Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 170054
AFDTFX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREAT FALLS MT
554 PM MST Tue Dec 16 2014

.UPDATE...
Updated evening forecast to increase the chances for snow tonight
for Southwest and Central Montana. Overall a swath of snow will
move through the Dillon/Bozeman and Townsend areas this
evening...and then spread into the Helena/Great Falls/Lewistown
area late this evening and overnight. In the Havre area...if the
system holds together...the snow should move up into this region
generally after 4 am. Overall snow accumulations will generally be
around 1/2 inch...but there will be a few areas that get around an
inch of snow.

In terms of fog...with increasing clouds and precip chances...the
threat for widespread dense freezing fog redeveloping in the Great
Falls/Stanford areas is diminishing. If the fog does not redevelop
in the next few hours...this portion might be dropped from the
advisory. However...in the Cut Bank area...visibilities continue
to remain near 1/2 mile...so I feel more confident about
widespread dense fog continuing in this region at times overnight.

The next evening update is planned for around the normal time of
9 to 930 PM. Brusda

&&

.AVIATION...
UPDATED 2350Z.
Light snow continues to push northward into Southwest MT. Expect
visibilities to generally be 1SM in areas of snow along with
mountains obscured. The snow will spread into the Helena/Bozeman
areas this evening...and then into the Great Falls/Lewistown areas
over night. The snow should move into the Havre area on Wednesday
morning. Areas of dense fog will continue to affect the Cut Bank
area until Wednesday morning. Brusda

&&

.PREV DISCUSSION...
/ISSUED 450 PM MST Tue Dec 16 2014/
Tonight through Thursday...Relatively quiet conditions over
central and north central Montana during the period. Areas of
freezing fog continue to cover portions of north central Montana
this afternoon...and those areas that did burn off will likely
see the fog redevelop late this afternoon or this evening. Have
extended the Freezing Fog Advisory for most of north central
Montana until 11 a.m. Wednesday.

Upper air pattern shows a high pressure ridge over the Rocky
Mountain states with a low pressure trough over the west coast.
Idiosyncrasies in the pattern are resulting in a split flow aloft
over southwest and central Montana...and small weather systems
embedded in the flow, and being steered into Montana, are being
sheared apart and weakened. Best chance for snow from these
systems will be over the southwest Montana mountains. Probabilities
decrease to only slight chances as the system moves to the
northeast over central Montana. As the high pressure ridge shifts
east and the trough begins to move inland, another system will
approach the area during the day Thursday. This again brings slight
chances for snow mainly over the west and southwest mountains.
Temperatures will remain near seasonal averages for this time of
year.

Thursday Night through Tuesday...A weak upper level ridge of high
pressure will keep the forecast area mostly dry Thursday night and
Friday. A shortwave trough will then move into the area Friday
night into Saturday, bringing an increased chance of mountain
snow. However, breezy westerly downslope winds will keep the
plains dry. The weather pattern will change somewhat beginning on
Sunday. An upper level ridge of high pressure will strengthen off
the California coast and an upper level trough of low pressure
will deepen from Hudson Bay into the Great Lakes. This will put
Montana under the influence of a strong northwesterly flow aloft.
The strongest winds aloft (60 to 70 kt at 700 mb) will occur
Sunday into Monday with the passage of a disturbance embedded in
the flow. This set- up will bring gusty westerly downslope winds
to much of the area, but especially over the plains of north
central Montana. In fact, some areas along the Rocky Mountain
Front could approach High Wind criteria (40 mph sustained/58 mph
gusts). Confidence is increasing that High Wind highlights may be
needed for this event. There is also significant moisture with
this disturbance, so have increased the chance for precipitation
out onto the plains. Although the winds will decrease somewhat
after the passage of the disturbance, the northwesterly flow aloft
will remain fairly moist, so have kept the chance for
precipitation above climatological normals across the area into
Tuesday. The breezy winds throughout the period will keep
temperatures around 10 degrees above normal. The warmest day of
the period appears to be Monday, when most lower elevation
locations should get well into the 40s. Coulston

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  21  37  25  37 /  60  30  10  10
CTB  17  34  22  36 /  10  10  10  10
HLN  20  31  18  33 /  80  20  10  20
BZN  22  31  14  30 /  90  20  10  20
WEY  22  31  17  28 / 100  40  20  30
DLN  20  33  17  33 / 100  20  10  20
HVR  15  31  19  35 /   0  30  20  10
LWT  21  38  22  39 /  60  40  10  10

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
FREEZING FOG ADVISORY until 11 AM MST Wednesday Cascade...
Eastern Glacier...Eastern Pondera...Eastern Teton...Judith
Basin...Northern Rocky Mountain Front...Southern Rocky Mountain
Front...Toole.

&&

$$

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