Area Forecast Discussion
Issued by NWS Great Falls, MT

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273
FXUS65 KTFX 240550
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
1050 PM MST Thu Feb 23 2017

Aviation Section Updated

.UPDATE...

Only update this evening was to increase PoPs to categorical from
the Great Falls area north-northeast to between the Sweetgrass
Hills and Havre to better reflect current radar returns and
observations. This area of enhanced snow shower activity has not
produced significant accumulation so far, but an additional 0.5 to
1 inch over the plains with 1 to 1.5 inches in the mountains are
not out of the question before forecast models indicate the
showers should decrease toward morning. This additional snow will
likely impact overnight driving, but snowfall will likely taper by
the morning commute, so am not looking to issue any winter weather
highlights at this time. Am not anticipating significant
additional snowfall outside of this area across the rest of North
Central Montana. Additional accumulating snow in Southwest Montana
will likely stay in the mountains. As cloud decks lower overnight
into Friday morning, areas of freezing fog will also reduce
visibility in some areas.
Coulston


&&

.AVIATION...
UPDATED 0550Z.

Regional analysis shows a slow moving area of low pressure over
eastern Idaho (near KIDA), while to the NE, high pressure was
located over southern Canada. North of the SFC low, an inverted
trough stretches north through much of central/SW MT. Within this
trough, areas of -SN have persisted much of the evening from KGTF to
KHVR. This area of -SN appears to be dissipating. In fact, latest
satellite imagery shows some decrease in the cloudcover across
central MT. Given the low level moisture in place there and lighter
winds, some BR/FG development will be possible. Confidence in BR/FG
isn`t high, though, given the lingering cloudcover, so for now I
just went with some decrease in VIS, but trends will be monitored
and TAFs will be amended if FG becomes more likely. Beyond tonight,
the question will be how far north the ID low can get before getting
pushed east by an incoming cold front (slated to arrive FRI
afternoon). The chance of snow with this low appears greatest for
KEKS/KBZN and later forecasts may need to lower CIGS/VIS at those
sites if new model guidance continues to trend further north with
the low. Otherwise, a cold front should sweep NW to SE across the
forecast area Friday afternoon/evening and may be accompanied by a
band of SHSN as it moves through. Martin

&&

.PREV DISCUSSION...
/ISSUED 505 PM MST Thu Feb 23 2017/

Tonight through Saturday...Disturbances to our south will continue
an unsettled weather pattern through much of the short term
period. A developing upper low in WY is currently bringing moist
easterly flow to the region...providing abundant cloud cover and
scattered snow showers through this evening. A sfc low then looks
to develop in southern ID sending an inverted trough across the
region...with more moisture...and scattered snow showers. This
setup lingers through Friday...continuing more periods of snow for
much of the CWA...with the best chances coming across the south.
Some periods of moderate snow will be possible in the
south...however...confidence is again low to moderate for exact
locations and impacts. Will hold off on advisories for now...but
portions of the south will have to be watched. Meanwhile isolated
to scattered snow showers...under cloudy skies...and below normal
conditions are expected for central and northern portions through
Friday. The mentioned sfc low in ID moves across southern MT
Friday night through Saturday...with a weak shortwave moving out
of Canada on Saturday. This will bring more scattered snow showers
to the region...especially across the south. Again light to
moderate snow looks possible with perhaps a few impacts to area
roads. Confidence is still only moderate as recent snowfall hasn`t
brought to many impacts. Advisories again will just need to be
monitored. Temperatures look to be below normal during the short
term. Most areas will see lows in the teens to single
digits...while highs get in the 20s and 30. Areas with lack of
fresh snow may not as cold as originally thought. Anglin

Saturday Night through Thursday...A series of weather systems will
move through Western Canada and the PacNW Sunday into early next
week. Most of the synoptic forcing with these systems skirts around
our forecast area to the west and south. Temperatures remain below
seasonal averages as we remain on the cold side of the mid-latitude
jet. Although moisture is limited through this period, a combination
of weak instability combined with orographic and minor shortwave
forcing will continue to produce widespread light snow showers
across much of the forecast area. Continued light snow accumulation
should be expected in mountain areas. Temperatures begin to moderate
by mid-week as the Pacific ridge axis flattens somewhat and moves
inland. The second half of the week is likely to be windy, with
temperatures near average and periods of snow continuing mainly
along and west of the Continental Divide. PN

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  14  29   9  30 /  80  40  20  20
CTB   7  24   5  26 /  20  20  20  10
HLN  15  29  10  30 /  30  30  30  20
BZN  15  28  10  28 /  30  30  30  40
WEY   6  21   3  19 /  50  50  40  40
DLN  11  24   7  24 /  40  50  30  30
HVR  16  31  12  28 /  80  30  20  20
LWT  16  30  13  28 /  40  30  30  30

&&

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

$$

weather.gov/greatfalls



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