Winter Storm Watch 1, NWS Missoula, MT

Area Forecast Discussion
National Weather Service Missoula MT
347 AM MST Wed Jan 9 2013

...A warm, windy and wet Wednesday will quickly be replaced by
snow and much cooler temperatures Thursday...

.DISCUSSION...Another surge of moisture in the warm sector of the
Pacific weather system moving through the Northern Rockies is
spreading another round of valley rain and high mountain snow
across the northern Clearwaters and northwest Montana this hour.
A weak mid level ridge is over the area behind yesterdays surge
so this moisture will take some time to spread south and east,
but most of north central Idaho and west central and northwest
Montana will see precipitation by this afternoon. In addition to
the precipitation, the winds will be a factor today with breezy to
locally windy conditions across the region. The highest winds are
expected over the Lower Hells Canyon and Camas Prairie regions
where gusts to 40 mph are expected once again today. Thanks to the
winds, temperatures will be quite balmy for January with readings
in the lower to middle 40s with some low 50s in the Lower Hells
canyon.

Everything changes this evening/overnight as a cold front swings
through the region from northwest to southeast. Expect a burst of
precipitation along the front with rain mixing with than changing
over to snow behind the front along with falling temperatures.
This will cause some travel impacts as wet roads could become
slushy and freeze overnight. The front is expected to stall over
southwest Montana and Lemhi county of Idaho by early Thursday
morning. Behind the front expect decreasing winds, but much cooler
temperatures and snow showers. In fact, high temperatures Thursday
could very likely be in the morning as cold air advection behind
the front will keep temperatures steady or slowly falling through
the day.

The area of most concern for late tonight and Thursday is
across southwest Montana along the stalled boundary. Locations
from Drummond through Sula in Montana to Dixie in Idaho and
locations south and east could see snow, moderate at times
through the day Thursday. Models are still struggling with amount
of moisture along the front for Thursday. Therefore, have not
issued any winter headlines at this time, but persons should be
prepared for snow covered roads in the above mentioned locations
for Thursday. Stay tuned to later forecasts as winter headlines
may be warranted.

The models are still in fairly good agreement with the trough of low
pressure continuing to swing through the Northern Rockies Thursday
night into Friday. The cold front continues to be stalled over
southwest Montana and Lemhi county which should keep snow showers
lingering over night into the early morning hours. The latest
runs have some arctic air spilling over the divide Thursday
evening. The arctic air appears to be fairly shallow so the extent
of strong easterly winds is still uncertain at this time.

The trough of low pressure will settle over the central part of the
continental United States leaving the Northern Rockies in a
Northerly to Northwesterly flow pattern by this weekend. This
pattern will keep a cold airmass in place causing temperatures
to be well below normal for this time year. Moisture will be
fairly limited but can not rule out lingering snow showers through out
the weekend and into early next week. The latest model runs are
starting to hint at the trough shifting to the east allowing the
Northern Rockies to become influenced by the ridge high pressure
over the west coast.

&&

.AVIATION...Another surge of moisture in the warm sector of a
Pacific weather system will spread valley rain and high mountain
snow across northwest Montana this morning. This precipitation
will gradually shift southwest over north central Idaho and
west central Montana this afternoon. Widespread mountain
obscurations should be expected with occasional airfield impacts
from lowered ceilings and visibilities in precipitation today. A
cold front will sweep from northwest to southeast this evening and
eventually stall across Lemhi county and southwest Montana
Thursday. Widespread snow showers with locally heavier snow along
the front is expected for Thursday. KSMN and KBTM will be most
impacted Thursday from lowered visibilities and ceilings from snow
with accumulations expected on runways.

&&

.HYDROLOGY...Ice jam flooding on the Lemhi river and especially
the Salmon river in Lemhi county is a growing concern for next
week. Warm temperatures today will help break up some of the ice
that built up from the recent cold spell. A return to cold weather
tomorrow through the weekend will likely cause ice growth again.
Confidence in temperature forecasts for the upper Salmon river
basin for next week is below average, but there is a good chance
that daytime temperatures will drop back into the low teens with
subzero temperatures by Sunday night. If this occurs, new ice
growth on top of the already dense ice may cause jams. The main
area of concern is from North Fork to Salmon.

&&

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

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