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Area forecast discussion 
National Weather Service Billings Montana
827 PM MST Friday Dec 19 2014

Update...

Minor update to expand slight probability of precipitation eastward through Miles City
before 06z. Once again radar echoes are extending further
eastward than like to see with a zero probability of precipitation. Not very confident of
any significant accumulations...but coverage does seem to be
increasing and there are still a few hours 06z...when inherited
forecast brings probability of precipitation into the Miles City area. Otherwise...the
inherited forecast appears to be in good shape. Aag

&&

Short term...valid for Sat and sun...

Pacific shortwave in Washington/or is moving east and will pass through the
region tonight into early Saturday. Brief window of forcing
should bring a band of mainly light showers from west to east
across our County Warning Area...with maybe an inch of snow accumulation over the
western mountains and the potential for a little light freezing rain
east of surface trough...ie east of Billings...where surface temperatures will fall
below freezing this evening. Subsidence/drying will spread in from
the west tomorrow morning behind this shortwave. Expect above
normal temperatures with highs back into the lower to middle 40s on
Saturday.

Impressive moisture plume approaching the Pacific coast extends
back toward 30n 170e...and will push through the Pacific northwest and
northern rockies over the next few days. GFS shows 305k mixing
ratios as high as 2-3 g/kg reaching our SW mountains westerly flow
aloft will favor the beartooth absarokas for a period of
accumulating snowfall beginning Saturday evening and lasting
through at least Sunday...after which there is some uncertainty as
to how fast moisture plume will shift south with flow aloft
veering northwest behind Sunday shortwave. Precipitation will be aided by
increasing instability Sunday/Sunday night with middle level lapse
rates expected to exceed 7 c/km and some locally heavy banding is
possible as heavier precipitation shifts to northwest aspects. Overall lots of
potential here so will issue a Winter Storm Watch for the
beartooth absarokas from Saturday evening into Monday morning.

At the same time there will be a period of enhanced winds along
the foothills as middle level flow increases and Lee side surface trough
deepens. Confidence at this point is high for advisory level gusts
at Livingston and Nye Saturday night into Sunday morning. As surface
low shifts east locations such as Big Timber and Harlowton should
also see increased winds. Cold advection/deeper mixing will allow
for gusty middle level flow to at least partially surface across lower
elevations Sunday through the day. GFS/NAM show 850mb flow on the
order of 40 kts...so 40+ miles per hour gusts are possible across much of the
lower elevations Sunday from late morning through the afternoon...
including Billings. Have adjusted wind speeds up some for this
period. Temperatures on Sunday will continue to ride well above normal
and with deeper mixing think some places will touch 50f...
especially with snow cover dwindling.

Aforementioned Sunday shortwave and increased instability will
allow for a better chance of showers across the plains Sunday and
Sunday night. Showalter indices near zero suggest some banded
showers are a possibility...and as flow turns to the northwest the Big
Horn Mountains should also pick up some snow accumulation. Given
the instability have raised probability of precipitation some in northwest flow favored areas
Sunday night.

Jkl

Long term...valid for Monday...Tue...Wed...Thu...Fri...

Beginning of the extended period has established northwest flow
aloft begin to back around on Tuesday which will support an onset
of gap flow winds. This Monday Tuesday time period has a minor
shower threat as moisture streams across the area but mechanisms
are not in place for significant precipitation and temperatures
are just above normal which means its not even a given that it
will be snow.

Windy pattern sets up Tuesday night and continues into Wednesday
for The Gap flow areas. With a positively tilted ridge into Idaho
and rounds of pressure falls over southwest Alberta a tight
pressure gradient is expected to develop. Pattern breaks down on
Wednesday evening as energy begins to approach upstream.

Possibility of a system producing weather Christmas evening and
Christmas day is still in the cards but models are not in good
agreement. GFS splits the incoming system with more energy heading
south while the European model (ecmwf) develops a more favorable surface low over
Wyoming and has a better trajectory for cold air in response to a
weak upper low near cyfo. Neither model lingers effects too long
into Friday as the system digs away from the area. Will not change
current messaging for midweek system but likely communicate
stronger tomorrow after another round of model runs. Borsum



&&

Aviation...

VFR conditions will generally prevail tonight and Saturday outside
of mountain obscurations. However...some showers will move west to
east across the area tonight and precipitation could come as a mix
of light rain...light snow and/or light freezing rain east of kbil
toward kmls. MVFR conditions are possible. However...we/ve encoded
that potential as only vcsh in taf forecasts until later trends in
radar data perhaps add confidence to coverage and intensity of the
precipitation. Schultz

&&

Preliminary point temp/pops...

Sat sun Monday Tuesday Wednesday Thursday Friday
-----------------------------------------------------------
bil 032/042 032/049 034/043 027/040 026/039 025/033 015/026
20/b 14/west 33/r 22/o 22/S 34/S 21/b
lvm 032/043 034/048 033/041 021/040 024/039 024/035 014/029
31/north 35/west 44/o 32/S 23/S 54/S 32/S
hdn 024/041 020/049 030/041 022/039 021/038 020/032 012/025
41/b 14/west 33/r 23/o 22/S 24/S 22/S
mls 025/041 022/044 029/039 024/036 023/036 020/029 013/023
23/west 15/west 22/o 22/S 32/S 22/S 22/S
4bq 023/042 021/044 029/039 024/038 022/038 020/031 014/025
23/j 04/west 32/o 22/S 32/S 23/S 22/S
bhk 021/042 021/041 027/035 021/033 020/032 016/027 011/018
13/west 05/west 22/S 21/b 32/S 22/S 22/S
shr 022/042 020/045 030/038 022/037 022/037 019/032 013/025
21/b 13/west 33/o 32/S 22/S 46/S 32/S

&&

Byz watches/warnings/advisories...
Montana...Winter Storm Watch in effect from Saturday evening through
Monday morning for zone 67.
Wyoming...none.

&&

$$
Weather.Gov/Billings

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