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Area forecast discussion 
National Weather Service Billings Montana
336 am MST Monday Dec 22 2014

Short term...valid for today and Tuesday...

Active weather pattern continues through the short
term...although most of our forecast area will be quieter than
Sunday. A northwest flow aloft with embedded pieces of energy will
move southeast across our area today. The main weather feature for
today will be a strong surface low pressure over eastern South
Dakota and Nebraska this afternoon into this evening. The low will
strengthen during the day resulting in tight pressure gradients as
far west as eastern Montana and Wyoming. This will result in windy
conditions across mainly the eastern half of our forecast area. will be windy across our entire area but not as much
as was the case Sunday. A strong vorticity maximum dives south out of
Canada and across the Montana/Dakota borders this afternoon and
evening. Moisture on the front side of the low will get pulled
north and westward wrapping back on the back side of the low
across mainly Fallon Carter and Carter counties. Although the
latest model runs have brought some of the heavier quantitative precipitation forecast amounts
west into the eastern portions of Custer and Powder River

Ggw radar is starting to fill in during the past couple of hours
with movement south/southeast. Timing brings the snow into Custer
and Fallon County around daybreak or slightly thereafter. Will not
make any changes to the Winter Weather Advisory for Fallon and
Carter counties which is currently in effect even though the snow
looks to start later than previously anticipated. Given that
models bring the quantitative precipitation forecast further west and wpc is bringing the 2-4 inch
snow amounts into eastern Custer and Powder River counties...will
add these two counties to the advisory with a start time of 9 am.
The system looks to be a quick mover with the snow expected to end
after midnight tonight. Elsewhere across the forecast
area...expect a windy day with a slight chance for a snow shower.

The low pulls east Tuesday with pressure gradients slackening
across our east. However...another surface low sets up across
southern b.C. During the afternoon and this will allow for Lee-
side troughing to re-develop across our area. Pressure gradients
tighten considerably by tomorrow evening/night across our western
areas and this will bring another round of strong winds to
Livingston and the beartooth/absaroka foothills. Have increased
winds in the grids to advisory levels but will hold off on any
statements with this issuance. Temperatures the next couple of
days will run at or slightly above normal. Hooley

Long term...valid for Wednesday...Thu...Fri...Sat...sun...

GFS continues to be the outlier for the Christmas system this
morning. The NAM window is now touching the Christmas time frame
and it and the sref mean are in support of the European model (ecmwf) solution
with the Canadian not too far off. Wpc guidance supports a blend
of ec/NAM for timing and precipitation and see no reason to
deviate from this as the ec has been the more consistent model
over the past few days. Consistency was good last night but this
dissipated when the 06z GFS came in and shifted its solution north
and slowed arrival of the colder Canadian airmass. The GFS
continues to hold onto this solution which holds off snow 18 hours
later than other models due to a secondary surface low that moves
out of Idaho and across central Montana Wednesday night into
Thursday morning. The other models have a more traditional surface
pattern with a surface low dropping into Wyoming in one piece
Wednesday evening. Both model sets bring precipitation and colder
air to the area...the GFS just brings it in for Christmas
afternoon and night while the preferred model consensus is for a
late Wednesday night into Thursday timing. While consistency
between the GFS and other models is poor the consistency between
the other model solutions is good. So will lean on this mix for
the current forecast package...but will leave some room for the
slower GFS possibility.

Wednesday will be a warm and in some locations a windy day. Zonal
flow aloft will support Lee side trough and potentially strong winds
for Livingston...Nye...Big Timber and possibly Harlowton. Pacific
moisture trapped in this westerly flow could bring a few snow
showers to the western mountains but by and large Wednesday will
be dry. Disturbance drops into the Pacific northwest Wednesday
afternoon and by Wednesday evening jet divergence increases and
pulls Canadian cold front southward and through the area
overnight. Expect to see snow develop over northern and western
zones before midnight...spreading south and east during the
overnight hours. As cold air deepens up and northerly winds
upslope into the mountains will see snow intensity increase. Looks
like the heaviest snow will be from 3 am through early afternoon.
2 to 4 inches of snow seems reasonable for much of the area by
Christmas afternoon...with locally heavier amounts possible. The
foothills around Red Lodge and Sheridan could go over 6 inches
based on latest quantitative precipitation forecast and snow ratio projections. Will probably need
some winter highlites for this system but will wait until we
hopefully get more model agreement before working that out.

Area will be under a trough in northerly flow on Friday which will
keep temperatures cool. Will keep low precipitation chances in the
forecast but wouldn't expect any accumulation with it. Saturday
will see some weak ridging aloft for dry conditions and a bit
warmer temperatures. Models hinting at a colder system for the end
of the weekend with the potential for high temperatures in the
single digits and lower teens for early next week. Still too much
uncertainty to drop temperatures that far right now but did trend
temperatures lower and added in a chance of snow for Sunday into
Monday to get foot in the door.

For Holiday travelers...Tuesday looks like the best travel day
across the northern rockies. Wednesday doesn't look too bad over
southern Montana either but areas north and west will be seeing
increasing snow chances. For Christmas evening and Christmas day
expect snow that could make travel difficult. Chambers


Unstable northwest flow over the area will keep shower activity
going through the day today in most locations. Precipitation will
be more concentrated along and east of a Miles City to Broadus
line where MVFR conditions will develop by middle day as snow
intensifies. Local IFR conditions are possible into this evening
for this region along with blowing and drifting snow. Periods of
snow in the mountains will Keep Mountain obscuration going into
tonight as well for all area mountains. For the rest of the area
local MVFR conditions can be expected near showers otherwise VFR
conditions will prevail. West to northwest winds will be gusty
again today with gusts 30 to 40kts across the region. Chambers

Preliminary point temp/pops...

Today Tuesday Wednesday Thursday Friday Sat sun
bil 042 025/039 026/041 028/031 012/025 012/029 018/027
2/west 20/north 02/r 77/S 31/b 11/b 23/S
lvm 037 014/034 023/039 025/031 012/028 011/031 018/030
4/j 20/north 02/r 88/S 52/S 12/S 23/S
hdn 040 022/037 019/040 026/031 010/025 010/028 015/027
3/west 30/b 12/r 77/S 32/S 11/b 23/S
mls 034 021/035 019/040 026/030 013/023 009/028 013/024
7/S 31/b 12/r 65/S 21/east 11/b 23/S
4bq 034 024/034 017/040 025/031 014/024 007/027 013/027
8/S 82/S 11/b 66/S 32/S 11/b 13/S
bhk 032 024/029 015/038 022/028 008/019 007/025 009/021
8/S 72/S 12/o 44/S 21/b 11/b 23/S
shr 038 025/036 018/041 026/033 012/026 008/031 013/029
4/west 41/b 01/b 77/S 42/S 11/b 14/S


Byz watches/warnings/advisories...
Montana...Winter Weather Advisory in effect from 9 am this morning to 5
am MST Tuesday for zones 32-36.
Winter Weather Advisory in effect until 5 am MST Tuesday for
zones 33-37.



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