Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 201225
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
525 AM MST Tue Feb 20 2018

.UPDATE...
Issued at 522 AM MST Tue Feb 20 2018

Decided to issue a wind chill warning for the lower elevations of
Natrona County since winds at KCPR and KTMH (Twenty-mile Hill)
have been reaching wind thresholds as well as wind chills cooler
than 30 below zero. This warning goes until 10 AM. The rest of the
area have calm to winds of 5 mph so will keep current SPS going
for extreme cold. Updated grids and zones are already out.

&&

.SHORT TERM...(Today through Thursday night)
Issued at 105 AM MST Tue Feb 20 2018

Imagery shows a large deep trough across most of the CONUS some
moderate ridging over the East Coast. The trough axis is cutting
through north central MT-western WY-and on through UT into the
Desert SW. The northern branch of the jet continues to dive down the
West Coast, rounding the horn over the srn Great Basin/Desert
Southwest, then heads northeast over CO and off through the central
and northern Great Plains. A nice upper level baroclinic
leaf/deformation zone and associated jet are moving out of WY and
into the Plains states. The surface has an exiting Arctic cold front
south and east of WY with strong high pressure building across the
forecast area and all points north.

Today into Wednesday morning, the main trough continues to translate
eastward with the trough axis oriented over the western border of WY
this morning. At the same time, as the surface front(s) continue to
push to the south and east, a few areas of lingering light
snow/flurries will remain over some locations, especially the
mountains, that can`t shake loose the clouds as the relatively
strong high pressure spreads across Wyoming. Extremely cold
temperatures are now being left in the wake of the exiting cold
front. With all the new fallen snow and H7 temps dropping to -22 to
-27 deg C for both Today and Wednesday, and depending on the amount
of clearing in the skies, expect extremely cold morning
temperatures...ranging from -5 to -30 deg F, only rising into the
single digits and lower teens during the day. The coldest air looks
to be trapped in the area`s Basins and across Yellowstone NP both
today and Wednesday.

Wednesday night through Thursday night, the main upper level trough
continues translating slowly to the east while expanding westward as
the West Coast jet and associated embedded shortwave dig deep down
along the coast. This general troughiness will prevail through the
end of the forecast period with small chances for light snowfall
remaining in the forecast across the western zones. And while it
will still be quite cold Wednesday, there are signs that some
moderation will begin across southern and eastern Wyoming as low
level winds return to west/southwest and skies let more sunshine
through with what looks like an inverted surface trough sneaking
into the area from the south. Thursday should see daytime
temperatures reaching all the way into the teens and lower 20s!

.LONG TERM...(Friday through Monday)
Issued at 105 AM MST Tue Feb 20 2018

For the most part, a longwave trough will remain in place across the
western U.S. for the extended. Temperatures are expected to below
average to much below average as a result. On Friday, a system in
the northern part of the mean trough could give the west a shot of
snowfall, but the main energy is progged to be with the southern
stream which could clip the southern part of the forecast area with
some snowfall as the bulk of the energy remains south of the area.
Maybe some flat ridging on Saturday, before another trough digs
southeast into the western U.S. Sunday and Monday. However models
differ quite a bit on timing/location/strength of this system. Thus
have gone slight/low chance PoPs across the west with little or no
PoPs east of the Divide. For now this system doesn`t seem to be
anything major.

&&

.AVIATION...(For the 12Z TAFS through 12Z Wednesday morning)
Issued at 105 AM MST Tue Feb 20 2018

West of the Continental Divide...KJAC, KBPI, KPNA and KRKS

Some lingering light snow through 12Z with local MVFR conditions.
Otherwise VFR conditions will prevail after 12Z through 06Z Wed.
Snow showers will occur in the western mountains through 03Z Wed
with some mountain top obscuration. Will have VCSH in the western
terminal sites in the afternoon into early evening.

East of the Continental Divide...KCPR, KCOD, KRIW, KLND, and KWRL

Lingering light snow in places through 14Z with local MVFR
conditions. After 14Z VFR conditions will prevail over the area.

Please see the Aviation Weather Center and/or CWSU ZDV and ZLC for
the latest information on icing and turbulence forecasts.


&&

.FIRE WEATHER...
Issued AT 105 AM MST Tue Feb 20 2018

A large and cold winter weather system has exited the region with
most all of the forecast area receiving significant snowfall.
However, extremely cold conditions will overtake the region in the
wake of the exiting cold front. Winds have quieted, but brutally
cold morning lows of 5 to 30 below zero will be common, especially
this morning and Wednesday morning. Smoke dispersion will be poor to
fair (east of the Divide) to good (west of the divide) both today
and Wednesday.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Wind Chill Warning until 10 AM MST this morning for WYZ020.

&&

$$

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


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