Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 240624
AFDRIW

Area Forecast Discussion
National Weather Service Riverton WY
1124 PM MST Thu Feb 23 2017

.UPDATE...Added 06z aviation discussion.

&&

.AVIATION.../06Z ISSUANCE/

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

A complex low pressure system will keep areas of MFVR/IFR ceilings
and light snow with widespread mountain obscurations across the area
through Friday morning.  Conditions will improve Friday afternoon
with scattered snow showers mainly confined to the mountains, VFR
conditions prevailing across the valleys. Clearing will continue
Friday night with a few snow showers mainly confined to the extreme
northwest corner of Wyoming 6z-12z Saturday.

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

The main area of upper level low pressure system will continue to
move east across the Central Plains tonight and Friday morning.
Widespread MVFR/IFR conditions will remain across the area with
areas of light snow, but winds will continue to diminish through
this morning.  Low level upslope easterly flow will shift westerly
Friday afternoon with snow ending, ceilings lifting and dissipating,
VFR conditions prevailing mid to late Friday afternoon.  Clouds will
continue to lift and scatter out Friday night. Southwest winds will
increase to 15-25 knots vicinity KCPR-50 SM NE KRKS 6z-12z Saturday.

&&

.PREV DISCUSSION... /issued 916 PM MST Thu Feb 23 2017/

UPDATE...All Blizzard Warnings were downgraded to Winter Storm
Warnings across southern Wyoming where winds have diminished and
snowfall intensity has decreased.

PREV DISCUSSION... /issued 229 PM MST Thu Feb 23 2017/

SHORT TERM...TONIGHT THROUGH FRIDAY

THE MAIN CIRCULATION ASCT WITH THE ONGOING SNOWSTORM EAST OF
THE DIVIDE AND OVER SWEETWATER COUNTY APPEARS TO HAVE DRIFTED SOUTH
TO THE CO STATE LINE. AT THE TIME OF THIS WRITING, THERE APPEARS TO
BE AT LEAST 3 DISTINCT SOUTHWEST NORTHEAST ORIENTED BANDS OF HEAVY
SNOWFALL CIRCULATING FROM NORTHEAST TO SOUTHWEST AROUND THIS CLOSED
LOW. ONE BAND LAYS OVER THE CITY OF RIVERTON, THE SECOND ONE BEHIND
IT ALONG BEAVER RIM, AND THE THIRD TRACKS ACROSS THE VCNTY OF
JEFFREY CITY. SNOWFALL HAS PILED UP TO 9 INCHES FROM RIVERTON TO
LANDER AND SINKS CANYON TO 7-8 INCHES NORTHWEST OF RIVERTON. AS
MUCH AS 12 INCHES HAS FALLEN SO FAR ON CASPER MOUNTAIN CLOSER TO
THE STORM CENTER AND 8 INCHES IN CASPER. THIS MODERATE TO HEAVY
SNOW SHOULD CONTINUE TO SWIRL AROUND THE LOW CENTER AS A LARGE
SWATH OF ISENTROPIC LIFT CIRCULATES AROUND THE LOW, STRONG QG
FORCING DIRECTLY ASCT WITH THE LOW, AND THE DEFORMATION ZONE ALL
CONTRIBUTE TO THE AMPLE SNOWFALL. BY 00Z THIS EVENING WE WILL LOSE
MOST OF THESE DYNAMICS AND WILL BE RELYING MORE ON PURE UPSLOPE
DRIVEN SNOW FROM THE LANDER FOOTHILLS AND SURROUND WIND RVR MTNS
TO THE GREEN MTNS. HOWEVER, THE EAST NORTHEAST WINDS THAT WILL
PREDOMINATE OVER THE CASPER AREA THIS EVENING ONCE THE LOW EXITS
THE AREA IS NOT A FAVORABLE FLOW FOR CPR/CPR MTN TO RECEIVE
UPSLOPE SNOWFALL. THAT IS WHY WE HAVE PAINTED A BIT MORE TOTAL
SNOWFALL FOR THE LANDER/EAST SLOPE OF THE WINDS AREA. SNOW TOTALS
BY FRIDAY SHOULD RANGE FROM 15 TO 16 INCHES FROM THE EAST SLOPE OF
THE WIND RIVER TO BEAVER RIM, UP TO 14 INCHES IN LANDER, 13
INCHES IN RIVERTON, 10 TO 11 INCHES IN CASPER, WITH CASPER MTN
EXPECTED TO RECEIVE 14 INCHES. THE LANDER AREA WILL CATCH UP FROM
BONUS UPSLOPE SNOW TOWARD THE END OF THE EVENT TONIGHT AND FRIDAY
WITH THE CASPER AREA SEEING MORE OF THEIR SNOWFALL FROM UPPER
LEVEL DYNAMICS. OUTSIDE OF THIS AREA, 4 TO 6 INCHES WILL BE FAIRLY
COMMON IN THE AREAS WITH ADVISORIES AND WARNINGS IN THE LOWER
ELEVATIONS AND 6 TO 12 INCHES IN THE MTNS, NAMELY THE OWL CREEK
AND BIG HORNS. SNOWFALL WILL BECOME FAIRLY LIGHT AFTER 15Z FRIDAY
WITH LIGHT, ISOLD POCKETS OF MODERATE SNOW EXPECTED TO CONTINUE
THROUGH THE REST OF THE DAY FRIDAY UNDERNEATH THE ELONGATED
EXTENSION OF THE UPPER TROUGH WEST OF THE EXITING CENTER WHERE
PIECES OF VORTICITY COULD CONTRIBUTE TO A FINAL COATING SNOW.

