Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 210523

Area Forecast Discussion
National Weather Service Great Falls MT
1020 PM MST Fri Jan 20 2017



Freshened overnight lows per latest observations and consensus of
short-term model guidance. With light winds tonight, patchy
freezing fog may develop after midnight where skies remain at
least relatively-clear. This includes the Helena Valley. While
most will be dry tonight, occasional light snow is possible in
far-southern portions of Southwest Montana due to weak
lift/sufficient moisture within the 700-300 mb layer ahead of a
shortwave trough approaching from the southwest. Any fresh snow
accumulation will likely be an inch or less through daybreak.
Skies will range from mainly clear in North-Central Montana to
partly to mostly cloudy farther south.


Updated 0520Z.

VFR conditions are mostly expected across Southwest and North
Central Montana over the next 24 hours, with a few exceptions noted
below. Scattered mid and high clouds will move over Southwest
Montana overnight ahead of an approaching disturbance, as North
Central Montana stays mostly clear. Decreasing winds should allow
patchy fog to develop across much of the area between 10Z and 18Z,
but recent history shows that the best chance for lower
ceilings/visibilities at terminals will be at KHLN, where MVFR
conditions are possible. Scattered snow showers will then start to
overspread Southwest Montana (including KEKS, KBZN, KHLN) after 12Z
and into portions of Central Montana (KGTF) after 00Z, while areas
north and east of KGTF only see increasing high clouds. These
showers will likely obscure mountain tops, with the main threat of
MVFR conditions in far Southwest Montana (KEKS, KBZN).


/ISSUED 415 PM MST Fri Jan 20 2017/

This afternoon through Sunday...The period starts with a closed
upper level low spinning just off the WA/OR coast. South of the low,
water vapor imagery shows a very pronounced 140+ kt jet crashing
into California. This jet will remain south of Montana through
Saturday, but then it will briefly edge north by Sunday as the upper
low opens up/weakens. Through Sunday, a couple of shortwaves
rounding the base of the upper low will move NE through our CWA.

For the rest of today and tonight, mostly dry conditions are
expected with weak ridging aloft and weak high pressure at the SFC.
The exception will be far SW MT along the MT/ID border where
scattered snow showers will remain possible through tonight. With
high pressure at the SFC and light winds tonight, fog will be
possible tonight along the Hi-Line and some of the SW valleys.

The first of the above-mentioned shortwaves moves through on
Saturday. This wave should be weakening as it approaches the area
with limited large-scale lifting expected. For this reason, the best
chance of snow will be across SW MT, with little, if any, precip
further north. Once again, the MT/ID border area will be favored for
the greatest coverage of snow and highest snowfall amounts (albeit,
fairly light).

The next s/w moves through on Sunday and the models suggest this
wave will be stronger with better large scale lift as the jet
tries to nose into the area. Through Sunday afternoon, it appears
the greatest chance of snow will still be across SW MT, coinciding
with the approaching area of better lift. It`s likely, though,
that precip with this wave will continue on beyond Sunday
afternoon and even reach areas further north (see the extended
discussion for more). At this time, significant impacts from snow
are not expected through the short-term period. But, along the
MT/ID border, there will likely be enough snow (and maybe even
some blowing snow) for at least somewhat slower travel, especially
on Sunday and is an area we will keep an eye on for any possible
future winter weather products. Temp-wise, no significant airmass
change is expected, but a gradual cooling trend is likely through
the weekend.

Sunday night through Friday...Medium range models are in reasonable
agreement through most of next week. Period starts with a longwave
upper level trough progressing inland across the western US early in
the week followed by a building upper ridge from the middle through
later portions of next week. Upper level jet and main shortwave
energy associated with the upper trough early next week moves by well
to the south of the region, however some weak shortwave energy does
lift NE across the area on Monday ahead of the trough axis, followed
by additional weak energy embedded within northerly flow behind the
trough axis Monday night through Tuesday night. Best chance for a
period of widespread precipitation will be with the energy ejecting
NE across the region on Monday with southern portions of the
forecast area having the best shot at some light snow accumulations.
Light snow showers will linger Tuesday and Tuesday night in the
northerly flow following the trough axis with drier conditions
moving in Wednesday through the remainder of next week. Temperatures
will cool to near or slightly below seasonal averages for the first
half of next week under the upper level trough. Temperatures
gradually moderate/warm late next week. Upper level ridge with
surface high pressure over the interior western US late next week
may lead to the redevelopment of strong temperature inversions in
the valleys of SW MT, resulting in slower warming and possible
redevelopment of air quality issues in some SW valley areas.


GTF  19  35  21  34 /   0  10  10  10
CTB  16  37  19  31 /   0  10  10  10
HLN  13  25  10  26 /  10  20  20  10
BZN   8  26   8  29 /  10  20  30  10
WEY   4  26   6  23 /  20  40  40  50
DLN  12  35  13  32 /  10  30  20  20
HVR  18  34  18  30 /   0   0  10  10
LWT  20  35  20  34 /   0   0  10  10



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