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Area forecast discussion 
National Weather Service Billings Montana
251 PM MST Sat Dec 20 2014

Short term...valid for sun and Monday...

Very active weather is in store for our region over the next 60
hours with several potential impacts.

First wind. Will make no changes to wind advisories in effect for
The Gap areas beginning at midnight tonight and continuing through
Sunday night. Gap flow will actually end with the onset of cold
advection tomorrow morning...but plenty of mixed wind potential
exists thereafter. Have added Big Timber to these existing
advisories as this location will be favored as the surface low tracks
east on Sunday. It is becoming more apparent that other areas
besides the usual foothill spots will see strong winds on Sunday.
Primarily this includes Harlowton and Judith Gap eastward into
Golden Valley County beginning late morning or early afternoon.
Shortwave expected to pass between 18-00z tomorrow with the top of
an increasingly mixed boundary layer tapping into 60-70 kts of west-northwest
middle level flow per the GFS. NAM is a bit weaker as usual. Given
the ideal timing of the passing of this energy feel that the
combination of subsidence during peak mixing will allow for strong
gusts to surface in our northwest parts...and have issued a high wind
watch for Wheatland and Golden Valley counties. Further east is a
trickier call of course...with late day subsidence perhaps being a
bit too late. That being said...would expect wind gusts to 50 miles per hour
to reach Billings during the afternoon...with gusts of 30-40 miles per hour
further east as we get into the evening. Gusty winds will continue
through Sunday night.

Mountain snowfall will increase tonight with Pacific moisture
advection already pushing through WA/OR/ID...with plume extending
well back into the Pacific. Trajectory of this plume with 2-3
g/kg mixing ratios on the 305k surface suggest a good period of
accumulating orographic snowfall through Sunday...with snowfall
rates aided by increasing instability tomorrow. Moisture plume
will begin to sag south as flow veers more northwest late
tomorrow...but instability will allow for snow showers to continue
especially on northwest aspects Sunday night into Monday. Will make no
changes to the Winter Storm Warning for the beartooth absarokas
where amts of a foot to foot and a half are expected. This veered
flow will also allow for snowfall to pick up over the big horns
Sunday night through Monday night...with several inches a good
possibility even as moisture becomes shallower.

Another interesting period of weather is emerging from the
developing active northwest flow. Latest model runs show a trowal
wrapping southward through our east Monday as upper low deepens
over the Dakotas. Impacts from this may include a period of modest
snowfall along with gusty northwest winds...maybe 1-2 inches and some
blowing snow...mainly east of Billings though if middle level flow
veers sufficiently we could see some banded snow showers in the
Billings area too. Will hit higher terrain to our east and
southeast hardest here...and have raised wind speeds to suggest
30-40 miles per hour gusts. This will include Sheridan. Anyone planning to
travel through areas like Lame Deer Divide and Aberdeen hill on
Monday should take note.

Finally...there is a slight risk of a little freezing rain east of
surface trough late tonight and early Sunday as Pacific shortwave moves
in. Surface temperatures are the key here and it is possible that temperatures will
warm into the middle/upper 30s as precipitation develops. Something to keep
watch of tonight.

Temperatures will remain above normal the next couple days with Sunday
being the warmer of the two. Highs will approach 50f tomorrow and
may get into the lower 50s in places if mixing is sufficient.
Monday will be cooler with highs near 40f in our west and maybe
staying in the upper 20s to lower 30s in our east as 850mb cold
pool swings through with northwest flow trowal.


Long term...valid for Tuesday...Wed...Thu...Fri...Sat...

Active weather pattern continues into the extended forecast as a
relatively quiet start to Tuesday finishes with the evolution of a
gap flow wind event. Positively tilted ridge axis shifts into the
area in response to energetic flow moving into southern British Columbia. Will
be a dry day with near normal temperatures. Winds accelerate
strongly Tuesday night and a strong gap flow wind pattern is set
up for Wednesday. Mex guidance showing strongest winds of the week
during this period and this look good with better mixing across
the plains driving warmer temperatures. Have kept strong winds in
The Gap flow areas and adjusted winds across the plains upward.

The devolution of the wind pattern and the arrival of push of
colder air bringing a chance for Christmas evening or Christmas day
snow is not as clear cut. One change from previous runs is the GFS
is pulling cold air into the area whereas before it had been
looking to stay warmer. European model (ecmwf) continues to show a better
potential for significant snowfall since a surface boundary will
be in place when a shortwave reaches the area. Bigger key is timing
between European model (ecmwf) and GFS remains out of sync and this is mostly due
to uncertainty of the location of a deep upper level low over the
Great Lakes or Ohio Valley.Still confidence is better for a cooler
Thursday with snow some time in the window between 18z Wednesday
and 00z Friday.

Friday the airmass dries out and moderates as wind reenters the
forecast but its not currently expected to become strong under
Saturday when pressure gradients set up again. Models reverse
roles for next weekend as the European model (ecmwf) is pessimistic on another shot
of snow. Borsum



Mountain snow will impact areas around klvm and over the
beartooth and Absaroka Mountains through the period. Expect
isolated showers to begin moving over the adjacent plains after
midnight. Winds will increase later this evening for favored areas
over the foothills. Low level wind shear will be possible for kbil
and kmls towards daybreak Sunday. Borsum


Preliminary point temp/pops...

Sun Monday Tuesday Wednesday Thursday Friday Sat
bil 034/049 034/040 025/036 027/040 025/031 016/028 011/029
23/west 34/west 21/north 24/o 66/S 22/S 11/b
lvm 035/046 032/037 016/035 024/039 023/034 010/029 012/031
34/west 44/j 22/S 35/S 66/S 32/S 11/b
hdn 026/049 031/039 021/035 020/038 021/031 012/028 007/028
26/west 34/j 31/b 23/S 66/S 22/S 11/b
mls 026/044 029/035 024/036 023/037 022/029 014/025 010/028
16/west 34/j 21/b 12/S 33/S 22/S 11/b
4bq 024/045 031/035 024/036 022/039 022/031 014/027 009/029
06/west 35/j 31/b 12/o 23/S 22/S 11/b
bhk 025/043 028/030 022/033 019/034 019/027 010/022 009/025
17/west 45/j 32/S 22/S 23/S 22/S 11/b
shr 025/045 029/036 019/033 021/038 022/031 012/030 008/030
03/west 44/west 31/b 13/S 66/S 32/S 11/b


Byz watches/warnings/advisories...
Montana...high wind watch in effect from Sunday morning through Monday
morning for zones 28-42-63.
Wind Advisory in effect from 5 am Sunday to noon MST Monday
for zone 41.
Wind Advisory in effect from midnight tonight to noon MST
Monday for zones 65-66.
Winter Storm Warning in effect from 6 PM this evening to noon
MST Monday for zone 67.



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