Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 220451
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1051 PM MDT Thu Sep 21 2017

.SHORT TERM...(This evening through Sunday)
Issued at 132 PM MDT Thu Sep 21 2017

Quite a bit of snow fell in the northwest mountains last night as
expected with widespread 6 to 12 inches in the Tetons and varying
amounts of 2 to 8 inches elsewhere. The Grand Targhee SNOTEL is up
to 15 inches for the event. Even the Jackson Valley had a period of
snow with a dusting in Jackson and an 1 to 2 inches in the north end
of the valley. Snow has continued today as the sern edge of our
baroclinic band hovers over the nw and wcntrl sections. Some of the
snow showers have been fairly heavy with large flakes and limited
visibility. Large upper low will begin developing over the next 48
hours and then move slowly begin moving ewd on Sunday and into the
extended period. Baroclinic cloud shield will likely become more n-s
oriented this evening but also it`s a fine line if the pcpn will
shift slightly wwd for a few hours as the trough edges a little ewd
attm. Either way, increasing qg forcing and developing upper jet
divergence will develop a moderate to heavy band of pcpn over much
of our nwrn zones after 06z into Friday morning for more moderate to
heavy snow. The snow will probably lighten again late Friday into
Friday afternoon but snowfall will likely be fairly significant
again when these dynamics come together. I think we have a decent
shot of another 3 to 6 inches in the Tetons with local amounts to 8
inches with heavier amounts as you head north and east. Overall 5 to
10 inches from the northern Wind Rivers to the mountains on the east
side of YNP over to the Absarokas. The Beartooth Plateau could see a
good foot. Not much on South Pass until the low starts moving east
Saturday. Will extend our advisory for the Tetons until 21z Friday
and issue a winter weather advisory for the Wind Rivers focusing on
the north half. The snow level will likely rise some this evening
but then lower again after midnight with heavier pcpn period and
slight ewd advance of the baroclinic band. The Cody Foothills will
also likely see a increase of cold rain later tonight into Friday
morning with snow levels down to 5.5 to 6K feet or so. Cody could be
close for a couple hours near and after sunrise. As the upper low
strengthens and moves from nrn NV towards wrn UT Friday night into
Saturday, the central sections east of the divide (and south) see
increasing chances of pcpn. QG forcing develops Friday night for
much of the central and srn sections and continue through the day
Sunday. In addition, a moist upglide covers Natrona and especially
Fremont County for much of this period. Widespread lower elevation
rain is expected much of the weekend with snow level down into the
foothills and getting down to around 6K feet or a little lower at
times during the early morning hours. Beefed up pops to likely
across much of the area, especially Fremont to cover this idea. The
I80 corridor will also likely see period of rain showers over the
weekend. More winter highlights are possible for the Wind Rivers,
sern Absarokas and possibly the Big Horns. Even the highest spots on
Casper Mountain could see some decent snow but only near the top the
way it looks right now.

.LONG TERM...(Sunday night through Thursday)
Issued at 132 PM MDT Thu Sep 21 2017

By Sunday night and into Monday, the slow moving amplified
positively tilted trough will try to cut off. The GFS cuts it off
over Utah with the low moving east into Colorado, the Euro keeps the
trough more open, and the Canadian is kind of the compromise
depicting the low closing off over Wyoming. For now have kept some
of the higher pops over Southern Wyoming closer to the h5 low with
the GFS also depicting a mid level circulation over Sweetwater
County Monday. Also, with this pattern, low level upslope flow will
enhance precip somewhat east of the divide from Lander to Casper.
Snow levels during this time frame should average around 7k. With
dynamics on the wan kept amounts light.  Then by Monday night,
precip will end from west to east as a positively tilted ridge
builds in from the west as the long wave trough flattens over the
west. The GFS and the Euro keep us under nw steering flow on the
front side of the ridge and the back side of the exiting trough with
the Canadian keeping us under a looser gradient. With nw flow, have
very low pops in nw mtns for late Tuesday. Temps will warm up in the
middle of the week as the ridge builds in.

&&

.AVIATION...(For the 06Z TAFS through 06Z Saturday)
Issued at 1051 PM MDT Thu Sep 21 2017

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

Precipitation shield blossomed over eastern Idaho and western
Wyoming Thursday evening. Jet dynamics associated with south-north
oriented 100kt+ jet have been a large player, with even some thunder
earlier this evening near KBPI and KPNA. These dynamics and this
enhanced band of precipitation should slowly drift east between 09Z-
12Z/Fri. Another round of precipitation will reach the west around
midday with shortwave rotating north across the region during the
afternoon. Therefore, expect IFR/MVFR conditions at KJAC to improve
slightly after 12Z/Fri before this second round of showers impacts
KJAC late in the day. Best dynamics and moisture will slide east
after 00Z/Sat with improving conditions. Overall, KBPI, KPNA, and
KRKS should remain VFR through the forecast period. Band of
precipitation sweeps across these terminals between about 11Z and
16Z/Fri, so there could be some brief MVFR ceilings. Best bet would
be at KBPI and KPNA. Winds will stay west or southwest at these
three terminals, before cold front drops south between 03Z-06Z/Sat
and winds begin to veer to the northwest and north. Mountain top
obscurations will be widespread throughout the period, with frequent
mountain obscurations over the far west mountains until 00Z/Sat.

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

Precipitation shield across the west will march east early Friday
with a period of rain showers possible at all but KCPR. Jet dynamics
the main driver in sustaining this band of precipitation. KCOD will
continue to see IFR/MVFR through most of the forecast period. Best
chance of heavier precipitation at KCOD will come between 11Z-
15Z/Fri and then again after 00Z/Sat. Other terminals will be VFR
until moisture in southwest flow overruns cold front in place east
of the Divide after 00Z/Sat. Best chance of MVFR will come after
about 03Z/Sat. Cold front will be nearly stationary south of these
terminals and don`t see it wobbling north during the day Friday, so
north wind around 10kts to prevail. Another northerly surge will
push south across the terminals between 00Z/Sat and 06Z/Sat with 10-
20kt winds anticipated. Best chance for mountain obscurations will
be along the east slopes of the Absaroka and Bighorn ranges during
the period. Mountain top obscurations will be widespread throughout.

&&

.FIRE WEATHER...
Issued AT 132 PM 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).

Copious rainfall (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 Friday. Widespread rain and foothill
snow will shift east of the divide this weekend. There will be windy
conditions in our sern zones Friday but min rh`s will only drop into
the 30s. Saturday and Sunday will be cool/cold and moist.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until 6 PM MDT Friday for WYZ001-002.

Winter Weather Advisory from 9 PM this evening to noon MDT
Friday for WYZ014-015.

Winter Weather Advisory until 3 PM MDT Friday for WYZ012.

&&

$$

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


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