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000
FXUS65 KTFX 021158
AFDTFX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREAT FALLS MT
500 AM MST Mon Dec 2 2013

UPDATED AVIATION DISCUSSION

.DISCUSSION...

Today through Tuesday Night...Moisture ahead of an approaching
Pacific weather system will continue to bring snow, heavy at
times, to the mountains of western and central Montana through
tonight. Snow there will decrease Tuesday into Tuesday night as
the system exits the area. This system will drag a strong cold
front south from the Canadian border (where it is currently)
through north central Montana by early afternoon, then through
southwest Montana by early evening. Gusty westerly winds will
shift northerly behind the front, bringing winds of 15 to 25 mph
with gusts as high as 40 mph to the area. Strong lift and
instability will initially be associated with the frontal
passage. Then, more sustained (though a bit weaker) lift will move
into the area overnight into Tuesday due to deepening of the
northerly upslope flow as the system begins moving east of the
area. These features will cause snow to be heavy at times, causing
reduced visibility in blowing snow and adding snowpack onto icy
roads (due to rapidly cooling temperatures). Am therefore
continuing the Winter Storm Warnings for all elevations of north
central Montana and elevations above 6000 feet in southwest
Montana, with Advisories for lower elevations there (for lower
expected snowfall amounts). Mountains will generally receive an
additional 8 to 16 inches. The plains of north central Montana will
likely see 5 to 9 inches, while the valleys of southwest Montana
will receive 2 to 4 inches. Temperatures will also rapidly fall
behind the cold front today and tonight, bringing single digit
highs for Tuesday and lows possibly as low as 10 below zero for
Tuesday night. Overall, though, winds and snow should decrease
Tuesday afternoon through Tuesday night.

With regards to the High Wind Warnings for portions of north
central and southwest Montana, strong winds have not developed
there as previously expected, so have cancelled those warnings.
Coulston

Wednesday through Monday...From Wednesday at least through the
weekend a strong upper ridge will be in the eastern Pacific which
will keep a cold northwest to north flow aloft over Montana.
Moisture associated with an upper trough over eastern Montana
should lead to isolated to scattered snow showers on Wednesday.
Otherwise expect a mostly dry period with the main threat for snow
over the mountains. Models start to bring some snow to the area by
late in the weekend or early next week but differ on the details.
Added a mention of snow in the forecast for the lower elevations
during this period. The GEM and GFS models bring another surge of
cold air into the area late in the weekend or early next week with
the GFS being quite a bit colder than the GEM. On the other hand
the ECMWF does not show this next cold air invasion. Took a
middle-of-the road approach to temperatures with these different
solutions. Blank

&&

.AVIATION...
UPDATED 1200Z.
A Canadian cold front had passed through KCTB and KHVR as of 12z and
will reach KGTF and KLWT between 15z and 18z, KHLN between 17z and
20z, and KBZN between 21z and 00z. Confidence is on the low side
with the timing of the front hence the rather wide window with
frontal timing mentioned above. Confidence is low with details of
ceilings and visibilities and timing of precipitation today and
tonight but there should be a general downward trend to ceilings and
visibilities. For the plains do feel confident that widespread IFR
ceilings and visibilities with areas of LIFR ceilings and
visibilities will have developed by this evening. Of course the
adjoining mountains and passes will be obscured in snow. Although
not mentioned in the taf for KHVR there will be a small threat for
brief freezing rain until about mid-morning over north central
Montana. For the southwest Montana valleys expect a period of
precipitation as the cold front moves through the area later this
morning through this afternoon. Precipitation will likely be rain
ahead of the front but should turn to snow behind the front.
Precipitation in the valleys might not last more than a few hours.
For the adjoining mountains expect general obscurement with
occasional pass obscurement in widespread snow. Blank

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  39   2   2  -8 /  60 100  90  50
CTB  29  -4   0 -11 / 100 100  70  40
HLN  43   4   6  -7 /  60 100  50  30
BZN  45   3   8  -9 /  80  70  50  30
WEY  35  -7   5 -20 / 100 100  40  50
DLN  45   5   9  -8 /  80  70  40  20
HVR  29   6   6 -11 /  90 100  90  60
LWT  39   1   4 -11 /  60 100 100  70

&&

.TFX WATCHES/WARNINGS/ADVISORIES...

WINTER STORM WARNING until 11 PM MST Tuesday Blaine...Cascade...
Central and Southern Lewis and Clark...Chouteau...Eastern
Glacier...Eastern Pondera...Eastern Teton...Fergus...Hill...
Judith Basin...Liberty...Meagher...Northern Rocky Mountain
Front...Southern Rocky Mountain Front...Toole.

WINTER STORM WARNING until 11 PM MST Tuesday FOR ELEVATIONS
ABOVE 6000 FEET FOR Beaverhead...Broadwater...Gallatin...
Jefferson...Madison.

WINTER WEATHER ADVISORY from 11 AM this morning to 11 PM MST
Tuesday FOR ELEVATIONS BELOW 6000 FEET FOR Beaverhead...
Broadwater...Gallatin...Jefferson...Madison.

&&

$$


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