High Wind Watch 19, NWS Great Falls, MT

Area Forecast Discussion
National Weather Service Great Falls MT
1045 AM MST Thu Nov 12 2020

...Aviation Section Updated...

.UPDATE...

Visible satellite imagery reveals mostly clear skies across the
forecast area this morning, with just patchy high clouds moving
into western Montana and scattered lower clouds capping the
mountain ridges. Temperatures dipped below zero along the HiLine
east of I-15. Overall forecast for a fair weather day remains on
track. Minor adjustments were posted to better match observed
trends, including some higher chances for light snow over the
Northern Rockies, and nudging a few degrees cooler for the
afternoon highs along the HiLine.

&&

.SYNOPSIS...

A high pressure ridge building into the western United States
will keep the region dry today, but increasing southwest winds
will blow over the plains and through north south oriented
valleys, possibly causing areas of blowing snow. Disturbances
will flatten the ridge through Saturday, further increasing
surface winds and bringing a prolonged period of snow to the
mountains, mainly above pass level, with just a chance of snow at
lower elevations. Temperatures will also warm to just below
normal, but the winds will make temperatures fell like they are
in the teens and 20s.

&&

.AVIATION...
1045 AM MST Thu Nov 12 2020 (12/18Z TAF period)

VFR conditions are expected to prevail over the next 24 hours at
most TAF locations; however, low level clouds and light snow over
KWYS will continue to lead to IFR/MVFR conditions through at least
the afternoon hours today. Mid- and upper level cloudiness will then
increase from west to east over the area between 06z-12z Friday as
moist, and strong westerly winds aloft overspread the Northern
Rockies. These strong westerly winds aloft will lead to further
increasing of the surface winds (from the breezy winds that are
observed today) after 12z-15Z Friday. In addition to increasing
surface winds there will also be increasing chances for mountain
wave turbulence and low level wind shear over/at most of the
region/terminals after 00Z Friday. - Moldan

Refer to weather.gov/zlc for more detailed regional aviation weather
and hazard information.

&&

.PREV DISCUSSION...
/ISSUED 430 AM MST Thu Nov 12 2020/

Today through Tonight...Clouds will decrease throughout the day
today as a high pressure ridge builds over the western United
States and puts the area under a more anticyclonic westerly flow
aloft. However, disturbances moving through this flow will flatten
the ridge and increase the winds aloft. The resulting surface low
pressure trough to the lee of the Rockies will combine with high
pressure over the Idaho/Wyoming border to create a surface
pressure gradient favorable for increasing south-southwesterly
winds. These winds should blow through north-south oriented
valleys and out onto the plains. Gusts in the more favored areas
will generally be 30 to 35 mph, but gusts in excess of 50 mph are
expected close to the Rocky Mountain Front. The building ridge and
downsloping winds will help warm temperatures around 10 degrees
over yesterday in most areas, still leaving them about 10 degrees
below normal. The breezy wind pattern will persist overnight in
these favored areas, which will keep temperatures near normal
there, but below normal elsewhere. The flow aloft will also tap
into Pacific moisture and combine with energy in the flow to start
a prolonged period of accumulating mountain snow, mainly above
mountain passes.

Friday through Sunday...An upper level low pressure trough will
then move into the Pacific Northwest on Friday and move across
Montana on Saturday. This will further increase the westerly winds
aloft and continue mainly mountain snow, but there will be a
chance of precipitation at lower elevations Friday night into
Saturday. However, the main impact at lower elevations will be
the increasing winds. Winds from just off the surface to near
ridge top level (850-700 mb) are forecast to be mostly between 35
and 45 kt, which could result in wind gusts at times of 40 to 55
mph over ridge tops and onto the plains. However, this latest
forecast model run has decreased these winds over previous runs.
As a result, confidence has decreased that winds will reach high
wind criteria. Will therefore hold off on high wind highlights for
now and stay with a strongly worded wind forecast. The mountains
will be most impacted by the persistent accumulating snow.
Snowfall totals of 4 to 7 inches are possible at mountain pass
level on the Rocky Mountain Front and in the West Yellowstone
area, but little accumulation is expected below these elevations.
Accumulation in the mountains on the Rocky Mountain Front could
be significant, as the current forecast indicates 1 to 2 feet of
snow are possible on the higher peaks, along with the strong
ridge top winds. Overall, though, temperatures will not cool
significantly with the passage of this system, as the westerly
flow aloft will stay tapped into the more moderate Pacific
airmass, rather than tapping into colder Canadian air. Snow and
wind will then decrease on Sunday, as the system exits the area.

Monday through next Thursday...A broad high pressure ridge will
spread east over the western and central portions of the United
States during this period. The resulting westerly flow aloft will
be not be quite as strong as the flow today through Saturday, but
it will be strong enough to keep breezy westerly downsloping winds
over the plains at times, while limiting the chance for
precipitation to the mountains. These winds should also help warm
temperatures above normal by Tuesday, when most lower elevations
could warm into the lower 50s. However, the trend is for
temperatures to cool slightly Wednesday into next Thursday, as
another low pressure trough approaches from the Pacific Ocean.
Coulston

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  34  23  42  28 /   0   0   0  10
CTB  31  19  38  23 /   0   0  10  30
HLN  35  18  43  28 /   0   0  30  40
BZN  32  12  41  24 /   0   0  30  60
WYS  22   8  30  18 /  10  10  70  90
DLN  29  12  35  21 /   0   0  60  70
HVR  26  19  40  25 /   0   0   0  20
LWT  33  18  42  26 /   0   0   0  10

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
None.
&&

http://www.weather.gov/greatfalls

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE GREAT FALLS MT
1247 PM MST THU NOV 12 2020

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0708 AM     BLIZZARD         3 SW CUT BANK           48.60N 112.38W
11/08/2020                   GLACIER            MT   ASOS

            SUSTAINED WINDS ABOVE 35 MPH WITH GUSTS UP
            TO 50 MPH COMBINED WITH FALLING SNOW TO
            PRODUCE BLIZZARD CONDITIONS FROM 708 AM TO
            1056 AM LST.

&&

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE GREAT FALLS MT
105 PM MST THU NOV 12 2020

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

1109 PM     BLIZZARD         2 SE BROWNING           48.54N 112.98W
11/07/2020                   GLACIER            MT   DEPT OF HIGHWAYS

            SUSTAINED WINDS UP TO 43 MPH WITH GUSTS UP
            TO 53 MPH COMBINED WITH FALLING SNOW TO
            PRODUCE BLIZZARD CONDITIONS FROM 11/7 1109
            PM TO 11/8 239 AM LST.

&&

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE GREAT FALLS MT
127 PM MST THU NOV 12 2020

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0758 AM     BLIZZARD         14 SW SIMMS             47.33N 112.09W
11/08/2020                   LEWIS AND CLARK    MT   MESONET

            SUSTAINED WINDS UP TO 40 MPH WITH GUSTS UP
            TO 49 MPH COMBINED WITH FALLING SNOW TO
            PRODUCE BLIZZARD CONDITIONS FROM 758 AM TO
            1058 AM LST.

&&