High Wind in Montana, December 18, 2016

Area Forecast Discussion
National Weather Service Great Falls MT
1035 PM MST Sat Dec 17 2016

.UPDATE AVIATION...

...INCREASING WINDS THROUGH MONDAY WITH IMPACTS FROM BLOWING AND
DRIFTING SNOW...ESPECIALLY ACROSS NORTH CENTRAL MONTANA...

.UPDATE...

940 pm Update: Increasing winds, and consequently increasing
temperatures, have spread into the Wind Chill Advisory area. As a
result, I have warmed their overnight lows and increased their
winds, leaving marginal Advisory criteria wind chills of 15 below
to 25 below zero. Will keep the Advisory in place, but am
anticipating potential for an early cancellation overnight. Have
also made some localized adjustments to temperatures, mainly to
warm widespread lows of 20s below zero in Southwest Montana, as
cloud cover appears to be moving into the area.
Coulston

703 pm Update: Early update this evening. Temperatures have
already reached their previously forecast lows for tonight.
Increasing winds are helping keep temperatures from reaching
their lows along the Rocky Mountain Front. However, they are
falling elsewhere, so have lowered lows 5 to 10 degrees there.
Also, the colder temperatures with 10 to 20 mph winds over our
eastern plains areas (Hill, Blaine, Chouteau, Fergus Counties)
have caused some areas there to have wind chills in the 20 to 30
below zero range already. Though temperatures there should
gradually warm after midnight with the influx of increasing west
winds, wind chills will likely not recover until late tonight.
Have therefore issued a Wind Chill Advisory until 5 am MST Sunday
for those areas. Will evaluate further potential updates.
Coulston

&&

.DISCUSSION...

Tonight through Monday...Large scale upper level trough shifts east
into the central CONUS tonight with an upper level ridge just
offshore shifting inland while flattening through Sunday before
additional shortwave energy currently upstream in the Gulf of AK
moves quickly across the Region on Monday. As the upper trough
shifts east the low level cold/arctic airmass will continue to
erode/moderate through tonight. Strongest warming will continue to
be focused over N-central MT where a lee-side surface trough is
extending south from Alberta and bringing increasingly breezy
conditions through tonight. Surface high pressure over SW MT will
keep the colder air locked in place across the SW MT valleys where
temperatures will fall back into the single digits and teens
below zero tonight. The lee-side trough continues to deepen
Sunday as weak shortwave energy tracks SE across the region. This
should bring an additional increase in winds across northcentral
MT with many areas seeing gusts of 30-40mph by Sunday afternoon.
Blowing and drifting snow impacts have become the main concern
going through the next couple of days winds increase. Already
seeing some drifting of snow in areas where winds have increased
to 20-30 mph along the Rocky Mtn front though so far visibility
is not being reduced significantly. With winds increasing tonight
through tomorrow will have a winter weather advisory for blowing
snow across much of North Central MT, beginning late this
afternoon across western and southern portions where winds will
increase first and expanding to eastern sections Sunday morning.
Shortwave energy currently in the Gulf of AK arrives Sunday night
through Monday bringing even stronger winds aloft and the
potential for high winds initially along the Rocky Mtn front and
adjacent areas, but also expanding across eastern portions of
north-central MT by Monday afternoon. Will highlight to potential
for strong winds across North-Central MT with a High Wind Watch
for Sunday night through Monday period. While confidence in winds
and at least low-moderate impacts from blowing and drifting snow
is fairly high, there are some factors that could mitigate the
severity of blowing/drifting snow. Snow pack of around 3-6 inches
generally covers the area where highest winds are anticipated.
With drifting of snow already underway, the snow pack may undergo
enough settling/redistribution for significant blowing/drifting
to occur. By the time the strongest winds arrive on Monday,
temperatures will also have risen to near or above freezing in
the areas where the highest winds are anticipated, potentially
developing a crust on the snow surface. Situation still needs to
be monitored as winds strengthen Sunday. Hoenisch

Monday Night through Saturday...This period begins with windy
conditions and temperatures near- to slightly above normal
(mid/upper 30s) as strong westerly winds promote downslope warming
and bring in a relatively milder Pacific airmass.  Forecast models
are maintaining decent continuity and agreement in showing a
progressive weather pattern for our area next week, as shortwave
trofs move across our region on Tues and Thur into Fri, with weak
high pressure ridging between these systems.  Better moisture for
more widespread light precipitation is expected with the Tues
shortwave, with highest amounts concentrated along the Rocky Mtn
Front and over southwest MT. Given the warmer airmass, snow levels
may move up to around 4000 ft on Tues aftn, meaning that some of the
precip over central MT may fall as rain/snow mix or even all rain.
But the surface cold front with this system will bring a quick
conversion back to light snow as it moves across our forecast area
Tues eve. Look for dry conditions from Wed aftn to Thur aftn, when
the second shortwave arrives.  Winds aloft are not as strong as
weather systems earlier in the week, keeping temps only in the upper
20s to low 30s as the shortwave passes.  Prolonged northwest flow
behind the shortwave will generate areas of snow showers through
Fri, especially in favored upslope areas of local mtn ranges. Weak
ridging returns on Sat, with mostly dry conditions, and highs only
in the 20s.
Waranauskas

&&

.AVIATION...
/Updated 0600Z/

Mainly VFR conditions...with a few mid to high level clouds...are
expected to continue across the forecast area tonight. Breezy
winds are starting to be found across area TAF sites...with some
gusts up to 25 knots possible through the overnight hours. Low
Level wind shear will be possible for many northern and central
sites Sunday morning...as upper level winds strengthen. Sunday
will then see more VFR conditions with increasing winds for most
sites. Gusts of 30 to 40 Kts are expected with higher gusts near
50 Kts possible near KCTB. With gusty winds and recent snow...some
reduced visibilities in blowing snow are possible...especially
from KCTB to KGTF to KLWT and north. Anglin

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF   3  22  19  34 /   0  10  10  10
CTB   5  22  19  31 /   0  10  10  20
HLN -16  12  11  32 /   0  10  10  20
BZN -16   8   8  27 /  10  10  10  20
WEY -17   2 -10  13 /  10  10  10  30
DLN  -7  12   4  23 /  10  10  10  20
HVR   2  18  17  35 /  10  10  10  20
LWT  -4  19  16  33 /   0  10  10  10

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until 11 PM MST Sunday Cascade...Central
and Southern Lewis and Clark...Eastern Glacier...Eastern
Pondera...Eastern Teton...Judith Basin...Liberty...Northern
Rocky Mountain Front...Southern Rocky Mountain Front...Toole.

High Wind Watch from Sunday evening through Monday afternoon
Blaine...Cascade...Central and Southern Lewis and Clark...
Chouteau...Eastern Glacier...Eastern Pondera...Eastern Teton...
Fergus...Hill...Judith Basin...Liberty...Northern Rocky Mountain
Front...Southern Rocky Mountain Front...Toole.

Winter Weather Advisory from 5 AM to 11 PM MST Sunday Blaine...
Chouteau...Fergus...Hill.

Wind Chill Advisory until 5 AM MST Sunday Blaine...Chouteau...
Fergus...Hill.

&&

weather.gov/greatfalls