Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 230630
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
1130 PM MST MON DEC 22 2014

.UPDATE...ISSUED A HIGH WIND WARNING FOR NORTHEAST JOHNSON COUNTY
UNTIL 6 AM TUESDAY. WIND GUSTS AT BYG TO 69 MPH AT 1047 PM.

&&

.AVIATION.../06Z ISSUANCE/

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

STRONG NW WINDS ACROSS THE INTERSTATE 90 CORRIDOR...VCNTY
KBYG...WILL GRADUALLY DIMINISH 09Z-15Z TUESDAY.  LOCAL BANDS OF SHSN
WILL MOSTLY DIMINISH BY 10Z WITH SOME SHSN POSSIBLY LINGERING VCNTY
AND EAST OF INTERSTATE 25 THROUGH AROUND 12Z.  OTHERWISE A RIDGE OF
HIGH PRESSURE WILL BUILD ACROSS THE AREA ON TUESDAY BRINGING IN A
DRIER MORE STABLE AIRMASS WITH WINDS BECOMING MAINLY LIGHT...10KTS
OR LESS...TUESDAY AFTERNOON.  A LEE SIDE SFC TROUGH WILL INTENSIFY
TUESDAY NIGHT WITH SW WINDS PICKING UP TO 15-25KTS VCNTY 50NE
KRKS-KCPR.

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

SCT-NMRS -SHSN OVR THE MTNS AND VCNTY KRKS WILL DISSIPATE THROUGH
THE EARLY MORNING AS A DRIER MORE STABLE AIRMASS PUSHES INTO WESTERN
WYOMING 09Z-15Z.  SOME PATCHY FOG MAY PARTIALLY OBSCURE THE WESTERN
VALLEYS THROUGH THE MORNING HOURS.  A RIDGE OF HIGH PRESSURE WILL
BUILD IN THOUGH THE DAY TUESDAY WITH UNLIMITED CIGS AND GENERALLY
LIGHT WIND PREVAILING TUESDAY AFTERNOON INTO TUESDAY EVENING.
INCREASING MOISTURE IN A WESTERLY FLOW WILL LIKELY BRING SOME
MID-LEVEL CIGS AT FL110-130 INTO THE FAR WEST...VCNTY KJAC-KAFO LATE
TUESDAY EVENING AND WEDNESDAY MORNING.

&&

.PREV DISCUSSION... /ISSUED 220 PM MST MON DEC 22 2014/

SHORT TERM...TONIGHT THROUGH TUESDAY

AS THE MAIN TROUGH MOVES EAST AND UNDERGOES RAPID CYCLOGENESIS IN
THE PLAINS THERE WILL STILL BE A FEW HOURS OF STEEP LAPSE RATES AS
THE COLD POOL SETTLES AT THE BACK EDGE OF THE TROUGH...FOLLOWED BY
SOME WRAP-AROUND MOISTURE AND SHORT WAVE THAT WILL BRING ANOTHER
ROUND OF LIGHT SNOW TO THE EASTERN THIRD OF THE FORECAST AREA
TONIGHT. IT WILL ALSO BE ACCOMPANIED BY SOME STRONG NORTHERLY WIND
WITH GUSTS OVER 50 MPH EXPECTED AROUND BUFFALO THIS
EVENING...ELSEWHERE EXPECT STEADY 15 TO 25 MPH WINDS ACROSS MUCH OF
THE AREA BETWEEN SUNSET AND MIDNIGHT WITH WIND AND SNOW TAPERING OFF
THEREAFTER.

ON TUESDAY...HIGH PRESSURE BUILDS INTO THE WEST WITH ANOTHER SNOW
SHOWER OR TWO POSSIBLE ALONG THE I25 CORRIDOR AND IN THE BIGHORNS.
ELSEWHERE...CLEARING AND COOL WITH DECREASING WINDS.

