High Wind in Montana, December 31, 2016

Area Forecast Discussion
National Weather Service Great Falls MT
440 AM MST Sat Dec 31 2016

Updated Aviation Discussion

.DISCUSSION...

Today through Monday...Upper level pattern this morning features
a high over low block in the eastern Pacific with upper level
ridge amplifying into AK. Downstream, a longwave trough extends
from the Arctic SE across Canada with deep/cold low centered near
Hudson Bay. Shortwave energy to impact the region later this
weekend is dropping S along the AK panhandle. Westerly winds
aloft increase across the region today in response to the
shortwave energy dropping S through BC. Surface pressure gradient
remains strongest along the E slopes of the Rockies, where a few
wind gusts to 60 mph are possible through this afternoon with
30-40 mph gusts across areas adjacent to the Rocky Mtn Front.
Shortwave energy descending S through BC today will continue S
into the Pacific NW tonight, carving out a deep positively tilted
trough from Central Canada SW into the Western US by Sunday night
and Monday. A cold airmass will surge south across the Canadian
border tonight and continue south through central and SW MT
Sunday. Moisture ejecting out of the deepening trough to the west
will overrun the cold air, bringing widespread snow to most of
the forecast area Sunday and Sunday night, while some enhancement
is also likely from upslope northerly winds at the surface veering
to easterly aloft Sunday night and Monday as the western US
trough elongates. Total snow accumulation from early Sunday
through Monday morning of 2 to 5 inches appears likely for much of
central and North-central MT with lighter amounts across far NE
and SW portions of the forecast area. In addition to the snow,
temperatures will fall through the day Sunday as the cold airmass
moves in and deepens with northerly winds gusting  20-30 mph
creating areas of blowing/drifting snow and contributing to wind
chill values well below zero by late Sunday. Will issue winter
weather advisories to highlight these hazards beginning late
tonight across northern zones and Sunday afternoon across SW MT.
Winds will gradually diminish Sunday night with snow diminishing
to light snow showers across most areas by Sunday afternoon.
Hoenisch

Monday Night through Saturday...A mid- to upper-level longwave
trough extending generally southwestward from central Canada to
North-Central and Southwest Montana looks to persist through
Thursday. Simultaneously, a surface ridge of arctic high pressure
should prevail over the CWA. Lingering snow showers over the CWA
Monday night, especially along northern aspects of mountains, should
dissipate by daybreak Tuesday as the aforementioned arctic high
builds into the area and stabilizes our atmosphere. On Wednesday and
Thursday, shortwave troughs embedded within the aforementioned
longwave trough look to dive southeastward over the region. Lift and
moisture ahead of these disturbances should trigger scattered snow
showers. On Friday and Saturday, the latest runs of the ECMWF and
GFS agree that mid-to upper-level flow will become zonal over our
CWA, allowing very cold air to retreat northward into Canada and
temperatures to begin moderating over our region. Both models also
suggest disturbances within the zonal flow will generate additional
snow showers, but differences exist regarding timing and amounts.
Thus, forecast confidence decreases for the Friday through Saturday
time frame. At this point, no major snow accumulations are expected
during the forecast period.

For Tuesday through Thursday, temperatures will tend about 15 to 30
degrees below-normal. At this time, Tuesday morning may be the
coldest, with lows reaching the -10 to -20F range in many places.
Despite a moderation in temperatures, readings still look to be
about 10 to 20 degrees below-normal Friday and Saturday.
Jaszka

&&

.AVIATION...Updated 1140Z.

Dry westerly flow aloft persists across the region through this
evening ahead of an upper level weather disturbance descending south
along the BC coast. West to southwest surface winds will remain
gusty through today at N-central MT terminals. Canadian cold front
sweeps south across the border tonight, likely reaching KCTB and
KHVR by around 06z, bringing a wind shift to the north and lowering
ceilings with light snow developing late tonight. Front proceeds
south late tonight, reaching KGTF, KHLN and KLWT by around 12z
Sunday. Hoenisch

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  31  14  15   0 /   0  70 100  80
CTB  29   7  12  -4 /  10 100  80  60
HLN  30  10  16   0 /   0  10  90  60
BZN  27   8  18  -2 /   0  10  60  80
WEY  18   1  16  -9 /   0  20  50  70
DLN  29   4  17  -6 /   0  10  50  60
HVR  30  12  15   1 /  10  80  80  50
LWT  32  10  15  -1 /   0  10  80  80

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from 5 AM Sunday to noon MST Monday
Broadwater...Cascade...Central and Southern Lewis and Clark...
Chouteau...Fergus...Jefferson...Judith Basin...Meagher.

Winter Weather Advisory from 11 PM this evening to noon MST
Monday Blaine...Eastern Glacier...Eastern Pondera...Eastern
Teton...Hill...Liberty...Northern Rocky Mountain Front...
Southern Rocky Mountain Front...Toole.

Winter Weather Advisory from 11 AM Sunday to noon MST Monday
Beaverhead...Gallatin...Madison.

&&

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