High Wind in Montana, February 13, 2018

Area Forecast Discussion
National Weather Service Great Falls MT
446 AM MST Tue Feb 13 2018

.UPDATE...
430am MST update : Extended the Wind Chill Advisory for Chouteau,
Fergus, Liberty, and Toole Counties until 8am MST, as current
observations continue to indicate wind chill values of -20 degrees
or colder. Very sharp temperature gradient is also occurring over
the Hi-Line, generally along and west of the Interstate 15
corridor where a 36 degree temperature difference was noted
between the cities of Shelby (-15 degrees) and Cut Bank (21
degrees). - Moldan

&&

.SYNOPSIS...

Chinook winds will cause blowing and drifting snow to persist east
of the Rocky Mountain Front and north of the Central Montana
mountains. In addition, a ground blizzard is expected to affect the
eastern slopes and plains of the Northern Rocky Mountain Front
region. Blowing and drifting snow will likely end this evening as
temperatures continue to warm into Wednesday morning. An Arctic
front will then move southward over our region late Wednesday
morning through Thursday, resulting in periods of accumulating snow
and another drop in temperatures.

&&

.AVIATION...
Updated 1127Z.

Mainly VFR expected next 24-hours. High pressure aloft will be
flattened gradually by a disturbance approaching Montana from the
Gulf of Alaska region, causing northerly to northwesterly flow aloft
to eventually shift to westerly over our area between 18Z/Tue and
00Z/Wed. Strong southerly surface winds are expected to affect the
north-south valleys of Southwest Montana, including KEKS, around and
especially after 18Z/Tue. In addition, gusty southwesterly to
westerly surface winds are expected over the North-Central Montana
plains during the TAF period. Here, pockets of blowing snow reducing
VSBY to MVFR or worse are also expected. The worst blowing snow and
lowest VSBY (down to VLIFR) will occur along the Northern Rocky
Mountain Front. Warming temperatures should allow the blowing snow
to end around 00Z/Wed. Lastly, mountain-obscuring snow should begin
to overspread the Northern Rocky Mountain Front between 06Z and
12Z/Wed.
Jaszka

&&

.PREV DISCUSSION...
/ISSUED 427 AM MST Tue Feb 13 2018/

Today through Thursday...Continued winter weather impacts are
expected over the short term, with strong winds leading to blowing/
drifting snow and then accumulating snow.

H500 pattern over the Northern Rockies will begin to back from light
north/northwest flow during the morning hours today, to increasing
westerly flow by 00z Wednesday. An energetic shortwave located over
Southeastern Alaska this evening will begin to dig south towards the
Pacific Northwest by 12z Wednesday, before sliding southeast towards
the Great Basin/Central Rockies by 12z Thursday. This upper level
track will allow very favorable divergence and subsequent lift to
overspread/move in across the Northern Rockies during the day on
Wednesday. Upper level support will remain strong through the
morning hours on Thursday as the trough axis approaches, with
eventual decreasing lift as subsidence settles in behind the upper
level trough axis Thursday afternoon. H700 cross barrier flow will
develop by the afternoon hours today, with wind speeds increasing to
around 50-70kts over the Rocky Mountain Front by 03-06z Wednesday.
These strong winds will persist over the Rocky Mountain Front
through approximately 15z Wednesday before gradually decreasing. 40-
55kt H700 winds will also begin to overspread the plains of North
Central Montana after 06z Wednesday, with these winds persisting
over the region into the afternoon on Wednesday. At the surface, lee-
side pressure trough will continue to sharpen through tonight, with
the pressure gradient along the Continental Divide remaining very
strong into the morning hours on Wednesday. Surface low moving
across Central Canada tonight through Wednesday morning will drag a
cold front from north to south across North Central Montana during
the afternoon hours on Wednesday, with the cold front pushing into
Southwest Montana by the early morning hours on Thursday.

Gusty to strong downsloping winds will continue to overspread North
Central Montana this morning, which will help to moderate
temperatures for today. Wind gusts in excess of 55 mph will continue
to occur along the Rocky Mountain Front through Wednesday morning
in response to the strong pressure gradient, increasing H700 cross
barrier flow, and strong ridgetop stability which will lead to
mountain wave activity. Wind gusts nearing 70 mph will be possible
along the Rocky Mountain Front, especially in wind prone areas.
These strong winds will likely lead to widespread blowing/drifting
snow through at least the evening hours today before
scouring/compaction of the snow field eventually limits the amount
of snow available to be blown around. For this reason, the Blizzard
Warning remains very valid through the evening hours tonight. The
gusty winds over the plains of North Central Montana will also lead
to at least patchy blowing/drifting snow, with some areas of
blowing/drifting snow being observed. Warming temperatures today
will eventually help to limit this blowing/drifting snow. Snow (the
stuff actually falling from clouds) then overspreads the region
Wednesday and continues into the morning hours on Thursday. Snowfall
accumulations of 2-4" appear possible across all of Central and
North Central Montana, with 4-6" possible along and west of the
Interstate 15 corridor from Great Falls points northward. Snowfall
accumulations in the mountains of Southwest and North Central
Montana of 4-8" are possible, with 6-12" possible along the
Continental Divide. Given the on-going blowing/drifting snow event,
have held off on any winter weather highlights with regards to this
potential accumulating snow event, however, future highlights will
likely be needed with future forecasts. - Moldan

Thursday night through Tuesday...Northwesterly to westerly flow
aloft is expected Thursday night through Saturday morning as high
pressure ridging aloft builds-in from the west, yet multiple
disturbances ripple through the flow aloft. In addition,
strengthening surface winds are forecast on Friday into Saturday,
especially along the Rocky Mountain Front, due in part to
strengthening flow aloft. This period will continue to be monitored
for potential high wind highlights. Later Saturday through Tuesday
of next week feature another longwave trough aloft and embedded
shortwave disturbances overspreading our area from the northwest. In
addition, another Arctic front is progged to cross the CWA from
north to south Saturday night into Sunday, allowing an overall
weakening trend in surface winds to begin across the forecast area.

Near-normal temperatures are expected Thursday night into Saturday.
Thereafter, a return to below-normal readings should begin Saturday
night and continue through Tuesday. Periods of primarily snow are
expected during the long term period, with the best potential
existing Saturday night into Sunday due to the aforementioned Arctic
frontal passage.
Jaszka

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  41  32  39   6 /   0  10  70  70
CTB  39  30  36   0 /   0  20  70  70
HLN  26  17  35  13 /   0  10  50  70
BZN  25  16  35  18 /   0  10  30  70
WEY  25  15  29  10 /   0  30  70  70
DLN  27  15  36  13 /   0  10  30  50
HVR  30  24  36  -3 /   0  30  70  70
LWT  43  28  37   8 /   0  10  40  70

&&

.TFX WATCHES/WARNINGS/ADVISORIES...

Winter Weather Advisory until 8 PM MST this evening Cascade...
Central and Southern Lewis and Clark...Eastern Glacier...Eastern
Pondera...Eastern Teton...Judith Basin...Liberty...Southern
Rocky Mountain Front...Toole.

Wind Chill Warning until 10 AM MST this morning Blaine...Hill.

Blizzard Warning until 8 PM MST this evening Northern Rocky
Mountain Front.

Wind Chill Advisory until 8 AM MST this morning Chouteau...
Fergus...Liberty...Toole.

Flood Warning for an ice jam until further notice central Broadwater
County near Townsend on the Missouri River.

&&

http://www.weather.gov/greatfalls