Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Riverton Wyoming
234 PM MST Thursday Dec 5 2013
Short term...tonight through Friday
Cloud decks will be increasing over the forecast area during the
later portion of the afternoon as a shortwave approaches the area.
Models indicating a fair amount of isentropic lift associated with
the shortwave that will help to initiate a bit more snowfall over
the western mountains and valleys. Model precipitation forecast is not as
excited...but enough support exists to leave snowfall over the
mountains and western valleys tonight. Lowered probabilities
slightly after collaboration with neighboring offices...but still
left chances for activity across the west.
Meanwhile...another frigid night expected across the area until the
continued influence of the Arctic airmass. Some areas may see some
cooler temperatures than what was experienced Wednesday night...with
similar temperatures expected elsewhere. This will leave another
cool day across the forecast area on Friday. Bumped up temperatures
a bit with how temperatures have been acting over the last couple
days across the area. Single digits will still be common across the
northern two thirds of the forecast area...with some 10-12 degree
values possible across Sweetwater County.
Otherwise...a fairly quiet period for areas east of The Divide.
Long term...Friday night through Thursday night
Only minor changes made to the forecast this afternoon. Models
remain in agreement through Tuesday...with digging shortwave moving
into The Four Corners over the weekend followed by another surge of
Arctic air Sunday night/Monday. Models begin to diverge by 00z
Wednesday...with the GFS being the slower and stronger solution. It
brings a moist...flat Pacific storm system into the northern rockies
on Tuesday...before splitting from the main flow into the Southern
Plains on Wednesday. The system is stronger compared to the European model (ecmwf)
because of ample jet support that the European model (ecmwf) is not resolving at this
time. The European model (ecmwf) does forecast this system to have the same track as
the GFS...so will have to continue to monitor these trends for the
next several runs. The GFS and European model (ecmwf) widely diverge by Thursday...as
the European model (ecmwf) breaks down a ridge across the eastern Pacific with a
strengthening long wave trough. The GFS is not as robust with this
solution and keeps a somewhat zonal pattern across the western U.S.
As it rebuilds the ridge over the Pacific.
Arctic airmass will remain in place through Monday morning...with
little modification occurring through this time. By Monday
afternoon...a northwest flow pattern will setup behind monday's
reinforcing Arctic system. This pattern will allow for mixing to
occur...as well as warming 700 mb temperatures. Bumped high temperatures
slightly...especially over northern Johnson County. Tuesday and
Wednesday will remain below average for this time of year...but will
be closer to normal.
Kept slight chance to chance probability of precipitation for Friday night into Sunday
morning...and snow amounts low...as the next shortwave moves into
the Desert Southwest. Reduced probability of precipitation slightly for Sunday...as well as
on Wednesday night/Thursday morning...across western portions.
Pushed the timing for the latter into Thursday...as it appears that
the forecast area will be under the influence of a transitional
ridge. The one forecast challenge...other than the items mentioned
above...will be the location of the polar front jet on Monday and
Tuesday...as well as Thursday...for possible strong gusty winds
across northern portions and the central basins.
East of The Divide...generally VFR conditions expected across the
terminals for much of the forecast period. Some terminals adjacent
to mountain ranges may see flurries or mist after 03z
tonight...especially around klnd. Otherwise...light wind and dry
conditions will persist through 18z Friday. After 18z...moisture
associated with a moving disturbance may move east of The Divide.
The main terminals to see much from this disturbance may be klnd
and kcod. Otherwise...some lowering cloud decks may be experienced
across the area.
West of The Divide...periods of light snowfall will remain possible
for some areas as a weak disturbance passes through the area.
Terminal forecasts are not aggressive with this feature...but do
continue light snow through much of the forecast
period...especially over kjac...kpna...and krks. Brief periods of
MVFR conditions will be possible with this snowfall through 18z.
Snowfall may get a bit more intense after 18z across the valleys.
Prevailing snow in reduced visibilities has been indicated at
kjac...and kpna. The potential for enhanced icing and turbulence
over the mountain passes will exist through much of the period.
Bitterly cold temperatures to be found across the area again
tonight. Additionally...a disturbance will impact the western
mountains and valleys. This disturbance will initiate additional
snowfall over the west...though snowfall accumulation is not
expected to be significant. The cold temperatures will persist
through the weekend across the area. Another wintry system may
impact much of the forecast area Saturday and into early
Sunday...especially the south and west.