Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 190527
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
1027 PM MST Sat Feb 18 2017

.UPDATE...

Satellite imagery this evening shows a splitting upper level
trough progressing into the interior Western US with main energy
approaching the 4-corners region while additional shortwave and
upper jet energy is moving in quickly behind the initial trough
and onto the OR coast with yet another upper level system upstream
around 150W. Diffuse shortwave energy associated with the
northern branch of the splitting trough will continue to stream NE
across the Northern Rockies and MT tonight with precipitation
mainly confined to areas along and west of the Continental Divide.
Snow levels will remain around 6500 ft through the rest of
tonight with accumulating snow mainly impacting areas near the
ID/WY border where a winter wx advisory remains in effect. Warmed
min temps tonight by several degrees across central MT where
combination of cloud-cover, light south to southeast low level
winds and mild SW flow aloft is keeping temperatures from falling
much this evening.



&&

.AVIATION...
Updated 0030z.

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 06Z and 21Z. This
disturbance will also bring increasing mid/high clouds to the plains
of North Central Montana after 06Z. A few showers may also move out
onto the plains after 12Z as the disturbance moves over the frontal
boundary in place over the area, but ceilings lowering to MVFR
levels is unlikely. Have also removed mention of fog from terminal
forecasts due to the increase of cloudiness.
Coulston

&&

.PREV DISCUSSION...
/ISSUED 530 PM MST Sat Feb 18 2017/

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

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  36  53  33  52 /  10  20  20  10
CTB  27  45  28  44 /  10  10  20  10
HLN  33  42  24  44 /  20  30  30  20
BZN  33  44  23  44 /  20  30  30  30
WEY  30  35  19  34 /  90  90  60  70
DLN  34  43  26  44 /  30  30  30  30
HVR  31  51  31  51 /   0  20  10   0
LWT  37  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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