Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 210810
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
210 AM MDT Thu Sep 21 2017

.SHORT TERM...(Today through Saturday night)
Issued at 215 AM MDT Thu Sep 21 2017

Imagery shows broad deep upper low/trof across the wrn 2/3rds of the
CONUS with modest high pressure ridge tot he east with the exception
of the embedded TS JOSE east of Long Island, NY. Strong upper jet
stretches from cntrl California through NV, nrn UT, ern ID and
across the nwrn half of WY. SFC has low pressure across much of the
wrn CONUS with a front located nearly under the jet from cntrl MT
through cntrl California. Moderate to heavy precipitation entering
wcntrl/nwrn WY this morning.

Significant changes in the forecast with respect to the
trof/embedded closed low across the wrn CONUS...trof axis and
elongation/digging of trof staying further west for a longer period
of time than originally through...is keeping warm air over the FA at
least through this evening. This will, in effect, inhibit much in
the way of accumulating snowfall below 9.5 ft. This has killed
the Watch/Warning/Advisories that were previously issued over the
last day or so. Snow accumulation should begin to lower below the
highest levels by Friday morning. In the mean time, copious
precipitation arrives this morning and continues in earnest
through Friday west of the Divide and across ncntrl WY before the
displacement of the upper trof ewd provides the impetus to shift
precipitation east of the Divide Friday night (finally). The
arrival of colder air will also coincide with this shift. Before
the colder air arrives, however, accumulating snowfall will mainly
be confined to the highest elevations above 9500 feet...with not
enough area covered really to issue an Advisory/Warning at this
time. That said, precipitation amounts from 0.5 inch to as much as
1.5 inches of liquid will be common from now through the day
Friday. As far as snowfall after temps get cold enough...now
expecting mainly sub- Advisory amounts below 9500 feet through the
day Friday...with Advisory amounts possible above 9500 feet over
prime locations such as the Wind River Mountains. From Friday
night through the end of the forecast period, the bulk of the
precipitation shifts east of the Divide with perhaps a quarter to
half an inch of liquid falling over portions of cntrl and srn WY.
Some snow will also be possible...although only light amounts
ranging from a trace to 2 inches from Friday night through
Saturday night...and most of that falling (with little if any
sticking to roadways) during the overnight/early morning periods.
Temperatures will be relatively cold for this time of year with
highs across the lower elevations ranging from the upper 30s to
upper 40s...20s and 30s in the mountains.

.LONG TERM...(Sunday through Thursday)
Issued at 215 AM MDT Thu Sep 21 2017

00Z global models are showing an upper low in the mean long-wave
trough over south-central Utah at 12Z Sunday. This upper low is
expected to slowly lift to around Grand Junction by 00Z Monday,
and then be across east-central Wyoming by 00Z Tuesday. This track
will keep good chances of precipitation mainly across the
southeast/eastern half of the forecast area Sunday and Monday.
Snow levels look to range between 6000 and 7500 feet with a trend
to the lower of that range on Monday. Could see localized advisory
amounts of snow in the central mountains, but confidence is not
high at this time.

In wake of this upper low, a cool, but relatively dry northwest
flow will across the area Tuesday. Except for a few snow showers
over the northern mountains, Tuesday looks to be quiet with a
warming trend.

Wednesday and Thursday, ridging begins to build/overspread the
West Coast/Northern Rockies with dry conditions and temperatures
returning to seasonal levels.

&&

.AVIATION...(For the 12Z TAFS through 12Z Friday morning)
Issued at 215 AM MDT Thu Sep 21 2017

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

Precipitation spreading across northwest Wyoming early this morning
in southwest flow aloft. Conditions will generally oscillate between
IFR and MVFR at KJAC today and tonight. Temperatures will remain
warm enough for precipitation to remain all warm through today and
probably through the evening at KJAC. There could be a mix of rain
and snow early Friday morning, but belief is that it will likely
remain rain through the forecast period. Mountain tops will be
obscured across western Wyoming through Thursday night. Best chance
for mountain obscurations will be north and west of a line from KAFO
to near KDUB to around KCOD. Snow levels around 10K feet MSL will
fall to as low as 7K ft MSL late Thursday night. KJAC southwest
surface wind will veer to the north during the mid-late afternoon
hours Thursday with the north flow continuing through the night. VFR
to prevail for most of Thursday at KBPI, KPNA, and KRKS. Possible
MVFR at KBPI and KPNA late Thursday afternoon as moisture slides
slightly east and ceilings lower. Higher likelihood of MVFR at KBPI
and KPNA comes after 00Z/Fri as jet dynamics and moisture become
more favorable for precipitation production. Precipitation may wait
until after 06Z/Fri to reach KRKS. As for wind, a tightening
pressure gradient and daytime mixing will lead to southwest 25G35kt
wind at KRKS, while KBPI and KPNA look to remain 10G20kts in the
afternoon. Winds decrease with the setting sun.

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

Conditions will remain VFR through the bulk of the forecast period
at all terminals except KCOD. Precipitation will be generally
confined to areas along and west of the Continental Divide through
00Z/Fri with a better chance of light rain late Thursday night from
KRIW to KWRL to KCOD. Northerly frontal push will provide a better
chance of MVFR ceilings at KCOD beginning late Thursday morning
through the remainder of the period. This frontal push provides
northerly flow in the Big Horn and eventually Wind River basins.
Speeds of 10-15kts will spread north-to-south in these basins during
the late afternoon and early evening. Besides KCOD, best chance for
low-end VFR or MVFR ceilings will be at KWRL beginning around
00Z/Fri. Have included VCSH at KRIW and KWRL late Thursday night as
moisture begins to spread east and jet dynamics should assist in
precipitation production. As for wind, occasional higher gusts of 16-
22kts are possible with a secondary northerly surge around sunset
Thursday on into the evening. At KCPR, dry conditions along with
gusty southwest wind 15G25kts are expected through about 23Z/Thu.
Northerly surge will then produces a northeast wind Thursday night.

&&

.FIRE WEATHER...
Issued AT 215 AM MDT Thu Sep 21 2017

Fuels at critical levels for most of the low elevation forecast
zones east of the Divide and across southern Wyoming. (Please see
the Fire Weather Forecast product for more information).

Today will be cooler across western and northern Wyoming...slightly
warmer over portions of central and souther Wyoming with minimum RH
values largely ranging above 20 percent everywhere. Wind will be
less gusty than previous day for most of the area except across
south central Wyoming where south to southwest winds will gust 30 to
45 mph at times. Copious rainfall (high mountain snow) is expected
west of the Divide and across northern Wyoming...with from a half
inch to and inch and a half of liquid possible through tonight.
Otherwise, very cool/unseasonably cold temperatures and relatively
high moisture with precipitation at times is expected to continue
through the weekend.

&&

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

&&

$$

SHORT TERM...Braun
LONG TERM...Murrell
AVIATION...Jones
FIRE WEATHER...Braun


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