Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 232202
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
302 PM MST Fri Feb 23 2018

.SYNOPSIS...

Strong winds along the Rocky Mountain Front and across the remainder
of Glacier County will continue through the night, leading to
significant blowing and drifting of snow across these areas. Across
Southwest Montana and the mountains of Central Montana, light to
at times moderate snow is expected to develop tonight and persist
into the day on Saturday. Snowfall accumulations over this
timeframe and area of two to six inches are expected. After a
brief lull in the strong winds Saturday afternoon, winds will once
again begin to increase along the Rocky Mountain Front and spread
east across the plains of North Central Montana Sunday morning,
potentially leading to more blowing and drifting of snow.

&&

.DISCUSSION...

This Afternoon through Sunday night...numerous forecast concerns
through this forecast period which are as follows: 1) Strong winds
leading to significant blowing and drifting of snow along the Rocky
Mountain Front and Eastern Glacier through Saturday morning, 2)
Moderate snowfall accumulations across Southwest Montana and along
the Continental Divide tonight through Sunday night, and 3)
Strong (potentially) high winds along the Rocky Mountain Front and
over the plains of North Central Montana Sunday morning and
afternoon, possibly leading to additional blowing and drifting of
snow.

Surface pressure gradient will continue to strengthen along the
Rocky Mountain Front through the early morning hours on Saturday as
a lee-side surface trough sharpens in response to a developing area
of low pressure across Central Alberta. By the mid-morning hours on
Saturday, the surface low over Alberta will begin to slide east and
south into Saskatchewan, which will allow the pressure gradient to
relax significantly. This relaxation in the pressure gradient will
allow winds to drop off along the Rocky Mountain Front and over the
adjacent plains of North Central Montana. For this reason, will keep
the current end times of 13z for the Blizzard Warning and Winter
Weather Advisory for blowing/drifting snow as is for the current
highlighted areas. Continued blowing and drifting of snow is
expected through the night tonight over the highlighted areas, with
the most severe impacts within the Blizzard Warning where numerous
road closures have already occurred today.

A fast moving and energetic shortwave with very impressive H700 frontogenic
forcing will cross Southwest Montana this evening and into the
day on Saturday. This disturbance looks to bring a quick shot of
2-6" for elevations above 6,500ft, which primarily will affect
mountain passes like Monida, Chief Joseph, Raynolds, and Targhee.
For this reason, have decided to issue a Winter Weather Advisory
for Beaverhead, Madison, and Gallatin Counties above 6500ft for
the expected snowfall, which will likely create some impacts to
travel over the aforementioned mountain passes due to snow
covered/slick roadways through Saturday afternoon. After a slight
decrease in snowfall intensity Saturday afternoon/evening,
snowfall intensities will begin to increase along the Continental
Divide and over the mountains of Southwest and Central Montana.
Additional winter weather headlights will likely be needed for
this second wave of accumulating snow, especially across Southwest
Montana and along the Continental Divide.

Another round of windy weather can be expected on
Sunday with winds possibly exceeding high wind criteria over a
large portion of North Central and Central Montana. Have kept the
Winter Storm Watch in effect for now, but the Sunday event will
also feature warmer temperatures for much of the region. This
raises the possibility that blowing snow may not become as
widespread as anticipated if temperatures are sufficiently high to
cause the existing snowpack to crust over. - Moldan/mpj

Monday through Friday...A broad low pressure trough will remain
over North America through this period, but some differences
develop among the models beginning on Wednesday. A progressive
weather pattern with a mostly westerly flow aloft with weak
embedded disturbances will persist through Tuesday night. The
strong westerly winds will continue to into Monday as a shortwave
trough swings southeast through the Pacific Northwest and into the
Great Basin. This will keep the best chance for snow in the
mountains of western and southwest Montana with decreasing blowing
snow. This shortwave will then split Monday night and Tuesday,
sending the strongest energy south toward the western U.S./Mexico
border through Wednesday and leaving only a chance of mountain
snow for the forecast area. As mentioned above, this is where the
divergence in forecast solutions develops. The ECMWF and Canadian
models dig a deep upper level low pressure area south along the
Canadian and U.S. Pacific coasts for Thursday through Friday. This
would bring an increasingly moist southwest flow to the forecast
area, as well as an increasing chance of snow over the entire
forecast area, especially for Friday. The GFS model maintains more
of a westerly flow aloft through the latter part of the work
week, which would keep the better chance for snow over the
mountains, as westerly downslope winds keep the plains mostly dry.
Will therefore go with a chance of snow near climatological
normals for this period and make adjustments as models progress
over the next few days. Model consensus temperatures do not vary
much through this period, keeping highs mostly 10 to 15 degrees
below normal (in the 20s) and lows 5 to 10 degrees below normal
(in the single digits to lower teens). Coulston

&&

.AVIATION...
Updated 1800Z.

VFR conditions prevail in general. However, strong west winds across
western portions of North-central Montana are causing drifting and
blowing snow. Blowing snow may cause reduced RVR at KGTF, with MVFR
visibility in stronger winds at KCTB through the overnight period.
Clouds increase across Southwest MT early Saturday with chances for
light snow at KBZN/KEKS late in the period. PN

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  13  30  15  41 /   0  10  10  20
CTB  13  31  14  37 /   0  10  10  20
HLN   5  27  15  39 /  20  20  20  50
BZN   5  25   9  34 /  10  30  30  30
WEY  -4  18  -1  21 /  50  60  60  70
DLN   1  21   4  32 /  20  30  30  50
HVR   3  25  10  39 /   0  10  10  20
LWT   8  28  14  38 /   0  10  20  10

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until 6 AM MST Saturday Cascade...
Chouteau...Eastern Pondera...Eastern Teton...Hill...Judith
Basin...Liberty...Toole.

Winter Storm Watch from Sunday morning through Sunday evening
Blaine...Cascade...Chouteau...Eastern Pondera...Eastern Teton...
Fergus...Hill...Judith Basin...Liberty...Toole.

Winter Weather Advisory from midnight tonight to midnight MST
Saturday night above 6500 feet for Beaverhead...Gallatin...
Madison.

Blizzard Warning until 6 AM MST Saturday Eastern Glacier...
Northern Rocky Mountain Front...Southern Rocky Mountain Front.

&&

$$

http://www.weather.gov/greatfalls



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