Area Forecast Discussion
Issued by NWS Glasgow, MT

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FXUS65 KGGW 171039

Area Forecast Discussion
National Weather Service Glasgow MT
339 AM MST Sun Dec 17 2017

Pattern shift underway expected to lead to snow by midweek and
much colder temperatures.

Upper trof moving across the western states splitting, leaving a
closed low over the desert sw while the northern portion heads
toward the Great Lakes. This results in zonal flow into western
Canada and the northern Rockies. With the westerly flow, skies
will be mostly sunny today, although should begin to see some
increase in clouds in the afternoon. Temperatures will be cooler
than last week but still generally mild.

Arctic low beginning to strengthen north of Hudson Bay, and will
push the Pacific jet slightly southward beginning tonight. This
will allow colder air to filter into the northern plains on
Monday. Moisture and lift on the leading edge of the cold air
could result in some light rain or snow spreading south through
the day.

Midweek sees the development of a potential winter storm system
with upper trof dropping out of the Gulf of Alaska and into the
Great Basin. Slight ridge develops over Montana on Tuesday with
warm moist airmass lifting across the edge of the colder Canadian
high. Models in pretty good agreement currently with developing
the surface low over Wyoming but keeping 700mb trof over northern
Montana, where the focus for the greatest precipitation could be
on Wednesday. Precipitation shield shifts southeast Wednesday
night as the system moves into the central Plains. Snow
probabilities suggest snow amounts anywhere from an inch or 2 in
the northeast corner of the state to 10 inches in the Little
Rockies. The most likely amounts are 2 to 4 inches north of the
Missouri River with lesser amounts south, from Tuesday afternoon
though Wednesday evening.

As the system clears the region Wednesday night, with clearing
skies and possible fresh snow cover, temperatures expected to drop
to near or below zero.

The models diverge on solutions for the remainder of the period,
but agree to leave the area under an unstable cold northwest flow.
This will keep cold air over the northern plains and could bring
some additional snow for the Christmas weekend. Ebert



VFR. Mid and high clouds will begin increasing coverage during the
late afternoon and overnight as a front sags southward from
Canada. A little rain or snow is possible behind the front on
Monday along with a drop to MVFR flight conditions.




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