Area Forecast Discussion
Issued by NWS Riverton, WY

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794
FXUS65 KRIW 231059
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
359 AM MST Thu Nov 23 2017

.SHORT TERM...(Today through Saturday night) Issued at 330 AM MST
Thu Nov 23 2017

Today will feature record breaking warmth and windy weather as
the gradient tightens out ahead of a rapidly approaching Pacific
trough. In fact high winds are expected today and tonight over the
Absaroka Mountains and Cody Foothills with cross sections
indicating mtn wave potential along with h7 winds of 75 knots and
RFQ dynamics to pull those winds to the sfc this morning. The
winds later in the day and tonight will be mainly due to RFQ
dynamics. High winds are also expected in the Green Mtns this
afternoon and tonight. Without a strong signature besides the RFQ
dynamics and 70 knot winds aloft, namely a weak h7 height
packing, unimpressive numbers in MOS guidance, and lack of
sufficient sfc gradient, will only mention stronger winds along
Outer Drive south of Casper in an SPS for this afternoon and
tonight.

Dense fog and low clouds will begin to dissipate over the
Upper Green River basin and Star Valley around 12 noon today.

This approaching shortwave wave is a fairly warm system which
will deliver snow above 8000 feet and the far west, pretty high in
elevation for this time of year, with up to 5 inches of wet snow
expected in some of the mtns. Rain will fall below 8k. The
associated strong cool front will max out the precip intensity
around 12Z Friday morning. The sfc pressure rises do not look as
impressive as they did with this front compared to last nights
model run with last nights 3 hourly sfc pressure rise showing a
rise of 11 mb and tonights only 6mb so will only forecast a short
spike in the winds in many areas Friday morning with the fropa.
Expect cooler and drier weather Friday and Saturday but still
above normal temps Friday and Saturday behind the front.

.LONG TERM...(Sunday through Thursday) Issued at 330 AM MST Thu
Nov 23 2017

Transitory ridge moves ovhd Sunday with warm thermal ridge aloft,
not unlike today. Another day of upper 50s to upper 60s east of
the divide and around 50 west. Weather will take a dramatic
downturn Monday into Monday night as system currently moving into
western Alaska dives se and moves across us. It should have a very
potent front as the cold upper low is reinforced by some Siberian
air as it moves into the far ern Pacific which may help to
redevelop it further south. The Euro is digging the srn part very
far south into nrn NM by Tuesday. There should be some development
swd but not ready to buy into the extreme ECMWF solution right
now. We`ll see increasing sw flow ahead of the system Sunday into
Sunday night with the Lander Foothills over to the wind corridor
susceptible to high winds ahead of the cold front which blasts ewd
across the area on late Sunday night through Monday. Winter
highlights will likely be needed out west with potential for
another significant mountain event which should at least be a
solid advisory and possibly warning (>12") amounts in favored
locales. Rain changing to snow across the I80 corridor Monday will
have the potential to cause travel concerns and the nrn/ern
sections may see a combination of strong north wind and a period
of snow. Most areas will see strong winds Sunday and/or Monday.
Conditions start to improve Tuesday (chilly but improving).
Tuesday night could be quite cold, especially for areas with fresh
snow cover and light wind as transitory ridge moves back into the
area. There may be enough of a gradient to keep some of the
favored sw flow areas up a little Tuesday night over what they
could be. We`ll have to watch the weather around the middle of
next week. Mean longwave ridge forms/retrogrades to near the west
coast leaving us susceptible to sewd moving disturbances. The Euro
is showing the strongest one on Wednesday which could hit the
nrn/ern zones with some light pcpn. The GEM and GFS have a farther
north disturbance on Thursday. So, they`re starting to show the
potential we`ll just have to watch the evolution with time. The
blend is dry right now but we`ll have to watch not only the pcpn
potential but winds and clouds with time.

&&

.AVIATION...(For the 12Z TAFS through 12Z Friday morning)
Issued at 330 AM MST Thu Nov 23 2017

|East of the Divide...KCOD/KCPR/KLND/KRIW/KWRL Routes

The main issue again today will be windy to strong west/southwest
wind and/or LLWS in the wind corridor from South Pass to Casper, lee
side of mountain ranges and adjacent foothills especially the Cody
Foothills. Otherwise VFR conditions with lee-enhanced high
cloudiness are expected. After 06Z Friday, a cold front will begin
to drop southward into northern Wyoming switching winds to the north
with the possibility of some mountain snow/valley rain showers.

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

Fog and low ceilings will be the main issue through Thanksgiving
morning. The worst conditions from fog are expected across the Upper
Green River Basin (KBPI and KPNA) where airfield minimums are likely
for parts of this morning. The far western valleys including KJAC
will have a combination of stratocu and some fog, but not expecting
KJAC to go down as hard as the Upper Green terminals. The
ceilings/fog are expected to lift/scatter out by midday with the
Upper Green probably the last one to improve to VFR. Tonight, an
approaching storm system will result in a chance of some mountain
snow/valley rain showers especially after midnight. The approach of
this system will result in breezy southerly winds at KJAC terminal
later today into the overnight hours. If the winds would decouple
tonight, then LLWS would be an issue at KJAC. After 06Z Friday, have
prevailed KJAC with some -SHRA, but kept ceilings in the low VFR
range as the south/southwest flow usually keeps ceilings higher than
what MOS guidance give us.

Please see the Aviation Weather Center for the latest information on
icing and turbulence forecasts.

&&

.FIRE WEATHER...
Issued AT 330 AM MST Thu Nov 23 2017

A gusty wind will develop tonight with high wind expected in the
Lee of the Absarokas today and tonight. High winds are also
expected for zone 289 this afternoon and tonight. Breezy to windy
conditions are still expected across other areas east of the
Divide. Near record high temperatures are expected today. Relative
humidity should remain above critical levels. Snow will fall late
tonight and Friday morning above 8000 feet in the far west with
up to 4 inches of snow possible above 8500 feet. Rain is expected
below 8000 feet.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
High Wind Warning until 5 AM MST Friday for WYZ002-003.

High Wind Warning from noon today to 5 AM MST Friday for WYZ019.

&&

$$

SHORT TERM...Lipson
LONG TERM...Skrbac
AVIATION...Murrell
FIRE WEATHER...Lipson



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