AS FOR WESTERN WYOMING, ONLY LIGHT SNOW IS EXPECTED FROM THE
ELONGATED TROUGH EXTENDING WEST TO THAT AREA WITH 4 TO 7 INCHES
EXPECTED IN THE SALT/WYOMING RANGES THROUGH FRIDAY, 1 TO 3
ADDITIONAL INCHES IN THE STAR VALLEY, 3 TO 4 INCHES IN THE TETONS
AND 1 TO 3 INCHES ELSEWHERE IN THE FAR WEST NORTH OF SALT RIVER PASS.

ONCE THE STACKED LOW CENTER MOVES SOUTHEAST OF THE CWA, WINDS WILL
DRASTICALLY DIMINISH EARLY THIS EVENING. COLDER AIR WILL CONTINUE TO
FILTER INTO THE AREA BEHIND THE EXITING STORM WITH LOWS IN THE
SINGLE DIGITS AND TEENS TONIGHT AND HIGHS IN THE TEENS AND 20S
FRIDAY.

LONG TERM...FRIDAY NIGHT THROUGH THURSDAY

Beginning of FCST, the upper levels will be characterized by a broad
deep and somewhat flat longwave trof across most of the CONUS with
WY under weak flow doldrums aloft and the main jet being directed
around two bookend closed lows/embedded trofs within the main
trof...one near OR and the other moving into the wrn Great Lakes.
Mid to upper level moisture will be modest but fairly evenly placed
over the FA. The SFC will show general high pressure around the
region with the stronger high(s) out over the Central/High Plains
and central Rockies/ern Great Basin with the weaker high (relative
lee-side trof) over northern WY. A modest P GRAD will then exist
between these areas with west to southwest winds gusting 20 to 30
mph across the higher ridge-lines, the eastern foothills and the
Wind Corridor. Isolated to scattered snow showers will be found
mainly across the western and northwestern mountains.

Saturday afternoon and night, the western upper SW trof swings over
the FA, lowering temps aloft and increasing convective parameters
and subsequent snow shower activity west of the Divide. 24 to 30 hr
total snow fall from Friday night through Saturday night look to
range from 2 to 4 inches in the Teton, Gros Ventre, and Bighorn
mountains, to lesser amounts most elsewhere. All this occurring with
relatively light wind does not look to be enough for any highlights
at this point.

