Winter Storm and Heavy Snow in Montana, December 19 to 20, 2017

Area Forecast Discussion
National Weather Service Great Falls MT
1035 AM MST Tue Dec 19 2017

Updated Aviation Discussion

.UPDATE...

Widespread light snow continues this morning over north-central
MT with area of best lift (overrunning the colder low level
airmass) occurring primarily south of the hi-line and and
generally along or north of of axis from Great Falls to Lewistown.
This zone of greater snow coverage will gradually lift back
northward this afternoon, with snow increasing again along the hi-
line and decreasing somewhat across areas further south.
Meanwhile, moderate to at times heavy snow is likely to continue
along the continental divide from Lincoln north to the Canadian
border today where orographic effects of moist Pacific flow
combine with overrunning occurring to the east of the divide. To
the south, snow will primarily be confined to the mountains with
cooler air not expected to shift south into the SW MT valleys
until tomorrow. All of this is handled very well by the previous
forecast with only minor adjustments made to temp and pops.
Hoenisch

&&

.SYNOPSIS...

An active weather pattern will continue to bring cold temperatures
and accumulating snows to Central and North Central today through
Wednesday. Cold and unsettled conditions continue through the
remainder of the week with another round of accumulating snow
expected Friday into Saturday. Very cold temperatures are
anticipated by next weekend.

&&

.AVIATION...
Updated 1735Z.

IFR conditions to prevail at north-central MT terminals through
Wednesday morning under widespread light snow. Area of widespread
snow will slowly shift northward tonight with coverage and intensity
of snow decreasing at KGTF and KLWT but low clouds/cigs will
probably linger this period. SW MT terminals will remain mostly dry
through tonight with snow showers primarily confined to the
mountains, leaving VFR conditions in place for KHLN and KLWT. A
weather disturbance tracking southeast from near Vancouver Island
will drag the band of Snow over northern areas back southeastward
late tonight through Wednesday morning. Hoenisch

&&

.PREV DISCUSSION...
/ISSUED 433 AM MST Tue Dec 19 2017/

Today through Thursday...Bands of light precipitation associated
with frontogenetic boundary continue across portions of Central
Montana this morning. Forecast area remains beneath a very moist
westerly flow aloft with models in good agreement expanding
snowfall coverage across all of Central and North Central
Montana through the day. Vigorous upper level shortwave trough is
still expected to sweep inland across the Pacific Northwest
tonight with models now expecting this system to dive southward
into the Great Basin on Wednesday. The associated surface low
pressure center is now progged to move to the northern portion
of Southwest Montana by 6 AM MST Wednesday, and models are
speeding up the southward progression of a strong Canadian cold
front. Widespread accumulating snows and areas of blowing snow
will accompany this weather system and I have opted to expand the
Winter Storm Warning to include Rogers Pass and MacDonald Pass in
Lewis and Clark County. However, please note that the city of
Helena is NOT included in the current warning products. It is
possible that Helena and much of Southwest Montana may need to
be covered with Winter Weather Advisories for late tonight
through Wednesday but I have held off on issuing these products
for now. Snowfall should taper off from northwest to southeast
Wednesday night and models indicate that mostly sunny but cold
conditions should be expected for Thursday with surface winds
becoming southwest to westerly as surface pressure falls in
Alberta. mpj

Thursday night through Tuesday...Main forecast concerns in the
long term is another decent shot for snow CWA wide (Thursday night
through Friday night), followed by well below normal
temperatures.

By Thursday evening, a negatively tilted H500 shortwave trough will
begin to move southeast from British Colombia and into the
Intermountain West/Northern Rockies. 12hr H500 height falls of ~90m
to 130m are expected to occur throughout the overnight hours on
Thursday/Friday morning across Eastern Washington/Northern
Idaho/Western Montana, with much of North Central and Southwest
Montana being beneath the favorable left exit region of the H500 jet
maximum. At the surface, an area of low pressure over Central
Saskatchewan will drag a cold front from northwest to southeast
across the region Thursday evening and into the morning on Friday as
it slides east southeast in Manitoba. The combination of favorable
jet dynamics/upper level divergence, surface cold front, and
moderate northwest orientated flow, will allow for an area of light
to at times moderate snow to develop Thursday night, persisting into
the afternoon/evening hours on Friday across portions of the CWA.
The moderate northwest flow is ideal for orographic lift over the
Little Belt Mountains, with snow potentially back building into the
lower elevations of Cascade County (ie Great Falls) as Froude values
fall to those that have been found to be ideal for this flow
scenario. At this time, snowfall accumulations of 2-4" appear
possible south and west of a KCTB to KLWT line, with generally 1-2"
northeast of this line. Mountains across Central and Southwest
Montana can expect 4-6", and possibly as high as 8". This snow is
expected to be of the light/fluffy variety, and should stack up
easily (given the cold air to work with and lift within a fairly
deep DGZ zone).

Following the snow on Friday, northwest flow aloft will dominate the
Northern Rockies region, including North Central and Southwest
Montana. While some light snow showers will be possible over the
Central Montana mountains due to this flow, most areas will be
quiet. At the surface, very strong high pressure will take over from
all indications, with NAEFS Mean MSLP anomalies of 2-3 standard
deviations throughout the weekend and 1-2 standard deviations on
Monday/Tuesday. This unseasonably strong high pressure combined
recent snowfall, will set the stage for very cold nights (single to
double digits below zero, with some cold basins in Southwest Montana
falling into the 20s below zero possibly) across the region, and
daytime highs that will struggle to reach the single digits above
zero and teens. - Moldan

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  31  18  20   2 / 100  80  80  30
CTB  25  11  13  -3 / 100 100  70  10
HLN  38  23  24   4 /  30  50  80  20
BZN  41  25  27   0 /  30  80  80  30
WEY  29  22  23  -8 /  50  80  80  40
DLN  40  24  25   3 /  20  40  70  40
HVR  29  18  20  -1 / 100 100  90  30
LWT  32  22  25   0 /  70  60  80  40

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
Winter Storm Warning until 11 AM MST Wednesday Eastern Glacier...
Eastern Pondera...Eastern Teton...Liberty...Toole.

Winter Storm Warning until 2 PM MST Wednesday Blaine...Cascade...
Chouteau...Fergus...Hill...Judith Basin...Meagher.

Winter Storm Warning until 11 AM MST Wednesday Central and
Southern Lewis and Clark...Southern Rocky Mountain Front.

Winter Storm Warning until 11 AM MST Wednesday Northern Rocky
Mountain Front.

&&

http://www.weather.gov/greatfalls

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE GREAT FALLS MT
639 AM MST TUE DEC 19 2017

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0635 AM     SNOW             4 SSE BYNUM             47.93N 112.29W
12/19/2017  M4.8 INCH        TETON              MT   CO-OP OBSERVER

            .35 MOISTURE WITH 4.8 INCHES OF NEW SNOW AND 3 INCHES
            FOR SNOW DEPTH.

&&

MWM