Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Riverton Wyoming
1003 PM MST Thursday Feb 11 2016
Short term...tonight through Friday (issued at 109 PM mst)
Satellite imagery shows the forecast area in northwest
flow...between an upper ridge over the West Coast...and a large
trough over the eastern 2/3 of the U.S. A weak shortwave located
over southwest Oregon around 18z...is expected to push through the
ridge...and across the forecast area late tonight and Friday
morning. The main impact from this system will be increasing middle
level cloudiness this evening and into the overnight hours. There
will be enough moisture for isolated to locally scattered snow
showers...mainly over the northwest and northern mountains. The
best shot for seeing the showers will be between midnight and 6
am. The chance of showers and middle level clouds will decrease from
west to east during the day Friday. No front to speak of with this
system...and actually the 700mb flow weakens...and the surface
pressure gradient weakens early this evening and remains weak on
Friday. Thus winds are expected to light in the lower
elevations...with still a west to northwest breeze in the
mountains...and the DuBois area...but not as nearly as windy as it
was this afternoon. Min temperatures will be tricky...and could
occur by midnight...as an extensive middle/high level clouds spread
over the area. Maximum temperatures on Friday...will once again be above to
well above average. However...timing of clearing...steep surface
inversions...and variations of snow cover will keep maximum temperatures a
challenge. For example...kcpr maximum temperature has been about 7-10
degrees cooler than what has been recorded in town over the past
couple of days. This seems to be from the deeper snowpack
west/southwest of Casper.
Long term...Friday night through Thursday
A fast moving pattern looks to be the rule for much of the long
range. That the models agree on. The challenge is the
timing...position and strength of the numerous shortwaves that will
move through the area. Otherwise everything is good. Shortwave
ridging will control the area through Saturday morning...bringing
mainly dry conditions. On Saturday afternoon into Saturday night a
shortwave will top the ridge and bring the chance of showers across
mainly western and northern portions of the area. With the warmer
temperatures...some of this could actually fall as rain in the lower
elevations. Winds will also increase with this system with some
gusty conditions across areas favored by northwest flow such as the
Wind River basin as well as Johnson County. This system should zip
out Saturday night. As for the chance of snow or rain bands Saturday
night...it is possible although looking less likely than last
night. Snow melt will continue with high temperatures climbing into
the 50s in some of the lower elevations.
Another system then moves into western Wyoming later Saturday night.
With some decent overrunning there could be a decent amount of snow
in the western mountains. However...with the warmer air some of the
valleys could have some mixing with or even changing to rain at
times. Then the models have some differences. The GFS shows the
precipitation jumping The Divide...probably in the form of
convective showers. Meanwhile...the European keeps the best lift
across Montana. For now...we made only minor changes to continuity.
Gusty winds will continue across the favored northwest flow areas as
well. With 700 millibar winds approaching 50 knots...there could
even be some concern for wind highlights. Mixing should get rid of
the inversions...and snowmelt should continue.
The models continue to diverge for Monday as well with the European
setting up a boundary across northern Wyoming with precipitation
continuing and the GFS rebuilding the ridge further to the east and
keeping the precipitation across eastern Wyoming...out of the
Riverton County Warning Area. Again...with the uncertainty few
changes were made.
A drying trend should then take place from Tuesday into Wednesday as
riding builds back across the area. Temperatures still look to be
above normal. Another...potential more potent system may then move
in for Wednesday night into Thursday...again depending on which
model you believe...with the European 12 to 18 hours closer than the
GFS. Again...with the uncertainty few changes were made to
continuity as the details can be fine tuned as we get closer to the
A weather disturbance will bring scattered snow showers to the
northern Wyoming mountains overnight until around 18z Friday. Some MVFR
conditions will occur in this area along with some mountain
obscuration. Will keep vcsh at the kjac terminal site until 14z.
Isolated snow showers are possible near kbyg through 18z with brief
MVFR ceilings possible. Otherwise VFR conditions over the rest of the
region through 18z Friday.
After 18z...VFR conditions will prevail over the region through 06z
Isolated to locally scattered snow showers are expected over the
northwestern and northern mountains late tonight into Friday
morning. Snowfall amounts will be light...with a dusting to an
inch of snow expected. Otherwise...dry conditions with increasing
middle level clouds are expected tonight. Windy conditions occurring
in the mountains and some adjacent foothills this afternoon...are
expected to gradually weaken overnight...with mainly light winds
in most areas Friday. The exception will be breezy west and
northwest winds on exposed mountain ridges and possibly adjacent
foothills...but not nearly as windy as what occurred Thursday.
Light winds...and a low sun angle will result in poor smoke
dispersal in most if not all of the area Friday.
A quick moving storm system...and associated cold front is
expected to push across the area Saturday and Saturday
night...with a chance of showers...especially over the north.
Expect breezy conditions to develop in wake of the cold front
Saturday afternoon into the overnight hours.