Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 041726
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1025 AM MST Mon Dec 4 2017

.SHORT TERM...Today through Wednesday night
Issued at 300 AM MST Mon Dec 4 2017

The snow continues to be ongoing, although the main action has now
shifted to East of the Divide. We still have some light echoes
across the west but they are decreasing and the reporting stations
are only reporting light snow for the most part. We will likely
cancel some of the northwestern zones with issuance this morning.
The storm has also underperformed across the northern Big Horn
Basin so we will also likely cancel those at issuance. With the
low moving into southwestern Natrona County, the heaviest snow is
now falling across Fremont, Hot Springs and Natrona Counties,
although the back edge is shifting south and east. At 2 am,
Natrona County is now getting into the heavier snow. We still
expect the worst conditions before sunrise with gradual
improvement through the morning with almost all now over in the
afternoon. Strong winds will continue across northern areas this
morning as well before gradually decreasing as the low moves away
to the east. Sunshine should also gradually increase through the
afternoon. Needless to say, with a colder airmass over the area
and the fresh snowpack in many areas, it will be sharply colder
than the previous few days.

Lows are a bit uncertain tonight since a lot depends on how much
clearing takes place. With the fresh snow, many areas will see the
coldest lows seen in a while. We did lower them a few degrees but
the day shift may want to think about lowering them some more
depending on how much clearing develops.

More settled weather then moves in through Wednesday. With some
weak clippers moving into the Plains states, there will be a
tighter pressure gradient in the east which will keep a gusty wind
going in that area. The models are again showing the Big Horn rain
forest effect with snow showers during every period. The models
always overdue this, so we kept this dry. Temperatures will
continue to be the contrast with areas that mix likely seeing
temperatures close to normal while basins will likely see cooler
temperatures with inversions setting up and the weak December sun
having trouble breaking the inversions.

.LONG TERM...Thursday through Monday
Issued at 300 AM MST Mon Dec 4 2017

Thursday will see a north flow aloft over the area with a few snow
showers over the Northern Bighorn mountains. Dry elsewhere. The
high amplitude ridge over Western North America is to remain
fairly stationary from Friday through Sunday. Dry weather is in
the forecast. The GFS brings in a weak cold front into the east
half of WY Sunday night into next Monday. Any snow showers would
be over the Northern Bighorn mountains. Otherwise dry Sunday night
and Monday. The ECMWF model hints at this cold front but for now
is weaker with the cooling. Leaning toward the GFS model at this
time. Temperatures will be the difficult issue as areas with new
snow cover are going to be colder than those places that missed
out on the snow. As the warm air aloft moves in basin inversions
will continue to enhance limiting the warming potential. Areas
that are mixed will see the milder temps with time as will the
higher elevations. Have attempted to adjust highs and lows to
better fit what is likely to occur. More changes will be needed
once snow cover is confirmed.

&&

.AVIATION...18Z Issuance
Issued at 1025 AM MST Mon Dec 4 2017

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

VFR to prevail for all locations except KRKS which will start out
this forecast period with MVFR/occasional IFR conditions until 20z.
Gusty winds will also mix with light snow at times along the
Interstate 80 corridor through 21z as this activity shifts south and
east this afternoon. Trapped stratus or patchy fog will be possible
in the western valleys later tonight, partially obscuring terrain
and mainly affecting the KJAC terminal after 08z Tuesday.

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

VFR conditions will prevail for most locations through the forecast
period. However, KCOD and KCPR will see a couple of hours at the
beginning of this forecast with MVFR conditions in lower CIGs/brief
VIS reduction in -SN. Otherwise, improving conditions expected
everywhere. Gusty west northwest winds are expected to develop
around the area later with the east slopes of the Absaroka
mountains, vcnty KCOD-KDUB, and also near KCPR tonight seeing
possible LLWS.

&&

.FIRE WEATHER...
Issued at 300 AM MST Mon Dec 4 2017

Snow will continue to end from northwest to southeast this morning
with most areas largely dry by the afternoon with only a few
leftover showers expected. Expect much cooler temperatures with a
gusty wind in many areas although strong winds should decrease
through the day. Relative humidity will remain well above critical
levels. Mixing and smoke dispersal will be generally fair to good.
Expect dry and colder weather for much of the week with inversions
likely developing in the basins and deeper valleys.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until 11 AM MST this morning for WYZ030.

&&

$$

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



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