Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 012349
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
449 PM MST Thu Dec 1 2016

UPDATED AVIATION

.DISCUSSION...

Thursday through Saturday...Water vapor and visible satellite
imagery shows a weak, slow-moving upper level low spinning over
central Montana at this time. An area of decent low/mid level
moisture is rotating south out of Canada at this time and is
beginning to move into the Hi-Line counties. Snow has already
begun to fall in the Havre and Cut Banks areas as low level
moistening is already underway along with some large-scale lift
associated with the upper low. This area of moisture/lift will
shift south through tonight with scattered to numerous snow
showers shifting south as well. Given the WNW flow in the low/mid
levels, there will also be an upslope component up against the
central Montana mountains, especially tonight, and this may act to
enhance snowfall amounts some. Given all this, I decided to expand
the Winter Weather Advisory to include all of Cascade and Judith
Basin Counties as well as adding Fergus County. The advisory now
includes the greater Great Falls and Lewistown areas. For now, I
left counties to the north out as snowfall amounts should be a bit
lower there, especially with the heavier snow coming during the
day as opposed to at night. For the advisory area, 1-2 inches of
snow is likely at lower elevations with isolated amounts up to 4
inches possible depending on where the heaviest snow showers set
up. The remainder of the Winter Weather Advisory for the Rocky Mtn
Front and SW MT remains unchanged as snow showers will continue
to impact this area through around Midnight or so. While this is
not expected to be a high-impact event, the timing of the snow
(ie. continuing on into rush hour), especially across central
Montana could make it a bit more impactful.

Friday dries out some, although isolated to scattered snow showers
will be possible, especially the mountains of SW MT. The other
main concern in the short term is the wind. An upper level ridge
over the region on Friday will quickly flatten as a s/w drops SE
out of Canada. This will allow for strengthening winds aloft which
will likely mix down to the SFC, especially by Saturday afternoon.
At the same time, a tightening pressure gradient is expected as
a SFC trough develops east of the Rockies. The tighter pressure
gradient combined with the stronger winds aloft should allow a
period of strong winds along and just east of the Rocky Mountain
Front. There, wind gusts in excess of 60-70 mph appear likely,
with lower winds over the plains/valleys. The models continue to
vary on how strong the wind will be aloft, so at this time we will
not be issuing any High Wind Watches or Warnings due to lower
confidence. But, watches or warnings may still be needed in later
forecasts. Martin

Saturday night through Wednesday...Models continue to depict a
significant change in the overall weather pattern to evolve late
this weekend through early next week as an upper level trough digs
from the Gulf of AK/Western Canada south into the western and
eventually central US. Models are also coming into better agreement
on the details of how this system evolves with a fast/moist Pacific
Jet streaming across the Northern Rockies Saturday night and Sunday
as the upper trough moves into western Canada. This looks to focus
an increasing amount of orographic enhanced precipitation over the
western MT Mtns with significant snow likely along northern portions
of the continental divide in the Bob Marshal/Glacier Park areas.
Windy conditions are also likely Sat night through Sunday for much
of the forecast area with the potential for strong winds existing
across western portions of north-central MT. Westerly winds
will limit snow chances for areas east of the continental
divide Saturday night and Sunday. Expect snow to become more
widespread across the entire forecast area late Sunday through
Sunday night as a shortwave moving through the base of the deepening
western Canada trough crosses the northern Rockies with a surface
low emerging across central/eastern MT by Monday morning. This
scenario would favor portions of north central MT for some moderate
snow amounts of several inches through Monday morning with a colder
airmass beginning to filter in behind the surface low Sunday night
through Monday morning. Cold surface high pressure with origins in
the Arctic then noses southward east of the Rockies through the day
Monday for falling temperatures with snow becoming less widespread.
The much colder airmass looks to spread south and settle across the
entire forecast area Monday night through Wednesday with daytime
temperatures Tuesday/Wednesday likely only in the single digits and
teens with some areas seeing subzero temperatures Tues/Wed morning.
Confidence is growing in the potential for multiple winter weather
hazards during the Sunday though early next week period with more
specific details likely to emerge over the following days. Hoenisch

&&

.AVIATION...
Updated 2349Z.

A pair of weather disturbances are moving southeastward over the
region and should exit our area between 12Z and 15Z/Fri. Behind
these disturbances, a high pressure ridge will build-in from the NW.
Expect periods of light to moderate snow, especially over north-
central MT. This snow should taper-off from NW to SE between roughly
06Z and 12Z/Fri. Within the snow, expect mainly MVFR/IFR CIGS and
VSBY, with brief LIFR possible in steadier snow. Also, mountain
obscuration is likely. Conditions should begin to improve
drastically region-wide after 12Z/Fri.
Jaszka

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  23  34  29  42 /  50  10  20  10
CTB  21  33  28  38 /  30  10  20  10
HLN  22  35  25  43 /  30  10  20  20
BZN  19  30  21  36 /  40  20  30  30
WEY   5  23   9  23 /  30  10  30  40
DLN  12  29  20  35 /  20  10  20  20
HVR  23  35  26  40 /  30  10  30  20
LWT  24  34  24  39 /  60  20  20  20

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until 5 AM MST Friday Beaverhead...
Broadwater...Central and Southern Lewis and Clark...Gallatin...
Jefferson...Madison...Meagher...Northern Rocky Mountain Front...
Southern Rocky Mountain Front.

Winter Weather Advisory until 5 AM MST Friday Cascade...Fergus...
Judith Basin.

&&

$$

weather.gov/greatfalls



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