Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 130451
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1051 PM MDT Thu Apr 12 2018

.SHORT TERM...This evening through Sunday

The main negatively tilted trough continues to dig over the far
western US with the base of the trough extending down into the
Desert Southwest and Old Mexico. The trough axis is expected to be
over the CWA by 12Z Friday, with the trough closing off over the
Central Rockies as indicated by the models, except the Euro which
only comes close to closing it off. A lead shortwave out ahead of
the main trough tracked northeast across NW WY this morning and
produced substantial snowfall over portions of Northwest Wyoming,
namely an estimated 10 inches over the higher of the Tetons and 6
inches at Colter Bay, likely with the help of a jet couplet. An
additional estimated 5 to 6 inches fell over the Tetons during the
day today for a storm total so far of around 10 to 15 inches. For
the rest of this wx event, it appears that the majority of the
energy will be to the north/northeast, and south of the CWA.
Currently, the precip that fell over Northwest and Northern WY has
moved north of the area to Montana. The mid level circulation
will move east, north of the WY/MT border during the remainder of
the afternoon. Then, cyclogenesis will take place with the mid and
upper low beginning to bomb over the vcnty of where
KS/CO/Nebraska come together, with the low almost vertically
stacked from 700 to 500mb, and the sfc low redeveloping well to
the east of there. The best QG forcing will be in the northern
portion of WY today and then will jump to our south Friday. Will
leave the winter highlights as they are across Western Wyoming as
well as the Big Horn Mtns/Johnson County, but decided to cancel
the advisory for the Cody Foothills and Northern Big Horn Basin
that was valid from 9 pm this evening through 9 am Friday as there
is doubt that the wrap around moisture from the bombing low will
make it that far west and then down into the Big horn Basin. In
addition, a northerly component to the wrap around flow is a
downslope flow across portions of the eastern side of the basin.
As for the other lower elevations east of the divide under a
winter weather advisory, Johnson County will get a bit more precip
as well as 30 mph sustained winds, so that is justified with 2 to
3 inches of snow forecast there.

As for the high wind warning for the Southern Wind Corridor, RKS is
blowing as it should, although Casper had a hard time getting
with it as the southwest winds finally arrived there within the
past 30 minutes, as the warm front had a tough time tracking
northeast of the city. The Casper airport had light variable winds
and 54 degrees, yet Casper Mtn was 51 degrees with a southwest
wind gusting to 37 mph. Casper is just now starting to blow with
southwest winds finally gusting to 22 knots, so will leave the
high wind warning alone for now. This slow movement of the warm
front is due to the fact that an asct weak sfc low is tracking
slowly east across Central Wyoming, keeping the warm front at bay.
KRKS is gusting out of the west at 43 mph. These winds are in
response to the tightening gradient ahead of the approaching
trough from the west.

Any additional showers that develop during the remainder of this
afternoon east of the divide could actually produce an isolated
lightning strike or two as there is a large swath of minus 2 lifted
indices over Central Wyoming with the bulls eye of instability
having been accurately predicted by the GFS this morning over much
of Fremont County. There is a bulls eye of 600 capes over Fremont
County as well. This instability is likely tied into the weak sfc
low.

With 5, 700mb height lines spaced 30 decameters apart over WY
Friday, northwest winds of 30 to 40 mph will be fairly common behind
the trough along with 30 to 50 mph nnw winds over Johnson County.

After this potent weather system departs to the east, a high
pressure ridge will build in from the west. That will result in dry
wx and warming temps. After a chilly day Friday with highs in the
40s at the lower elevations east of the divide Friday and 30s west,
we will see temps warm from the 50s Saturday and 60s Sunday east,
and the 40s Saturday and 50s Sunday west. The only fly in the
ointment during this period will be a chance of rain/snow showers in
the far west Saturday night as a weak shortwave tracks across the
area.

&&

.LONG TERM...Sunday night through Thursday night

Overview...Models continue to trend with the ridge shifting eastward
Sunday night and H7 temps warming considerably into Monday. A strong
cold front associated with the next trough will enter western WY
late Monday afternoon and quickly pass over the forecast area Monday
night/Tuesday morning. This will be followed by another ridge that
builds over the region Wednesday, only to be followed by a digging
trough that will make its way over the Four Corners region. Moisture
associated with this system could begin to make its way over
portions of the forecast area by Thursday night.

Discussion...H7 temps will increase Sunday night into Monday, with
readings reaching +5C to +6C east of the Divide (+7C is still
possible for portions of Johnson, Natrona and eastern Sweetwater
counties). Widespread 60s will occur, but may need to bump toward
the lower 70s as these areas will be very mixed due to very windy
conditions. In addition, fire concerns will have to be watched as
RH values drop into the teens Monday afternoon. Rain and snow
showers will develop across the west behind the cold front Monday
afternoon, spreading across the rest of the forecast area Monday
night into Tuesday. Windy conditions will continue across much of
the area Tuesday, in the wake of the cold front. Despite the ridge
on Wednesday, temperatures will remain seasonal. Gusty afternoon
winds will develop over the Wind Corridor. Thursday looks to be
the quietest day of the period, however, will have to keep an eye
on the storm track of this system. If it tracks further north,
then temperatures will certainly be impacted. Also, there could be
an increase chance for afternoon showers. For now, it looks like
most of the impact will be outside of the forecast period.

&&

.AVIATION...For the 06Z TAFS through 06Z Saturday
Issued at 1050 PM MDT Thu Apr 12 2018

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

Scattered rain and snow showers will occur overnight from KBYG to
KCPR. Only isolated rain or snow showers will occur elsewhere. Then
periods of light snow will occur from KBYG to KCPR after 12Z Friday
through 00Z Saturday. Areas of MVFR to IFR conditions will occur
overnight through 06Z Saturday from KBYG to KCPR. Scattered rain or
snow showers will develop Friday afternoon in central WY with local
MVFR conditions after 21Z Fri through 02Z Sat. Gusty wind will be
common at the terminal sites through 01Z Saturday.

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

Snow will occur through 13Z in the far west from KJAC to Kemmerer
with MVFR to IFR conditions and some mountain obscuration.
Conditions will improve to VFR at KJAC airport around 13Z. Scattered
snow showers will develop over the Wind River mountains and the
Absaroka mountains Friday afternoon into the evening. Will put VCSH
at KRKS airport from 00Z to 06Z Saturday. MVFR conditions will occur
in the snow showers. Gusty wind will occur at most terminal sites
through 01Z Saturday.

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

&&

.FIRE WEATHER...

A cold front will move across the area during the remainder of today.
Strong winds are likely for many areas, especially from Rock
Springs through Casper. However, with somewhat lower temperatures
and higher relative humidity, fire conditions should improve a
bit. Snow will fall across much of the west, with some rain and
snow showers east of the divide followed by much colder
temperatures. Friday at this point looks chilly, windy and
showery. Mixing and smoke dispersal will generally range from good
to excellent. Expect a warming trend and mostly dry weather for
the weekend.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...

Winter Weather Advisory from 9 PM this evening to 6 AM MDT
Friday for WYZ013-023.

Winter Storm Warning until 6 AM MDT Friday for WYZ001-002-012.

Winter Weather Advisory from 9 PM this evening to 3 PM MDT
Friday for WYZ010-011.

Winter Weather Advisory until 6 AM MDT Friday for WYZ014-015-024.

Winter Weather Advisory until 3 PM MDT Friday for WYZ008-009.

&&

$$

SHORT TERM...Lipson
LONG TERM...LaVoie
AVIATION...AR
FIRE WEATHER...Lipson



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