Winter Storm in Montana, February 19, 2017

Area Forecast Discussion
National Weather Service Great Falls MT
323 AM MST Sun Feb 19 2017

.DISCUSSION...

Today through Monday...Negatively-tilted shortwave trough moving
ashore the Pac NW and far-northern CA this early morning will
advance across our CWA late this afternoon through early Monday
morning. The best chance for precipitation, which will likely be in
the form of wet snow, will exist along the Continental Divide and
along the south/west aspects of mountains in Southwest and Central
MT courtesy of west-southwesterly and moist upslope flow. Thus, the
winter weather advisory for portions of Southwest MT will continue
through 5 PM this evening, especially given Monida Pass along I-15
is closed until further notice per MT DOT. Elsewhere, isolated
freezing drizzle cannot be ruled-out along eastern portions of the
Hi-Line this morning, including Havre. In addition, large-scale lift
generated by the aforementioned shortwave trough combined with
sufficient moisture and some instability should trigger some widely
scattered rain showers over the valleys and plains, especially this
afternoon and early evening. There may also be enough instability
for a few short-lived and weak thunderstorms over Southwest and
southern portions of Central MT this afternoon and early evening.

Tonight into early Monday morning, a shortwave ridge will build
briefly into our region, allowing dry weather to return to most
places. Later Monday morning through afternoon, the coverage of
precipitation should increase across the CWA as another shortwave
trough approaches from the Pac NW. Abundant cloud cover and above-
normal temperatures will persist through Monday.
Jaszka

Monday Night through Sunday...The period starts with yet another
surge of moisture running into the western US in fire hose
fashion, aided by a 140+kt jet. Some of this moisture will likely
reach parts of our area, especially the western/southwestern
mountains, however it will be short-lived as an upper level
trough, currently south of the Aleutian Islands, moves into the
western US by by WED/THU. This trough should bring about a pattern
change from the milder, more spring like conditions of late to
more typical late-winter conditions. The models remain in decent
agreement through about Thursday, but 12Z guidance continues to
diverge by next weekend.

As the incoming plume of moisture runs into the western US, it will
be proceeded by a strong warm front late Monday into Tuesday. Light
precip developing ahead of this front will move over cooler air at
the SFC which may lead to a brief window of a wintry mix Monday
night into early Tuesday morning for any location that can drop down
near or just below freezing Monday night. Perhaps some light icing,
but too early to get specific on that. It is something we'll keep an
eye on in later forecasts, though. Otherwise, the warm air aloft
should allow SFC temps to warm through the day with rain vs snow
showers being mainly elevation-dependent. Additionally, an isolated
thunderstorm also can't be ruled out Tuesday afternoon across SW MT
south of the front in the warm sector. Regardless, the models do
agree that a strong Pacific cold front will cross the area Tuesday.
Of note, the GFS hints at a fairly potent mid/upper level jet max
moving through at the same time as the front while the EC/CMC are
less aggressive. Either way, a period of stronger wind gusts is
possible with the passage of the front itself and potentially with
any shower activity associated with the front, especially across SW
MT. Given the setup, the potential exists for wind gusts of 50-60+
mph across the higher terrain of SW MT as the front moves through.

In the wake of Tuesday's front, a northerly flow develops bringing a
return to more normal late-winter temps areawide. This northerly
flow will likely be accompanied by enough moisture for some light
rain/snow at times through next weekend. However, with the main
storm track looking to stay well south of the area, it appears
rain/snow amounts will remain on the lighter side through the
extended period. Although, the MT/ID border area will be closest to
the storm track/better lift, and snowfall amounts will likely be
greatest there.
Martin/Cassell

&&

.AVIATION...
Updated 0535Z.

VFR conditions are generally expected over the next 24 hours.
However, the moist southwest flow aloft will continue mountain top
obscuring precipitation. A disturbance in the flow will bring areas
of rain/snow with possibly brief MVFR conditions to the Southwest
Montana terminals (KEKS KBZN KHLN) between through 21Z. This
disturbance will also bring increasing mid/high clouds to the plains
of North Central Montana. A few showers may also move out onto the
plains after 10Z, mainly along and east of a KGTF-KHVR line, as the
disturbance moves over the frontal boundary in place over the area.
MVFR ceilings are unlikely with these showers, but cooler air in
place in the KHVR area may allow light freezing drizzle to briefly
mix with rain/snow between 14Z and 18Z. Initially, was not concerned
about fog, but with fog being reported at CYQL just over the border
from KCTB, cannot rule out brief MVFR level fog until 09Z, after
which, easterly winds should shift more westerly and downsloping.
Coulston

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  51  32  52  38 /  20  10  30  20
CTB  41  27  44  31 /  10  10  10  20
HLN  44  23  44  29 /  30  30  20  30
BZN  44  23  44  32 /  40  40  30  30
WEY  33  19  35  30 /  90  70  70  70
DLN  41  26  43  35 /  40  30  30  40
HVR  49  30  51  33 /  20  10  10  10
LWT  52  29  52  36 /  20  20  20  20

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until 5 PM MST this afternoon
Beaverhead...Gallatin...Madison.

&&

weather.gov/greatfalls