Sunday, after the passage of the SW trof, modest ridging builds
across WY with weak SFC high pressure and light wind for the most
part. The only chance for precip will come across the
western/northwestern mountains as mildly convective afternoon
showers occur ahead of the next embedded SW trof moving through the
western portion of the main longwave trof...into western WY by
Monday morning. Snowfall potential west of the Divide will begin to
increase accordingly late overnight Sunday and Monday as upper level
flow and forcing also increases. Trof passage with this system will
also take some time...perhaps until Wednesday morning. These
circumstances together with continued modest moisture may allow for
at least some Advisory conditions over some preferred locations out
west...although overall it does not look particularly brutal. Monday
night, frontogenesis will occur to the north of WY as the back-end of
the upper trof approaches...offering a chance for precip/snow east
of the Divide with upper trof/frontal passage Tuesday afternoon and
evening. This should be a quick hitter east of the Divide with
perhaps an inch or so through Tuesday night.

Wednesday through Thursday, post frontal with high pressure at the
SFC and NW flow aloft will keep the northwest portion of the FA
slightly unstable along with a little extra mountain lift keeping
the mountains across northwest WY under small POPs through the end
of the FCST. Nothing significant here...and even temperatures
throughout the forecast period will remain below seasonal...so no
quick melt-offs planned.

AVIATION.../00Z ISSUANCE/

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

A strong low pressure system will meander eastward across southern
Wyoming through mid-day Friday. The main impacts with this storm
will be continued widespread IFR/LIFR conditions in low CIGS and VIS
with higher terrain obscurations persisting mainly across the
southern portions of the forecast area. Near blizzard-like
conditions with N to NNE winds 20-40kts will be decreasing across
the region from near KRKS to points east. Elsewhere, occasional
MVFR/IFR shsn may occur, but VFR conditions are expected to prevail
through 08Z before MVFR in lower CIGS and possible VIS in -SHSN
become more probable near KJAC and KPNA. All precip chances should
be on the decrease by the afternoon period on Friday.

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

A strong low pressure system will meander eastward across southern
Wyoming through mid-day Friday. This system will continue widespread
IFR/LIFR conditions in moderate to locally heavy snow and low CIGS
with persistent mountain obscurations through the evening period.
Near blizzard conditions with N to NNE winds 20-40kts will be on the
decrease from south of a KCPR-KLND line. KCOD-KPOY routes will be
least affected with primarily VFR, occasional MVFR conditions.
Conditions will gradually start to improve this evening as the
surface low weakens with decreasing winds and snow slowly tapering
off. However, widespread MVFR/IFR conditions and mountain
obscurations are expected to continue through at least 18z Friday
before more significant improvement is noted.

FIRE WEATHER...

A SIGNIFICANT WINTER STORM WILL CONTINUE TO IMPACT MUCH OF CENTRAL
AND WESTERN WYOMING TODAY WITH AREAS OF MODERATE TO HEAVY SNOW.
LIGHT SNOW WILL LINGER THIS AFTERNOON AND THROUGH THE DAY FRIDAY,
BEFORE ENDING FRIDAY EVENING. STRONG NORTHERLY WINDS WILL OCCUR WITH
THIS SNOW FOR THE REST OF THIS AFTERNOON, OVER THE WIND
RIVER MOUNTAINS, SOUTHEASTERN FREMONT AND EASTERN SWEETWATER
COUNTIES. LIGHT TO MODERATE SNOW WILL ALSO OCCUR WEST OF THE DIVIDE
DURING THIS TIME, ENDING FROM SOUTH TO NORTH FRIDAY EVENING. COLDER
AIR WILL FILTER IN BEHIND THE WINTER STORM WITH HIGHS IN THE TEENS
AND 20S AND LOWS 10 BELOW TO 10 ABOVE BY THIS WEEKEND.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...

Winter Storm Warning until 11 AM MST Friday for WYZ007-010-011-
014-015-017>020-022-028>030.

Winter Weather Advisory until 11 AM MST Friday for WYZ003-005-
009.

&&

$$

UPDATE...AEM
SHORT TERM...Lipson
LONG TERM...Braun
AVIATION...AEM
FIRE WEATHER...Lipson


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