High Wind in Montana, December 29 to 30, 2016

Area Forecast Discussion
National Weather Service Great Falls MT
450 PM MST Thu Dec 29 2016

...AVIATION SECTION UPDATED...

.DISCUSSION...

Tonight through Sunday...Looks like 2016 will be ending with an
active weather scenario in place for much of Montana. Frontal
passage and strong upper level winds will bring windy conditions to
north central Montana and the Rocky Mountain Front (RMF) overnight.
Areas of snow are also possible at lower elevations with widespread
snow and blowing snow anticipated along the Continental Divide. High
Wind Warning remain in place for western portions of the north
central plains and the RMF but have lowered strongest gusts over the
plains to 65 mph while 80 mph along the RMF still seems reasonable.
Have also left the Winter Weather Advisory for snow and blowing snow
in place for the RMF. Snow accumulations over the plains and in the
southwest valleys will likely be less than an inch so no highlights
appear to be needed for these areas.
Somewhat quieter weather is expected Friday afternoon through
Saturday afternoon although breezy to windy conditions will
likely continue. Next active weather scenario arrives New Years
Eve and New Years Day. A Canadian cold front will push south of
the border late Saturday night with a good likelihood for
widespread accumulating snows of 1 to 3 inches expected over the
plains by Sunday night. Gusty north winds and snowfall will
combine for areas of snow over the plains with high temperatures
expected to top out only in the teens on Sunday. At this time am
anticipating that Winter Weather Advisories will be issued for
this system in the next day or two. mpj

Sunday night through Thursday...The long term period starts with a
broad upper level trough being carved out across the western US. The
models are in fairly good agreement regarding the evolution of the
trough as it progresses slowly east/south next week. That said,
differences do begin to show up Tuesday. While most models agree we
will stay in a NW/N flow aloft, there are differences regarding the
track of individual shortwaves that drop south out of Canada. The
track is important because it could mean the difference between
renewed shots of cold air and a continued chance of snow or drier
conditions with moderating temps.

The main forecast challenges in the extended, then, are how cold
will it get in the wake of the New Year's weekend cold front and
whether we stay cold or moderate. Forecast-wise, I expect lingering
snow showers on Monday as the weekend system pulls away. While
additional snowfall amounts look to be generally light on Monday,
there could be some higher amounts on north-facing slopes with some
upslope enhancement possible. The focus then turns to temperatures.
A strong SFC high (maybe 1040-1045mb) builds in during this time
which should allow for lighter winds across much of the area. At the
same time, 1000-500mb thickness values crash to potentially as low
as around 500dm (maybe even a bit lower). This should allow for
another period of very cold temperatures areawide. The main limiting
factors for how cold it gets will likely be clouds and wind. There
appears to be good agreement among models and analog guidance to
suggest widespread values of -10 to -20 across the area Monday
night/Tuesday morning and possibly again Tuesday night/Wednesday
morning. There are some indications temperatures could go lower, but
given it is still 5/6 days away, I didn't want to go too cold too
quickly, especially given that the models vary on the amount of
cloudcover.

Beyond Wednesday, the models differ on staying cold vs moderating,
as mentioned above. The official forecast shows a gradual moderating
trend as most models agree on this idea, but it still looks to stay
colder than normal. Precip-wise, a couple of shortwaves may move
through the area with at least some snow, especially for favored
northerly flow upslope areas, but confidence in precip next week is
lower given the model differences regarding the track of each s/w.
Martin

&&

.AVIATION...Updated 2350Z.

A low pressure trof over the Eastern Pacific will move onshore
tonight and winds aloft will continue to strengthen as this trof
approaches. Strongest surface winds will be across the Northern
Rockies and adjacent plains where gusts to 80 kts are possible.
Elsewhere, gusts of 50 to 60 kts are possible tonight. A cold front
will drop across the Canadian Border around 12z Friday. This front
will move south through the region and winds will become much
lighter behind the front. Snow and mountain obscuration is expected
over the Northern Rockies through Friday morning. Areas of light
snow and MVFR conditions will develop behind the front. Best chances
for snow will be across Southwest Montana, including KBZN and KEKS.

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  29  32  18  27 /  20  30  20   0
CTB  24  28  11  27 /  20  20  10   0
HLN  21  30   9  24 /  20  40  20   0
BZN  19  31   5  24 /  10  50  30  10
WEY   8  22  -6  15 /   0  40  50  10
DLN  18  30   3  22 /  10  30  30   0
HVR  25  30  14  27 /  10  10  10  10
LWT  28  32  13  30 /  20  40  10  10

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
High Wind Warning from 8 PM this evening to 8 AM MST Friday
Cascade...Eastern Glacier...Eastern Pondera...Eastern Teton...
Judith Basin...Liberty...Toole.

High Wind Warning until 8 AM MST Friday Northern Rocky Mountain
Front...Southern Rocky Mountain Front.

Winter Weather Advisory until 8 AM MST Friday Northern Rocky
Mountain Front...Southern Rocky Mountain Front.

Flood Advisory for Madison County until 245 AM MST Saturday

&&

weather.gov/greatfalls