Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 171736
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
1036 AM MST Fri Nov 17 2017

...Updated Aviation Discussion...

.UPDATE...

There are three areas of interest for this update. Temperatures
are cooling aloft with a passing trough axis, producing weak
convective instability. First, snow showers will continue through
this afternoon over the Northern Rockies. The Winter Weather
Advisory remains in effect mainly for travel over Marias Pass,
where MTDOT cams indicate snow covered roads from Two Medicine to
Essex. Second, an area of radar reflectivity between Fort Benton
and Havre suggests light snow developing along a stationary
surface front. There is no evidence that this precipitation is
reaching the ground. However, With surface temperature remaining
below freezing, light snow or freezing rain cannot be ruled out,
and has been added to the forecast for the rest of this morning.
Lastly, the winter weather advisory for southwest Montana has
been cancelled. Current observations reveal little in the way of
snowfall. Scattered snow showers are expected this afternoon, but
will move quickly and produce little new snowfall. The rest of the
forecast remains on track with increasing winds and declining
precipitation overnight. PN

&&

.SYNOPSIS...

An upper level trough of low pressure will shift east across the
Northern Rockies and Montana today for cool and unsettled
conditions. Snow showers will primarily be focused over the areas
near the continental divide as well as the mountains of southwest
Montana. Drier conditions move into the region this weekend with
temperatures near seasonal averages through Saturday, then warming
somewhat with increasing winds Sunday.

&&

.AVIATION...
Updated 1736Z.

Cold, moist and somewhat unstable westerly flow aloft continues over
the region through this afternoon before drier and more stable
conditions move into the region tonight. Bands of snow will continue
to affect areas near the Continental Divide and over southwest MT
today with periods of MVFR/IFR conditions and mountain obscurations.
Likewise, a stalled weak frontal boundary is situated over north-
central MT roughly between KGTF and KHVR to near KLWT, bringing
predominantly VFR conditions with brief MVFR reductions in snow
showers through late this afternoon. Additionally, breezy west winds
will affect most terminals this afternoon and evening. Decreasing
cloud-cover is expected tonight as the airmass stabilizes. CC

&&

.PREV DISCUSSION...
/ISSUED 430 AM MST Fri Nov 17 2017/

Today through Sunday...Upper trough axis extending south from
western Canada through the Northern Rockies this morning,
progresses east across the area today, followed by a period of
upper level ridging tonight through Sunday. Cold, moist and somewhat
unstable flow aloft within the trough axis will continue to produce
bands of snow showers across the western and SW MT mountains today,
though models have backed off with the coverage of of snow across SW
MT with large scale forcing having largely shifting SE of the
forecast area. HAve adjusted the winter weather advisory for SW MT
to primarily focus on areas near the ID/WY border where snow showers
will increase some in coverage and intensity later today. Snow
showers will also continue to affect areas along the continental
divide north to the Canadian border with winds also producing
blowing/drifting snow hazards along the Rocky Mtn Front. Incoming
upper level ridge will bring drying and more stable conditions
tonight through Saturday with decreasing cloud-cover and temperatures
remaining close to seasonal averages. Mid level westerly flow
increases some on Saturday for windy conditions across the north-
central MT plains, then increases further by late sunday as the
upper level ridge, de-amplifies. Lee-side trough of low pressure
deepens further Sunday as well, for increasing potential of strong
winds, initially along the Rocky Mtn Front, and possibly extending
east into adjacent western portions of north-central MT by late
Sunday. Hoenisch

Sunday night through Friday...A progressive pattern is still
expected during this period. High pressure ridging aloft looks to
exit the CWA to the east Sunday night and be followed by the passage
of a shortwave trough and Canadian cold front Monday. High pressure
ridging aloft should then become re-established over the region
Tuesday through Wednesday. However, multiple and subtle weather
disturbances may traverse the ridge during this period. This ridge
may then be broken-down by a shortwave trough and Pacific cold
frontal passage on Thanksgiving. Another high pressure ridge aloft
should then build-in from the west on Friday, but model guidance
differs on how quickly this will happen. During this progressive
pattern, atmospheric profiles look to be favorable for periods of
strong, gusty downslope winds. This may especially be the case along
the Rocky Mountain Front and during Sunday night, as well as
Wednesday.

The aforementioned pattern will be conducive to periods of upslope
precipitation, mainly in the form of snow, along the Continental
Divide and the Southwest MT mountains. Elsewhere, the best potential
for precipitation looks to be on Monday, courtesy of the Canadian
cold frontal passage, and on Thanksgiving into Friday, along and
behind the aforementioned Pacific cold front. Overall, high and low
temperatures will tend to be near or above-normal during the period.
However, some slightly below-normal readings are possible Monday and
Friday, courtesy of the previously-mentioned cold frontal passages.
Jaszka

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  41  29  41  30 /  10   0   0   0
CTB  38  25  38  25 /  20   0   0   0
HLN  38  24  40  20 /  10  10  10   0
BZN  35  16  35  15 /  50  10   0   0
WEY  31   9  27   7 /  80  20  10  10
DLN  35  15  33  13 /  20   0   0   0
HVR  37  20  39  23 /  20  10   0   0
LWT  38  24  39  25 /  20   0   0   0

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until 3 PM MST this afternoon above 4500
feet for Northern Rocky Mountain Front.

&&

$$

http://www.weather.gov/greatfalls



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