Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 120527
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1027 PM MST Wed Jan 11 2017

.SHORT TERM...Tonight through Saturday (Issued at 124 PM MST)

Imagery shows broad longwave trof across the CONUS with an upstream
axis stretching from WA to the southwest over the EPAC. Southern
branch of the jet across CA and the desert SW into CO and then
central Plains with the northern branch well north across Canada and
into the northeast CONUS. A mesoscale jet streak with warmer air
aloft is also moving across the northern border area of WY/MT. SFC
has a cold front through the region east of the Divide with
comparatively very cold air entrenched...much warmer to the west
ATTM.

Moderate to occasionally heavy snow continues west of the Divide
while east of the Divide...snow associated with the frontal passage
has left the area except for light areas of post frontal
flurries/occasional light snow. The main exception to this is a
seeder/feeder condition across northern WY...including the Cody
Foothills and northern Big Horn Basin...which continue with bouts of
significant snowfall as warm air aloft is creating mid/upper level
unstable conditions with lapse rates from 7.5 to 8.5 producing
clouds and precipitation falling through a very cold lower layer of
clouds. 6 to 12 inches has already been report over portions of this
area...with convective snowfall rates over 3 inches/hr at some
locations. Upgraded the Advisory for the northern Big Horn Basin to
a Warning earlier this morning.

The current weather making system will finally wind down later this
afternoon and tonight with an upper level split flow pattern
beginning. Dialed back the ending of the warnings/advisories from
5am Thursday to 11pm tonight as most significant snowfall should be
well out of the area by that time. Northwest WY will, however, still
be under some mildly convective conditions with light snow
showers/flurries continuing into Thursday morning.

From Thursday through the end of the forecast, the FA will remain
dry for the most part with seasonally cold temperatures (nothing too
extreme) and relatively light wind. The lone exception will occur
Thursday afternoon/evening as the southern branch of the jet and an
associated jet streak moves quickly across southern WY providing
enough mid/upper forcing and overrunning to produce some light
precip/snow mainly along and south of I80. Wind should not be an
issue with this system and snowfall through the afternoon and evening
should remain below 2 inches for most locations with the heaviest
amounts near the border with Colorado.

.LONG TERM...Saturday Night Through Wednesday

Cold dry high pressure will be over western and central Wyoming
with relatively light wind. with much of the western and central
valleys and basins seeing lows from zero to -15.  The high pressure
will remain in place Sunday and Monday with winds remaining rather
light.  Temperatures expected to gradually increase.  The High
pressure ridge will slowly shift east as a rather strong pacific low
pressure trough approaches the west coast resulting in some
increased cloud cover in the west and brisk winds developing through
the wind corridor from Rock Springs to Casper on Tuesday with more
significant warming taking place.  Tuesday evening will see some
light snow develop in the west with clouds spreading east and
southwest winds continuing to increase along and east of the
continental divide and through the above mentioned wind corridor. By
sunrise snowfall expected to become rather widespread and increase
in intensity across the western mountains as strong southwest winds
increase east of the continental divide and significant warming
continues.

&&

.AVIATION.../06Z ISSUANCE/

West of the divide...KBPI/KJAC/KPNA/KRKS Terminals

VFR conditions will occur at most terminal sites through 06Z Fri.
KRKS airport may see a few snow showers until 12Z with MVFR
conditions at times. Then a short duration of snow is forecast to
move across southern Wyoming after 23Z through 07Z Fri. MVFR to
IFR conditions will occur and this includes the KRKS airport.

East of the divide...KCOD/KCPR/KLND/KRIW/KWRL Terminals

VFR conditions will prevail through the period. Patchy fog may
occur in the northern Big Horn Basin until 17Z with local MVFR or
IFR conditions. This may impact the KCOD airport. An area of snow
will develop in southern WY around 23Z. This snow will move northeast
into southern Natrona county Thursday evening. Will have VCSH at
KCPR airport for now. MVFR conditions will be found from KRKS to
near KCPR in the snow area.

&&

.FIRE WEATHER...

Fire Danger low for all of WY through, at least, the weekend. Cold
seasonal temperatures, good existing snow pack, and relatively light
winds will characterize the next several days. Smoke dispersion will
also remain poor for all locations through the weekend.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...None.

&&

$$

SHORT TERM...JB
LONG TERM...CMB
AVIATION...AR
FIRE WEATHER...JB



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