Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 200515
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1115 PM MDT Thu Oct 19 2017

.UPDATE...
Issued at 809 PM MDT Thu Oct 19 2017

Updated to allow the Red Flag Warning to expire.

&&

.SHORT TERM...(This evening through Sunday)
Issued at 307 PM MDT Thu Oct 19 2017

...Strong winds with low relative humidity Friday...
...Snow with strong winds Friday afternoon and night in the mountains...

Ahead of a Pacific low pressure trough, strong winds are expected
developing late Friday Morning continuing through Friday evening.
Southwest winds of 25 to 35 mph with gusts to 45 mph are
expected. Local wind gusts to 55 mph may occur around Clark.
Relative humidity is expected to drop to 17 to 23 percent in the
afternoon.

The Pacific low pressure trough is expected to bring areas of
moderate to heavy snow showers to the mountains developing
Friday morning and becoming more widespread Friday afternoon
and night. Snowfall of 3 to 5 inches is expected above 8500 feet
with areas of 5 to 8 inches above 9500 feet. The snow level will
fall into the valleys during the evening with 1 to 2 inches possible
below 8500 feet. The snow showers will be accompanied by strong west
winds of 30 to 40 mph with gusts to 50 mph on the passes and higher
ridges.

Snowfall will taper off Late Friday night and Saturday.  Wind
will continue to blow across the south into central areas while
decreasing to the north as cold air moves south behind a front.
Temperatures will drop.  West flow will bring another Pacific
low pressure trough into the west with increased snowfall on Sunday
and winds increasing east of the divide becoming quite windy south and
central and along the east slopes of the mountains.

.LONG TERM...(Sunday night through Thursday)
Issued at 307 PM MDT Thu Oct 19 2017

Beginning of fcst, post frontal and under brisk wnw flow aloft with
the main body of a 130-150kt jet lying just to the north over MT. A
strong P GRAD will lay roughly along the Divide with west to
southwest SFC winds gusting 35 to 50 mph across the Wind Corridor.
Additionally, Sunday night through Monday morning another "typical"
near or high wind pattern across the Absaroka (possibly the Wind
River Mountains) develops with strong upper level wind speeds and
good mountain wave production combine with also strong cross barrier
temp/pressure/omega gradients. Have therefore greatly increased wind
speeds/gusts across those areas over that provided by the standard
model output. Otherwise, precip chances move and end from west to
east mainly across nrn WY Monday with ridging, warming and drying
returning to the FA as well as most of the wrn CONUS...lasting into
next week. Wednesday will likely be the warmest day of the week...at
least 3 to 8 deg warmer than what is currently being dealt (H7 temps
ranging from 7.5 to 10.5 east of the Divide - model depending)
...so this area will obviously need to be nudged with (modest)
increases over the next several days. Otherwise, good overall model
agreement through most of the rest of the forecast period shows
that the next WX system does not affect WY until perhaps Wednesday
night or Thursday morning. For a day 7 - 8 rather large un-focused
system, modest POPs look OK ATTM...especially when taking the a
possibly broad spatial scale into account.

&&

.AVIATION...(For the 06Z TAFS through 06Z Friday night)
Issued at 1114 PM MDT Thu Oct 19 2017

A strong cold front will sweep west across the area between 18Z
Friday and 03Z Saturday with the bulk of the precipitation occurring
over the far west. Before frontal passage, LLWS in many locations
this morning will transition to windy/strong south to southwest
surface wind. In wake of the front, winds will become more west to
northwest, and remain quite windy for a few hours. The heaviest
precipitation with this system is expected with frontal passage over
far western Wyoming between 19Z and 23Z. This could result in MVFR
to brief IFR conditions at the KJAC terminal with rain changing to
snow. MVFR conditions could persist at times through the night at
KJAC. Elsewhere the period of precipitation, and overall chance of
precipitation is too low to prevail precipitation.

&&

.FIRE WEATHER...
Issued AT 307 PM MDT Thu Oct 19 2017

...Strong winds with low relative humidity Friday...
...Snow with strong winds Friday afternoon and night in the mountains...

Ahead of a Pacific low pressure trough, strong winds are expected
developing late Friday Morning continuing through Friday evening.
Southwest winds of 25 to 35 mph with gusts to 45 mph are
expected. Local wind gusts to 55 mph may occur around Clark.
Relative humidity is expected to drop to 17 to 23 percent in the
afternoon.

The Pacific low pressure trough is expected to bring areas of
moderate to heavy snow showers to the mountains developing
Friday morning and becoming more widespread Friday afternoon
and night. Snowfall of 3 to 5 inches is expected above 8500 feet
with areas of 5 to 8 inches above 9500 feet. The snow level will
fall into the valleys during the evening with 1 to 2 inches possible
below 8500 feet. The snow showers will be accompanied by strong west
winds of 30 to 40 mph with gusts to 50 mph on the passes and higher
ridges.

&&

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

$$

UPDATE...Murrell
SHORT TERM...Baker
LONG TERM...Braun
AVIATION...Murrell
FIRE WEATHER...Baker



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