Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 220602
AFDTFX

Area Forecast Discussion
National Weather Service Great Falls MT
1101 PM MST Thu Feb 21 2019

AVIATION SECTION UPDATED...

.UPDATE...

Main update this evening was to lower lows quite a bit across most
of the area. The breezy westerly winds will take a while to spread
east from the Rocky Mountain Front. As a result, the clear skies,
light winds, and widespread snow cover will allow temperatures to
fall into the teens below zero for most lower elevations; some
higher valleys (Lincoln, Wisdom, West Yellowstone, etc.) will
likely fall into the 20s below zero. Winds over the plains will
approach 10 mph, but they should stay below it; as a result, wind
chill temperatures will mostly fall into the 20s below zero, but
it will be more so due to the temperatures themselves, rather than
the wind influence. Will therefore hold off on any wind chill
highlights for the time being. Cooler temperatures will also help
cause patchy fog to form across much of the area, so have
increased fog coverage overnight as well.  Coulston

&&

.SYNOPSIS...

Clear skies tonight and light winds will allow temperatures to
fall below zero across most of the region, with the coldest
temperatures in the valleys of Southwest Montana. Temperatures
moderate on Friday, but with increasing southwest winds and areas
of drifting snow along the Rocky Mountain Front. A new cold front
moves through the region Saturday night, with a prolonged period
of snow and colder temperatures for central and southwest Montana.


&&

.AVIATION...
Updated 1100 pm MST Thu Feb 21 2019

VFR conditions are expected for most of the forecast area for much
off the TAF period. Although, some patchy fog may form over some of
the eastern plains (KHVR) and in some southwest valleys (KBZN KEKS),
but the terminal most likely to have periods of MVFR conditions will
be KBZN between 09Z and 16Z. Otherwise, an increasing westerly flow
aloft on Friday will cause southwest winds and mid level cloudiness
to increase along the Rocky Mountain Front and adjacent plains
(including KCTB and KGTF). This may cause snow to drift onto some
runways there. Clouds will increase ahead of the next disturbance
diving southeastward out of British Columbia/Alberta especially
after 6Z but any snowfall should hold off until after 12Z with the
exception of areas immediately along the Northern Rocky Mountain
Front where snow showers along with mountain obscuration may occur
primarily after 6Z. rcg/Coulston

&&

.HYDROLOGY...

Emergency management reported flooding from an ice jam on the
Madison River near Ennis. Ice from the river has already caused
minor property damage to a fence line adjacent to U.S. Highway
287. A flood warning has been issued through Friday afternoon.

Ice jamming on the Missouri River south of the City of Cascade has
caused minor flooding to develop across portions of Southwest
Cascade County. This minor flooding has led to the closure of Sheep
Creek Road. With continued cold temperatures in the forecast, a
flood advisory is in effect for the this region until further
notice.

On the Jefferson River between Twin Bridges and Silver Star, an ice
jam is possible. Weather conditions still remain prime for new ice
jams to form in this area. As a result a flood watch is in effect
for this area until further notice.

Anyone near rivers should be on the look out and report any new
ice jams.

&&

.PREV DISCUSSION...
/ISSUED 405 PM MST Thu Feb 21 2019/

Skies have cleared as high pressure has settled across the
forecast area. Temperatures will drop below zero for most areas
with light winds. The coldest temperatures are expected below -20F
for the southwest valleys. Pressure falls are already noted over
the Northern Rockies. Southwest winds will develop by early Friday
along the Rocky Mountain Front as the surface pressure gradient
strengthens. Wind gusts to 40 mph could cause localize reductions
in visibility due to blowing snow, while drifting snow could
drift across roadways in some areas.

A strong westerly flow aloft develops on Saturday as a cold front
crosses the area. This will begin a prolonged period with much
colder temperatures and snow. -PN

With respect to temperatures Saturday night through Tuesday
night...another Arctic front is poised to surge south across North
Central and portions of Southwest Montana Saturday through
Saturday night, before stalling somewhere across Southwest
Montana/Northern Idaho (the location of where this front stalls
will be critical to where the heaviest snow falls Sunday afternoon
through Tuesday morning). ~1040mb Arctic high will then build
south towards Montana Sunday night/Monday morning, which will set
the stage for the coldest temperatures of the period by Monday
morning. At this time, low temperatures Monday morning of the
teens below zero to 20s below zero are expected across all of
North Central Montana, but some locations reaching 30 below zero
are entirely possible if clearing skies occurs. Temperatures
across the region will be well, well below normal over this
timeframe.

Finally with respect to accumulating snow Saturday through Tuesday
morning...snow is expected to develop along/with the Arctic front
Saturday and Saturday night. A surge of Pacific moisture from Sunday
through Tuesday across Southwest Montana combined with increasing
synoptic ascent with the approach of an upper level disturbance and
forcing from the stalled front will set the stage for the potential
of significant snowfall accumulations across portions of
Southwest/Central Montana. At this time, areas south of a Helena to
White Sulphur Springs line have a chance of seeing accumulating snow
from Sunday through Tuesday, with the best chances for significant
snowfall accumulations residing along and south of the I-90
corridor. Where the Arctic front stalls will be critical to where
the heaviest snow falls. - Moldan

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF -10  25  12  22 /   0   0   0  50
CTB -11  23   8  17 /   0   0  10  40
HLN -15  23   5  25 /   0   0   0  60
BZN -18  20   2  26 /   0   0   0  30
WEY -27  16  -5  19 /   0   0  30  80
DLN -17  18   1  23 /   0   0   0  30
HVR -16  16   2  16 /   0   0   0  20
LWT -16  24  10  23 /   0   0   0  20

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from 4 AM Friday to midnight MST Friday
night Eastern Glacier...Northern Rocky Mountain Front...Southern
Rocky Mountain Front.

Flood Watch through Friday afternoon for Jefferson River in Madison
county.

Flood Warning through Friday afternoon for the Madison River near
Ennis in Madison County.

Flood Advisory through Saturday afternoon for the Missouri River in
Cascade County.

&&

$$

http://www.weather.gov/greatfalls



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