Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 230534
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
1033 PM MST Sun Jan 22 2017

UPDATED AVIATION

.UPDATE...

845 pm...Freezing fog has become quite dense in spots this evening
across North Central Montana, especially along the north and east
facing mountain slopes due to the northeast winds. Light snow has
already started to move into this area as well. Have therefore
increased the chance of snow and issued a Freezing Fog Advisory
until 11 am Monday. The combination of potentially icy roadways and
bridges and light snow accumulation will make the Monday morning
commute quite a bit more difficult. Have also added mention of
patchy freezing fog to the valleys of Southwest Montana with the
increasing potential for light snow. Remainder of the forecast
appears to be on track.  Coulston

455 pm...Quick update to add fog to all of North Central and
Central Montana through Monday morning.
Coulston

&&

.AVIATION...
Updated 0533Z.

Pesky low clouds and occasional freezing fog are expected to persist
through at least 14Z/Mon for much of North-Central MT, including
KCTB/KHVR/KGTF. Expect IFR, occasionally worse, in these areas.
Light snow should be fairly persistent in North-Central MT throughout
the TAF period. For Southwest MT, light snow should be more
intermittent, with the best potential for snow after 18Z/Mon. Expect
at least MVFR/IFR CIGS/VSBY in any snow. Mountains will tend to be
obscured during the period.
Jaszka

&&

.PREV DISCUSSION...
/ISSUED 432 PM MST Sun Jan 22 2017/

This afternoon through Tuesday...Water vapor imagery pretty much
tells this story this afternoon. A well defined, deep upper level
low continues to spin just off the WA/OR coast at this time. On the
south side of the low, a strong upper level jet continues to run
right into the state of California. On the leading edge of this jet,
a mid-level warm front is steadily progressing NE through the Pac NW
at this time. Broad, diffluent flow aloft combined with warm air
advection along the warm front and large scale ascent associated
with the jet appears to be driving the precip across the region at
this time.

At the SFC, high pressure has been nosing south into central Montana
and has been associated with a very shallow, moist layer near the
SFC. This has led to a persistent stratus deck from Cut Bank to
Havre. It has shown very little movement this afternoon and will
likely remain in place the rest of this afternoon, producing patchy
freezing fog and reduced visibility at times. The 12Z TFX sounding
shows quite a bit of dry air in the mid/upper levels at this time.
However, continued moisture advection should continue to moisten the
column areawide which, combined with the large scale ascent
should allow light snow to break out CWA wide by tonight as the
aforementioned warm front lifts NE through the area.

Timing-wise, radar and webcams suggest snow is already beginning to
reach the SFC across SW MT at and above pass level as of 2pm. The
snow will take a bit longer to reach the valley floors, though,
due to the drier air at the SFC. Here, this will likely limit the
coverage of snow through tonight. From Great Falls to Lewistown,
the snow should arrive this evening, then reach the Hi-line by
Midnight. The warm front will stall over central Montana on
Monday, then more or less return south as a cold front later
Monday into Tuesday. This will allow a moist, northerly flow to
keep light snow going across much of the area through Monday
night. The snow should then begin to taper off from north to south
on Tuesday as drier air filters in from the north.

Regarding snowfall amounts, I expect a general 1-3" across much of
the area, although for reasons mentioned above, the SW valleys may
only see an inch or less. The greatest snowfall totals are expected
across far SW MT along the MT/ID border where 4-8" is expected. Of
note, there still exists the possibility of some mesoscale banding
of the snow in this area on Monday, but the models continue to
differ on the handling of the mid-level low as it tracks through
southern ID and into Wyoming. Interestingly, most models have
trended further south with the low track, but the Canadian insists
on a more northerly track. For this reason, confidence in heavier
snowfall rates/totals in this area remains low. Given the
confidence issues, we`ll continue to hold off on any winter
weather highlights for now in this area until the models can come
into better agreement. Regardless, travel impacts are likely over
Monida and Raynolds Passes, but how significant remains to be
determined. Elsewhere, lighter snowfall amounts/rates should keep
impacts at a minimum. That said, coming after a relatively milder
stretch, travel will certainly be impacted with the fresh coating
of snow. One last note. Lingering low level moisture later Monday
or Tuesday combined with drying aloft may actually allow some
freezing drizzle to develop, mainly south of a Great Falls to
Lewistown line. We`ll continue to monitor this possibility, but
for now opted to leave out of the forecast as not all models show
this. Martin

Tuesday night through Sunday...Northerly flow aloft will persist
across the region through late this week with weak embedded
shortwave energy bringing a slight chance for snow showers Tuesday
night through Wednesday. Temperatures begin on the cool side,
slightly below seasonal averages, but gradually warm late this week
into next weekend as an upper level ridge amplifying along the west
coast gradually shifts inland. Warming aloft will initially be
realized across the plains of N-central MT first as a lee-side
surface trough of low pressure brings breezy W/SW winds late this
week into the weekend. Surface high pressure situated over the
interior western US late this week will lead to temperature
inversions in SW MT valleys where warming will be subdued with some
air quality impacts also possible. By late next weekend, models
indicate the upper ridge will flatten as energy moves into western
Canada, resulting in strengthening winds and continued mild
temperatures across N-central MT. Hoenisch

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  15  27  13  25 /  80  80  50  20
CTB  14  25  11  24 /  80  80  40  20
HLN  11  22   6  19 /  70  20  30  20
BZN  13  25   5  19 /  50  30  50  30
WEY  14  24   0  18 /  80  70  70  20
DLN  14  30  10  24 /  60  30  30  20
HVR  16  27  13  23 /  60  60  40  20
LWT  15  25  13  21 /  70  70  60  30

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
Freezing Fog Advisory until 11 AM MST Monday Cascade...
Chouteau...Eastern Glacier...Eastern Pondera...Eastern Teton...
Fergus...Judith Basin...Liberty...Northern Rocky Mountain
Front...Southern Rocky Mountain Front...Toole.

&&

$$

weather.gov/greatfalls



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