT
321 AM MDT SAT APR 25 2015

...Spring storm will bring SNOW to Mountain Passes & high valleys
across southwest Montana tonight...

.DISCUSSION...The upper level wave that brought snow to portions
of southwest Montana including Homestake Pass last night will be
departing to the east this morning. A large upper level low, as
seen swirling counter-clockwise in water vapor satellite imagery
centered over southwestern British Columbia and northwest
Washington State, will slowly shift eastward today. Showers will
be on the increase across central Idaho and into northwest-
western Montana by early this afternoon. Have added a chance for
isolated thunder along the Idaho and Montana border as there will
be marginal instability there this afternoon. The Flathead,
Mission, Missoula and Bitterroot Valleys may be able to avoid much
of the shower activity through this evening.

The upper level trough pinches off into a secondary system over
southern Idaho tonight. At the same time, a mid-level low develops
across western Wyoming with troughiness extending north and west
into southwestern Montana. This will bring wet snow to mountain
passes including Lolo, Lost Trail, Bannock, Gilmore and Homestake
by Sunday morning, and even to the high valleys above 5000 feet
including Butte. Generally 1 to 2 inches is expected currently,
while Lost Trail, Georgetown Lake and the higher elevations south
of Butte may be able to see a little bit more. The bulk of the
precipitation will fall between midnight and 11 am MDT. Area
roadways will become slick at times during this period. There is a
chance that Lookout Pass may be able to see light snow as recent
weather models have increased their precipitation amounts there,
but confidence is also lower in that vicinity. There is also a
chance that snow makes it down to 4000 feet across the southern
Bitterroot Valley from Darby southwards, but probably would only
stick to grassy surfaces.

The northern trough swings through western Montana Sunday
afternoon and will bring scattered rain showers to the region.
Have lowered temperatures across southwest Montana as cloudiness
and precipitation may linger into Sunday afternoon reducing any
chance for any substantial solar heating to occur. An upper-level
ridge builds in behind this system Sunday night bringing an end to
the precipitation.

Dry and warmer weather is on the way for the first part of next
week. Valley highs could reach the mid 70s or higher by the time
the upper-level ridge is directly overhead, which looks to be
Tuesday. In fact, current guidance would bring the warmest temps
yet this season for Missoula by Tues/Wed.

The flow aloft shifts to southwesterly for the latter half of
next week, bringing a more moist airmass back into the region and
allowing several weak disturbances to move through along with a
general cooling trend. The details remain fuzzy at this point, but
current indications point to Friday and Saturday having the best
chance of showers along with near to slightly below-normal


.AVIATION...Showers will be on the increase today across central
Idaho by 25/1800z and into western Montana by 25/2000z. There may
be just enough instability to support an isolated thunderstorm
with small graupel mainly north and west of Superior, MT in the
mountains and also south and east of Elk City, Idaho. There could
be shower-induced winds to 15 knots mainly at KSMN and KBTM
between 25/1900z and 26/0000z. For KGPI, showers should mainly be
situated over the surrounding terrain and may not affect the
terminal at all. A storm system brings rain and snow to the region
after 26/0000z continuing through at least 26/1800z Sunday. KBTM
has the highest chance of seeing the lowest ceilings and
visibility due to snow, especially between 26/0900z and 26/1800z. KSMN
will see lower ceilings with mostly rain. Expect obscured terrain
along the Idaho & Montana border into western Montana during the
time period, while the Glacier National Park region may be spared.



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