LONG TERM...TUESDAY NIGHT THROUGH MONDAY

A TRANSIENT RIDGE BUILDS IN FOR TUESDAY NIGHT FOR DRY WEATHER
EVERYWHERE. THEN ON WEDNESDAY...THE NEXT UPSTREAM SHORTWAVE TROUGH
WILL APPROACH THE PACIFIC COAST. AS A RESULT...LIGHT SNOW WILL BREAK
OUT IN NORTHWEST WYOMING. AS THE TROUGH APPROACHES...THE GRADIENT
WILL RETIGHTEN AND A RESULTANT SURFACE LEE SIDE TROUGH WILL DEVELOP
WITH SOUTHWEST WINDS INCREASING FROM THE GREEN MTNS TO CASPER. THE
TROUGH WILL THEN DIG OVER THE GREAT BASIN WEDNESDAY NIGHT WITH THE
WEST SOUTHWEST DIFLUENT UPSLOPE FLOW INCREASING IN THE FAR WEST OUT
AHEAD OF THE APPROACHING TROUGH FOR INCREASING SNOWFALL IN THE FAR
WEST. 12Z THURSDAY IS WHERE THE GFS AND THE ECMWF BEGIN TO DIVERGE.
THE GFS CLOSES THE TROUGH OVER SOUTHERN IDAHO AND TRACKS IT EAST
ACROSS SOUTHERN WYOMING...ROUGHLY ALONG THE NORTHERN SWEETWATER
COUNTY BORDER DURING THE DAY THURSDAY. THIS IS A GREAT TRACK FOR
WIDESPREAD SNOWFALL ACROSS THE ENTIRE CWA CHRISTMAS DAY GIVEN THE
FAVORABLE POSITION OF THE LEFT FRONT QUAD OF THE JET ALONG WITH
AMPLE Q VECTOR FORCING AND WEAK ISENTROPIC LIFT FROM THE NORTH. THE
SFC LOW...AS PROGGED BY THE GFS...WILL FORM OVER SE COLORADO AND
PROVIDE THE LOW LEVEL UPSLOPE FLOW AND RESULTANT SNOWFALL WITH THE
HELP FROM THE SEEDER FEEDER ACTION FROM THE CLOUDS ABOVE...LINKED TO
THE AFOREMENTIONED UPPER AIR DYNAMICS. THIS TRACK IS FARTHER SOUTH
THEN THE NORTHERN WYOMING ROUTE OF THE PREVIOUS GFS RUN. THE EURO
KEEPS THE TROUGH OPEN AND FOCUSES MOST OF THE ENERGY TO THE SOUTH
OVER THE FOUR CORNERS AREA. WILL KEEP LIKELY POPS EVERYWHERE THU FOR
NOW. SNOWFALL WILL SLOWLY DECREASE THURSDAY NIGHT AS THE TROUGH
MOVES EAST. EXCEPT FOR ISOLATED MTN SNOW SHOWERS ON FRIDAY DUE TO A
VORT LOBE IN THE NW FLOW ON THE BACKSIDE OF THE MAIN TROUGH...EXPECT
MAINLY DRY CONDITIONS. ON SATURDAY THE FLOW WILL BACK TO WESTERLY
AGAIN RESULTING IN INCREASING UPSLOPE SNOWFALL FOR THE FAR WEST IN
RESPONSE TO A TROUGH MOVING ONSHORE TO THE PACIFIC NORTHWEST. THE
SNOWFALL WILL INTENSIFY SATURDAY NIGHT AS THE TROUGH DIGS OVER THE
PAC NW. FOR SUNDAY AND MONDAY...THE GFS AND THE EURO ARE
SURPRISINGLY SIMILAR IN CARVING OUT THE TROUGH OVER THE GREAT BASIN
AND ATTEMPTING TO CLOSE IT OFF OVER THE VCNTY OF SLC. THE GFS PLACES
THE H7 LOW RIGHT OVER CENTRAL WY SUNDAY. HAVE HIGH CHC POPS OVER THE MUCH
OF THE CWA SUNDAY AND SUNDAY NIGHT. THIS WILL BE A COLDER SYSTEM
THAN THE CHRISTMAS STORM WITH -20C H7 TEMPS BEING PULLED SOUTHWEST
FROM THE HUDSON BAY AREA BEHIND THE ASCT COLD FRONT SUNDAY NT. TEMPS
WILL WARM WED WITH ABOVE FREEZING TEMPS IN MANY AREAS EAST OF THE
DIVIDE AHEAD OF THE SNOW EVENT ON CHRISTMAS EVE. THEN EXPECT
SEASONABLY COLD TEMPS THROUGH SUNDAY. BEHIND AND AFTER THE COLD
SYSTEM SUNDAY NIGHT...MANY AREAS WILL STRUGGLE TO GET ABOVE THE
TEENS MONDAY.

