Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 130532

Area Forecast Discussion
National Weather Service Riverton WY
1032 PM MST Thu Jan 12 2017

.SHORT TERM...Tonight through Sunday

Imagery shows split flow developing with a broad longwave trof
across the CONUS and an upstream axis stretching from north central
MT to the southwest over central CA. The southern branch of the jet
crosses southern CA and the desert SW into the UT/wrn CO/srn WY
confluence and then east into central Plains with the northern
branch staying well north across Canada and into the northeast
CONUS. A mesoscale jet streak/baroclinic leaf, which is part of the
jet mentioned above, is currently moving from nrn UT and into sw
WY/nw CO. This feature will keep snow chances around southern WY
through this evening, especially along and south of I80. Wind should
not be an issue with this system and snowfall through the afternoon
and evening should remain below 2 inches for most locations with the
heaviest amounts near the border with Colorado. Otherwise, northwest
WY will also keep some lingering isolated, but decreasing, mildly
convective light snow showers/flurries around today and patchy fog
around tonight.

The rest of the forecast should most remain dry across the FA with
light wind and seasonally cold temperatures. Nothing too extreme

.LONG TERM...Sunday night through Thursday

Beginning of FCST, split flow still in place with a large/broad trof
across the wrn CONUS and a weak close low in the base of the trof
over the srn Rockies/srn Plains and a relatively strong jet diving
down the west coast. WY will start out under weak flow conditions
aloft, little moisture, no precipitation except for an isolated
mountain shower across the far northwest (maybe). Seasonal to
seasonally cold highs/lows throughout the forecast period with
little wind until Tuesday, and even then we are just increasing LL
sw flow a bit...with gusts in the 25 to 35 mph range...nothing
highlight worthy. The next weather system of any note does not look
to arrive until Tuesday night as modest ridging across the wrn CONUS
allows EPAC moisture and warming back into wrn WY along with a ridge
cresting jet. This precip/snow maker will then keep the west wet and
white through the end of the forecast period...and beyond...and
will be highlight worthy for sure. A pattern change back to cold
Arctic looks possible by the next weekend. However, long range
pattern trends look to have a relatively progressive 4/5 wave
pattern in place, so hopefully any cold snaps coming through the
region will be of short duration.


.AVIATION.../06Z Issuance/

West of the Divide...KBPI/KJAC/KPNA/KRKS Terminals

Cloud shield and associated light snow across southwest Wyoming
has slid off to the southeast late this evening. The clearing sky
across western Wyoming means that low clouds and fog will be the
main concern for the remainder of the night. Light snow at KRKS
has left the boundary layer moist and the amount of additional
clearing will determine if fog forms. Right now, belief is that
cloud shield will be just to the southeast of KRKS which will allow
for good radiational cooling. Have gradually lowered visibility to
IFR/MVFR overnight at KRKS. Other terminals have gone SKC and cannot
see more than scattered mid-cloud drifting north overnight. Expect
that KBPI and KPNA could see a period of low clouds and fog but
confidence is not more than 50/50. Decided to stick with VFR but
would not be surprised to see MVFR at these two terminals. VCFG and
scattered low clouds may hold at KJAC, so have continued with VFR
forecast. Expect all terminals to be VFR by 18Z/Fri. Some mid-cloud
will drift north across KRKS Friday afternoon, but don`t see
precipitation being an issue. Any cloud cover will clear from
northwest-to-southeast Friday night with the potential again for some
VCFG. Wind speeds throughout the period will be less than 12kts.
Foothills and buttes will be occasionally obscured because of the
low clouds and fog.

East of the Divide...KCOD/KCPR/KLND/KRIW/KWRL Terminals

Cloud cover across central Wyoming has pushed off to the southeast
this evening leaving a generally clear sky. Have continued with
VCFG at several terminals with snow covered ground and good
radiational cooling underway. however, don`t think the boundary
layer is moist enough for widespread fog or MVFR conditions since
we had little melting and no precipitation on Thursday. Patchy fog
that does develop in the basins will dissipate by 18Z/Fri and expect
VFR through the remainder of the period. Wind speeds through Friday
evening will be less than 12kts.



Fire Danger low for all of WY through, at least, the weekend. Cold
seasonal temperatures, good existing snow pack, and relatively light
winds will characterize the next several days. A short period of
light snow will occur across southern Wyoming this afternoon and
evening. Smoke dispersion will also remain poor for all locations
through the weekend.





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