Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Glasgow Montana
819 PM MST Sat Dec 27 2014
Short term...for northeast Montana...tonight through Monday...
forecast on track with little change to the inherited forecast
this evening. Echoes on radar have largely been virga during the
afternoon...however...observations over North Dakota do show light
snow and steadier radar echoes exist upstream that will pivot
across the northeast zones during the next several hours. Some of
this activity is shallow enough that it may be underneath the
radar beam so radar observations may not prove to be the most
valuable tool here. Nevertheless...00z NAM captured this activity
well and so will keep chance for light snow in those areas.
Other change was to shift steadiest snowfall accumulations ever
slightly to the south based on latest guidance. Steadiest amounts
still likely to be south of a line passing through Hays and
Jordan. Current highlights express the latest thinking rather
well. Snow will spread across the southwest zones this evening and
become steadier Sunday morning...before tapering off during the
afternoon and evening. Maliawco
Main forecast challenge will revolve around the next anticipated
accumulating snow storm expected to impact our County Warning Area beginning late
tonight and continuing through Sunday afternoon.
Currently the responsible upper-level disturbance is traversing
the Rocky Mountains of British Columbia. Excellent model agreement
continues to track this disturbance southeastward over the
mountains of western Montana by late tonight or early Sunday
morning where it eventually gets washed out by the larger-scale
upper-level Arctic trough descending southward over much of the Continental U.S..
Period of greatest impact will likely be 09z overnight tonight
through 00z Sunday afternoon. Latest consensus model precipitation
estimates brings 2 to 3 inches to many location in our Petroleum
and Garfield counties. Some portions or higher elevations near the
Little Rockies could also receive 2 inches of new snowfall. Chose
to go ahead and issue a Winter Weather Advisory for those areas.
The accompanying cold front will filter in some breezy
northeasterly winds but most of this will be after the snow fall.
But...this could still lead to periods of blowing ground snow. The
advisory seems to be good enough for now. Looking at the sref
model plumes for snowfall...some 15z data suggests a few outliers
with higher snow amounts.
Given recent fairly good run to run consistency between
differing model quantitative precipitation forecast output...will proceed with a blend of the
NAM...GFS...ec...CMC...sref depictions which place most of the
snowfall along and south of a Hays to Jordan line. Bmickelson
Long term...Monday night through Saturday...
the long range period begins with a broad long-wave trough over
the Continental U.S. With a Sharp Ridge off the West Coast. As strengthening
Hudson Bay low maintains trough over the East...Ridge folds over
into western Canada and cut off low forms in the Desert Southwest.
Arctic high slides south of the region on Tuesday with warmer air
spreading east of the Canadian rockies as the upper ridge folds.
However with upper flow remaining from the northwest due to the
strong Hudson Bay low...the warm air will be slow to move
east...and cold air will linger a the surface for Wednesday.
Warmer air gets a push to the east on Thursday with a slight
change in the upper flow in advance of a shortwave dropping
through central Canada. Moisture stream across The Rockies pushes
into Montana...and with Arctic high pressure providing lift from
upslope flow and cold air advection...expect another round of snow
followed by very cold temperatures for the weekend. Best chances
of snow again expected across central Montana but some potential
for a little accumulation across the eastern plains. Ebert
Aviation forecast is a bit complicated for this taf cycle. Chances
for snow from the west are expected to increase during the
overnight and early morning hours. Models indicate either MVFR or IFR
ceilings or worse after 10z in snow. Chances seem a bit better for
kggw and kgdv to be in worse conditions...while kolf and ksdy will
likely not see these worse conditions. Will keep and eye on
observations to be ready to amend as needed. Bmickelson
Winter Weather Advisory until 5 PM MST Sunday for Garfield...