Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 100440
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
940 PM MST Fri Dec 9 2016

.UPDATE...
Current winter weather advisories remain in effect with no
changes at this time. Initial wave of moisture and light snow
remained over southwest MT earlier this evening, and based on
latest IR satellite imagery and surface observations, that
moisture/precip as dissipated and moved east. Leading edge of next
wave of moisture, associated with the shortwave trof moving into
western WA/OR, is already in central Idaho and western Beaverhead
County. Main part of that moisture plume arrives later tonight
through tomorrow aftn, bringing more widespread light snow for all
parts of our forecast area. Latest models slightly reduced the
liquid water/QPF values with the next plume so total snowfall
amounts through Sat aftn came out around 0.2 to 0.5 inch less in
most areas.
Waranauskas

&&

.AVIATION...Updated 0009Z.

One band of -SN is moving NE away from the forecast area at this
time, but may bring a brief period of -SN to KHVR/KLWT through
around 02-03z. A second band of snow has already begun to move into
SW MT (note recent obs at KDLN/KEKS). This band of snow will
gradually move east/north through tonight, then push east of the
area late Saturday afternoon. This band of snow will bring
widespread IFR conditions and mtn obscurations to the area, with
occasional LIFR conditions possible. The models differ some on where
the heavier snow will develop and confidence is low regarding
if/when any TAF site will drop to LIFR. So for now, have opted to
leave LIFR conditions out of any TAF until confidence increases.
Significant snowfall amounts are not expected at any TAF site, but
there will be enough to potentially create some issues for airport
ground operations. A gradual decrease in the snow is expected from
west to east after 18z/SAT. Martin

&&

.PREV DISCUSSION...
/ISSUED 509 PM MST Fri Dec 9 2016/

Tonight through Sunday...Bands of very light snow or flurries
continue to weaken this afternoon across north central MT while
radar/observations to the W/SW show the next batch of moisture and
precipitation to impact the region spreading east across Idaho.
Incoming plume of Pacific moisture will bring light snow to SW MT by
this evening with additional bands of light snow developing to the
north across Central MT late tonight through Saturday morning as
shortwave energy now moving into WA state tracks into the Northern
Rockies and MT. Areas along the Hi-line eventually see light snow re-
develop Saturday morning and continue through Saturday afternoon as
this energy tracks across the northern tier of MT. Total liquid
equivalent precipitation amounts through the next 24 hrs ranges from
0.05"-0.15" over the plains and valleys, with 0.15"-0.25" in the
Mtns. With snow to liquid ratios generally around 20:1 or better
expected, this should yield about 1 to 3 inches of snow at lower
elevations with 2 to 5 inches across the mountains over the next 24
hrs with the bulk of the snowfall across central/northern areas
occurring later tonight through Saturday morning. Have expanded the
winter weather advisory across northern counties beginning tonight at
midnight. Main impacts continue to be reduced visibility and
deteriorating road conditions in periods of light snow. Most areas
will continue to see light winds through Saturday with the exception
of some gusty SW winds over the Mtns of SW MT near the ID border.
The westerly flow aloft will begin to erode the cold airmass from
portions of SW MT tonight through Saturday, with some potential for
the cold airmass to briefly retreat north across portions of central
MT Sunday morning, allowing temperatures to rise into the 20s. This
looks be very short-lived however for much of north-central MT as
the cold airmass surges back south Sunday afternoon. During this
period (Saturday night through Sunday afternoon) precipitation will
diminish across all but the western Mtns, where orographic enhanced
snow showers will continue. Hoenisch

Sunday night through Friday...Northwest flow aloft over the Northern
Rockies will bring a continuing chance for snow Sunday night.
Another Canadian cold front pushes southward through North Central
Montana by Monday morning with high temperatures expected to drop
back into the single digits above zero while Southwest Montana
remains in the 20s. By Tuesday, a deep upper level closed low is
expected to be centered over central Manitoba with continued
northerly flow aloft continuing to push the surface cold front
through Southwest Montana along with areas of light snow and gusty
northerly winds. The closed low gradually drifts eastward across
Canada by late Tuesday night into Wednesday morning. From here
models diverge on what happens next. GFS solution keeps cold nw flow
over the region through the end of the work week...with limited
disturbances bringing some chances for light snow at
times...especially Thursday. The GFS solution then brings a more
active pattern by next weekend. The Euro solution has a different
thought. As the low moves out Tuesday night...a weak frontal
boundary could bring some overrunning and light snow to the southern
half of the CWA...the the remainder stays mainly dry and slightly
warmer for Wednesday into Thursday morning. Euro solution is then
dry until Friday when it indicates a shortwave trough moves across
the region and brings widespread snow...with some warming
conditions. The next weekend time period then looks cold with some
lingering light snow. Really tough to say which one wins out at this
point...tried to compromise and place some slight pops in a few
areas to account for either solution with this afternoons update.
Also lowered temperatures for mid next week...given the expected
snowfall and push of arctic air. mpj/anglin

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  -3  11   9  26 /  60  40  10  20
CTB  -7   2  -2   7 /  40  50  10  20
HLN   6  26  15  30 /  50  40  10  20
BZN   9  28  15  30 /  70  40  40  20
WEY  22  31   7  22 /  90  90  50  40
DLN  11  30  15  29 /  70  40  40  20
HVR  -2   5  -5   8 /  50  70  30  20
LWT   0  16   8  27 /  50  50  20  20

&&

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

Winter Weather Advisory until 5 PM MST Saturday Blaine...
Chouteau...Eastern Glacier...Eastern Pondera...Eastern Teton...
Hill...Liberty...Northern Rocky Mountain Front...Southern Rocky
Mountain Front...Toole.

Winter Weather Advisory until 5 PM MST Saturday Beaverhead...
Gallatin...Madison.

&&

$$

weather.gov/greatfalls


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