Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 081233
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
533 AM MST Thu Dec 8 2016

.AVIATION SECTION UPDATED...

.AMENDED DISCUSSION BELOW REGARDING WINTER WEATHER HIGHLIGHTS...

.DISCUSSION...

Today through Saturday...Northwesterly mid- to upper-level flow and
the arctic surface high pressure ridge will persist over our CWA
this morning. During this afternoon and evening, still expect
diffluent southwesterly flow aloft and associated lift ahead of a
low- to midlevel shortwave trough approaching from the Pac NW to
allow surface pressure falls to commence over the forecast area.
Also, a surface warm front accompanying this disturbance should move
northward into Southwest MT by late morning and then near the I-90
corridor by daybreak Friday. This pattern will permit weak
isentropic lift of warm, moist air to occur over the arctic air,
ahead of the front, and set the stage for a widespread snowfall this
evening into Friday morning. This will especially be the case over
our Southwest and Central MT counties, where the isentropic lift
looks to have a higher moisture content.

After daybreak on Friday, snow looks to become more scattered over
the CWA for a time as the aforementioned disturbance weakens while
moving farther eastward and flow aloft becomes more zonal due to a
rather flat mid to upper-level high pressure ridge building-in from
the Pac NW. This shift in the flow aloft will cause isentropic lift
to end over our area. Thus, upslope flow looks to become the main
trigger for snow. On Friday, the aforementioned warm front should
move farther northward toward the Central MT mountains. With
southerly surface flow south of the front and mainly northeasterly
to easterly flow north of the front, places such as the south
aspects of Southwest MT mountains and the north/east aspects of the
Rocky Mountain Front and Little Belts will likely experience
scattered upslope snow showers. By Friday afternoon, more widespread
snow should begin to overspread the region from the west as another
low-to midlevel shortwave trough approaches from the Pac NW.

As this disturbance continues eastward, the widespread snowfall will
persist Friday night into Saturday morning. By Saturday afternoon,
the best potential for lingering snow showers will be along the
Continental Divide and over the Southwest MT mountains due to
upslope flow. Elsewhere, quieter conditions are expected due to a
shortwave high pressure ridge building-in from the Pac NW. At the
surface, the aforementioned warm front is expected to stall near the
Central MT mountains Friday night into Saturday. Thus, this front
will continue to separate arctic air to its north from milder air to
its south. Did not adjust the winter weather advisory that takes
effect later today and continues into Saturday for essentially the
southern two thirds of the CWA. However, winter weather highlights
may be needed for the Rocky Mountain Front, including Marias Pass,
for Friday evening into Saturday. Will let subsequent shifts review
more model data before making a final decision. Snow totals during
the period should range from 1-4 inches on the plains and in the
valleys to mainly 4-6 inches in the mountains of Southwest MT and
along the Rocky Mountain Front. However, localized totals of 6-12
inches cannot be ruled-out along the Rocky Mountain Front and
Southwest MT mountains.

As for temperatures, a slow moderating trend is expected in North-
Central MT between today and Saturday. However, temperatures will
still be about 15 to 30 degrees below normal during the period. In
Southwest MT, temperatures will be about 20 to 30 degrees below
normal today. By Saturday, temperatures here are expected to be near
normal.
Jaszka

Saturday Night through Wednesday...Flow aloft across Central and
Southwest Montana will generally be from the west Saturday night but
a shortwave moving through the flow will bring increased chances of
precipitation to the region by Sunday. The passage of the shortwave
will bring an associated cold front across the Canadian border by
late Sunday, and temperatures will take a downward direction again.
Another disturbance will move into the area Monday and this will
keep cold air across northern portions of the region. Snow will be
moving out of the area by Tuesday but the cold air already in place
will result in continued cold temperatures. High pressure will build
over the zones and the air mass will dry until another shortwave
brings snow across the Rockies again late Wednesday night and
Thursday. The surface boundary between cold air north and warmer
south will likely be lying across Southwest Montana and this area
would be expected to receive the most snow.
Zelzer

&&

.AVIATION...Updated 1142Z.

An upper level low pressure disturbance is approaching Central and
Southwest Montana. This will bring snow and lowered cigs to the
region tonight. MVFR/IFR cigs are possible this evening as the snow
moves east and north across the region. However, locations in the
north and northeast, including KHVR, should remain VFR through the
next 24 hours.

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF   0  -7   3  -1 /  10  40  40  60
CTB  -4 -11  -4  -7 /  10  20  40  50
HLN   7  -2  19  11 /   0  50  30  60
BZN  12   4  26  19 /   0  40  30  50
WEY  11   6  27  16 /  20  60  50  70
DLN  13   9  33  21 /  10  30  20  40
HVR   0  -7   1  -3 /  10  10  40  50
LWT   4  -6   6   0 /  10  30  40  60

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from 11 PM this evening to 5 PM MST
Saturday Broadwater...Cascade...Central and Southern Lewis and
Clark...Fergus...Jefferson...Judith Basin...Meagher.

Winter Weather Advisory from 5 PM this afternoon to 5 PM MST
Saturday Beaverhead...Gallatin...Madison.

&&

$$

weather.gov/greatfalls



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