Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 212133
AFDTFX

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

.SYNOPSIS...

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.

&&

.DISCUSSION...

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

&&

.AVIATION...
Updated 1735Z.

VFR conditions will prevail for all terminals across North Central
and Southwest Montana through 12z, when-there-after MVFR conditions
may be realized across Southwest Montana due to lowering ceilings
from precipitation. Otherwise, few to scattered mid-level clouds are
expected across the region through the afternoon hours, with
increasing mid-level cloudiness during the evening and overnight
hours as the next disturbance begins to move into the Northern
Rockies. Mountain obscuration is likely across Southwest Montana and
along the Continental Divide this evening and into the morning hours
on Sunday. Westerly winds will be breezy and gusty today and this
evening, but increase in earnest early Sunday morning. LLWS and
mountain wave turbulence is likely over the region, especially
across the North Central and Central Montana terminals of KCTB,
KHVR, KGTF, KLWT, and KHLN. - Moldan

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  45  61  40  57 /  40  10  10   0
CTB  41  58  39  55 /  40  20  10   0
HLN  42  63  43  59 /  20  30  20   0
BZN  40  65  38  58 /  30  20  30  10
WEY  33  44  30  47 /  80  50  60  10
DLN  39  57  35  55 /  30  10  20   0
HVR  40  64  37  58 /  50  20   0   0
LWT  39  59  36  53 /  40  10  10   0

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
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...
Toole.

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

&&

$$

weather.gov/greatfalls



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