Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 200522
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
1020 PM MST Thu Jan 19 2017

UPDATED AVIATION SECTION

.UPDATE...

Most of tonight`s forecast appears valid. Adjusted overnight lows
upward by several degrees for Southwest Montana, where temperatures
have been cooling more slowly than forecast. Expect this trend to
continue with abundant cloud cover tonight. Also increased southerly
winds in the Madison River Valley, in and near Ennis, as the MSLP
field looks to remain favorable for breezy winds there. Scattered
and primarily light snow and/or rain showers remain possible
tonight, especially over the eastern half of our Central and
Southwest Montana counties. This is due to deep lift/sufficient
moisture within the right entrance region of an upper jet streak
propagating eastward over the area and ahead of a mid- to upper-
level shortwave trough approaching from the west. However, liquid
precipitation amounts should be less than 0.10" and any fresh
snowfall should generally be an inch or less.
Jaszka

&&

.AVIATION...
Updated 0520Z.

VFR conditions are generally expected at the terminals through at
least the next 24 hours. A moist southerly flow aloft will continue
mainly mountain top obscuring precipitation (mostly snow) through
14Z, while the terminals will have mostly mid and high level
cloudiness. Clouds and precipitation will then decrease after 14Z as
drier air moves into the forecast area. Patchy fog may form in some
of the southwest valleys, but it will likely not directly impact the
terminals there. Breezy southwest winds over the plains will also
decrease throughout the period.
Coulston

&&

.PREV DISCUSSION...
/ISSUED 450 PM MST Thu Jan 19 2017/

This afternoon through Saturday... A more active pattern
continues to try to establish itself across the central and
southwestern MT region. The primary longwave trough persists over
the West Coast ushering southwest flow across the area. A series
of disturbances within this SW flow will continue to push
northeast across the area through tonight, which will result in
some snow (possibly mixed with rain early) mainly across the
higher elevations of southwest and central MT, with little if any
accumulations, while the north-central Plains will remain dry
with residual downslope winds still in play. A stronger
disturbance pushes through during the day Friday that will help
result in at least partial clearing, at least briefly, and bring
drier conditions to most areas late Friday through Friday night,
especially over the north- central Plains. Weak upper flow with
weak surface ridging Saturday morning gives way to slowly
increasing westerly flow by later in the day, with mid- and high-
level clouds and possibly some flurries developing again mainly
toward the western mountains. Cassell

Saturday Night through Thursday...At the start of the long term
period, the models continue to show the jet stream shifting well
south of Montana and remaining fairly active across the southern US.
Locally, this would put the Pac NW/northern High Plains beneath a
broad trough with somewhat quieter weather and seasonally cool
temperatures. By mid to late next week, there is good agreement in
the models that a very amplified upper level ridge will build into
western North America with a N/NW flow regime setting up across
Montana. While the models agree on the large-scale pattern, as usual
there are differences regarding various shortwaves that will move
through the area.

Initially within the broad trough, a large upper level low will
likely get cutoff from the flow over the eastern Pacific (just off
the coast of WA/OR) over the weekend. Within the SW flow ahead of
the low, several shortwaves will likely eject quickly NE through the
northern Rockies through early next week before the flow becomes
northerly. However, as the shortwaves approach Montana, the models
suggest they will weaken/get sheared off. This would favor SW MT for
precip with a lowering chance as you go north. Once the northerly
flow commences, embedded shortwaves will then drop south out of
Canada, but for now those look fairly weak/moisture-starved and for
now I don`t expect much precip out of those.

As for sensible weather, the models indicate a stronger s/w may lift
NE through the area Sunday night through Monday night with good
moisture and sightly better large scale lift. This may actually be
the best chance for a bit more widespread coverage of snow across
the CWA before drier weather returns. Of note, as the low level flow
turns northerly, there may be a bit of an upslope component to the
precip by Monday, possibly enhancing snowfall amounts a bit across
parts of central Montana. However, the models don`t all agree on
this idea and I kept QPF/snowfall amounts on the lower side until
the models show better agreement. Based on this, then, it appears
accumulations will mostly fall into the light category for most
areas SUN/MON, with slightly higher amounts likely across far SW MT
where lift will be maximized. If there ends up being more of an
upslope enhancement Monday, QPF/snowfall amounts may need to be
adjusted upwards some in later forecasts. As mentioned above,
temperatures will slowly drop through early next week with MON/TUE
likely the coldest days of the period. A gradual moderating trend
may then ensue by late-week as the ridge builds back in. Martin

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  28  37  19  32 /  20  10  10  10
CTB  24  37  15  32 /  10   0  10  10
HLN  23  28   9  25 /  30  20  10  10
BZN  22  30   7  28 /  30  30  10  10
WEY  14  23   3  24 /  50  30  20  20
DLN  17  31  11  30 /  20  20  10  20
HVR  28  37  20  31 /  10  10  10   0
LWT  29  37  17  33 /  40  20  10   0

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
None.
&&

$$

weather.gov/greatfalls



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