Area Forecast Discussion
Issued by NWS Riverton, WY

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246
FXUS65 KRIW 112024
AFDRIW

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

.SHORT TERM...Tonight through Saturday

...WINTER STORM WARNINGS CONTINUE FOR MOST OF WESTERN WYOMING IN
ADDITION TO THE CODY FOOTHILLS AND THE NORTHERN BIG HORN BASIN
THROUGH THIS EVENING...

...WINTER WEATHER ADVISORIES CONTINUE FOR PORTIONS OF CENTRAL AND
SOUTHERN WYOMING THROUGH THIS AFTERNOON AND EVENING...

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.../Z ISSUANCE/

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

Periods of snow in the west through 09Z. KJAC airport will stay in
the snow through 06Z Thurs. MVFR to IFR conditions will occur
through the period. The other terminal sites in SW WY will see
improving conditions with VFR conditions. Expect a gusty wind at
many terminal sites through 02Z. Mountains will be frequently
obscured. 09Z thru 15Z local fog with shallow low stratus expected
to develop in valleys and basins along rivers and lakes with local
IFR ceilings and visibilities. Please see the individual terminal
forecasts for more details.

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

In northern WY areas of snow will occur through 06Z with MVFR to IFR
conditions prevailing. Conditions will be improving in the north
after 06Z as the snow ends and cloud heights rise. Expect VFR
conditions in most places by 06Z Thurs. For the central area...VFR
conditions expected. KCPR terminal will see improving conditions 00Z
to 93z.  KRIW and KLND should remain VFR.  Mountain snow will
continue along the continental divide with some obscuration also
expected through the period.  09Z thru 15Z local fog with shallow
low stratus expected develop in valleys and basins along rivers and
lakes with local IFR ceilings and visibilities.  Please see the
individual terminal forecasts for more details.

&&

.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...
Winter Weather Advisory until 5 PM MST this afternoon for
WYZ028>030.

Winter Storm Warning until 11 PM MST this evening for WYZ001>004-
012>015-023>027.

Winter Weather Advisory until 11 PM MST this evening for WYZ016.

&&

$$

SHORT TERM...99
LONG TERM...99
AVIATION...Team Riverton
FIRE WEATHER...Team Riverton



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