Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 040552
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
1150 PM MST Sat Dec 3 2016

Aviation Section Updated.

.UPDATE...Mostly cloudy skies, breezy to windy conditions and
nearly steady temperatures will prevail across the forecast area
overnight. Widespread snow is already developing in the mountains
along the Continental Divide and it appears favorable for isolated
to scattered snow showers to move out over the plains tonight and
through much of Sunday. The going forecast package is generally
on the right track but I have made updates to nearly all forecast
parameters in order to fine-tune and tweak several locations based
on observations, latest models and border changes brought about by
neighboring offices. Despite the shortening time-frame for the
approaching storm system, models are still showing way more
discrepancies than I like to see this close to an event. However,
overall, I believe that the forecast and all highlights appear to
be on the right track. May be worth noting that changes I made
this evening resulted in increased snow accumulation amounts for
Glacier National Park but slightly lower accumulations for the
mountains of Southwest Montana while the remainder of the forecast
area remained generally similar. mpj

&&

.AVIATION...
Updated 0550Z.
Clouds increase overnight from the west as attention turns to winter
weather developing by the end of this period. VFR conditions prevail
through Sunday morning at area terminals, with mountains becoming
obscured overnight. A cold front will bring a wind shift and snow to
KCTB Sunday afternoon. Snow will be over the mountains surrounding
KHLN/KBZN/KEKS late Sunday afternoon, possibly developing at these
terminals with IFR conditions around 00Z. Timing of decline in
flight category remains uncertain. PN

&&

.PREV DISCUSSION...
/ISSUED 445 PM MST Sat Dec 3 2016/

Sunday through Monday...An active short-term period is expected as
we transition into a much colder weather pattern across much of the
US. Locally, a s/w trough will move through the area late tonight
through Monday bringing snow and breezy conditions. The models
appear to have solidified onto generally the same solution through
Monday with more of an open wave aloft leading to a quicker
movement of the storm. This leads to some concerns regarding how
much moisture the storm will be able to work with and for this
reason, there remains some differences in precip amounts and the
exact track of the low. At this time, the expected track of the
SFC low will likely favor the mountains as well as the favored
upslope areas of central Montana for the heaviest snow. It is
these areas that we upgraded the Winter Storm Watch to a Winter
Storm Warning and is where we expect the most significant impacts.
Elsewhere, snow will likely create some impacts, but impacts may
not be quite as significant as in the warning areas.

The snow will begin along the Rocky Mountain Front tonight, then
spread east across the plains/valleys Sunday/Sunday night. At lower
elevations, the relatively milder temps today and Sunday should
offset accumulations on roadways initially, and some rain may even
mix in at the onset, but a sharp cold front will quickly send
temperatures plummeting into the teens Sunday night which may lead
to a layer of ice forming beneath the snow which will only
exacerbate travel issues. Across central Montana and the higher
terrain of SW MT (including passes), a period of gusty winds is
expected on Sunday which will lead to blowing/drifting snow in
spots as well as significant reductions in visibility at times.
Adjustments in snow amounts are likely over the next 12-24 hours
as the storm moves in, so check back for later updates. In the
wake of the snow, an Arctic airmass will move in and impacts from
the snow will likely continue even after the snow ends on Monday.
Martin

Monday night through Friday...Strong west flow will be moving
through a broad trof over Central and Southwest Montana. Cold air
will be settling into the zones and the Tuesday morning lows will be
much colder than the previous day. This will begin a period, lasting
through the remainder of the forecast period, where temperatures
will be well below seasonal normals. High pressure will already be
building aloft Monday night and Tuesday although the flow aloft will
be unsettled and scattered snow showers will remain over higher
elevations. The high pressure will continue to build Tuesday night
and Wednesday, and the air mass will continue to dry. Temperatures
will remain cold and, with snow on the ground and clearing skies,
very cold temperatures are expected Tuesday through Thursday
mornings. The latest GFS/EC now have cold air continuing into next
weekend, along with another chance for widespread snow Thursday
night and Friday. Zelzer/Brusda

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  29  39  14  20 /  20  30  70  60
CTB  28  35   9  14 /  20  40  70  30
HLN  26  41  19  27 /  20  30  40  40
BZN  24  37  17  23 /  20  30  50  30
WEY  15  28   6  17 /  50  80  90  40
DLN  24  36  14  21 /  10  40  50  20
HVR  25  38  13  18 /  10  30  40  30
LWT  24  38  15  18 /  20  30  70  60

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from 8 AM Sunday to 5 PM MST Monday
Central and Southern Lewis and Clark...Jefferson.

Winter Weather Advisory from 8 AM Sunday to 5 PM MST Monday
Beaverhead...Madison.

Winter Storm Warning from 1 PM Sunday to 5 PM MST Monday
Cascade...Judith Basin...Meagher.

Winter Weather Advisory from noon Sunday to 5 PM MST Monday
Broadwater...Gallatin.

Winter Weather Advisory from 2 PM Sunday to 5 PM MST Monday
Chouteau...Fergus.

Winter Weather Advisory from noon Sunday to 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.

&&

$$

weather.gov/greatfalls



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