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Area forecast discussion 
National Weather Service Billings Montana
358 PM MST Monday Dec 22 2014

Short term...valid for Tuesday and Wednesday...

Unsettled northwest flow over the region with a better area of
wrap around snowfall in southeast Montana where accumulations of
1/2 inch to an inch have been reported with observations showing
20 to 30 miles per hour winds. Radars show snow still moving into the area
with rap forecasting additional snowfall through the evening
hours. Will keep the advisory for Fallon and Carter counties but
echos are decreasing for Custer and Powder River counties so will
cancel the advisories for those areas.

Tuesday is a quieter weather day across the region as heights
build and the airmass dries out. A series of disturbances moving
through British Columbia begin a round of leeside troughing late in the day with
will promote some mixing but temperatures will not rebound past
the 30s. Leeside troughing strengthens Tuesday night which sets up
gap flow winds for Livingston and Nye areas. Guidance shows Wind
Advisory criteria winds developing after midnight so will not
issue a highlight yet but will begin messaging about a period of
strong winds along i90 late Tuesday into Wednesday.

Energy continues to stream across Alberta and British Columbia Wednesday with a
stronger wave moving into Washington and Oregon during the day.
This maintains the favorable pressure gradients and increases
mixing so temperatures will be warmer by a few degrees. Approach
of the disturbance Wednesday night allows a cold front to move
north to south through Montana with The Heights falling. The
alignment of the front moving into southern Montana after midnight
and the dynamics of the upstream wave increasing the moisture in
the airmass will allow for the development of snow and
frontogenesis. Expect snow to begin for areas around Judith Gap
and Harlowton Wednesday evening and spreading south overnight. By
late Wednesday night expect most areas to see some snow falling
with the exception of Sheridan County which may still be waiting a
few hours for it to develop.

The favorable ingredients late Wednesday night should yield 1 to 3
inches of snowfall by daybreak with snow continuing with the
higher amounts expected west of Billings...southeast Montana
accumulations will be closer to an inch. Winds becoming upslope
will help snowfall rates increase in the late night hours with a
good rate of snowfall expected at the very end of the short term
forecast period. Billings has not had an inch or better of
snowfall on Christmas day since 1988 and Sheridan since 1996 so
this will be a notable event. The record snowfall for Billings for
Christmas day of 4.5 inches could be threatened. Borsum

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

The main focus in the extended remains the snow on Christmas day.
The latest runs of the GFS and European model (ecmwf) have come into better
agreement with each other. The 12z NAM is maybe slightly faster
than the GFS/ECMWF...but the Gem is the main outlier...maintaining the
faster solution. Have trended the snow amounts up
slightly...generally 3 to 5 across much of the plains...with
amounts increasing to west into the mountains. Winds will be out
of the north for much of Christmas day...which sets up a god
upslope flow event for the mountains. BUFKIT soundings are showing
a very moist dendridic growth layer through much of the day...so
expecting good snowfall rates out of this system.

Given that the area will be under cloud cover with falling
snow...have kept temperatures Christmas day a little cooler than
guidance. Throughout the extended...temperatures will remain
cold...with highs generally in the 20s to lower 30s...and lows
dropping into the teens.

Friday things should dry out as the system moves into the Central
Plains. Saturday should be another quiet day across the
area...before we see the next trough move into the area with
another round of snow Sunday. The GFS is a bit faster with this
second system...but at this point am going with a blend of the
models given that this is at the end of the forecast period.
Reimer
&&

Aviation...

Light snow showers will continue across the area through
midnight...with the heavier snowfall from Miles City to Broadus
and east. Generally VFR conditions will prevail. Some snow showers
will produce some brief localized MVFR conditions...and continue
to obscure the mountains. Winds will remain gusty through middle
evening. Reimer
&&

Preliminary point temp/pops...

Tuesday Wednesday Thursday Friday Sat sun Monday
-----------------------------------------------------------
bil 025/039 026/042 027/028 012/025 012/029 018/027 008/019
20/north 01/b 78/j 51/b 11/b 23/j 32/j
lvm 014/034 023/042 025/026 012/028 011/031 018/030 009/023
20/north 02/west +9/j 62/j 11/north 23/j 32/j
hdn 022/037 019/042 026/027 010/025 010/028 015/027 008/019
30/b 11/b 58/j 52/j 11/b 23/j 32/j
mls 021/035 019/042 026/027 013/023 009/028 013/024 006/016
41/b 11/east 66/j 41/east 11/b 22/j 22/j
4bq 024/034 017/042 025/026 014/024 007/027 013/027 007/019
52/S 11/b 47/j 42/j 11/b 13/j 32/j
bhk 024/029 015/038 024/025 008/019 007/025 009/021 003/013
72/S 11/b 45/j 41/east 11/b 22/j 22/j
shr 025/036 018/042 025/026 012/026 008/031 013/029 010/022
41/b 01/b 48/j 52/j 11/b 13/j 32/j

&&

Byz watches/warnings/advisories...
Montana...Winter Weather Advisory in effect until 5 am MST Tuesday for
zones 33-37.
Wyoming...none.

&&

$$
Weather.Gov/Billings

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