Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 210836
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
136 AM MST Sat Jan 21 2017

.SHORT TERM...Today through Monday night

Imagery shows broad negatively trof still across the wrn CONUS and
into the central Plains with jet well to the north and south of the
WY. Nose of the southern jet well into and through the Desert SW
with the strongest portion of this jet still offshore nrn
California. SFC has weak high P across WY with lower P and a cold
front east over the central Plains. While precip has mostly ended
across WY...light snow showers are slowly feeding toward western WY
from UT/ID.

Relatively warm weather and sunshine Friday contributed to raising
dew points across WY...with crossover temperatures easily reached
and surpassed Friday evening and this evening over the cold ground
snow across many lower elevations sites...so patchy dense and
somewhat transient fog will be around through this morning.

This morning through most of tonight, the broad but still digging,
negatively tilted trof and another weak embedded SW will cover and
lift through the FA with light snow showers continuing across
portions of western/southwestern WY. With the stronger portion of
the jet, mentioned above, moving through the west central CONUS a SW
ridge will build into the the Great Basin and across WY through most
of Sunday. Forcing, moisture and instability across the FA at this
time will remain limited/modest until at least late Sunday
afternoon. Then, increasing moisture from the EPAC, but partially
wrung out by the Sierra, will arrive across the wrn/swrn FA along
with the nose of the strong jet. This will greatly increase precip
chances across west/southwest WY Sunday night with a much better
chance for significant snowfall lasting through the day Monday
across the western mountains. A Winter Storm Watch has now been
issued for western WY beginning late Sunday afternoon and lasting
through Monday afternoon.

Monday, as another embedded wave moves across the FA,
frontogenesis will take place Monday afternoon near the northern
border with MT and then push swd Monday afternoon through the end
of the forecast period...with accumulating snow possible east of
the Divide and increasing across southern WY and the I80 corridor.
Advisory level conditions now look quite possible across portions
of central and southern WY during this timeframe.

.LONG TERM...TUESDAY THROUGH SATURDAY

Synopsis...A low pressure system will lift out of the central
Rockies across the central Plains on Tuesday. Areas of light to
moderate snow will continue across central Wyoming Tuesday
morning, gradually tapering off Tuesday afternoon and evening.
Cool and unsettled conditions will keep isolated to scattered
mountain snow showers over the area on Wednesday into Thursday. A
ridge of high pressure will build into the area Friday and
Saturday with milder conditions in the mountains and east, while
steepening inversions keep temperatures cold with areas of morning
fog in the western valleys and lower central basins.

Discussion...Synoptic pattern Tuesday morning features broad upper
trough from the west coast into the High Plains.  Lead shortwave
will be ejecting out of Colorado/New Mexico with surface low
expected to be over SW Kansas 12z Tuesday, moving NE across Kansas
through the day.  Overall GFS and ECMWF have trended slightly
further south with this track.  Northerly upslope flow across
central and eastern Wyoming will gradually weaken through the
afternoon and evening along with decreasing moisture.  Additional
snow amounts across much of the north and central look to be in the
1 to 3 inch range on Tuesday.

ECMWF and GFS continue to trend slower with progression of upper
ridge across the western U.S. Wednesday through Friday, holding west
to east oriented upper trough over area into Thursday before folding
ridge across the area on Friday.  Warm advection aloft and light
vertical wind profile will set up favorable conditions for steep
inversions and fog formation across snow-covered western valleys and
central basins by Friday morning.

&&

.AVIATION.../12Z ISSUANCE/

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

Conditions will remain VFR through about late morning Saturday
before shortwave and associated moisture moves across the region.
Best chance for MVFR conditions will be at KJAC Saturday
afternoon, and perhaps for a brief period at KRKS late in the day.
Less favorable pattern for snow at KBPI and KPNA and have stuck
with VCSH at those terminals. Departing shortwave will leave
little forcing and moisture to work with after 00Z/Sun. Expect
conditions to improve to VFR at all terminals. However, believe
that if KJAC does receive some light snow low clouds could form
in Jackson Hole late Saturday night. Currently have a BKN020
ceiling to reflect this possibility. As for wind, there will be
a brief period of gusty west-northwest wind at KBPI and KPNA
toward 00Z/Sun. KRKS will blow 15-25kts between 20Z/Sat and at
least 02Z/Sun. Mountain top obscurations will be frequent through
about 06Z/Sun, with lingering obscurations around KJAC until 12Z.

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

Main concern will be fog until about 16Z/Sat. Enough melting
of snow Friday afternoon to moisten the boundary layer. With
clouds departing east, the clearing sky has allowed for good
radiational cooling and subsequent fog formation. Some sporadic
IFR/MVFR conditions possible at KRIW and KWRL until that time.
Could still see MVFR at KLND during the 11Z-15Z/Sat time frame.
KCOD and KCPR look to remain fog free. Fog should dissipate by
17Z/Sat leaving VFR conditions for the remainder of the period.

&&

.FIRE WEATHER...

Fire Danger low for all of WY through the next week. Below seasonal
to seasonal temperatures, a good existing snow pack, and increasing
chances for measurable snowfall over the next week will keep fire
danger very low. Heaviest snowfall of the period will begin Sunday
night west of the Divide, expanding east of the Divide by Monday.
Smoke dispersion fair to good west of the Divide this afternoon,
poor to fair east of the Divide.


&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Winter Storm Watch from Sunday afternoon through Monday
afternoon for WYZ001-012>014-023>025-027.

&&

$$

SHORT TERM...Braun
LONG TERM...AEM
AVIATION...CNJ
FIRE WEATHER...Braun


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