Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Riverton Wyoming
209 PM MST Tuesday Dec 10 2013
Short term...tonight through Wednesday
The last of the parade of clippers is now working its way through
the County Warning Area. The main snow has now shifted to
northeastern portions of the County Warning Area. Some snow and
blowing snow is occurring with sharply reduced visibility.
However...it should be short lived so we will stick with an Special Weather Statement
rather than any highlights. Elsewhere...any mountain showers should
continue to wane as surface high pressure and flat ridging build
across the area. We continue to have gusty winds with the tight
pressure gradient...although not as strong as yesterday. These
should also subside tonight as the gradient lessens and high
pressure builds into the cowboy state. Low temperatures will be
tricky tonight and a lot will depend on how much clearing there is.
For now...we made few changes to continuity although our thinking is
that the lows will be the lowest temperatures will be.
As for Wednesday we should finally have a precipitation free day
with the surface high near the area. 700 millibar winds are still
decent but the pressure gradient is a bit less so there will still
be the pesky gusty winds across the wind corridor...but not as
strong as today. Most areas of the country would call this
windy...in Wyoming we call it Wednesday. As for high
temperatures...they should be cooler in the basins with less wind
with the high pressure. Areas that mix should see warmer
Long term...Wednesday night through Tuesday
Sat imagery showing nearly same upper (and lower) pattern as
yesterday...with eastward leaning/favoring lw trough over Continental U.S....with
embedded sws through the Great Lakes and points S...with another now
cruising through our state. Stout wrly middle level flow is providing for
mountain wave clouds to be visible now that the cirrus shield is
clearing from north to S. Points west are experiencing an epac high...with
col still rotating off the Southern California/northern Baja California coast. Surface set-up in Wyoming
nearly the same as yesterday too...W/hi W/SW...l NE...and relatively
tight p gradient cutting diagonally from northwest to southeast through the forecast area.
Lr models continuing in remarkable agreement through most of the forecast
period...the general weather idea remains the same...through the
beginning of the next work week the forecast area will continue under mostly
nwrly to occasional zonal upper level flow (w/ridge intrusion).
With the good blocking high out west and favorable northwest to west middle to upper
flow with several embedded quick moving upper level disturbances moving
through the rather stagnant large scale flow...this will then keep
the sws from digging and amplifying the the western portion of the lw
trough...leaving the Arctic cold only able to intrude to our east for
the most part. Over the forecast period...the northwestern to western mountains and
big horns at time will be favored for persistent light precipitation/snow
from Friday through the end of the period. Light precipitation due to a couple
of factors...either a relative lack of overall moisture...or a lack
of deep moisture (only shallow)...or a lack of good upward vertical velocity with
modest moisture...or any combination of these. Mostly dry everywhere
else west/o terrain sticking up high enough to lift and precipitate the
shallow moisture. Otherwise...warming and relatively stagnant surface pattern
will keep surface press gradient on the higher side across the usual wind
corridor...with middle to upper-level winds being able to get down to the
ground adiabatically in the afternoons...will help reinforce the
surface winds just enough in the afternoons that we should see a
period of rather gusty conditions with winds out of the west to SW
between rks and cpr gusting to 25 to 30 kts kts most
afternoons...perhaps 35 to 40 kts on Thursday/sun/Mon afternoons. The upper
ridge-lines of the wind rivers and absarokas...in addition to east/west
oriented canyons/gaps will also gust on the higher side of what is
currently being forecast. Will adjust as needed. Otherwise...expect high
temperatures warming and ranging from the middle 20s to lower 40s over the
period...with low temperatures improving from below zero just above
zero...into the teens and twenties for many areas except the Upper
Green river basin and higher elevations of the mountains which will
still drop into the single digits.
West of The Divide...kjac-kbpi-kpna-krks routes.
Snow will end by 00z tonight with VFR conditions expected for the
through 00z Thursday. A gusty wind will occur at krks terminal site
through 00z...then decreasing. The wind will increase at krks again
tomorrow afternoon but not as strong as on Tuesday.
East of The Divide...kcod-kcpr-klnd-kriw-kwrl routes
VFR conditions will occur through 00z Thursday at the terminal
sites. Some light snow will occur over the northern mountains and
foothills through 06z. Expect MVFR to local IFR conditions in these
locations. The snow will decrease and end after 06z with VFR
conditions prevailing. A gusty wind will occur at the kcpr terminal
site until 01z...then decrease. The wind will increase again
tomorrow but should not be as strong as on Tuesday.
Any snow will end this evening with dry weather prevailing from late
tonight through Thursday as high pressure builds across the area.
Strong winds east of the absarokas and across the southwestern wind
corridor will subside tonight. Winds will increase again Wednesday
afternoon across the forementioned areas but will not be as strong
as on Tuesday.