Area Forecast Discussion
Issued by NWS Glasgow, MT

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682
FXUS65 KGGW 220102
AFDGGW

Area Forecast Discussion
National Weather Service Glasgow MT
602 PM MST Thu Nov 21 2024

.DISCUSSION...

KEY MESSAGES:

 - Wintry mix Friday morning through Saturday afternoon, then
   change over to all snow by Saturday evening. Winter Weather
   Advisory has been issued for areas north of the Missouri River.

 - Model trend for forecast snow amounts has decreased a bit with
   data released today for the weekend storm. Two to five inches
   north and trace to 2 inches south.

 - A well-below normal period of temperatures are in store
   beginning Sunday evening.


WEATHER PATTERN OVERVIEW:
Today, an Alberta clipper will flatten out the upper level ridge
and introduce some light precipitation or virga, mainly for our
northern zones. Might be a little bit of freezing rain or wintry
mix, but amounts and impacts will be light today.

As the clipper moves toward the Great Lakes region on Friday, a
short-lived shortwave ridge of high pressure builds in its wake
across Montana. Consequently, this warm advection in the low
levels will result in a warm layer of air aloft setting up across
central Montana and extending into portions of NE Montana. An
organized low pressure system over Washington and Oregon will
slowly move and eject a short-wave impulse in our direction. This
will increase chances for a wintry mix beginning Friday morning,
including freezing rain as surface temperatures are expected to be
in the 20s. North of the Missouri, the forecast soundings are
more likely to support frozen precipitation, thus allowing a quick
1-2 inches of snowfall accumulating by Friday night along the
International Border.

By Saturday, this split off impulse will strengthen a bit and
provide the upper-level divergence needed to support a moderate
snowfall event through Sunday. Given the latest forecast trends
of decreasing snowfall amounts north of the Missouri River with
the latest run, a headline was still needed, but the winter
weather advisory will suffice, expecting 2 to 5 inches of snow
over a few days.

From Monday, flow aloft will be northwesterly, ushering in an
Arctic air mass, leading to well below normal temperatures as we
round out November. Wind chill values at the night time are
expected to remain above -20F for now, so likely no headlines for
cold weather as of yet.


FORECAST CONFIDENCE & DEVIATIONS:
There is a low to medium confidence in impacts as a result of ice
accumulations from the freezing rain threat Friday afternoon
through Saturday morning. As of now, the area with the greatest
freezing rain threat appears to be US Highway 2 and MT-Highway
200. Sleet is also possible north of US Highway 2.


&&

.AVIATION...

LAST UPDATED: 0100Z

FLIGHT CAT RANGE: VFR - IFR

DISCUSSION: Incoming storm will lower ceilings across the area.
MVFR ceilings will begin impacting KGGW around 15Z and expand to
other terminals around 21Z tomorrow afternoon. Snow will move
into the area around 21Z and bring visibilities that could drop
to IFR until the late evening.

FREEZING RAIN: There is a roughly 10 to 20% chance that light
freezing rain could impact KGGW and KOLF from 18-06Z Friday
afternoon and evening before the transition to all snow.
Accumulations are expected to be less than a tenth of an inch if
they do occur, but this could still lead to ICING for aircraft.
This is not quite high enough chance to be handled by a Prob30 in
the TAF, but if it occurs will still be significant.

WIND: Light to calm and E to SE or variable through this evening.
turning N after midnight and increasing to 5 to 10 kts through
the morning. Veering NE to E in the afternoon through Friday
night.

GAH


&&

.GGW WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from 9 AM Friday to 5 PM MST Sunday for
Central and Southeast Phillips-Central and Southern Valley-
Daniels-Eastern Roosevelt-Northern Phillips-Northern Valley-
Sheridan-Southwest Phillips-Western Roosevelt.


&&

$$

weather.gov/glasgow