Winter Storm Watch 13, NWS Great Falls, MT

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREAT FALLS MT
1055 AM MST Fri Dec 19 2014

UPDATED AVIATION DISCUSSION

.UPDATE...

Webcams this morning show periods of snow occurring across the
southwest, including Monida and Raynolds passes, Big Sky and West
Yellowstone. Raised near-term PoPs for today and tonight based on
the NAM to reflect this early activity. Snow accumulation of 1 to
3 inches is expected for these areas through Saturday morning. Winter
weather advisories will not be issued for tonight, although
travelers through this area should expect snow covered roads.
Winds speeds along the Rocky Mountain front were raised along the
Rocky Mountain Front as a lee trough begins to form. Areas of
light freezing rain remain possible tonight for areas generally
north and east of the Bears Paw Mountains. Nutter

&&

.AVIATION...
UPDATED 1755Z.
A cold front will sweep from west to east through the forecast area
this evening and it will be followed by a weather disturbance aloft.
Expect generally VFR conditions however mountains will be obscured
in snow until late tonight. Have moderate confidence in the timing
of the weather and how low ceilings will go but for the plains have
low confidence in the type of precipitation. For instance at
KGTF/KLWT/KHVR precipitation could be either rain or snow or even
freezing rain. Should freezing rain develop do not think it would
last more than an hour. Over the plains skies should clear rapidly
behind the weather disturbance.

BZN taf...as mentioned above have moderate confidence in the timing
of the threat of precipitation. The cold front should move through
the area during the 04z to 05z time frame but should not notice much
of a change with winds. That is the winds should remain light.
Although there is a prob group for MVFR conditions in snow from 04z
to 08z..there could even be a brief period of IFR conditions in snow
during this same time frame. Should these IFR conditions develop
they would last no more than an hour. Also note that nearby
mountains could be obscured in snow during the 04z to 08z time
frame. Blank

&&

.PREV DISCUSSION...
/ISSUED 438 AM MST Fri Dec 19 2014/

Tonight through Sunday...A high pressure ridge will be over the
Rockies and moving over the zones during the day. To the west, a low
pressure trof will move over the West Coast during the day. Decent
dynamics develop as the trof crosses the Rockies although this will
be a fast-moving feature and little time for development of weather.
Best lift with this system will be Friday night over the southwest
zones. However, a chance for mixed snow and rain are expected
tonight over central and northern portions of the CWA. Will include
some freezing rain in the grids after midnight as the surface
conditions cool. The upper level trof will move over the zones by
early Saturday and weak ridging will be in place to the west and
moving across the zones during the day. A strong jet at upper levels
will be over the Pacific Northwest Saturday and spread east over the
zones Saturday night and Sunday. Mid-level winds will increase to
45+ kts Saturday night and Sunday. This will bring very strong winds
across the Rockies and adjacent plains. Some stability develops at
lower levels after midnight and the will bring the best chance for
strong winds spreading east. Currently, have some gusts above
58 mph over the Rockies but because of uncertainty will hold off on
statements. Some strong winds as well at higher elevations will also
occur across Southwest Montana by Sunday and will have to be
monitored. Widespread precipitation will develop with the approach
of a shortwave associated with these strong winds Saturday night
and, once again, the chance of freezing rain will develop and will
be included in the grids after midnight into Sunday morning. Some
higher snow accumulations are expected at higher elevations in the
southwest zones but widespread heavy snow is not expected. Zelzer

Sunday night through Friday...A rather active weather pattern is
setting up for much of the region for next week. At this time...we
are not expecting significant snow storms...but rather 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 lower 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 MT on Tuesday.

In terms of precipitation...Southwest MT will continue to see
precipitation Sunday night and into Monday morning...with scattered
showers over North Central MT. The precipitation becomes fairly
light and scattered Monday night through Tuesday. Then a fairly
strong storm system will move through the region Wednesday afternoon
through Christmas Day. At this time...it looks like Central and
Southwest MT 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.
Brusda

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  42  28  40  34 /  10  30  10  20
CTB  40  26  36  29 /  10  10   0  20
HLN  37  22  36  28 /  30  50  10  40
BZN  35  18  33  27 /  30  60  10  50
WEY  33  20  32  27 /  60  60  40  70
DLN  38  20  35  31 /  50  50  10  50
HVR  39  26  39  26 /   0  30  10  20
LWT  44  25  41  29 /   0  30  10  30

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

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