Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 122142

Area Forecast Discussion
National Weather Service Great Falls MT
242 PM MST Tue Dec 12 2017

An upper level ridge of high pressure will be over the region
through Friday, bringing mostly quiet weather to the region. A
weak, fast-moving system will bring light snow to southwest
Montana during the day Wednesday, with snow accumulations up to 1
inch. Another system arrives for Friday and will bring increasing
chances for rain or snow to the region.



Tonight through Thursday...

Winds will diminish through the evening as a passing disturbance
responsible for today`s strong winds and fire weather threat moves
away from the area. Expect some clearing especially over northern
and eastern parts of our area, but some clouds will remain along
the Continental Divide and much of southwest Montana. Models seem
to be catching up to the next fast-moving system in northwest flow
that will move across western and southwestern Montana during the
day Wednesday. Decent dynamics combined with good (but not
excellent) moisture in the dendritic snow growth layer suggest up
to a quick inch of snow for most of the mountains and valleys of
Southwest MT, with localized higher amounts to 3 inches on
northwest-facing slopes. This system will move through primarily
during the daytime hours, so there will likely be at least some
travel impacts. Yet another system will come immediately on its
heels, dropping south from Alberta and Saskatchewan over north-
central and northeast Montana. Models suggest very little moisture
to work with despite decent dynamics, so precipitation should be
limited to minimal amounts and primarily over the higher
elevations such as the Bears Paw and possible the Little Belts
Wednesday night. High pressure at the surface and aloft move
overhead Thursday, resulting in continued strong inversions. Fog
will be likely especially over the southwest valleys Thursday
morning, especially if there is fresh snow from Wednesday`s
expected minor snow event. CC

Thursday night through Saturday...A very slow change in the weather
pattern is expected to begin, as the upper-level high pressure ridge
will start to break down. Because of how strong the ridge is, it is
going to take a few disturbances to knock it down. Strong winds are
possible on Friday ahead of the precipitation. Expect snow over much
of the CWA Friday into Saturday, with the potential for some rain to
mix-in at lower elevations. Minor snow accumulations remain possible
as a shortwave trough traverses the region Friday evening into
Saturday morning. Thereafter, quieter weather is expected as another
high pressure ridge aloft builds-in from the west.

Sunday through Tuesday...Both the ECMWF and GFS continue to depict a
transition day on Sunday, with breezy southwesterly winds developing
as the aforementioned ridge is broken-down by another disturbance
that looks to remain north of Montana. These downslope winds will
warm temperatures a bit over North-Central Montana and allow only a
small chance for a passing shower over the plains. For Monday into
Tuesday, the aforementioned forecast models still differ on when the
next storm system will arrive from the Gulf of Alaska region. Thus,
while forecast confidence remains low during this time period,
precipitation probabilities may increase over much of the CWA.


Updated 1726z.
VFR conditions will prevail over the region through tonight, though
increasing high clouds in association with a Chinook Arch cloud will
bring mainly BKN skies for much of the afternoon and possibly
extending into the evening. The two primary aviation impacts through
tonight will be the potential for patchy morning fog in the Helena
and West Yellowstone areas. The second problem is strong surface
winds through this evening over the Plains of North Central MT. The
winds should diminish after 03z Wed. After 12z Wed a fast-moving
system moves through west-central and southwest Montana bringing the
potential for a period of light snow and MVFR cigs to terminals from
KHLN to KBZN and KEKS.  CC/Brusda


GTF  26  46  23  46 /  10  20  10   0
CTB  23  47  20  45 /  10  10  10   0
HLN   9  33   5  25 /  30  40  10   0
BZN   3  28   0  23 /  30  60  20  10
WEY  -5  23  -6  18 /  20  40  20   0
DLN   7  35   3  28 /  30  40  10   0
HVR  23  48  19  45 /   0   0  10   0
LWT  24  42  21  41 /   0   0  20  10


High Wind Warning until midnight MST tonight Blaine...Chouteau...

High Wind Warning until 6 PM MST this evening Eastern Glacier...
Liberty...Northern Rocky Mountain Front...Toole.


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