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fxus65 kbyz 131005 

Area forecast discussion
National Weather Service Billings Montana
305 am MST Wed Dec 13 2017

Short term...valid for tdy and Thu...

A north-northwest flow will be over the region today. Jet will dive S into west
Mt, bringing jet divergence to the area, while a second jet will
move southeast through the Dakotas bringing divergence to the area as
well. Shortwave energy and moisture will move S through the
western zones. Kept similar timing for precipitation chances as
the previous forecast with a dry period until 15z, then a slight
chance of rain/snow showers over the west. The best moisture and lift
will be over the area this afternoon for a likelihood of rain/snow
showers in klvm and numerous snow showers in the western
mountains. A couple inches of snow are possible over the Paradise
Valley. It will be breezy with cold advection over the area, and
temperatures will be much lower than they were yesterday. Highs
will be in the upper 30s to 40s.

The shortwave energy will move S of the area this evening as other
shortwaves move S into east Montana. Spread low pops across the area this
evening with moisture from both waves over the region, then had
pops mainly in the east overnight. Very little snow accumulation is
expected tonight. Went with windy conditions in the eastern zones
after 06z as strong winds aloft move into the area. Kept a chance
of snow showers over the east Thursday morning as shortwaves move out
of the area. Temperatures will hold in the 30s over the east on
Thursday, with 40s elsewhere. On Thu. Night, the surface pressure
gradient again becomes favorable for gusty winds in The Gap areas
and Big Timber. Local guidance suggested winds will stay below
highlight criteria, so increased winds to just under criteria.

Long term...valid for Fri...Sat...sun...Mon...Tue...

Ridge crashing Pacific storm system will move into the area
Friday. Ahead of the system expect warm windy and mainly dry
conditions through the afternoon hours. Scattered showers move
into the area Friday evening as a cold front moves through. Models
have been steadily increasing quantitative precipitation forecast/pops for Friday night into
Saturday morning. The draw back is the dry air ahead of the system
that will have to be moistened up before precipitation can make it
to the ground, along with persistent gusty west to northwest winds
that will add additional downslope subsidence to work against
precipitation. On the positive side there is a narrow feed of
Pacific moisture pushing through the area overnight which will
help, to go along with increased dynamics, so have increased
precipitation potential to likely for the mountains with 30 to 40
pops in the lower elevations. The dry air should help wet bulb
down temperatures to allow some light snow accumulations over the
lower elevations (generally an inch or less) with 3 to 6 inches
for the mountains. The north slopes of the beartooth foothills
could see low advisory numbers (2 to 4 in) as winds turn
northerly for 3 to 6 hours with favorable upslope instability.

Saturday looks blustery with temperatures in the 35 to 45 range
and isolated to scattered showers, mainly in the west, and
northwest winds in the 20 to 30 mph range. The potential exists for
some mountain induced snow bands persisting into the afternoon
before precipitation dissipates Saturday evening.

Sunday looks mainly dry with energy generally staying north and
south of the area. Temperatures look to be in the 40s.

More uncertainty creeping into the forecast for the Monday through
Wednesday time frame as a transition to more winter-like
conditions gets underway. In general this period will see a chance
for mainly snow showers each day with highs in the upper 30s to
lower 40s. There looks to be a Canadian cold frontal boundary
stalled out somewhere across our area during this period that
should be the main focus for light snow/flurries, but exactly
where that boundary sets up is uncertain, thus the broadbrushed
low pops.

For the end of the week and the Christmas travel period, GFS and
its ensembles continue to advertise a strong Arctic intrusion to
last through Christmas. The GFS ensembles are showing strong
probabilities (70 to 80 percent) of temperatures below 10 degrees
across the area from Thursday afternoon (21st) through the 28th.
The European model (ecmwf) is milder, but still cold, during this period as it
shunts coldest air to the east of the area. The Canadian remains
in between the other models. The key to this Arctic plunge is the
track of a strong central Pacific storm systems from Tuesday
through Thursday. The GFS drives this system north into the
Aleutian Islands dislodging the Arctic air down the Rocky Mountain
front, while the European model (ecmwf) holds this feature further southeast over
the Gulf of Alaska leading to a less intense cold plunge. Still a
lot of time for this system to evolve, but if you have travel
plans toward the end of next week keep an eye on the forecast as
chances are increasing for a drastic change to the weather
pattern we have been in. Chambers



Scattered rain/snow showers will move into western mountains and
foothills by mid day with local MVFR conditions near precipitation
areas. Occasional mountain and pass obscuration expected with this
shower activity. Otherwise VFR conditions will prevail. Expect
northwest winds to gust around 30kts east of a Roundup to Sheridan
Wyoming line today. Chambers

Preliminary point temp/pops...

Tdy Thu Fri Sat sun Mon Tue
bil 047 030/045 031/053 030/040 022/042 029/045 026/044
1/b 20/u 00/b 44/west 10/u 12/west 22/west
lvm 039 027/043 030/046 028/037 017/039 028/042 028/043
6/west 30/north 00/north 54/west 11/b 12/west 22/west
hdn 048 028/043 026/054 030/041 021/043 027/046 025/043
1/b 21/b 00/b 54/west 10/u 12/west 22/west
mls 045 028/038 024/053 030/041 022/043 028/043 025/041
0/b 21/north 10/b 42/j 11/u 12/west 22/west
4bq 044 029/037 024/053 029/040 022/042 027/045 025/042
0/north 32/j 00/b 42/west 11/b 12/west 22/west
bhk 039 026/033 021/049 027/038 022/040 026/041 023/039
0/north 43/j 10/b 32/j 11/b 12/west 11/east
shr 046 025/040 021/053 028/039 018/041 023/047 021/044
0/b 21/north 00/u 46/west 21/u 11/b 22/j


Byz watches/warnings/advisories...

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