Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 232026

Area Forecast Discussion
National Weather Service Riverton WY
226 PM MDT FRI SEP 23 2016

.SHORT TERM...Tonight Through Monday

Upper low spinning over the nern corner of UT as of early this
afternoon. New speed max is rotating nwd around the base of the
upper low and helping to form a baroclinic leaf over our srn zones.
A little further west than most of the guidance has. Main trend
again today is slower upper low movement and also farther south. It
now looks like the low may very well take the srn track and mover
over the southern end of the Wind Rivers later tonight and pop out
over the lower elevations of ern Fremont County or wrn Natrona
County before sunrise. The system is then forecast to lift into sern
MT by Saturday afternoon. Impressive upper divergence with our jet
maximizes over our Fremont/Srn Park Counties this afternoon into
early this evening before lifting further nwd. Increasing QG forcing
lifts into the north overnight along with favorable nw quadrant of
our upper low. Heaviest rain and higher elevation snow favors the
nwrn mountains through tongight before moving over to the Bighorns
Saturday. The nrn Bighorns may still see some significant pcpn
tonight also. Dry punch as shown by the models since yesterday is
actually occuring underneath our cooling tops at the same time the
rain is increasing. Winds up to 40kts as the dew points cratered and
the rain began. As the low moves into Central Wyo...we`ll get
another swath of cold rain across Sweetwater County and I80 with
some areas changing to snow later tonight into Saturday...especially
the higher parts along I80 and Sweetwater County. Steady to
significant pcpn continues in the nern zones centered on Johnson
County into Saturday with heavy mountain snow and still some decent
rain in the foothills. Lots of low clouds and fog potential will
continue into Saturday morning. Gradual improvement later Saturday
into Saturday night as the low moves off to the northeast. A
clearing, cold night Saturday night with many areas having to worry
about freezing temperatures with the west obviously being well below
freezing. Slow warmup Sunday into Monday and dry ridge builds in
from the west.

.Long Term...Monday Night Through Friday

Medium-range forecast models in much better agreement today
regarding the overall evolution of the synoptic pattern. Ridging
will be in place Monday night and Tuesday in the wake of the
departing upper low. This will leave dry conditions and warmer more
seasonal temperatures for Tuesday. A piece of energy cut-off from
the main flow over southern Arizona will slowly drift north Tuesday
and Wednesday, reaching southern Wyoming late Wednesday. Both the
GFS and ECMWF paint light precipitation across the south Wednesday
evening and overnight. This seems plausible and have at least
trended in the direction of slight chance of showers across the far
south and western mountains. Ridging builds back across the forecast
area Thursday and Friday ahead of next upper low approaching the
Pacific Northwest. 12Z/ECMWF, however, does slow this open wave and
keeps more showery activity for Thursday. Expect increasing
southwest flow aloft to boost temperatures to seasonal normals or
just above, and to generate at least breezy conditions in the wind
corridor. Probably will be enough moisture in this pattern to at
least necessitate slight chance precipitation chances across the
western mountains during the late afternoon and early evening.


.AVIATION.../00Z Issuance/

Upper low near the ID/UT/WY triple-point will trudge slowly
northeast across the forecast area tonight. While the trend has been
to slow this system, models are consistent in taking this low to
northeast Wyoming by 18Z/Sat and on into MT and the western Dakotas
by 00Z/Sun. Axis of convection will slowly shift east this afternoon
and a dry slot will work into southwest Wyoming. Jet streak may
still serve to generate isolated thunderstorms around KCPR.
Southerly wind of 10-20kts over the central and southwest terminals
will slowly veer to the southwest as shortwave and precipitation
axis shifts east. Speeds of 15-30kts expected at KRKS and KCPR this
afternoon. Mountain top obscurations will be widespread through at
least 18Z/Sat, with mountain obscurations across far northern
Wyoming. There will be some MVFR visibilities with the strongest
convection from KRKS to KRIW pushing east toward KCPR until around
00Z/Sat. Early evening trend still appears to be for a quick lull in
the action as upper low tracks east almost directly through the
center of the forecast area tonight. Rain will increase late this
evening and continue overnight at KJAC, KBPI, and KPNA. Model
forecast soundings for these terminals are saturated from the
surface to at least 400mb beginning around 04Z/Sat and continuing
until 10Z/Sat. There are likely to be occasional MVFR conditions
from around 04Z-08Z/Sat, if not until 12Z/Sat, at these three
terminals. Low-end VFR expected across the basins east of the Divide
overnight until 16Z-18Z/Sat. The exception will be KCOD where IFR or
lower conditions will persist through late Saturday morning due to
low-level upslope flow. To the west, should be improving conditions
during the same 16Z-18Z/Sat time period as less favorable northwest
flow kicks-in. This will lead to increasing northwest wind 15-25kts
at KBPI, KPNA, and KRKS during the morning. Breezy northwest wind
and gradually drier air will work into the forecast area from west-
to-east Saturday afternoon. Conditions will improve to VFR at KCOD.
Thermal trough hangs around through much of Saturday, so KJAC will
be the only terminal with SCT sky cover through 00Z/Sun. Other
terminals will keep BKN mid-deck through at least 22Z/Sat.



Fire danger low all areas through at least the weekend. A large
upper low is slowly progressing fron the Great Basin into Wyoming
tonight through Saturday. Copious moisture has made its way into the
the forecast area and will remain around the region through the day
Saturday. Significant rain will continue in the mountains through
tonight and in the Bighorns through Saturday where another 1 to 1.5
inches of rain is possible with higher elevation snow. Snow levels
will drop from around 10000 feet to near 8000 tonight through
Saturday morning with some mountain locations seeing between 2 and
12 inches tonight through the day Saturday. Heaviest snowfall
potential looks to be found over the western Wind River
mountains...Tetons...southern Absarokas...and especially the Big


.RIW Watches/Warnings/Advisories...
Flash Flood Watch through late tonight for WYZ001-002-012.

Winter Weather Advisory from 6 PM this evening to 6 PM MDT
Saturday for WYZ008-009.



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