Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Great Falls Montana
1045 PM MST Fri Dec 19 2014
Update...quick update to grids to add small areas of slight chance
of freezing rain after receiving reports of spotty light freezing
rain and mix of rain/snow in the Helena valley and rain with a temp
of 26f at ft Belknap. The main cell causing the Helena precip has
moved east of town, but there is still a small area of moisture to
the west heading toward town. In the ft Belknap area, the precip
band is moving steadily to the east-northeast, so any freezing rain
will be limited by the short duration of the precip over any given
Previous update...(855 pm) another night of quiet weather conditions
across central and southwest Montana. Latest satellite imagery shows the
shortwave trof that was in western Montana this aftn has moved slightly
quicker than some forecast models were indicating. The trof axis is
currently between Great Falls and Lewistown and making steady
progress eastward. The moisture plume accompanying the trof has
been thinning as it moves along, as evidenced by very light and
spotty showers seen on latest radar imagery. Even with the few
spots where radar echoes may be a bit better defined, it appears
that little if any precipitation is reaching the ground, especially
in the lower elevations. Given the system's quicker progress and
minimal precipitation coverage, main adjustment this evening was to
reduce the chances of precipitation by 20-30% over most of the
forecast area between now and 6am Saturday. Rest of the current
forecast is in good shape.
Tonight through Sunday...
an upper level short wave trof and associated Pacific cold front
will move through the region overnight. This will case periods of
light snow across the southwest. Warmer air in the southwest flow
ahead of this system could lift over a colder airmass in place
over eastern portions of north-central Montana. This creates a
possible light freezing rain scenario tonight. Have added freezing
rain to the forecast, but otherwise do not have enough confidence
to issue any advisories. Will continue to monitor once light
precipitation begins to spread through this area tonight.
Surface pressure falls are already causing wind gusts to pick up
at Deep Creek along the northern rocky mtn front. Short term
guidance added confidence that wind gusts will briefly reach near
60 mph at isolated locations in this area early Saturday as the
front passes. Issued a High Wind Warning to cover this situation,
even knowing that gusts will taper Saturday. The High Wind Warning
continues on through Sunday as a much stronger downslope wind
signature appears with a strong Lee trough and synoptic
subsidence. Issued a high wind watch extending east off the Rocky
Mountain front on Sunday to evaluate the possibility of a more
widespread high wind event Sunday.
An atmospheric river event is developing over the Pacific
northwest Saturday night and Sunday. This affects mainly southwest
Montana...but heavy snow will accumulate along the Rocky
Mountains, shifting from north to south through Sunday. Snow
accumulations at pass level remain below 8 inches in 24 hours, so
issued a Winter Weather Advisory for snow and blowing snow above
5000 feet. Downsloping winds and mild temperatures will keep any
snow accumulations within and closely adjacent to the Rocky
Mountain front. In the southwest, snow accumulations could exceed
8 inches from 06z Sunday through 06z Monday. Snow levels are
forecast to rise above 5000 feet on Sunday, with rain possible in
the valleys. Will need to monitor forecast snowlevels before
upgrading to Winter Storm Warning or dropping back to Winter
Weather Advisory. Nutter
Sunday night through Friday...a rather active
weather pattern is setting up for much of the region for next
week. There will be several accumulating snowfall events that will
cause impacts for daily commuters and travelers. Through much of
the extended...the gfs13 and ec are in pretty good agreement.
Temperatures will be near/a bit above normal Monday and into
Tuesday...but then begin to turn colder Wednesday through Friday.
For now I used a blend of the colder models for temperatures...but
temperatures could be several degrees colder than currently
forecast should the snowpack become a bit deeper than expected.
Additionally...there is the potential for another high wind event
over portions of north central Montana on Tuesday.
In terms of precipitation...southwest Montana will continue to see
precipitation Sunday night and into Monday morning...with scattered
showers over north central Montana. The precipitation becomes
light and scattered Monday night through Tuesday. Then a another
fairly strong storm system will move through the region Wednesday
afternoon through Christmas day. At this time...it looks like
central and southwest Montana will see the brunt of this storm. At lower
elevations...there will be a swath of 3 to 5 inches of snow...with
around 8 to 10 inches in the mountains. It is too early to Pin-Point
the exact area that will see this heavier snowfall...but right now
that swath of heavier snow is forecast along a Helena to Bozeman
line. This line of heavier snow could shift a bit over the next few
days as later model runs get a better handle on the storm track.
Light mixed precipitation consisting of rain, freezing rain and snow
will affect khln and khvr. Little to no ice accumulation is expected
at khln and khvr and as temperatures warm above freezing tonight the
threat for freezing rain will diminish. At kbzn, temperatures are
cold enough that light snow showers may occur through 08z. Any snow
accumulation will be less than one-half inch. A surface cold front
associated with a weak upper-level weather system has quickly moved
across central Montana this evening. Winds have shifted southwest at
most taf sites with gusts up to 15 kts. At kbzn, light and variable
winds will continue. While VFR conditions will be predominate, a
brief period of MVFR cigs/vis are possible in and near precipitation
at kbzn, khvr and khln through 08z. Some fog is also possible at
khln, but confidence is low, thus have added vcfg to the taf for
late tonight into Saturday morning. Mountains will also remain
obscured through Saturday morning. Conditions will improve after 12z
as a weak ridge of high pressure moves across the Treasure state.
Mid and high level clouds will begin to increase late Saturday
afternoon and evening as the next Pacific weather system approaches.
Preliminary point temps/pops...
gtf 29 41 35 48 / 30 0 20 30
ctb 27 37 30 43 / 10 0 20 20
hln 23 37 30 47 / 30 10 70 50
bzn 21 35 29 45 / 60 10 60 60
wey 22 33 28 39 / 70 50 80 70
dln 22 37 33 47 / 40 10 60 50
hvr 26 40 27 46 / 40 10 40 40
lwt 27 43 31 48 / 40 10 30 50
Winter Weather Advisory from 5 PM Saturday to 5 am MST Monday
for elevations above 5000 feet for central and southern Lewis
And Clark...Jefferson...northern Rocky Mountain front...southern
Rocky Mountain front.
High wind watch from late Saturday night through Sunday
afternoon eastern Glacier...eastern Pondera...eastern Teton...
High Wind Warning from 2 am Saturday to 5 PM MST Sunday northern
Rocky Mountain front...southern Rocky Mountain front.
Winter Storm Watch from Saturday afternoon through late Sunday
night for elevations above 6000 feet for Beaverhead...Gallatin...