Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 170930

Area Forecast Discussion
National Weather Service Riverton WY
230 AM MST Tue Jan 17 2017

.SHORT TERM...Tuesday through Thursday night

For the next couple of days, there looks to be little change in the
general weather pattern. Across the basins and deeper valleys,
inversions will continue to hold and bring more cold temperatures
along with light winds. The temperatures in continuity looked fairly
reasonable so we made few changes. Meanwhile, with a tightening
pressure gradient some locally windy conditions will develop across
the usual suspects...a.k.a. the Lee of the Absarokas and from the
Red Desert through Casper. At this point, the 700 millibar winds do
not look strong enough for any wind highlights but it will have to
be watched. It will make these areas significantly warmer than the
basins, however. In addition, we have someone hanging around like
that friend that won`t stop crashing on your couch, the dreaded fog
monster. With warm air aloft, we will likely see fog that next
couple of nights in the same areas as the previous nights. We will
continue the Special Weather Statement this morning.

As for any chance of precipitation, the models continue to slow down
the progress of the next system moving in from the west. The models
now have the area dry through Wednesday night. As a result, we
trimmed POPS back once again and future shifts may want to remove
them completely. Even on Thursday, much of the day could end up
mainly dry as the best upper level support may not move in until
Thursday night and even then it looks split. There are some
differences on Thursday night as to how much QPF can spread east of
the Divide with the NAM more aggressive. For now, we compromised with
some slight chance POPS spreading East of the Divide later Thursday

.LONG TERM...Friday through Tuesday

As the weather system moves through the region Friday, some light
snow in the mountains will continue. Scattered light snow expected
in the western valleys. Not much activity east of the divide aside
from some light snow in the Bighorn mountains. Saturday looks
mostly dry at the lower elevations east of the divide. Scattered
light snow for the western mountains and valleys on Saturday.
Sunday looks dry for most areas. The western mountains could see
some light snow over the higher peaks. The next weather system
moves into the Western U.S. Sunday night sending moisture toward
Western WY starting Sunday night through Monday. The GFS is faster
with the onset of snow than the ECMWF model by about 6 hours. Will
gradually increase the snow chance Sunday through Monday out west.
The south may also see some light snow Monday afternoon. Monday
looks dry east of the divide. Monday night and Tuesday are looking
unsettled per the ECMWF model with the trough over the Rockies.
The GFS is a little faster with this trough and wants to dry
things out Tue morning. Will side with the ECMWF model and have a
low chance of snow over much of the region Mon night and Tue.
Plenty of time to watch this unfold. As for temps, not a lot of
day to day change Friday through Monday with mainly teens and 20s
with some 30s in the east zones. The snow covered basins will
still stay on the cold side but should have some recovery each day
as a strong inversion should not occur with this unsettled weather



Warm air advection over a bitter cold surface air mass will
strengthen the surface inversion in basins and valleys. Where fog
eventually develops, it should be slower to much slower to
dissipate/lift due to a stronger surface inversion. A limiting
factor for fog development, and its thickness remains a marginal
amount of boundary layer moisture. KRKS and KRIW terminals followed
by KBPI have the best chance of fog development with a good chance
of airfield minimums occurring this morning. Elsewhere, confidence
is not as high so have limited MVFR/IFR conditions. Will monitor
satellite/observations trends to decide at 12Z issuance on how hard
to hit the fog. Fog is not expected at KCOD or KCPR with VFR
conditions through the period. Could see marginal LLWS on the lee
side of the Absaroka Mountains/Cody Foothills this morning and again
tonight, but latest model guidance have trended weaker with a
limited spatial extent. Thus have decided to remove it from COD
terminal for now.



More of the same for most areas today. Inversions will hold in the
basins and valley through at least Wednesday with cold temperatures
and poor mixing and smoke dispersal. Breezy to windy conditions will
develop across the southwestern Wind Corridor as well as in the Lee
of the Absarokas and may improve mixing. The next chance of snow
will likely hold off until Thursday and possibly until Thursday





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