Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 240550
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
1040 PM MST Mon Jan 23 2017

Aviation Section Updated.

.UPDATE...

Certainly a changing situation this evening. First, the snow that
was lingering over Central Montana has mostly diminished as of 8
pm, and any additional snow overnight will be light, so we have
dropped the Winter Weather Advisory. While low clouds will
continue over most of the area overnight with the focus of
additional snow over Southwest Montana, clearing skies along the
Rocky Mountain Front and east into the Cut Bank and Conrad areas
have caused temperatures to fall faster than originally expected
and allowed areas of freezing fog to form. In fact, Cut Bank has
been reporting visibility as low as 1/2 mile at times; however,
visibility there has been quite variable, suggesting more patchy
fog. Partial clearing is possible elsewhere across North Central
and Central Montana, so have gradually spread the patchy freezing
fog wording east across the plains overnight. Have also lowered
low temperatures in this clearing area along the Rocky Mountain
Front, as much of the area has already reached previously forecast
overnight lows. Gusty northerly winds in Southwest Montana will
also cause areas of blowing snow overnight, so have kept wording
indicating this in the forecast.
Coulston

&&

.AVIATION...
Updated 0540Z.

A weather system passing through Wyoming is producing light snow as
far north as KEKS/KBZN. MVFR conditions prevail at these terminals
through much of the night, with mountain obscuration across most of
SW Montana. Conditions gradually improve Tuesday. Strong north winds
at KEKS will decrease slightly overnight as the surface pressure
gradient weakens. Fog and low clouds are creating a difficult
terminal forecast for North-central Montana, especially
KCTB/KGTF/KLWT where model guidance has been of little value. Will
have to settle for tempo groups to cover periods of lower cigs and
fog, then gradual improvement after sunrise Tuesday. PN

&&

.PREV DISCUSSION...
/ISSUED 500 PM MST Mon Jan 23 2017/

This afternoon through Wednesday...The upper level low that has
been sitting off the WA/OR coast over the past couple of days has
begun to move south and will eventually open up/weaken, leaving a
broad trough across the west. The trough axis will begin to
progress east over the next couple of days as well which will push
the 160kt upper level jet over the central Rockies S/E as well.

As the upper level low moves ashore, a new mid/upper level low
will begin to develop over southern ID and progress east into the
central Plains by Tuesday. The models have really had a harder
time handling this low over the past few days. That said, it still
appears that as the low deepens through this evening, there will
be a window for steadier snow to develop across far SW MT along
the MT/ID border, possibly aided by some mesoscale banding north
of the low track. However, the quick movement of the low should
limit snowfall amounts in this area. Thinking a general 2-4" can
be expected here, mainly impacting Monida, Raynolds, and Targhee
Passes as well as the West Yellowstone area. If any mesoscale
banding can develop, then there may be a brief window of heavier
snowfall rates. However, confidence remains somewhat lower
regarding this potential. In addition, several inches of snow
shouldn`t cause significant impacts over the passes and for these
reasons, we`ll continue to hold off on any winter weather products
for those areas.

Further north, the warm front that moved through last night/this
morning has pretty much lost its upper level support which is
allowing a moist, northerly low/mid level flow to take over. Short
term guidance and latest radar imagery suggests there will be one
more push of decent moisture across southern sections of central
Montana (ie. from Great Falls to Lewistown) through tonight. Weak
upslope flow should allow light snow to continue in this area
through tonight before drier air begins to work in from the north
on Tuesday. Snowfall rates/amounts should remain light through
tonight due to a lack of better forcing, but given the somewhat
persistent light snow through tonight and in an attempt to avoid
confusion, we`ll keep the Winter Weather Advisory for going for
our central counties through 6am Tuesday. However, later shifts
may need to cancel the Advisory early if the snow ends sooner than
forecast.

For Tuesday/Wednesday, northerly flow will persist across the
region. With some lingering low/mid level moisture, north-facing
slopes may continue to see some light snow during this time, with
generally drier conditions for the valleys/plains. Temperatures
will remain a bit below normal through Wednesday. Martin

Wednesday night through Monday...Overall a generally quiet
weather pattern...with a slight warming trend...is expected for
the extended forecast. Northwest flow and a broad scale trough
will be found Wednesday night into Thursday across the region.
This combined with lingering moisture could bring a few flurries
to light snow showers across the region. Best chances look to be
across the southwest...however...anywhere that receives snow will
see light accumulations at best. A large ridge then builds into
the region Thursday night...diminishing chances for any snow. This
mentioned ridge then takes over for Friday through Saturday. Dry
air under this ridge will provide for mostly sunny skies and dry
conditions. Ridge starts to flatten on Sunday...possibly bringing
some breezy winds at times to the region...but continued dry
conditons. A weak disturbance and cold front look to push through
on Monday...however...moisture appears limited with this
feature...and mainly dry conditions...with some gusty winds are
expected at this time. And even though an isolated snow shower
could be possible...left out mention for now. Temperatures are
expected to start off below normal on Thursday into Friday. The
mentioned ridge then begins a warming trend on Saturday...and
continues through Monday...with highs in the 30s to low 40s
possible for the lower elevations. The one hindering factor for
the warming trend will be found across the valleys as light flow
under the ridge could bring back an inversion set up. This would
mainly affect the southwest and the Helena Valley for Friday and
Saturday. Anglin

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  10  23  11  28 /  20  10  10  10
CTB   0  23  10  29 /  10  10  10   0
HLN   7  22   5  22 /  40  30  20  10
BZN   4  20   1  20 /  50  30  20  20
WEY   5  19  -7  15 /  70  50  20  30
DLN   3  19   5  22 /  50  30  20  10
HVR  10  21   7  24 /  10  20  20  10
LWT  10  20   8  26 /  70  20  20  10

&&

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

$$

weather.gov/greatfalls


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