AVIATION.../00Z ISSUANCE/

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

AREAS OF GUSTY NORTHWEST WIND WILL REMAIN...ESPECIALLY VCNTY
KDUB-KRIW AND KBYG...THROUGH ABOUT 12Z TUE WITH DECREASING WINDS
THEREAFTER. -SHSN ACTIVITY WILL REMAIN MOSTLY ISOLD-WDLY SCT WITH
MORE NMRS SHSN OBSCURING THE SURROUNDING MTNS THRU 03Z. SOME
WRAPAROUND MOISTURE IS EXPECTED TO PUSH BACK FROM AN UPPER LOW OVER
THE PLAINS INTO EASTERN WYOMING TONIGHT...AND THIS MAY BRING MORE
WIDESPREAD MVFR/IFR -SN AS FAR WEST AS THE BIGHORN RANGE TO VCNTY
KCPR...IN ADDITION TO THE GUSTY 15 TO 30KT WINDS...GUSTS OVER 45KT
EXPECTED NEAR BYG...COULD ALSO RESULT IN AREAS OF BLSN.

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

AN UPPER LEVEL DISTURBANCE WILL DIVE SOUTH ALONG THE DIVIDE EARLY IN
THE PERIOD...EXITING SOUTH OF INTERSTATE 80 THIS EVENING. SNOW
SHOWER ACTIVITY WILL EVENTUALLY WANE...IMPROVING CONDITIONS
FOLLOWING THE DISTURBANCE THIS EVENING. CLEARING CONDITIONS AND
DIMINISHING WIND WILL FOLLOW OVERNIGHT WITH MOST AREAS EXPECTED TO
REMAIN FOG FREE DUE TO THE CONTINUED NORTHWEST BREEZES...HOWEVER
CONDITIONS WILL EVENTUALLY BECOME CALM ENOUGH IN THE FAR WESTERN
VALLEYS FOR SOME PATCHY FOG...ESPECIALLY NEAR KJAC.

FIRE WEATHER...

GUSTY WIND WILL REMAIN ACROSS MUCH OF THE FORECAST AREA...WITH WIND
WEAKENING DURING THE OVERNIGHT HOURS. TEMPERATURES WILL CONTINUE TO
COOL TUESDAY BEFORE EXPERIENCING A SLIGHT WARMING ON WEDNESDAY. SNOW
SHOWERS WILL TAPER OFF AFTER SUNSET WITH A PERIOD OF SNOW LINGERING
ACROSS CASPER DISPATCH PAST MIDNIGHT. THE NEXT APPROACHING TROUGH
WILL MOVE INTO NORTHWESTERN WYOMING WEDNESDAY AFTERNOON AND
EVENING...AND WILL SPREAD EAST THROUGH THE OVERNIGHT HOURS INTO
CHRISTMAS DAY.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
HIGH WIND WARNING UNTIL 6 AM MST TUESDAY WYZ010.

&&

$$

UPDATE...AEM
SHORT TERM...ALLEN
LONG TERM...LIPSON
AVIATION...AEM
FIRE WEATHER...ALLEN






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