Winter Storm in Montana, December 29 to 30, 2017

Area Forecast Discussion
National Weather Service Great Falls MT
1030 PM MST Thu Dec 28 2017

...Updated Aviation Discussion...

.UPDATE...

Evening update has been published, with no major changes made to
the on-going forecast or headlines. Water Vapor shows a better
push of Pacific moisture moving into the Northern Rockies from the
Pacific Northwest over the past couple of hours, with cooling
cloud tops per IR satellite images over this same time frame.
Surface observations across North Central Montana (KHVR and KCTB)
have generally shown an overall lowering in ceilings, with both
sites reporting light snow as of this update.

Synoptic lift should continue increase through the midnight hour
across Central and North Central Montana as the left exit region
of an upper level speed maximum moves overhead. At the surface, a
cold front moving into North Central Montana from Canada will
provide a reinforcing shot of cold air for North Central Montana
and additional frontogenetic lift. The combination of increasing
synoptic/frontogenetic lift and increasing Pacific moisture,
should allow for precipitation to expand in areal coverage across
North Central Montana between ~03-06z. The aforementioned region
of precipitation will then sag south into Central Montana near the
midnight hour as the cold front moves south, before stalling
along a Lincoln, to Helena, to White Sulphur Springs line (with
the precipitation extending ~50 to 75 miles north/south of this
line). - Moldan

&&

.SYNOPSIS...

Widespread accumulating snowfall is expected across most of
North-central and Southwest Montana through Saturday. Snowfall
will be nearly continuous in mountain areas. One round of light
snow spreads across North-central Montana tonight, with a second
moderate to heavy round of snow Friday night into early Saturday.
Warmer temperatures mainly south of interstate 90 will limit snow
accumulation over those southwest valleys.

&&

.AVIATION...
Updated 0530Z.

MVFR/IFR and occasionaly LIFR conditions are expected for the
entirety of the 2906/3006 TAF period. Areas of light to moderate
snow will continue overnight and through the day on Friday,
especially across Central Montana. A brief lull in snow is possible
across extreme North Central Montana (mainly affecting the KCTB and
KHVR terminals) late Friday morning through the early afternoon
hours, before main area of snow moves back north. Mountain
obscuration is likely across both Southwest and North Central
Montana. -CC/Moldan

&&

.PREV DISCUSSION...
/ISSUED 440 PM MST Thu Dec 28 2017/

Tonight through Saturday....The overall forecast for widespread
accumulating snow and continuing cold temperatures remains on
track. Radar, web-cams and surface observations show that steady
snow has developed over the Rocky Mountains under a continuing
moist westerly flow aloft. Expect snow to continue over northern
portions of the Continental Divide through Saturday, accumulating
one to potentially three feet. Radar also shows an area of light
snow across our northeast counties, primarily associated with a
jet exit region aloft and low-level ascent over the colder air.
Snow has been generally light, but is expected to become more
widespread overnight with a passing weak shortwave aloft. Surface
pressure also rises with strengthening northerly winds, thereby
lifting snow into the upslope regions along I15 and Great Falls to
Lewistown. Snowfall may taper and possibly stop along the Hiline
through mid-day Friday with a passing shortwave ridge, but is
likely to continue over and along the fortified stationary front
through Central MT. A more vigorous shortwave trough arrives early
Saturday morning. This next wave will produce stronger ascent and
a period of widespread moderate to possibly heavy snowfall.
Impacts to travel will become significant Saturday morning.
Snowfall is expected to continue through Saturday, tapering into
the evening. However, surface winds are forecast to strengthen
later Saturday, producing critically low wind chill values along
with the potential for blowing and drifting snow. Hence, even as
snow ends late Saturday, hazardous conditions persist well into
Sunday. Southwest Montana, mainly south of Interstate 90, remains
relatively mild through the period. Snowfall amounts in southwest
valleys will be minimal, if any, and possibly could see rain or
rain/snow mix. However, mountain areas of Southwest Montana could
still see heavy snowfall along with strong gusty winds. PN

Saturday night through Thursday...Overall very cold temperatures
are expected to start the period, especially over North Central
MT, followed by a slow warming trend. Snow will diminish from west
to east on Saturday night allowing for very cold temperatures to
develop by Sunday morning over North Central MT. Afternoon
temperatures are likely to remain below zero over the north on
Sunday. Another cold morning is expected on Monday, but an upper
level ridge will start to rebuild over the Pacific Northwest
starting on Monday afternoon. As a result, warmer Pacific air will
slowly try to move into the region from the West. Usually the
warm air takes a few days to move into the region, especially over
our northeastern sections in the Havre/Harlem areas. Overall,
little to no precipitation is expected much of next week. The GFS
does have a chance for rain/snow by next Thu while the EC is dry,
thus went with a low pop mainly over the western mountains and
Southwest MT for now on Thursday. Brusda

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  -5  -2 -11  -3 /  90 100 100  60
CTB -10  -7 -19  -9 / 100 100 100  80
HLN   8  12  -4  16 / 100 100 100  40
BZN  24  27   4  24 /  70 100 100  40
WEY  22  26  20  27 /  90  90  90  70
DLN  29  40  24  34 /  30  60  70  30
HVR  -6  -4 -15 -10 / 100  90 100  90
LWT  -1   1  -9  -1 / 100 100 100  60

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
Winter Storm Warning from 5 AM Friday to 5 PM MST Saturday
Broadwater...Central and Southern Lewis and Clark...Jefferson.

Winter Storm Warning until 5 PM MST Saturday Eastern Glacier...
Eastern Pondera...Eastern Teton...Liberty...Northern Rocky
Mountain Front...Southern Rocky Mountain Front...Toole.

Winter Weather Advisory from 5 AM Friday to 5 PM MST Saturday
Beaverhead...Madison.

Winter Storm Warning until 5 PM MST Saturday Cascade...
Chouteau...Fergus...Judith Basin...Meagher.

Winter Storm Warning until midnight MST Saturday night Gallatin.

Winter Storm Warning until 5 PM MST Saturday Blaine...Hill.

&&

http://www.weather.gov/greatfalls