Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 161755 AAA
AFDRIW

Area Forecast Discussion...UPDATED
National Weather Service Riverton WY
1055 AM MST Tue Jan 16 2018

.SHORT TERM...Tuesday through Thursday night

For the most part, things are fairly tranquil across the humble
Riverton weather abode early this morning. However, notice how I
used the modifier "for the most part." We do have one concern this
morning, this being the return of the dreaded fog monster. So far,
this has been mainly across the western valleys, where Jackson and
Afton have been in and out of the soup much of yesterday and into
this evening. Meanwhile, East of the Divide, there has been less in
the basins. At this point, it does not look terribly widespread so
we kept the definition as patchy. The other concern is temperatures,
especially in the valleys where warm advection will likely lead to
some inversions. It does not look to be very cold, especially due to
the lack of recent snow that will limit albedo somewhat.
Nevertheless, we did keep temperatures below guidance in the basins.
Areas that mix will see some increasing southwest flow that will
warm temperatures as a shortwave approaches from the west.

And speaking of that shortwave, other than some gusty winds across
the southwestern wind corridor, effects look to be minimal. The wave
looks like it will split and with limited moisture to work with we
kept the forecast dry. It could be enough to mix out some of the
western valleys though and keep fog from reforming.

At this point Wednesday looks to be a dry and quiet day with a gusty
wind continuing across the favored area from Rock Springs through
Casper. However, 700 millibar winds do not look strong enough for
high winds.

Things may get more active toward the end of the period. The models,
especially the GFS, continue to slow down the progress of the next
trough and cold front into western Wyoming (big Surprise, note
sarcasm. As a result, we removed POPS across the west. At this
point, the heaviest precipitation would likely hold off until after
midnight and into Friday. Meanwhile, mainly dry conditions should
hold East of the Divide. However, with a tightening pressure
gradient and increasing 700 millibar winds, it will become breezy
and even downright windy for many areas Thursday into Thursday
night. With the aforementioned 700 millibar winds eclipsing 50 knots
in some areas, we may have to watch out for the possibility of high
wind highlights. The main areas will be the usual suspects, the
corridor from the Green Mountains into Natrona County as well as the
Absarokas and Cody Foothills.

.LONG TERM...Friday through Tuesday

A positively tilted trough will push across the Rockies Friday
and Saturday. The associated cold front is expected to push
east/southeast across western Wyoming Friday morning and then
across the rest of the forecast area Friday afternoon/evening. A
moist southwest flow, jet streaks and frontal boundary could
result in a period of significant snowfall across the west Friday.
Ahead of the front, strong to high wind will be possible in the
central wind corridor. Precipitation will gradually break out
Friday afternoon/evening east of the Divide as the cold front
pulls across the rest of the forecast area. Initially the
precipitation east of the Divide could begin as rain, but should
turn over to snow by Friday evening in wake of the front. The
ECMWF is farther north with the 700mb low than the GFS and
Canadian models which results in the ECMWF producing a prolonged
period of snow across central and south-central parts of the
forecast area Friday night into Saturday. On the other hand the
GFS and Canadian models also some precipitation, but quickly push
it out most of the forecast area by Saturday morning. Still plenty
of time to watch this system evolve for areas east of the Divide,
and will keep high chance PoPs for the event as some snowfall is
still expected, but exact timing/amounts remain in question.

A transitory ridge is expected Saturday night and Sunday with
mainly dry conditions. A moist westerly flow is then expected to
develop Sunday night at the base of an upper low/trough pushing
into the Pacific northwest. Disturbances in the flow and a moist
westerly flow should result in a good chance of snow across the
west possibly spilling east of the Divide Monday as a disturbance
in the flow push across the region. The mean trough is then
expected to form across the Rockies Tuesday with some inclement
weather continuing mainly west of the Divide.

Temperatures will above average Friday, but will turn to seasonal
to slightly below average for the rest of the extended.

&&

.AVIATION...(For the 18Z TAFS through 18Z Wednesday) Issued at
1055 AM MST Tue Jan 16 2018

West of the Continental Divide...KJAC, KBPI, KPNA and KRKS

Erosion of MVFR/IFR ceilings are occurring over the Upper Green
River Basin with low clouds lingering over the Jackson Valley
through at least 21Z this afternoon. Fog with LIFR conditions
around the Afton area should dissipate by 21Z this afternoon. Any
improvement is expected to be temporary with steep inversions
remaining over the area through the day. Thus, forecasting
MVFR/IFR conditions to persist through at least 21Z this afternoon
at KJAC. Once fog and low clouds lift in the afternoon, a weak
upper level disturbance and some mid- level clouds should hinder
the redevelopment of fog and lower ceilings through 12z Wednesday
through 18Z Wednesday.

East of the Continental Divide...KCPR, KCOD, KRIW, KLND, and KWRL

Satellite fog imagery showed a persistent area of MVFR ceilings
across the Wind River Basin that will likely persist through at
least 20z this afternoon. Once these ceilings dissipate this
afternoon, VFR conditions with unlimited ceilings are expected to
prevail through 18z Wednesday. Fog, low cloud redevelopment
should be hindered as a weak upper level disturbance moves across
the area.

&&

.FIRE WEATHER...

Mainly dry weather will continue through Thursday. Inversions will
continue over the western valleys with areas of fog around.
Inversions are also likely over portions of the Basins east of
the divide. A gusty breeze will develop across the southwestern
wind corridor from the Red Desert through Natrona County. Mixing
and smoke dispersal will be generally poor.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...NONE.

&&

$$

SHORT TERM...Hattings
LONG TERM...Murrell
AVIATION...Meunier/Lipson
FIRE WEATHER...Hattings



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