Area Forecast Discussion
Issued by NWS Billings, MT

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FXUS65 KBYZ 040231
AFDBYZ

Area Forecast Discussion
National Weather Service Billings MT
731 PM MST Sat Dec 3 2016

.UPDATE...
Water vapor imagery shows a shortwave exiting into the Dakotas and
moist westerly Pacific flow thru the Pac NW ahead of deepening
trof off the BC coast. Isentropic ascent will begin to spread
west-aspect pcpn to the Beartooth Absarokas later tonight, with a
period of snowfall to persist thru Sunday night with passage of
Pacific trof. Heaviest pcpn expected to occur tomorrow afternoon
and night as ascent and instability increase. Analysis of latest
models suggest enough moisture/forcing for warning level snowfall
(12 inches) especially for west aspects including the mountains
near Cooke City. Have already upgraded the watch to a winter
storm warning.

Other issue tonight is wind in the gap areas. Pressure rises in
the wake of aforementioned exiting shortwave have reduced the
winds at Livingston and Nye significantly, and have updated winds
for the remainder of the evening. Pressure falls already starting
again in NW MT and this will translate east thru the night, so
would expect a return of gap flow before sunrise tomorrow. Will
make no changes to wind advisory in effect.

&&

.SHORT TERM...valid for Sun and Mon...

Little in the way of changes for the short term period once again
today. Active weather pattern will continue with broad cyclonic
flow across the region. Windy conditions in gap areas continue to
look on track through tonight and Sunday ahead of approaching
Arctic air.

The arctic cold front will begin to impact the area Sunday
spreading rain and snow showers across the region in the
afternoon. The coldest air will impact the long term period, but
affects in falling temperatures and accumulating snow will be felt
as early as Sunday night for the plains. Best snow potential still
appears to be late Sunday and overnight Sunday night. Area
mountains will get a healthy shot of snowfall from Sunday through
Monday. Temperatures will fall from Sunday through Monday...as
cold airmass drops south.

Uncertainty remains relatively high for snow amount with continued
differences among models. NAM and ECMWF favor a more typical
arctic passage with at least a short period of upslope flow in
area mountains...a period of heavier precip and band of more
persistent snow somewhere across the southern zones. GFS however
seems precip across the region in pieces...more like a
disorganized trof and cold frontal passage.

Greatest concerns at this point will be flash freezing roads
Sunday night, as snow melts and refreezes as cold air drops into
the area. This will cause treacherous driving conditions for the
Monday morning commute.

Have chosen to hold off on issuance of highlights at this
time, due to continued model uncertainty and plenty of time
before snow gets going in the area. Do expect at least advisories
will be needed across the area...if only for icy roads and blowing
snow, but still some question on whether will need warning in
Beartooth/Absarokas. AAG

.LONG TERM...valid for Tue...Wed...Thu...Fri...Sat...

Coldest air of the season so far moves in on Tuesday as a cold
airmass originating from the arctic continues to sink south into
the region behind an exiting upper low. 850 mb temps fall below
-20 C correlating to highs in the single digits with low
temperatures below zero for many locations on Tuesday and
Wednesday. Just how low temperatures go is still somewhat
uncertain as model spread continues. Have kept a slight chance to
chance of snow showers across the area on Tuesday with models
hinting at some precipitation with northwest flow aloft, and weak
upslope flow at the surface. Heights begin to build aloft on
Wednesday as shortwave ridging moves into western Montana.

Winds will remain elevated over the eastern Plains Tuesday and
Wednesday. Wind chills continue to be a concern as 15 to 20 mph
winds will allow for periods of wind chills below -30 F.

The arctic airmass begins to moderate and retreat on Thursday as
the ridge axis shifts farther east. Uncertainty continues with
the temperature forecast as models are struggling with when to
bring in downsloping winds. The latest model runs actually keep
the area cooler than previous runs on Thursday with highs in the
low 20s west, to single digits across the far east.

The next disturbance moves into the region from the west beginning on
early Friday and into Saturday bringing another shot of snow for
the area. Currently have gone with a forecast blend for temperatures
Friday into Saturday with temps gradually warming into the low
30s. However, there is great uncertainty in the temperature
forecast with high model spread. STP

&&

.AVIATION...

VFR conditions will prevail through Sunday for terminals in
south central-southeastern MT and north central WY. Southwest
winds near KLVM will remain strong at times through Sunday,
gusting to around 40-50 kt. Some LLWS possible near KSHR this
evening. Mountains may be obscured overnight. BT
&&

.PRELIMINARY POINT TEMP/POPS...

    Sun     Mon     Tue     Wed     Thu     Fri     Sat
-----------------------------------------------------------
BIL 030/043 021/024 005/008 905/004 903/018 013/033 021/035
    22/W    76/J    32/J    11/B    11/B    22/J    22/J
LVM 032/041 020/022 001/009 905/005 901/021 019/038 024/036
    45/W    85/J    32/J    11/B    11/B    23/J    32/J
HDN 026/045 022/026 005/009 904/003 907/018 009/032 019/034
    22/W    66/J    32/J    11/B    11/B    12/J    22/J
MLS 024/042 020/025 003/007 905/003 906/011 001/022 012/028
    22/W    66/J    31/E    10/B    00/U    12/J    32/J
4BQ 023/041 020/026 002/008 906/003 908/014 002/027 015/032
    21/E    67/J    31/E    11/B    00/U    11/E    22/J
BHK 021/039 018/024 002/006 905/002 910/007 902/017 011/025
    20/B    56/J    31/E    10/N    00/B    11/E    31/B
SHR 021/043 021/025 002/010 908/007 910/016 008/033 017/033
    21/E    67/J    32/J    11/B    01/B    11/B    12/J

&&

.BYZ WATCHES/WARNINGS/ADVISORIES...
MT...Wind Advisory in effect until 6 PM MST Sunday FOR ZONES
      65-66.
     Winter Storm Warning in effect from midnight tonight to noon
      MST Monday FOR ZONE 67.
WY...None.

&&

$$
weather.gov/billings


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