Area Forecast Discussion
Issued by NWS Glasgow, MT

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FXUS65 KGGW 180539

Area Forecast Discussion
National Weather Service Glasgow MT
1039 PM MST Sun Dec 17 2017


Changes to the forecast were focused on the first 6 or so hours
with winds. With sundown and the passage of a surface trough
winds have lighten much faster than anticipated; Especially, with
the speed increase that was seen for about 4 hours before dark.
Most other things look on track for this evening. GAH

Afternoon Update (1pm MST):
The main areas of edit revolve around the upcoming precipitation
event around midweek. Morning model solutions are producing
significantly more snowfall than the morning runs did. Should this
increase in totals persist through the coming forecast cycles, it
is not unreasonable to expect a Winter Weather Advisory for much
of the region along and north of the Missouri. Otherwise, the
narrative over the forecast is relatively intact, as a strong
cooling trend is in store for the region, and by late next
weekend, some temperatures well below zero seem a bit more than
possible. Bigelbach

Morning Update (9am MST):
The focus of the morning update was on updating the PoP and Sky
forecasts through Monday by incorporating newest short-term
guidance. Precipitation potential remains in the grids for tonight
through tomorrow, with best chances for snow accumulation
remaining in the northern tier of zones. Sky and wind grids were
nudged to match up with neighbors and bring in newer guidance.
Otherwise the short term was left mostly alone. Bigelbach

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



LLWS: Low level winds shear will be possible in the mid-morning


SYNOPSIS: A fast moving disturbance will push through the region
overnight. This will bring a dense cloud deck with lowering
ceilings through tomorrow. Ceilings may dip into MVFR levels at
KOLF, and approach them at the other sites but generally KGGW,
KSDY, KGDV should remain in VFR. Snow early in the morning will
be possible at all sites, and could transition to rain by late
morning to early afternoon.

WIND: West-southwest at 10-15 kts through the evening...
decreasing to 5-10 kts after midnight... Veering to the northwest
and increasing to 10-20 kts tomorrow after the disturbance





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