Winter Storm in Montana, February 20 to 21, 2017

Area Forecast Discussion
National Weather Service Great Falls MT
511 PM MST Mon Feb 20 2017

Aviation Section Updated

.DISCUSSION...

Tonight through Wednesday...Another stout atmospheric river is
currently pulling moisture as far inland as southern MT this
afternoon...bringing periods of moderate to heavy snow to these
regions. Meanwhile...a weak shortwave is trying to tap into some
of this moisture...and bring scattered showers to central and
northern portions. WV imagery showing the struggle of this weak
shortwave to pull up moisture...as the bulk of the moisture
is remaining in the south currently. The rest of the evening will
see continued snow in the south...with scattered rain and high
elevation snow in the central and north. Can not rule out an
isolated thunderstorm this evening with steep lapse rates across
central portions...however...abundant cloud cover is limiting
available instability. Precip then diminishes for the most part
across the northern half tonight...while continuing across the
south. A secondary shortwave could then bring a burst of moderate to
heavy snow to the south early Tuesday morning. This may further
deteriorate conditions...however...with current web cams in the
south showing current snow causing limited impacts...will continue
the advisory. This last push of snow will be the heavier and more
impactful snow of the advisory period...and travelers and outdoor
enthusiasts should be careful. This wave then pushes across the rest
of the CWA while bringing a pseudo warm front with it for Tuesday.
Models indicate somewhere across central to north central MT a band
a precipitation will develop along this stalling boundary. Timing
and location will determine the impacts of this band. Currently the
band develops late morning through the afternoon on Tuesday. This
would bring a brief shot of snow...but then lingering warm air could
transition snow to rain along this band. Brief freezing rain may be
possible at times...however...impacts will be limited. In addition
weak instability could provide for a few isolated thunderstorms by
Tuesday afternoon. Another weak wave then moves across the area
Tuesday night...turning our stalled front into a cold front...and
bringing widespread snow showers to the area. This snow could then
linger into Wednesday under northwest flow. Timing will be key again
on Wednesday. Right now looks like northern and central portions
could see accumulating snow Wednesday morning...then just enough
warm air may transition to a rain/snow mix Wednesday afternoon. This
will have to be monitored as the potential for moderate precip may
allow for temperatures to struggle to get above freezing. This could
bring more moderate snow accumulations to mentioned northern and
central portions. As mentioned...temperatures will remain mild
through Tuesday. Wednesday looks to be near normal...with highs in
the 30s and 40s.
Anglin

Wednesday night through Friday...The Wednesday disturbance looks
to get washed out by a cut off low in southern WY Wednesday night
into Thursday. This could linger scattered snow showers across the
area during the time period. Much cooler temperatures compared to
the recent warm spell are also expected....with highs only in the
20s by Thursday. Friday, snow looks to taper off in most North-
Central MT locations. Farther south, snow should become more
probable in Southwest MT as surface high pressure builds south-
southeastward from the Canadian Prairies, causing the
aforementioned front to advance southward and eastward. This
potential widespread accumulating snow will have to be monitored
closely.

Saturday through Monday...Below-normal temperatures will prevail
during this period courtesy of a predominantly northwesterly flow
aloft. Mid- to upper-level troughing should persist Saturday into
Sunday morning, while later on Sunday, shortwave ridging may briefly
crest over our region. By late Sunday evening into Monday, another
mid- to upper-level trough looks to overspread the CWA from the
northwest. This pattern looks to allow weakening surface high
pressure to reside over the CWA Saturday. Thereafter, a weak lee
surface trough should materialize. Additional snow showers are
expected during the period, especially on Saturday and Monday.
Jaszka

&&

.AVIATION...
Updated 0011z.

For starters, forecast models are really struggling with the
evolution of conditions over the next 24hrs with more variability
than would be expected for any given TAF period. As for the setup,
moisture continues to stream into the area from off the Pacific,
accompanied by embedded shortwaves. The timing of each shortwave as
well as the associated surface features and precip is where the
models differ. For now, thinking the ongoing shower activity
impacting KHVR/KGTF/KLWT/KHLN will begin to diminish this evening.
The next shortwave then arrives late tonight/TUE AM with another
round of rain/snow showers. In general VFR conditions look to
prevail through TUE afternoon, but some models do bring in lower
MVFR or lower CIGS by tomorrow and this trend will need to be
monitored in later forecasts. Lastly, in the wake of this evening's
shortwave, a short period of gusty winds is likely for most TAF
sites. Martin

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  35  48  31  41 /  40  60  40  60
CTB  29  39  26  35 /  20  20  30  60
HLN  32  43  26  36 /  30  40  30  20
BZN  34  47  25  36 /  30  30  30  30
WEY  32  36  18  28 /  90  80  60  60
DLN  33  42  26  34 /  40  30  20  20
HVR  34  46  29  40 /  30  20  50  50
LWT  37  50  30  39 /  20  60  30  50

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until 8 AM MST Tuesday Beaverhead...
Gallatin...Madison.

&&

weather.gov/greatfalls

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE GREAT FALLS MT
555 PM MST MON FEB 20 2017

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0425 PM     NON-TSTM WND GST MONIDA                  44.56N 112.31W
02/20/2017  M57 MPH          BEAVERHEAD         MT   DEPT OF HIGHWAYS

            PEAK WIND GUST REPORTED OVER MONIDA PASS. WIND
            ACCOMPANIED BY FALLING AND BLOWING SNOW AND REDUCED
            VISIBILITY

&&

MARTIN