Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 042150

Area Forecast Discussion
National Weather Service Great Falls MT
250 PM MST Sun Dec 4 2016

...Periods of Snow through Monday Followed by Significantly Colder
Air This Week...


Sunday through Tuesday...An active and complex short term period
will bring multiple hazards to the local area. As of 21Z, a potent
s/w was noted on water vapor imagery dropping slowly SE out of
British Columbia/southern Alberta. A dry slot developed ahead of
this s/w, allowing a bit more clearing and slightly milder temps
across central Montana. In turn, this has allowed weak instability
to develop with MUCAPE values on the order of 100-200 j/kg noted on
latest LAPS data across much of central Montana. This combined with
large scale lift ahead of the s/w is fostering the development of
bands of convectively-driven snow showers from Cut Bank to Havre.
These showers will cause significant reductions in visibility and
could put down a quick half inch to an inch of snow. These bands of
snow showers should continue to sink south through the afternoon,
but may weaken some with the loss of daytime heating.

In the wake of the snow showers, a more stratiform band of snow that
has developed in southern Alberta will likely drop south into
central Montana. It is this area of moisture that, combined with
some upslope enhancement, should bring most of the accumulating snow
to central Montana. The models weaken this area of precip as it
drops south, but radar imagery from Environment Canada suggests
there hasn`t been any weakening yet. The models, especially the NAM,
have insisted on a drier solution with this system, but the NAM,
especially, appears to be a bit more of an outlier overall and
QPF/snowfall amounts were generally kept the same (ie. not showing
the drier solution suggested by the NAM). That said, some of the SW
MT valleys will probably end up a bit drier with less of the upslope
enhancement that some areas of central Montana will likely see.
Decent moisture in the snow growth region combined with some weak
lift will likely produce a period of moderate to heavy snow from Cut
Bank to Great Falls to Lewistown this evening/tonight and I still
expect a general 2-5 inches at lower elevations during this period,
with higher amounts of a foot or more in the mountains.

Across SW MT, most of the snow is expected this afternoon/tonight as
the above mentioned s/w moves through. For the valleys, though,
moisture doesn`t look as good as further north and am generally
expecting lower impacts compared to locations further north. Still,
with sub zero temps moving in behind the snow, at least some impacts
are expected areawide, even for areas that don`t see as much snow.
In addition, there will be some blowing snow at times, especially
across central Montana and the higher terrain of SW MT (including
passes). This will add to the level of impact at times.

As mentioned above, a surge of Arctic air will move south into the
area in the wake of the current system. This will send temps
plummeting into the teens tonight, with sub zero temps likely by
Tuesday morning. It should be noted that the cold temps combined
with wind speeds of 10+ mph will likely lead to wind chill values
of -20 to -30 at times, especially Monday night through at least
Thursday. We`ll hold off on any Wind Chill highlights for now, but
a Wind Chill Advisory will likely be needed for parts of the area
later this week. Martin

Tuesday night through Saturday...High pressure will build over
Central and Southwest Montana Tuesday night as the low pressure trof
continues to move east. The air mass will dry and winds will
continue to weaken as the ridge builds. The ridge will be over the
Northern Rockies Thursday with the next low pressure trof over the
Eastern Pacific. Moisture ahead of this trof will be moving
underneath the ridge and across the Rockies. This will bring
increased chances of precipitation mainly across the Rockies and
Southwest Montana. The trof will move on shore late Thursday night
and Friday morning. Moisture will increase ahead of this system
however some splitting of the trof will occur and dynamics over the
forecast region will be quite weak. Most snow will be over areas
affected by west flow aloft.  High pressure will build again
Saturday however a shortwave will be moving into the area and keep a
chance of precipitation in the forecast. Temperatures will begin the
period well below normals but the air mass will trend warmer through
the period. Temperatures, by  Saturday, will be near seasonal
normals again. Zelzer


Updated 1800Z.
Widespread IFR conditions are likely to develop by 00z Monday as a
cold front moves south through the region. Expect light snow to
affect most terminals this evening. Some runway ice is
snow melts on the warm pavement initially...then freezes. The snow
will gradually diminish in intensity towards 12z Mon but light snow
showers will continue through 18z especially across Central MT.
Mountains will be obscured through the period. Brusda


GTF  15  19  -3   5 /  80  60  20  20
CTB  11  13  -7  -1 /  90  30  20  20
HLN  18  25   9  17 /  40  30  20  20
BZN  16  23   6  16 /  50  20  30  30
WEY   2  14  -1  13 /  80  20  50  50
DLN  11  21   6  16 /  40  10  50  50
HVR  16  18  -3   4 /  30  20  20  20
LWT  15  18  -1   3 /  70  60  20  20


Winter Weather Advisory until 11 AM MST Monday Broadwater...
Central and Southern Lewis and Clark...Jefferson.

Winter Weather Advisory until 11 AM MST Monday below 6000 feet
for Beaverhead...Gallatin...Madison.

Winter Storm Warning until 11 AM MST Monday above 6000 feet for

Winter Storm Warning until 5 PM MST Monday Cascade...Judith

Winter Weather Advisory until 5 PM MST Monday Chouteau...Fergus.

Winter Weather Advisory until 5 PM MST Monday Eastern Glacier...
Eastern Pondera...Eastern Teton...Toole.

Winter Storm Warning until 5 PM MST Monday Northern Rocky
Mountain Front...Southern Rocky Mountain Front.


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