Area Forecast Discussion
Issued by NWS Great Falls, MT

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000
FXUS65 KTFX 192345
AFDTFX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREAT FALLS MT
445 PM MST Fri Dec 19 2014

Updated Aviation

.DISCUSSION...

Tonight through Sunday...
An upper level short wave trof and associated Pacific cold front
will move through the region overnight. This will case periods of
light snow across the Southwest. Warmer air in the southwest flow
ahead of this system could lift over a colder airmass in place
over eastern portions of north-central Montana. This creates a
possible light freezing rain scenario tonight. Have added freezing
rain to the forecast, but otherwise do not have enough confidence
to issue any advisories. Will continue to monitor once light
precipitation begins to spread through this area tonight.

Surface pressure falls are already causing wind gusts to pick up
at Deep Creek along the northern Rocky Mtn Front. Short term
guidance added confidence that wind gusts will briefly reach near
60 mph at isolated locations in this area early Saturday as the
front passes. Issued a high wind warning to cover this situation,
even knowing that gusts will taper Saturday. The high wind warning
continues on through Sunday as a much stronger downslope wind
signature appears with a strong lee trough and synoptic
subsidence. Issued a high wind watch extending east off the Rocky
Mountain front on Sunday to evaluate the possibility of a more
widespread high wind event Sunday.

An atmospheric river event is developing over the Pacific
Northwest Saturday night and Sunday. This affects mainly southwest
Montana...but heavy snow will accumulate along the Rocky
Mountains, shifting from north to south through Sunday. Snow
accumulations at pass level remain below 8 inches in 24 hours, so
issued a winter weather advisory for snow and blowing snow above
5000 feet. Downsloping winds and mild temperatures will keep any
snow accumulations within and closely adjacent to the Rocky
Mountain Front. In the southwest, snow accumulations could exceed
8 inches from 06Z Sunday through 06Z Monday. Snow levels are
forecast to rise above 5000 feet on Sunday, with rain possible in
the valleys. Will need to monitor forecast snowlevels before
upgrading to winter storm warning or dropping back to winter
weather advisory. Nutter

Sunday night through Friday...A rather active
weather pattern is setting up for much of the region for next
week. There will be several accumulating snowfall events that will
cause impacts for daily commuters and travelers. Through much of
the extended...the GFS13 and EC are in pretty good agreement.
Temperatures will be near/a bit above normal Monday and into
Tuesday...but then begin to turn colder Wednesday through Friday.
For now I used a blend of the colder models for temperatures...but
temperatures could be several degrees colder than currently
forecast should the snowpack become a bit deeper than expected.
Additionally...there is the potential for another high wind event
over portions of North Central MT on Tuesday.

In terms of precipitation...Southwest MT will continue to see
precipitation Sunday night and into Monday morning...with scattered
showers over North Central MT. The precipitation becomes
light and scattered Monday night through Tuesday. Then a another
fairly strong storm system will move through the region Wednesday
afternoon through Christmas Day. At this time...it looks like
Central and Southwest MT will see the brunt of this storm. At lower
elevations...there will be a swath of 3 to 5 inches of snow...with
around 8 to 10 inches in the mountains. It is too early to pin-point
the exact area that will see this heavier snowfall...but right now
that swath of heavier snow is forecast along a Helena to Bozeman
line. This line of heavier snow could shift a bit over the next few
days as later model runs get a better handle on the storm track.
Brusda/Blank

&&

.AVIATION...
UPDATED 0000Z.
A surface cold front associated with a weak upper-level weather
system will sweep from west to east through the forecast area this
evening. Conditions will deteriorate from west to east this evening
and tonight with mountains obscured in snow until late tonight. Have
moderate confidence in the timing of precipitation, but forecast
confidence is low for flight category cigs/vis. At KHVR, freezing
rain is possible late this evening, then should changeover to a
rain/snow mix. Rain and snow showers are expected at KGTF and KLWT
with light snow developing at KHLN and KBZN between 01z and 03z.
This is a fast-moving system with conditions improving by 12z
Saturday.

BZN TAF...as mentioned above have moderate confidence in the timing
of the threat of precipitation. Winds will remain light and variable
despite the cold frontal passage that is expected by 04z. Although
there is a prob group for MVFR conditions in snow from 03z to
07z..there could even be a brief period of IFR conditions in
steadier snow during this same time frame. Should these IFR
conditions develop they would last no more than an hour. Mountains
will become obscured later this evening and remain obscured through
Saturday morning. Blank/MLV

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  29  41  35  48 /  30   0  20  30
CTB  27  37  30  43 /  10   0  20  20
HLN  23  37  30  47 /  30  10  70  50
BZN  21  35  29  45 /  60  10  60  60
WEY  22  33  28  39 /  70  50  80  70
DLN  22  37  33  47 /  40  10  60  50
HVR  26  40  27  46 /  40  10  40  40
LWT  27  43  31  48 /  40  10  30  50

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
WINTER WEATHER ADVISORY from 5 PM Saturday to 5 AM MST Monday
FOR ELEVATIONS ABOVE 5000 FEET FOR Central and Southern Lewis
and Clark...Jefferson...Northern Rocky Mountain Front...Southern
Rocky Mountain Front.

HIGH WIND WATCH from late Saturday night through Sunday
afternoon Eastern Glacier...Eastern Pondera...Eastern Teton...
Liberty...Toole.

HIGH WIND WARNING from 2 AM Saturday to 5 PM MST Sunday Northern
Rocky Mountain Front...Southern Rocky Mountain Front.

WINTER STORM WATCH from Saturday afternoon through late Sunday
night FOR ELEVATIONS ABOVE 6000 FEET FOR Beaverhead...Gallatin...
Madison.

&&

$$

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