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Area Forecast Discussion
Issued by NWS Great Falls, MT
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FXUS65 KTFX 010545
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
1045 PM MST Tue Jan 31 2017

...Update to Aviation...

.UPDATE...

Snow continues to fall across the southern half of the area at
this time, however recent SFC obs suggest the intensity has come
down some, with mostly light snow being reported as of 9pm. With
the SFC front basically parked over SW MT, light to, at times,
moderate snow is expected to continue through the remainder of the
night. But, again, the intensity should continue to gradually
diminish as drier air works in aloft. Across central Montana,
there appears to be one last band of snow that stretches from
Augusta to Raynesford. Decreasing precip to the north is likely
due to drier air gradually working in aloft. This band of precip
still appears to be primarily driven by low to mid level
frontogenesis and possibly by large scale lift ahead of the s/w
dropping south out of Canada. Models suggest this better lift will
shift south some overnight, potentially aiding in continued light
snow across SW MT, but the drier air moving in may ultimately tend
to limit the coverage of precip with time. Additional light
accumulations of snow are expected along and south of Great Falls,
with the highest amounts across SW MT.

Of note, despite the decreasing intensity of the snow across SW
MT, it is likely that significant impacts will still be felt
across much of the warning area and no adjustments to the warning
are needed at this time. For the areas in the Winter Weather
Advisory, impacts likely won`t be nearly as significant, but still
worthy of keeping the advisory going. Martin

&&

.AVIATION...
Updated 0545Z.

Widespread snow will continue overnight south of a line from KHLN to
KGTF to KLWT with widespread IFR/MVFR conditions and mountain
obscurations. A few bands of light snow along and north of this line
are possible overnight and Wednesday morning, producing mainly brief
MVFR conditions, but VFR conditions are expected to prevail. Snow
over the southern half of the forecast area will gradually diminish
in coverage during the day on Wednesday but low clouds will persist
and areas of light snow could continue to affect forecast terminals
with IFR/MVFR conditions off and on through late Wednesday evening.
mpj

 &&

.PREV DISCUSSION...
/ISSUED 250 PM MST Tue Jan 31 2017/

Tonight through Thursday...Frontal boundary continues to slowly
drift south this afternoon...bringing snow to much of the CWA.
There seems to be a sharp cutoff to the snow along this front.
Current snowfall line is south of a line from Augusta to Great
Falls to Lewistown. Feel this line will slowly progress southward
tonight...with snow diminishing to only flurries or brief light
snow behind it. With abundant moisture and a broad trough it makes
it hard to completely pull pops behind the front...however...made
sure to lower them to low chance or slight chance. Snow is still
occurring in at least parts of each winter highlight...which makes
it difficult to expire ones that are seeing snow on a diminishing
trend. Decided to continue them as is for now...but some may
need to end early by the evening update. By Wednesday morning
most of the snow will be confined to the southern portions of the
CWA. Models indicating a brief lull in snow may then be found
Wednesday afternoon. Then as a large upper level disturbance
approaches the western US...southwest flow lifts this front back
north and brings the next rounds of snow to southern and some
central portions Wednesday night into Thursday. Just how far north
this snow makes it is still uncertain. Most models indicate as
far north as perhaps Helena is the likely solution. NAM however
brings a weak cut off low across the region...which allows snow to
get as far north as the Great Falls region. NAM is the outlier
and tend to side with the general agreement towards the other
models. That being said...left in the slight pops for the further
north solution just in case. Temperatures still look to get well
below normal starting tonight and continuing through much of the
work weak. Below zero temps may be found Wednesday morning...with
even colder temps possible Thursday morning. Felt the model blends
were a bit high so lowered them for Wednesday and Thursday.
Daytime highs also look to be cold...some areas only in the low
teens. Anglin

Thursday night through Tuesday...The timing over which snow exits
southwest MT Thursday night will depend on the pace and track of
a shortwave trough passing through the state. Model solutions
have not reached a consensus on this feature, though the NAM is
notably slower than most of the ensemble guidance provided by the
GFS and ECMWF. Once this wave moves through early Friday, weak
ridging develops briefly over the interior west. Most lower
elevation areas will catch a dry period on Friday, although
chances for snow continue in mountain areas. Moisture increases
again from the west on Saturday as a strong upper low comes
ashore and diffuses across the PacNW amid a strong zonal flow.
Cloudy conditions prevail through the weekend with periods of
mountain snow. Temperatures moderate on Sunday, providing slight
chances for rain at lower elevations. Chances for precipitation
continue into next week, with a trend toward colder temperatures
arriving Tuesday.
PN

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF   3  12  -6  12 /  70  30  10  20
CTB   1  13  -6  12 /  20  20  10  10
HLN   5  14  -5   9 /  70  50  30  40
BZN   5  15  -5  10 /  80  60  40  60
WEY  10  20   4  20 /  90  80  70  80
DLN   8  16   2  13 /  70  70  50  60
HVR   3  17  -3  14 /  10  10  10  10
LWT   4  11  -5  12 /  70  30  10  20

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
Winter Storm Warning until 5 PM MST Thursday Gallatin...Madison.

Winter Weather Advisory until 5 PM MST Wednesday Cascade...
Central and Southern Lewis and Clark...Fergus...Judith Basin...
Southern Rocky Mountain Front.

Winter Storm Warning until 5 PM MST Wednesday Broadwater...
Jefferson...Meagher.

Winter Weather Advisory until 5 PM MST Thursday Beaverhead.

&&

$$

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