Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Riverton Wyoming
1103 PM MST Thursday Mar 6 2014
Short term...tonight through Friday
Infrared satellite imagery indicating the next band of cooler cloud tops
now across eastern Idaho and about to enter western Wyoming. Ksfx
radar showing the best reflectivites heading toward Star Valley
late this afternoon. Kafo has managed the lower 40s while kjac and
Jackson have only made it to 35-40f. Still...mainly valley rain
expected through about sunset before temperatures cool enough to mix
with and eventually change to all snow. By 03z/Friday the best dynamics
and moisture shift to southern Lincoln and Sweetwater counties.
Divergence-q fields and some upper level jet both support some precipitation
enhancement across this area. Timeframe for the best chance of snow
will come from about 02z-05z/Friday along the I-80 and US-30
corridor. Will issue a Special Weather Statement for slick roads
and accumulations of up to around one inch in this region by
midnight. East of The Divide...weak frontal boundary has already
slid south through the northern Big Horn basin and northern
Johnson County. This boundary will continue to sag slowly south
tonight. Have bumped rain/snow chances upward in Johnson and
Natrona counties behind this feature but do not feel this will be
necessary in the Big Horn basin. Again...div-q fields shift east
along with some jet support to assist in producing some very
light accumulations around kcpr. Favored time period appears to
be from 10z-14z/Friday but upslope is weak and moisture limited.
Other locations east of The Divide will remain dry through the
night. We will be on the backside of the trough by late Friday
morning. However...unstable northwest flow...500mb cold pool and
enough surface heating will combine to generate scattered
rain/snow showers in the mountains and downstream areas east of
The Divide. Light snow will persist across the west in northwest
flow aloft. Snowfall will be confined to the higher terrain and
will be minimal. Highest winds of 15-25kts Friday will be from
the west-northwest and begin during the late morning from the
Upper Green across Sweetwater County. Otherwise...have trended
wind speeds back somewhat across much of the forecast area.
Long term...Friday night through Thursday night
The beginning of the forecast period continues to look the best for
the cowboy state. The system from Thursday and Friday will quickly
make its way southward out of the forecast area by sunrise Saturday.
In its wake...a ridge of high pressure will build across the region
for the weekend. This will bring the first taste of the long awaited
effects of Spring to Wyoming. High temperatures of 50s and even
lower 60s can be expected east of The Divide. West of The Divide
will generally be in the 40s.
Unfortunately...these conditions will be short lived...as the next
storm system makes its way over the northern and central rockies.
Impacts from this storm will occur across the far west by late
Sunday afternoon...before spreading east of The Divide late Monday
afternoon. The GFS remains the faster solution...bringing the
associated cold front through Monday afternoon...while the European model (ecmwf)
brings it through Monday night into Tuesday morning. Any dynamics
with this system will move southward during this time...so it does
not seem that any rain/snow with this system will be significant.
The exception to this will again be western portions...where favored
upslope flow will occur...at least at the beginning of the event.
Winds will begin from the west-southwest...with a 120kt jet...
turning to the northwest by early Tuesday morning behind the cold
front. This should produce a persistent light snow across
northwest portions...especially across Yellowstone. The other
portion that will be significant...will be the continuing
precipitation and above freezing temperatures. These will
exacerbate the already saturated ground in the Star Valley and
Jackson Hole areas.
The models continue to diverge by Wednesday and Thursday. For
now...indications are that it will remain cooler and dry...with the
European model (ecmwf) forecasting a strong ridge to build in during this time. The
GFS is slower with this feature...building the ridge over the
Pacific northwest...and slowly transitioning eastward over The
Rockies by Saturday.
East of The Divide...kcod/kcpr/klnd/kriw/kwrl routes
A cold front will continue to push across the region tonight with
isolated to locally scattered rain and snow showers. By 12z...the
main potential will be for upslope induced light snow...mainly
around kcpr Friday morning...with less activity elsewhere but still
possible. MVFR ceilings are likely Friday morning across Natrona and
Johnson counties...with localized IFR areas. Parts of the Big Horn
basin could also see some fog develop later tonight...but counting
on middle level clouds to keep fog to a minimum. Otherwise...isolated
to scattered mountain snow showers could push southeast into the
lower elevations at times...especially Friday afternoon. Drier
conditions and decreasing cloudiness are expected Friday night.
Winds will generally be light northerly or variable today.
West of The Divide...kbpi/kjac/kpna/krks routes
Fog will be the main concern across the Upper Green river basin
Friday morning...while MVFR ceilings and possibly lingering light
snow could impact kjac Friday morning. During Friday afternoon the
main concern will be mainly snow showers developing over the
mountains and pushing southeast into the lower elevations. Drier
conditions and decreasing cloudiness are expected Friday
night...with the possibility of fog in the western valleys and
Moisture off the Pacific continues to stream into areas along and
west of The Divide. Little if any rain or snow is expected east of
The Divide. Locations from Rock Springs to Casper will see gusty
west-southwest surface winds through this afternoon. An upper level
disturbance will swing across the region this evening and early
Friday. Therefore...the best snow and rain/snow mix will shift from
far West Wyoming to southwest Wyoming tonight. Zones 280 and 300 may
get a quick shot of snow just prior to sunrise Friday. There will be
some gusty northwest wind Friday afternoon in the Upper Green river
basin and in zone 281. Otherwise...northwest flow aloft will just
create some very light snow across the northwest mountains. A ridge
of high pressure will bring dry and unseasonably warm conditions for
Saturday and Sunday. Would not be surprised to see some agricultural
burning going on east of The Divide in the snow-free areas this
weekend as temperatures climb into the 50s and lower 60s.