Area Forecast Discussion
Issued by NWS Great Falls, MT

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210
FXUS65 KTFX 182152
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
252 PM MST Sat Feb 18 2017

.DISCUSSION...

Tonight through Monday...An elongated upper level trough now
moving across the length of California continues to direct
significant clouds and moisture across the interior western U.S.
Clouds thinned across much of central MT today in the wake of a
minor shortwave disturbance. The next upstream shortwave is
located over western Idaho, poised to move across our region
tonight. This system will bring rain and mountain snow into the
central and southwest mountains. Temperatures remain mild in the
southwest flow. However, light rain re-developing over southwest
MT this evening is expected to change over to snow down to pass
level. Up to 3 inches of new snow may be possible on the primary
southwest passes, so the winter weather advisory continues for
this area. Impacts from snow accumulation are not expected to
cover large areas, but those traveling through the area should be
prepared for variable driving conditions. Areas of mixed
precipitation lift north-northeast into central Montana Sunday
afternoon along with the passage of a weak Pacific front. Short-
term models indicate that weak instability may accompany this
frontal passage. Thus, a few lightning strikes over higher terrain
cannot be ruled out in heavier showers. Most precipitation
dissipates Sunday night. Another Pacific system arrives Monday, as
westerly flow maintains precipitation over the western and
southwest mountains. Temperatures remain above average through
Monday. PN

Monday Night through Saturday...The period starts with yet another
surge of moisture running into the western US in firehose 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
being mainly elevation-dependent. On Tuesday/Wednesday, the models
are struggling with the track of a SFC low that is progged to move
through the area. Consequently, this leads to a lower confidence
forecast regarding chance of precip. 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

&&

.AVIATION...
Updated 1754z.

A warm front noted from near KGPI to KGTF to KLWT will continue to
lift north this afternoon. Moisture/lift along the front will
continue to lead to light showers from KCTB to KHVR/KLWT over the
next few hours until the front clears through the area. However,
significant reductions to VIS/CIG are not expected at any of those
sites this afternoon. Otherwise, moisture moving up from the south
will keep SCT/BKN low/mid level clouds over all sites through the
current TAF cycle. Across SW MT, there may be enough moistening in
the low levels for SHRASN to develop SUN AM and I have introduced
VCSH at KBZN/KEKS for this reason. Lower CIGS/VIS will be possible
at both sites during that time and later forecasts may need to lower
conditions if confidence increases. The only other issue of note is
the potential for BR/FG tonight across central MT, especially KHLN,
KCTB, KHVR. Martin

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  34  53  33  52 /  10  20  20  10
CTB  27  45  28  44 /  10  10  20  10
HLN  28  42  24  44 /  20  30  30  20
BZN  29  44  23  44 /  30  30  30  30
WEY  30  35  19  34 /  80  90  60  70
DLN  32  43  26  44 /  30  30  30  30
HVR  31  51  31  51 /  10  10  10   0
LWT  35  55  30  51 /   0  20  20  10

&&

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

&&

$$

weather.gov/greatfalls



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