Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 022201

Area Forecast Discussion
National Weather Service Great Falls MT
301 PM MST Fri Dec 2 2016


Tonight through Saturday...A fairly strong and moist northwest
flow aloft will keep snow showers with blowing snow and gusty west
winds along the Rocky Mountain front. Elsewhere, scattered light
snow showers will move across North Central and Southwest Montana
overnight, especially in the mountains, as a frontal boundary
moves through the area. Very little accumulation is expected at
lower elevations, but mountains could receive 2 to 4 inches.
Breezy westerly winds will also persist over the plains into
Saturday, which will limit precipitation there. These winds will
also help keep temperatures a bit above normal.

Saturday night through Monday...As the much talked about winter
system gets closer, details are becoming a bit more clear on the
evolution and impacts to central and SW Montana, although there
is still some uncertainty.

A nearly zonal flow aloft at the start of the period will quickly
transition into a broad trough across the western/central US by
early next week. This transition will feature several hazards for
the local area. At the SFC, the models bring an area of low pressure
through Montana late Saturday night through Monday. The track of
this low will be key in the evolution of the heaviest snow and
greatest impact to the forecast area and is the area the models have
struggled with the most over the last few days.

The biggest change in the 00z/12z guidance has been to keep
more of an open wave moving through aloft as opposed to a closed
low. Because of this, the models aren`t showing as deep of a SFC low
and this also leads to a faster solution with the low. This may
ultimately lead to slightly lower snowfall amounts across the area,
although the models continue to insist on several inches of snow
across much of the area. Regarding the track of the low, the open
wave now appears to favor more of a NW to SE track across our
forecast area as opposed to a SW to NE track. While the models all
show this general idea, they still differ some on the exact track
and placement of heaviest snow. Model consensus plus analog guidance
and input from WPC places the highest confidence in heavier snowfall
amounts from the Rocky Mountain Front SE through parts of central
and Southwest Montana. In the higher elevations, 1-2 feet is likely,
especially along the Rocky Mountain Front. Several inches are likely
at lower elevations, but again the areas that see the highest
amounts will depend on where the SFC low tracks.

The combination of moderate to heavy snow and, at times, the
potential for blowing/drifting snow will likely lead to hazardous
conditions at times for parts of the area Sunday through Monday. For
these reasons, confidence is high that advisories and/or warnings
will be issued for parts, if not all, of the area over the weekend.
For this reason, am going to issue a Winter Storm Watch for the
areas that we feel most confident in significant impacts. For now,
this will include areas roughly from Glacier NP through Great
Falls/Lewistown and points south. This may need adjustment in later
forecasts if confidence increases for areas to the north. This has
the potential to be the first winter storm to bring widespread
impacts to much of the area and everyone is encouraged to review
winter safety tips, especially given the relatively mild fall with
less winter weather than normal.

Sunday night through Thursday...The trof across the west will
continue to deepen and slow Monday night and Tuesday. However,
dynamics will be to the south and precipitation may be light or
intermittent. The trof will move east of the zones Wednesday.
However, the ECMWF is slower than the GFS with this movement.
Thus, the ECMWF holds low pressure over the zones longer than the
GFS. Do not have a preferred solution at this time so Will blend
this period. As the high pressure builds, Central and Southwest
Montana will remain underneath unsettled northwest flow aloft and
a chance of snow will continue. The air mass will begin to trend
warmer Wednesday night as the high pressure builds. However,
moisture moving underneath the ridge will keep at some showers
over the region, mainly the western mountains and Southwest
Montana. The chance of showers will increase Thursday as moisture
ahead of another to the west moves on shore. Zelzer


.AVIATION...Updated 1828Z.

High pressure aloft will generally continue VFR conditions over the
forecast area through at least 18Z Saturday, unless otherwise
mentioned. However, the high pressure ridge will gradually be broken
down by an approaching low pressure trough, bringing increasing high
and mid level cloudiness to the area, along with scattered mountain
obscuring showers that may lower ceilings to MVFR levels at times
after 04Z in Southwest Montana and over the eastern plains areas
(KHVR KLWT). Gusty southwest to west winds will also increase
through the period. Winds of 15 to 25 kt with gusts to 35 kt will be
common along the east slopes of the Rockies, with some areas
reaching 50 kt closer to the Rocky Mountain Front. Winds will be a
bit lighter elsewhere south and east of the east slopes. Coulston


GTF  28  40  31  38 /  30  10  20  40
CTB  28  38  28  34 /  20  10  20  50
HLN  26  39  26  38 /  20  20  20  40
BZN  21  35  25  35 /  40  20  20  40
WEY   8  24  15  25 /  30  40  40  60
DLN  18  33  22  34 /  30  20  20  40
HVR  27  41  27  37 /  40  20  20  40
LWT  26  38  27  37 /  30  30  20  40


Winter Storm Watch from Sunday morning through Monday afternoon
Beaverhead...Broadwater...Cascade...Central and Southern Lewis
and Clark...Eastern Glacier...Eastern Pondera...Eastern Teton...
Fergus...Gallatin...Jefferson...Judith Basin...Madison...Meagher.

Winter Storm Watch from Saturday evening through Monday
afternoon Northern Rocky Mountain Front...Southern Rocky
Mountain Front.


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