Area Forecast Discussion
Issued by NWS Riverton, WY

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107
FXUS65 KRIW 171744
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1045 AM MST Fri Nov 17 2017

.SHORT TERM...(Today through Sunday night)
Issued at 200 AM MST Fri Nov 17 2017

Imagery shows upper trofs ovr/near each coast and a ridge between
with the ridge axis stretching from the wrn Great Lakes through the
srn Plains and into nrn MX. Upstream trof axis lies from BC to nwrn
California and into the EPAC. Very strong jet (120-150kts) stretches
from cntrl California into wrn/srn WY with an associated embedded
shortwave also moving through WY. The SFc has general high P across
the Desert SW and lower(ing) pressure to the north with developing
lows/fronts/trofs from the Front Range of CO and ovr WY/MT/ID. One
cold front cuts diagonally from ne to wsw across the FA. Light to
occasionally moderate precip (both liquid and frozen) now across
much of the area along and west of the Divide in addition to a few
locations over cntrl/nrn WY east of the Divide.

Today/tonight: Lee side cyclogenesis takes place in the trof area
east of the Front Range of CO with a couple of fronts moving through
WY to the south and east. Precip west of the Divide will enhance
through the morning hrs west of the Divide while chances
develop/increase east of the Divide (rain and/or snow showers)
near/along these moving boundaries and especially under the most
favorable region of the jet across srn WY. As the upstream trof axis
continues to approach and temperatures aloft decrease, lapse rates
will continue to increase through the daytime hrs with convective
chances for thunder (snow) across the the FA...especially srn/cntrl
WY and areas west of the Divide. Post frontal winds will remain
quite gusty out of the west to northwest through today and into the
evening...with gusts from 25 to 45 kts (locally higher) across the
Wind Corridor...highest over ern Sweetwater County. Currently,
Advisories and Warnings cover all the mountain areas along and west
of the Divide in addition to the Star and Jackson Valleys...ending
at 5 PM today. The upr trof axis passes through this evening/tonight
along with another front and clears out the precip from nw to se.

Saturday through Sunday night: Saturday, the upper levels become
nwrly while high pressure west and lee troffing east
creates/maintains gusty west to southwest winds over the usual
spots...nrn/cntrl mountains and foothills (G45kts) in addition to
the Wind Corridor(30-35kts). Sunday will see a similar wind pattern
with possible High Wind conditions across the Absaroka Mountains and
Cody Foothills Sunday night with strong SFC P, T, and Omega Grads
under increasingly favorable mountain wave production aloft ahead of
the next approaching jet/SW trof. Precip should remain out of the FA
through this portion of the forecast...not returning until later
Monday.

.LONG TERM...(Monday through Friday)
Issued at 220 AM MST Fri Nov 17 2017

Moisture in a NW flow aloft for Monday with the GFS bringing a
weather system into Idaho by 00Z Tue. ECMWF model has the moisture
in the NW flow but hangs back the weather system into Wash/Oregon
area. Precipitation for the west and northern Bighorn mountains
Monday through Monday night. Could see some light precip in the
southwest zones as well. Dry elsewhere Monday. The GFS has dry
weather for Tue while the ECMWF model keeps the west in precip. Have
chosen the drier solution for now and see how things look tonight.
GFS/ECMWF models bring some light precip Tue night into Wednesday
in the west with dry weather east and south. Some light precip
may linger in the west Thursday as well, while dry weather
prevails elsewhere. GFS is bringing a cold front into WY for next
Friday with light precip developing. ECMWF is drier with this cold
front. Will side with GFS model for now. High temperatures will
be at or above normal through the period with 40s and 50s and
mainly 40s in the west.

&&

.AVIATION...(For the 18Z TAFS 18Z Friday through 18Z Saturday)
Issued at 1045 AM MST Fri Nov 17 2017

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

The weather system moves through the region tonight and exits to the
east around midnight. Periods of snow will occur into late evening
in the valleys and mountains. MVFR/IFR conditions are expected
across the area with the mountains frequently obscured.
Conditions will improve in the after 00Z with VFR conditions
prevailing by 06Z Saturday. A gusty wind will occur today as most of
the terminal sites with the wind decreasing after 03Z. Please see
individual terminal forecasts for more details.

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

VFR conditions to prevail tonight. Scattered rain and snow showers
will occur today into the late evening. MVFR and the occasional IFR
condition will occur due to lower ceilings and vsbys. A gusty wind
will be common in many locations today with the wind decreasing
after 03Z. VFR conditions will prevail over the region after 06Z
Saturday.

&&

.FIRE WEATHER...
Issued AT 200 AM MST Fri Nov 17 2017

Fuels remain at critical levels across over portions of Fremont,
Johnson and Natrona Counties.

Fire weather conditions will be relatively low today and Saturday
with seasonal to below seasonal temperatures and minimum RH values
ranging above 30 percent at all locations. Additionally, the entire
forecast area will see some chance of precipitation today with
moderate to heavy snow possible in the mountains along and west of
the Divide. Moderate precipitation possible across southern Wyoming
with only light precipitaion expected for most locations east of the
Divide. Winds, however will remain quite gusty today...especially
across Sweetwater County with gusts 35 to 50 mph possible. Saturday
will see no precipitation and it will be cold and drying out.
Sunday, some enhancement to fire weather conditions can be expected
as, although temperature warm slightly, the surface/boundary layer
dries out further, afternoon RH values drop into the upper teens to
mid 20s (percent), and winds gust out of the southwest 25 to 35 mph
at times across the critical fuel areas (see above).

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until 5 PM MST this afternoon for WYZ001-
002-013-015.

Winter Storm Warning until 5 PM MST this afternoon for WYZ012-
014-023-024.

&&

$$

SHORT TERM...Braun
LONG TERM...AR
AVIATION...AR
FIRE WEATHER...Braun



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