Winter Storm Warning 3, NWS Great Falls, MT

Area Forecast Discussion
National Weather Service Great Falls MT
1010 AM MST Sun Feb 12 2023

...AVIATION SECTION UPDATE...

.SYNOPSIS...

While weather across Southwestern Montana will remain quiet today,
winds will be on the increase across the North-Central Plains as
a weather disturbance approaches from the west. Winds will peak
overnight Sunday night into Monday morning across the plains, with
some snow expected to begin across Southwestern Montana Monday
afternoon and evening.

&&

.UPDATE...

The morning update has been published. The primary change was to
begin the High Wind Warning for the Rocky Mountain Front and
eastern Glacier County immediately. Winds are already gusting up
to and in excess of 55 mph along the immediate eastern slopes of
the Rocky Mountain Front, with further increase expected later
this morning and afternoon. The period of strongest winds is
still expected this evening through Monday morning. Only minor
changes were made to the remainder of the forecast per current
trends. -RCG

&&

.AVIATION...
1005 AM MST Sun Feb 12 2023 (12/18Z TAF Period)

Strong low level and sfc winds to be primary aviation weather impact
for this TAF period. Mountain wave expected to impact areas along
and adjacent to the eastern Rocky Mountain Front primarily after
dark tonight and into early Mon morning. Strongest winds expected
to occur at KCTB and vcnty with sustained values of 30 to 40 kts
likely and gusts in excess of 50 kts at times. Elsewhere, sfc winds
will generally strengthen this afternoon and especially tonight.
Wind speeds should peak tonight and only slowly weaken during the
day Monday. Otherwise, VFR will prevail this afternoon with
thickening CI at times.

Refer to weather.gov/zlc for more detailed regional aviation
weather and hazard information.

&&

.PREV DISCUSSION...
/ISSUED 237 AM MST Sun Feb 12 2023/

Today through Monday...primary forecast concern throughout the
period will be the threat for High Winds from late this afternoon
through Monday afternoon, and accumulating snow across all
elevations along the Continental Divide and across Southwest and
portions of Central Montana, mainly along and south of the Montana
Hwy 200 corridor, from Monday afternoon through early Tuesday
morning.

H500 ridge over the Pacific Northwest and western portions of the
Northern Rockies this morning will be flattened out by a potent H500
shortwave, currently over the Gulf of Alaska, which will dig to the
southeast over British Columbia and the Pacific Northwest by Monday
morning and then the Great Basin through Tuesday morning. At the
surface, lee-side surface trough will begin to sharpen rapidly this
afternoon and through Monday morning across the plains of Central
and North Central Montana as a strong (~980-985mb) surface low moves
from the Gulf of Alaska (this afternoon) across Northern Alberta
(near midnight tonight) to Northern Saskatchewan (Monday afternoon).
Surface pressure gradient along the Rocky Mountain Front, while
strong already this morning, will become very strong at around 17.5-
22.5mb/km by the evening hours tonight and persist into the morning
hours on Monday. This very strong surface pressure gradient combined
with increasing cross barrier flow of 50-70kts (between H850-700)
and a strengthening inversion near ridgetops along the Rocky
Mountain Front will set the stage for very strong west winds from
the late afternoon through Monday afternoon, especially beneath
mountain wave activity. At this time it appears that the most
intense winds will occur from ~04z this evening through 16z Monday
along and west of the US Hwy 89 corridor across the Blackfeet
Nation. NBM probabilistic data suggests a 60-90% chance that wind
gusts will exceed 64kts across the aforementioned region of the
Blackfeet Nation, with Hi-Res and NBM deterministic guidance
suggesting several mountain waves developing and propagating
eastward across Glacier County. Would not be surprised if the
Browning area sees several periods of sustained winds of 50-60mph,
with gusts approaching 90 mph, this evening and Monday morning. A
High Wind Warning remains in effect from 00z Monday through 22z
Monday for the Northern and Southern High Plains adjacent to the
Rocky Mountain Front, and Eastern Glacier/Western Toole/Central
Pondera Counties. Considered adding the Southern Rocky Mountain
Front and Eastern Glacier Park Region to the High Wind Warning as
the wind orientation appears favorable for the Gleason and St Mary
RAWS to hit high wind criteria, but decided to hold-off for now
given these areas stubbornness to "verify".

Focus then shifts to the precipitation/winter side of the
approaching shortwave. Increasing ascent and intense H500 height
falls of ~130m to ~180m will begin to overspread Southwest through
North Central Montana from Monday morning through Monday night,
which combined with a plume of Pacific moisture advecting east ahead
of an eastward advancing Pacific cold front will lead to the
development of precipitation (i.e. snow) along the Continental
Divide and across Southwest and portions of Central Montana (mainly
along and south of the Montana Hwy 200 corridor). Snowfall
accumulations of generally 1-3" remain possible across the
aforementioned lower elevations over this timeframe, with 3-6" of
snow at pass level and around 6-12" in the mountains of Glacier
National Park and the Gallatin and Madison Counties. A brief period
of moderate to heavy snow could develop along and immediately behind
the eastward advancing Pacific cold front Monday evening/night,
mainly along and south of the US Hwy 12 corridor across Central
Montana. This brief period of heavier snow could lead to significant
visibility reductions, especially along the busy I-90 corridor from
Bozeman to Butte. - Moldan

Tuesday through next weekend... Winds will begin to decrease
rather quickly after sunrise Tuesday as the axis of the upper
level trough begins to push east and a high pressure system begins
to slide southward out of Canada. As this high pressure system
pushes to the south, some upslope snow is expected along the
eastern slopes of the Rocky Mountain Front and on the northern
slopes of the Little Belt and Snowy Mountains. Snow will continue
to fall across Southwestern Montana as well on Tuesday from a
combination of the upper level trough and the cold front that will
be surging southward through the area. One thing to keep an eye on
across Southwestern Montana will be the possibility for a brief
period of gusty winds as colder air surges through some of the
North-South oriented valleys.

After an active weather day on Tuesday, the rest of the work week
will be relatively quiet as upper level ridging builds into the
Northwestern US. As this ridge builds in, expect temperatures to
warm a few degrees each day, though some of the valleys could be
slow to warm up thanks to some valley inversions. These warmer
temperatures across the plains will be accompanied by some
occasionally breezy winds, but expect dry conditions to close out
the work week. As we head into the weekend, another upper level
trough may push through the area, but uncertainty remains as to
where this system may track and how strong/widespread any winds or
precipitation would be. Ludwig

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  50  42  51  27 /   0   0  20  40
CTB  48  40  49  23 /   0   0  10  30
HLN  44  28  45  25 /   0  10  40  70
BZN  37  18  40  17 /   0   0  40 100
WYS  37   4  35   7 /   0   0  30 100
DLN  35  18  37  17 /   0   0  40  90
HVR  41  33  45  20 /   0   0  10  20
LWT  47  32  47  23 /   0   0  30  70

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
High Wind Warning until 3 PM MST Monday for Eastern Glacier,
Western Toole, and Central Pondera-Northern High Plains-Southern
High Plains.

Winter Storm Watch from Monday afternoon through Tuesday evening
for Big Belt, Bridger and Castle Mountains-East Glacier Park
Region-Elkhorn and Boulder Mountains-Gallatin and Madison County
Mountains and Centennial Mountains-Gates of the Mountains-Little
Belt and Highwood Mountains-Snowy and Judith Mountains-Southern
Rocky Mountain Front-Upper Blackfoot and MacDonald Pass.

&&

http://www.weather.gov/greatfalls

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