Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 241659
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
959 AM MST Tue Jan 24 2017

.SHORT TERM...TODAY THROUGH THURSDAY NIGHT

WINTER STORM WELL UNDERWAY ATTM WITH SNOWFALL OCCURRING ON BOTH
SIDES OF THE DIVIDE. TWO CIRCULATIONS ARE EVIDENT AT THE TIME OF
THIS WRITING, ONE OVER THE IDAHO/UTAH/WY TRIPLE POINT AND ONE
TRACKING NNE ACROSS FREMONT COUNTY JUST AS THE GFS HAD INDICATED
YESTERDAY. CENTRAL WYOMING, NAMELY THE WIND RIVER BASIN AND ADJACENT
LANDER FOOTHILLS, ARE SEEING STEADY MODERATE TO HEAVY SNOWFALL WHERE
ISENTROPIC LIFT IS OCCURRING ALONG WITH QG FORCING WHICH IS LIKELY
SEEDING A LOWER DECK OF UPSLOPE ENHANCED CLOUDINESS. THE HEAVIEST
BAND OF SNOW IS JUST TO THE EAST AND SOUTH OF RIVERTON WITH THIS
BAND OF HEAVY SNOWFALL CIRCULATING AROUND THE CORE OF THE LOW WHICH
APPEARS TO BE ABOUT 20 MILES SOUTH OF RIVERTON. 4 TO 6 INCHES OF
SNOWFALL HAVE FALLEN SO FAR IN THE RIVERTON LANDER AREA. RIW
VAD WIND PROFILE IS INDICATING A CLASSIC WARM AIR ADVECTION PATTERN.
WITH THE RELATIVELY LONG RESIDENCE TIME OF THIS LOW, THE WIND RIVER
BASIN COULD EASILY SEE UPWARDS OF 6 TO 8 INCHES SO WE UPGRADED THE
ADVISORY FOR THE WIND RIVER BASIN AND THE LANDER FOOTHILLS TO A
WARNING. EVEN A 9 OR 10 INCH SNOW TOTAL JUST SOUTH OF RIVERTON WHAT
NOT BE OUT OF THE REALM OF POSSIBILITIES. AS THE LOW CONTINUES TO
TRACK NORTHEAST, THE WRAP AROUND WILL EFFECT SOUTHERN JOHNSON COUNTY
AND NORTHERN NATRONA COUNTY WHERE 2 TO 4 INCHES OF SNOWFALL IS STILL
EXPECTED SO THE SPS FOR THAT AREA SHOULD STILL SUFFICE. SOUTHERN
NATRONA COUNTY AS WELL AS THE GREEN/GRANITE MTNS WILL BE ON THE
WRONG SIDE OF THE LOW TO GET WARNING SNOW AMOUNTS. THE REST OF THE
HILITES APPEAR ON TRACK. THE SNOW SHOULD END FROM NNW TO SSE TODAY
AS THE SHARP BACK EDGE TO THE BL MSTR SHIFTS SSE ACROSS THE CWA BY
THIS AFTERNOON COINCIDENT WITH THE LOW Propagating AND ACCELERATING
EAST TO THE CENTRAL PLAINS. THE ASCT TROUGHINESS WILL LINGER BEHIND
THE MAIN STORM SYSTEM THROUGH THURSDAY. AS A RESULT, SNOW SHOWERS
WILL LINGER IN THE MTNS TONIGHT, AND IN THE WESTERN MTNS WEDNESDAY
THROUGH THURSDAY. THEN SKIES WILL FINALLY CLEAR THURSDAY NIGHT WITH
A FOLD OVER RIDGE SHIFTING SOUTH OVER THE AREA WITH WARM AIR
ADVECTION STREAMING IN FROM THE NW WHICH IS AN IDEAL SET UP FOR
EXTENSIVE VALLEY AND BASIN FOG FOR THURSDAY NIGHT/EARLY FRIDAY
MORNING. TEMPS WILL BE COLDER BEHIND THE EXITING SNOWSTORM BY MID
WEEK.

.LONG TERM...FRIDAY THROUGH TUESDAY

Mean ridge builds into the Western U.S. with one potential sewd
moving shortwave on Monday. GFS is more aggressive with this
shortwave than the ECMWF. Lean a little towards the GFS at this
time. A few very weak disturbances Friday through the weekend may be
just enough for ocnl clouds which could disrupt our diurnal cycle
for overnight lows and fog. Will leave in our fog areas into the
weekend for now.  A transitory ridge may then move overhead Tuesday
into Wednesday. Main effect from this pattern will be the return of
the basin and valley inversions, especially those areas with deep
snow and new snow cover from our ongoing system. The Wind River
Basin will definitely see the return of decent inversions and a
possible periods of fog. There could be some high clouds streaming
swd at times to help temper the inversions but otherwise, cold to
bitter cold overnight lows in the basins/valleys with cold highs.
The mountains and foothills will warm over the weekend and into
early Monday ahead of our potential Monday shortwave. The Cody
Foothills may get quite windy ahead of this shortwave late Sunday
night into Monday morning so increased the winds there.

&&

.AVIATION.../18Z ISSUANCE/

WEST OF THE DIVIDE...KBPI/KJAC/KPNA/KRKS ROUTES

Widespread MVFR/IFR/LIFR conditions in snow and/or low ceilings are
expected to continue into this evening. A slow, gradual improvement
is expected from north to south through the forecast period, however
MVFR/IFR ceilings, light snow or flurries, and even some patchy fog
early Wednesday will linger across the forecast area...especially
over near and east of KRKS. The JAC terminal has the best shot at
VFR conditions sometime after 12z Wednesday.

EAST OF THE DIVIDE...KCOD/KCPR/KLND/KRIW/KWRL ROUTES

Widespread MVFR to occasional LIFR conditions are expected to
continue through until this evening. Slowly improving conditions are
expected from north to south this afternoon and tonight...with VFR
to MVFR conditions more commonplace Wednesday morning. That said,a
moist boundary layer and light wind could result in fog forming
early Wednesday across much of the area if/when/once the stratus
deck lifts/scatters out. VCFG is present in some TAFs to account for
this.

&&

.FIRE WEATHER...

ACCUMULATING SNOW WILL DIMINISH FROM NORTH TO SOUTH TODAY.
LINGERING SNOW SHOWERS WILL CONTINUE IN THE MOUNTAINS TONIGHT WITH
SNOW SHOWERS CONTINUING IN THE WESTERN MOUNTAINS TONIGHT THROUGH
WEDNESDAY AND THURSDAY. THEN SKIES WILL CLEAR AND STRONG
TEMPERATURE INVERSIONS WILL ESTABLISH THEMSELVES THURSDAY NIGHT AS
VALLEYS AND BASINS BECOME COLD AND WARM AIR ADVECTS IN FROM THE
NORTHWEST RESULTING IN POOR SMOKE DISPERSAL AND VALLEY AND BASIN
FOG.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until 5 PM MST this afternoon for WYZ014-
015-019-020-022-026-028>030.

Winter Storm Warning until 5 PM MST this afternoon for WYZ018-
023>025-027.

Winter Storm Warning until noon MST today for WYZ012-013-017.

Winter Weather Advisory until noon MST today for WYZ002-005-007-
016.

&&

$$

SHORT TERM...Lipson
LONG TERM...Skrbac
AVIATION...Braun
FIRE WEATHER...Lipson


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