Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 220158

Area Forecast Discussion
National Weather Service Great Falls MT
758 PM MDT Sat Oct 21 2017


Only change to current forecast was to lower tonight`s lows about
4 to 7 degrees. Winds have decreased significantly before clouds
have moved into the area, allowing temperatures to fall below
previously forecasted lows. This brings most of the lows over the
plains into the mid to upper 30s. However, am expecting the winds
to increase after midnight as clouds increase, which should help
stop the decrease in temperatures and allow them to warm back up
(into the 40s over the plains). Otherwise, the overall forecast
trend seems to be on track, with increasing precipitation over the
mountains and a chance of showers over the plains ahead of a
Pacific cold front. Also, strong lower- and mid-level winds are
still on track to bring windy conditions to much of the forecast
area on Sunday.



A fast moving Pacific cool front will move through the region on
Sunday. As a result, expect strong winds to develop over much of
the region later tonight and then continue to be strong through
Sunday. Additionally rain and snow will develop tonight over the
Rocky Mountain Front and across the mountains of Southwest MT,
which will continue into Sunday as well. Drier and slightly
warmer air moves into the region for Monday and Tuesday.


Updated 2355Z.

VFR conditions will persist east of the Rockies at least through
the day on Sunday, but mountain obscuration and occasional MVFR
conditions are likely in the western and southwest mountain
areas. A disturbance in the strong and moist westerly flow aloft
will bring rain and mountain snow showers to the mountain areas
between 03Z and 21Z. Only a few showers will move out onto the
plains between 03Z and 12Z ahead of a Pacific cold front, but
strong westerly downslope winds will develop from west to east
behind the front between 09Z and 15Z and clear out the skies.
Winds will be slower to mix down into the valley terminal areas.
Wind gusts of 45 to 65 kt will be common over the plains and at
ridge top level after 09Z, while gusts of 30 to 40 kt will
develop in the southwest valleys after 14Z.


/ISSUED 555 PM MDT Sat Oct 21 2017/

Tonight through Monday...main forecast concerns in the short term
is 1) accumulating snow tonight/early Sunday morning along the
Continental Divide and over Southwestern Montana, and 2) the high
wind event Sunday.

A fast moving shortwave ahead of a main H500 speed max, will
quickly move across the Northern Rockies this evening. This
shortwave will be the triggering mechanism for precipitation
first along the Continental Divide and over Southwest Montana,
before chances overspread the plains of North Central Montana
during the later evening/early morning hours tonight/tomorrow.
Snow levels will increase immediately east of the Continental
Divide throughout the night as the low and mid-levels warm due to
strong downslope warming, which will keep precipitation in the
form of rain generally below 6,000 ft. The nose of the impressive
H500 jet will then reach the Continental Divide shortly after
midnight tonight, which will provide additional synoptic and
orographic lift across this region. Snow could become moderate to
heavy at times for elevations above 6000 feet, with snowfall
totals of several inches expected near pass level (Marias Pass),
to as high as 4 to 8 inches atop ridgetops by Sunday morning.
While driving impacts due to the falling snow across the Rocky
Mountain Front will be marginal, am concerned about the impacts
the combination of the snow and wind will have on hunters in
wilderness areas due to this being the opening weekend of deer
and elk season. For this reason, have issued an SPS talking about
the accumulating snow and expected whiteout conditions due to the
strong winds.

By Sunday morning, focus then turns to the high wind event (which
will affect most of the CWA through the evening hours of Sunday),
as cross barrier flow becomes very strong. Expect widespread wind
gusts of 75 mph along the Rocky Mountain Front, with isolated
gusts up to 85 mph being possible at ridgetops. Across the
remainder of North Central and into Central Montana, widespread
wind gusts to 65 mph appear likely, with some isolated 75 mph
wind gusts being possible. These strong winds will make travel
very difficult on north and south orientated roads through Sunday
evening, and hazardous for high profile vehicles. Hunters and
those recreating outdoors will need to be aware of and prepared
for the possibility of falling trees, especially if they are near
recent burn scars. - Moldan

Monday night through Friday...The main concerns during this
period are a possible wind event for portions of Central Montana
WED, then the potential of sharply colder temps and accumulating
snow WED night/Thursday.

The period starts with an anomalously strong ridge (for this time
of year) building into the western US. Under mostly sunny skies
and, for some, a downsloping SW wind, this should allow temps to
warm into the 60s/70s at lower elevations TUE/WED (although,
increasing clouds may tend to keep it not as mild along the Hi-
line by WED). The breezy/mild conditions may also lead to
elevated fire concerns.

Those increasing clouds will signal what is expected to be a
potent shortwave moving over the ridge and crashing into the
Northern Rockies/Northern High Plains late Wednesday/Thursday.
This wave has origins over the Bering Sea and it should be noted
that the models continue to struggle this far out with its
track/evolution and associated impacts. The forecast, then, puts
a bit more weight on the previous forecast to avoid drastic
changes one way or the other. Ahead of the shortwave, another
wind event will be possible, especially portions of Central MT.
This is then followed by a strong cold front. It should be noted
that the potential exists for a fairly drastic temp change from
WED to THU in the wake of the front (possibly a 20-30 degree
drop). This system will also be accompanied by at least some
rain/snow, but how much and how impactful the snow is will very
much depend on how quickly the system moves through. MARTIN


GTF  39  61  40  57 /  40  10  10   0
CTB  35  58  39  55 /  40  20  10   0
HLN  38  63  43  59 /  20  30  20   0
BZN  36  65  38  58 /  30  20  30  10
WEY  25  44  30  47 /  80  50  60  10
DLN  35  57  35  55 /  30  10  20   0
HVR  36  64  37  58 /  50  20   0   0
LWT  35  59  36  53 /  40  10  10   0


High Wind Warning from 6 AM to 10 PM MDT Sunday Blaine...
Broadwater...Cascade...Central and Southern Lewis and Clark...
Chouteau...Eastern Glacier...Eastern Pondera...Eastern Teton...
Fergus...Hill...Jefferson...Judith Basin...Liberty...Meagher...

High Wind Warning from midnight tonight to 10 PM MDT Sunday
Northern Rocky Mountain Front...Southern Rocky Mountain Front.


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