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Area forecast discussion 
National Weather Service Billings Montana
327 am MST Sat Dec 27 2014

Short term...valid for today and sun...

Our messaging focus remains on snowfall Sunday through Monday with
a couple inches of fluffy accumulation likely.

As for today...temperatures will warm a bit as 500-mb flow backs a
bit and downslope low-level flow moderates the air mass. Highs are
still expected to stay in the 20s f in most areas though. Winds in
the foothills at Livingston and Nye are still on track to increase
by middle to late morning with gusts to 50 miles per hour for this afternoon and
evening...a bit later start and end to that than earlier believed.
That/S because the shortwave trough crossing the Pacific northwest
looks a bit slower to arrive based on observational trends and the
00 UTC model guidance. We still capped forecast gusts at 50 miles per hour at
this juncture based on 700-mb wind speed analogs and a statistical
model run locally here at National Weather Service Billings. Patchy blowing snow stayed
in the forecast in and near the foothills too and was also carried
into the evening hours now given the longer duration of winds.

Now for the snowfall...we are still expecting accumulations from 3
to 6 inches in most areas Sunday into Monday with the most intense
snowfall likely between noon Sunday and midnight Sunday night. The
snow will be low-density and fluffy so we have patchy blowing snow
in the forecast now where and when wind speeds exceed about 15 miles per hour
since near-ground drifting will be easy at the very least. This is
also the kind of snow that often causes very low visibility as one
meets and passes other vehicles /known to some as snow fog/ and so
that may be one of the biggest travel impacts. There/S still a bit
of uncertainty in the timing of snowfall onset and how many inches
of snow will actually fall though...and for that reason we Haven/T
issued any headlines /advisories or winter storm watches/ with the
morning forecast package.

Where technical details are concerned...the non-GFS model set from
00 UTC stayed a bit slower with the arrival of the Arctic front on
Sunday and we nudged the forecast toward that idea...though not to
the extent of the slow outlier 00 UTC NAM solution. Qg-forcing was
stronger than on earlier model runs...but the early-day portion of
that will be somewhat wasted before the Arctic front arrives. Once
that frontal passage occurs by afternoon...there looks to be about
6 hours with both notable 500-300-mb q-vector convergence and low-
level upslope/Post-frontal winds along with frontogenesis. That is
why we have gone with categorical probability of precipitation beginning at 18 UTC Sunday.
The forcing aloft wanes by Sunday evening but upslope flow will be
rather deep at that point and forecast soundings have the vertical
motion driven by that upslope contained in a rather deep dendritic
snow growth zone. A blend of Cobb and roebber methods supported US
using snow-to-liquid ratios between 20 and 25 to 1 Sunday evening.
That on its own would have increased snow totals a bit...but there
was also a small reduction in quantitative precipitation forecast in the 00 UTC consensus of model
output that ended up meaning our total snow amount forecast didn/T
change much. That consensus suggests areal-average quantitative precipitation forecast totals will
be around 0.20 inches...but given subtle trends in some individual
model solutions we are a bit concerned that may drop a bit. Add in
the solid 6-hour difference in snow onset between the GFS and non-
GFS model solutions and we were not comfortable hoisting headlines
30+ hours out when advisories are the most likely result over most
of the area. Schultz

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

Monday snow event will be beginning to taper off for southeast
Montana but should continue for the western half of the area. Will
be a bitter cold day as Arctic air settles into the region and
accumulations of another inch or two should be possible especially
for places like Harlowton...Livingston and Big Timber and the
foothills. The upper level trough axis is not contributing much to
the dynamics of the snowfall by this point as the real energy is
shifting into the northern Great Basin but it is a slow moving
system so the airmass is still pretty moist.

High temperatures are going to be single digits Monday and Monday
night will be very cold with numerous areas teens below zero or
even lower. Only thing keeping temperatures that warm is that
cloud cover still should be around but the fresh snow cover and
high surface pressures will be in place.

Upper trough axis moves south Tuesday and becomes a cut off low
over the western Great Basin which becomes blocky to the northerly
flow aloft streaming through Alberta into Montana. This produces
some ridging and while the Arctic is shifting south in response
to the northerly flow pressures remain high over the region.
Expect another day with temperatures well below normal but it will
be dry. The cold temperatures will not fall as far Tuesday night
as a leeside trough brings a downslope pressure gradient and some
mixing. This pressure gradient remains in place through Thursday
as the upper low over the Great Basin contributes to the blocking
flow. Currently have temperatures approaching freezing by
Thursday.

Another evolving story is strong confidence among the models that
another Arctic surge with an extended period of snow is coming for
Friday and the weekend. Models have been so consistent in
producing snowfall for 3 straight days for most of the area. Could
be approaching double digit snow depths for the first week of
January which has only happened in 1972 and 1979. Borsum

&&

Aviation...

Mostly clear conditions this morning will be yielding to
increasing winds over the foothills and increasing clouds for
western areas this afternoon. By late afternoon some showers will
be reaching areas near klvm with mountain obscurations possible.
Borsum

&&

Preliminary point temp/pops...

Today sun Monday Tuesday Wednesday Thursday Friday
-------------------------------------------------------
bil 027 015/024 901/002 914/902 908/026 013/031 017/026
1/north 3+/S +5/j 21/I 00/north 11/north 23/j
lvm 026 014/024 903/007 924/004 916/025 008/035 018/031
1/q 6+/S +5/j 31/b 00/north 11/b 23/j
hdn 031 010/026 901/003 916/001 913/022 008/030 013/026
1/b 2+/S +3/j 21/b 00/u 11/b 23/j
mls 028 014/022 906/002 915/002 906/023 012/028 015/023
1/b 28/S 72/j 10/b 00/u 11/b 23/j
4bq 029 011/026 902/006 914/003 911/022 010/030 016/027
0/b 18/S 93/j 11/b 00/u 11/b 23/j
bhk 027 009/019 908/000 915/001 906/021 011/027 014/022
1/b 17/S 51/east 10/b 00/u 11/b 23/j
shr 028 008/025 901/006 918/000 913/025 008/034 015/031
0/b 17/S +5/j 31/b 00/u 11/b 24/j

&&

Byz watches/warnings/advisories...
Montana...none.
Wyoming...none.

&&

$$
Weather.Gov/Billings

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