Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 211823 AAA
AFDRIW

Area Forecast Discussion...UPDATED
National Weather Service Riverton WY
1223 PM MDT Thu Sep 21 2017

.UPDATE...
Issued at 515 AM MDT Thu Sep 21 2017

A quick glance at SNOTELS showed some 4 to 6 inch snowfall
amounts above 9000 feet in the Tetons and Togwotee Pass area.
Teton Pass webcam looked like 1 to 3 inches had fallen with the
road becoming almost snowpacked before they plowed it. 06Z model
data showed snow levels around 7500-8500 feet across NW WY before
rising to 8500 to as much as 10000 feet as warm air surges
northward, especially over Teton County. Models in decent
agreement keeping a cold pool across the central/northern
Absaroka as well as Northeast/eastern sections of Yellowstone
National Park which would keep snow level around 8500 feet.

Based on the ongoing trends this morning, decided a winter weather
advisory was needed across the northwestern mountains through the
day Friday. However decided the Teton/Gros Ventre Mountains only
needed as , but with all models showing a surge of warm air of +2
to +5C by this evening over Teton County elected to end their
advisory at 3 PM this afternoon.

Updated WSW, ZFP, and grids are out.

&&

.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 18Z TAFS through 18Z Friday morning) Issued
at 215 AM MDT Thu Sep 21 2017

A large southwest northeast oriented swath of stratiform
precipiation lies across Western and Northern Wyoming. The
southeast leading edge of this precip shield runs from Cokeville
in Western Lincoln County, northeast to Dubois, and to Lovell in
the Northern Big Horn Basin. This shield of precipitation is
fairly stationary at the moment along with individual
precipitation elements tracking northeast within this area of
precip. For the rest of this afternoon, through tonight, and
Friday through 18z, expect the leading edge of this precipitation
band to slowly spread southeast to eventually encompass the rest
of Western and Central Wyoming including all area east of the
divide. Expect large areas of MVFR/IFR conditions within the
precip band, with the lowest cigs and vsby expected in Northwest
WY with KJAC and KCOD experiencing the lowest cigs/vsby at times
through this time frame. Expect mainly north northeast winds
across the north half of the area and southwest winds in the
south, with the associated surface boundary along the wind shift
line oscillating north this afternoon, then south after 01z this
evening, and then north again after 18z Friday. The winds in the
vcnty KCPR will likely shift to the southwest by 21z this
afternoon, then shifting back to northeast after 01z this evening,
then back to southwest by 18z Friday.

&&

.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...
Winter Weather Advisory until 6 PM MDT Friday for WYZ001-002.

Winter Weather Advisory until 3 PM MDT this afternoon for WYZ012.

&&

$$

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


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