Area Forecast Discussion
Issued by NWS Glasgow, MT

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FXUS65 KGGW 182157

Area Forecast Discussion
National Weather Service Glasgow MT
257 PM MST Mon Dec 18 2017


Afternoon Update (3pm MST): Focus of the update was to the system
coming tomorrow through Wednesday. Snow totals have been updated
to match closer to WPC guidance. With continued increasing totals
without decreases in the interim, and run-to-run consistency
increasing the confidence of the impacts for this system, the
Winter Storm Watch has been upgraded to a Warning around the
Little Rockies and Hwy-191, and an Advisory for much of the rest
of the region outside of Prairie and Wibaux Counties, where
accumulations do not look to be as significant, and where some
rain may also preclude some accumulation by pre-wetting the
ground. Temperatures continue to be adjusted downwards in the near
term to account for the expected snowpack, but temperatures near
the end of the period have been brought up some from the past
couple of forecast packages, as some guidance have been starting
to resolve a return to warmer weather, thus decreasing confidence
in such deep cold.


Morning Update (9am MST): No significant changes to the forecast
for the morning package. Minor tweaks to temperature and wind
grids comprised the main areas of updating this morning. Bigelbach

PREVIOUS DISCUSSION: A shift in the overall pattern is expected
to lead to accumulating snow by midweek, followed by much colder

A split upper trof is currently moving across the western CONUS,
with the northern portion heading across the northern Plains
states toward the Great Lakes, and the southern portion in the
form of a nearly entirely cutoff low over the Sonoran desert
region of the US and Mexico.

A surface boundary has set up over northern Montana, bringing
cloudy skies and a mixture of rain and snow showers this morning.
These will continue off and on throughout the day as this trof
slowly moves south and east through the region, and some locations
across far northeast Montana may see between half an inch to an
inch of snow.

Just behind this, a low pressure system will push into the area
from the northwest, allowing cold air to push into Montana from
the north. This system will bring snow to the region beginning as
early as Tuesday afternoon, which will persist into Thursday
morning. Currently, the highest snowfall amounts are anticipated
to be across the higher terrain of the Little Rockies. Areas of
northeast Montana north of the Missouri River should also see
higher snowfall amounts due to the surface boundary expected to
set up as cold air tries to push down from the north side of this
storm. Current forecast snow amounts look to be 5 to 8 inches over
the Little Rockies, and 3 to 5 inches over the rest of the area
north of the Missouri, with generally 2 inches or less south. As
these totals are anticipated from Tuesday morning through
Wednesday afternoon, a Winter Storm Watch has been issued for this
time period. Winds may also be somewhat breezy as this front
passes through, and we will keep an eye on that as the storm
approaches to determine how significant impacts from that will be.

Cold air will drop significantly as the system clears out from the
area, and with fresh snow cover, temperatures will drop to near or
below zero Wednesday night.

Cold northwest flow will keep temperatures cold, and there is
increasing model consensus regarding the probability of an upper
level low diving down in the Great Basin region from the Pacific
Northwest on Thursday. So have a chance of snow south of the
Missouri for Thursday night into Friday as this system passes,
although current thinking is that the precipitation will remain to
our south and over the Rockies with this storm.

Beyond this, model guidance diverges, although at this time it
looks like northeast Montana will remain cold under a generally
unstable northwest flow regime through Christmas. Hickford




SYNOPSIS: A cold front draping over the area will continue to
bring snow to the northern sites and some rain and snow mix to the
southern sites through the afternoon hours, before a transition to
all snow begins in the evening. By then, however, the
precipitation should move out and provide a break for the region
until the potentially significant precipitation producing system
makes its way towards eastern Montana tomorrow. Visibility in
snowfall will bounce around between 3/4 SM and 3 SM. Ceilings
will start in MVFR category and lift some to VFR and last through
tomorrow morning before crashing at most sites back to MVFR late
in the afternoon.

WIND: West winds at 10-20 kts with gusts to 30 kts will decrease
to 5-10 kts overnight. In advance of the system arriving tomorrow,
winds will flip to the east-southeast late in the morning and pick
up to 10-15 kts lasting through the afternoon.



Winter Weather Advisory from 5 PM Tuesday to 5 PM MST Wednesday
for Central and Southern Valley...Daniels...Dawson...Eastern
Roosevelt...Garfield...McCone...Northern Valley...Richland...
Sheridan...Western Roosevelt.

Winter Storm Warning from 11 AM Tuesday to 5 PM MST Wednesday
for Central and Southeast Phillips...Southwest Phillips.

Winter Weather Advisory from 11 AM Tuesday to 5 PM MST Wednesday
for Northern Phillips...Petroleum.


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