Area Forecast Discussion
Issued by NWS Riverton, WY

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272
FXUS65 KRIW 121700
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1000 AM MST Sun Nov 12 2017

.SHORT TERM...Sunday through Tuesday night

The trough that brought the light snow last night and this morning
has now exited stage east. High pressure will now control the
weather through Monday and bring above normal temperatures and dry
conditions. There will be some passing high clouds from time to
time but otherwise expect a nice day today. The one exception will
be some patchy morning fog in portions of Natrona and Johnson
Counties.

The progressive, zonal pattern looks to continue through Tuesday
night. The next Pacific trough and front will approach the west on
Monday. Monday still looks dry all day everywhere. However, a gusty
southwest wind will develop ahead of the front. Right now, mid level
winds to not look sufficient for high winds across the southwestern
wind corridor, but it will be fairly blustery. This will also allow
for a rather warm day, with high temperatures in the normally
breezier areas possibly reaching 60 degrees. All areas will see
above normal temperatures however.

The aforementioned front and trough will move into the state Monday
night. As expected, the progression of the front as slowed so now
the chance of any snow out west should wait until after midnight on
Monday night. As for the amounts of snow, this looks fairly similar
to the previous couple of systems in that they are fairly quick
hitters. The models were overdone with the QPF with the last system
and maybe doing it again with this system. The heaviest snow would
be Tuesday morning when the best upper level support and left front
quadrant dynamics approach the area. We also still have the
possibility of some rain mixing in initially since temperatures aloft
do not cool down until Tuesday morning. Probably nothing more than
an advisory if any highlights are needed. Any snow should taper off
relatively quickly Tuesday afternoon with dry conditions Tuesday
night.

Another area of concern is wind East of the Divide Monday night and
Tuesday. First will be the wind prone areas around the Cody
Foothills, especially around, you guessed it, Clark. The NAM is more
bullish with 700 millibar winds as high as 65 knots before cold
advection kicks in late at night. Also, some concern across the
southwestern Wind Corridor Monday night and Tuesday, mainly pre-
frontal. However, mid level winds are borderline. This will have
to be watched though. With the strong downsloping flow, any
showers would be limited to the north and mountains. And like the
previous system, with the Pacific origin of the system,
temperatures will not fall much since any really cold air is still
locked up in Canada.

.LONG TERM...Wednesday through Sunday

Overview...A strong Pacific cold front will move across Wyoming
Thursday afternoon-evening.  Mild and dry conditions will precede
the front on Wednesday.  Southwest winds Wednesday night and
Thursday with the possibility of high winds along the eastern
Foothills and from South Pass to Casper.  Snow will increase across
the western mountains Wednesday night and Thursday, with wet snow or
a rain and snow mix likely in the valleys ahead of the front.
Snowfall rates should be highest Thursday afternoon and evening
across the west with the frontal passage, snow levels dropping to
the valley floors.  The front will spread chances of rain and snow
east of the Divide Thursday night with some light snow accumulations
possible by sunrise Friday.  Dry and cooler conditions will prevail
Friday with some lingering chances of light snow across the west.  A
warming trend follows next weekend, dry conditions expected on
Saturday; chances of mountain snow, valley rain/snow returning to
the west on Sunday.

Discussion...The current upper air pattern will persist into the
early part of the extended period.  This pattern features Rex Block
upstream over the Pacific, high amplitude ridge over the central
Pacific with cyclone on the east flank off the NW coast, broad ridge
downstream centered over Mexico. Our main weather maker will be a
shortwave moving over the top of the high central Pacific ridge and
beginning the southward slide across Alaska.  This system is
expected to take up residence off the NW coast on Tuesday, before
getting kicked inland across the Interior West/Rockies Thursday-
Friday.  The main forecast problem has centered around the timing of
this system, the GFS on the fast side of guidance, ECMWF on the slow
side.  The two solutions have shown a decrease in spread over the
last few runs with an overall trend toward slower progression.  The
other noticeable trend has been toward a more split system with lead
shortwave ejecting ahead of the main trough into the northern High
Plains, followed by main trough passage Thursday night-Friday
morning.  The lead shortwave will likely rob the main trough of some
of its moisture, so this scenario does not appear quite as
impressive for snow amounts across the west, looking more like
advisory amounts or less. The slower trends also favor a frontal
passage late Thursday, so high temperatures were warmed especially
across central and southern Wyoming which should remain in the
warm (and windy) sector for most of the day. Ensemble means and
GFS 5-wave continue to show a slight retrogression of blocking
pattern over the Pacific by next weekend with broad ridge
expanding across most of the U.S. This will bring a return to
above normal temperatures Saturday and Sunday with the 8-14
outlook not showing a whole lot of change through Thanksgiving
weekend - above normal temperatures with W-SW flow bringing
Pacific moisture and chances of precipitation into the west.

&&

.AVIATION...18Z Issuance

VFR conditions will prevail through the forecast period at all
terminal sites. A gusty wind will occur at KCPR airport this
afternoon, decreasing by 01Z. VCFG KWRL 11Z-15Z Monday morning.

&&

.FIRE WEATHER...

High pressure will control the weather through Monday. A gusty
breeze will develop this afternoon in the Lee of the Absarokas as
well as across the southwestern Wind Corridor from Rock Springs to
Casper. Relative humidity will remain above critical levels. Mixing
and smoke dispersal will be generally poor abut fair to good from
Rock Springs to Casper.
&&

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

$$

SHORT TERM...Hattings
LONG TERM...Meunier
AVIATION...Braun
FIRE WEATHER...Hattings



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