000
FXUS65 KRIW 020126
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
626 PM MST SUN DEC 1 2013

.UPDATE...ISSUED TO CORRECT THE HIGHLIGHTS AND ADD THE LONG TERM
FORECAST TO THE DISCUSSION.

.SHORT TERM...TONIGHT THROUGH MONDAY

THE SNOWFALL IS WELL UNDERWAY IN FAR WESTERN WY WITH AT LEAST 5
INCHES REPORTED SO FAR AT RENDEZVOUS PEAK. ISENTROPIC
LIFT...UPSLOPE AND STRONG WESTERLY DIFLUENT FLOW ARE ALL
CONTRIBUTING TO THE SNOWFALL. EVEN SOME LOW CLOUDS AND PATCHY
LIGHT PRECIP ON BILLINGS RADAR ARE INDICATED ACROSS THE NORTHERN
TIER OF WY DUE TO ISENTROPIC LIFT. SNOWFALL WILL ALSO DEVELOP IN
THE BIG HORN MOUNTAINS TONIGHT. THE SNOWFALL WILL CONTINUE
MODERATE TO HEAVY IN THE WEST FOR THE REST OF TONIGHT AND THROUGH
TOMORROW WITH AS MUCH AS 2 TO 3 FEET EXPECTED IN THE TETONS. THEN
THE STRONG PACIFIC COLD FRONT WILL CREATE A HEAVY SNOWBAND MOVING
FROM NORTH TO SOUTH LATE MONDAY ACROSS THE WEST. THE WRF QPF
PORTRAYS THIS HEAVY FRONTAL BAND OF SNOWFALL AS AN ERASER AS IT
SCOURS OUT THE ONGOING HEAVY UPSLOPE SNOWFALL BEHIND IT AFTER THE
FROPA IN THE WEST. AFTER THIS FROPA...SNOWFALL WILL LOSE ITS
INTENSITY OUT WEST BUT WILL STILL ACCUMULATE SOME DURING THE
REMAINDER OF MONDAY NIGHT. IN THE MEANTIME...SNOWFALL WILL BEGIN TO
WORK ITS WAY SOUTH ACROSS THE BIG HORN BASIN AND BEGIN TO DEVELOP
OVER THE OWL CREEK MTNS AS WELL. ALL WINTER STORM WARNINGS OUT
WEST ARE STILL IN GOOD SHAPE.

THE HIGH WATCHES HAVE BEEN UPGRADED TO WARNINGS EXCEPT THE WATCH
FOR THE CODY FOOTHILLS REMAINS A WATCH SINCE MOUNTAIN WAVE/FROUDE
NUMBERS NOT VERY IMPRESSIVE AND WILL LET THE NIGHT CREW DECIDE
WHETHER TO UPGRADE OR NOT. THIRTY FIVE KNOT WINDS ARE INDICATED
FOR CPR FOR 18Z MONDAY IN THE MET MOS GUIDANCE. USUALLY ANY WIND
GUST AT OR ABOVE 30 KNOTS IS A SURE BET FOR STRONG WIND. WITH A
15MB SURFACE PRESSURE GRADIENT FROM GCC TO RKS...5...3 DECEMETER
H7 HEIGHT CONTOURS...BEING UNDERNEATH THE RIGHT FRONT QUAD OF THE
JET MAX AND 70 KNOT H7 WIND SPEEDS JUSTIFIES THIS HIGH WIND
WARNING. ALSO DECIDED TO INITIATE A HIGH WIND WATCH FOR THE LANDER
FOOTHILLS BASED ON THE ABOVE REASONING AS WELL AS A FROUDE NUMBER
CLOSE TO ONE. THIS WATCH WILL RUN FROM 15Z MONDAY THROUGH 00Z
TUESDAY.

THE AIRMASS WILL MIX OUT WELL IN AREAS THAT HAVE WIND...ESPECIALLY
IN AREAS THAT DOWNSLOPE OFF THE DIVIDE SUCH AS LANDER WITH A HIGH
OF 58 ON MONDAY WHICH IS JUSTIFIED WITH AN H7 TEMP OF -4C.

FIRE WEATHER...
HEAVY SNOW IS EXPECTED IN WESTERN WYOMING TONIGHT AND MONDAY IN
WESTERN WYOMING. VERY STRONG WINDS ARE EXPECTED IN AREAS EAST OF
THE DIVIDE AS WELL AS IN SWEETWATER COUNTY TONIGHT AND MONDAY.
ARCTIC AIR WILL PLUNGE SOUTH MONDAY NIGHT AND TUESDAY. BITTER
COLD WEATHER AND LIGHT SNOW IS EXPECTED FOR THE REMAINDER OF THE
WEEK.


.LONG TERM...MONDAY NIGHT THROUGH SUNDAY

HEAVY SNOW AND STRONG WIND EXPECTED OUT WEST THROUGH THE EVENING AS
PACIFIC FRONT BLASTS ACROSS THE AREA. SNOWFALL RATES OF 1 TO 2
INCHES PER HOUR ARE EXPECTED ALONG WITH STRONG TO VERY STRONG WINDS.
SNOW WILL BEGIN TO TAPER OFF IN THE FAR WEST FROM NORTH TO SOUTH
THEN OVERNIGHT. THE ATTENTION THEN TURNS TO AREAS EAST OF THE DIVIDE
AS THE ARCTIC COLD FRONT DROPS QUICKLY SOUTH MONDAY EVENING IN THE
NORTH HALF. IN THE SOUTH...THE PACIFIC FRONT MOVES ACROSS THE RED
DESERT FOLLOWED QUICKLY BY THE ARCTIC FRONT ON TUESDAY. A BURST OF
MODERATE TO HEAVY SNOW IS QUITE POSSIBLE ALONG THE FRONT FOR A 3
HOUR PERIOD AND POSSIBLY LONGER IN THE MOUNTAINS AND AROUND CASPER
MOUNTAIN. EACH MODEL RUN IS SLIGHTLY TO CONSIDERABLY DIFFERENT WITH
THE DETAILS. TODAY THE GFS HAS THE MONTANA UPPER LOW DROPPING SSWD
INTO ERN IDAHO AND WRN WYO ON TUESDAY AND THEN WEAKENING AND
SWINGING ACROSS US DURING THE DAY WEDNESDAY. SHALLOW UPSLOPE
CONTINUES EAST OF THE DIVIDE DURING THIS TIME. IT OBVIOUSLY TOO COLD
FOR OPTIMAL DENDRITIC SNOW CRYSTAL GROWTH BUT FORCING FROM THE UPPER
LOW AND MODERATE UPSLOPE WILL LIKELY KEEP AT LEAST SOME LIGHT SNOW
GOING AT TIMES WITH LOCALIZED MODERATE BURSTS. TOUGH CALL ON TOTAL
AMOUNTS BETWEEN TUESDAY AND WEDNESDAY BUT WILL GENERALLY GO 2 TO 4
INCHES IN THE LOWER ELEVATIONS WITH 4 TO 8 INCHES IN THE MOUNTAINS.
THE N TO NW FACING SLOPES OF THE BIGHORNS COULD SEE OVER A FOOT AND
EVEN CASPER MOUNTAIN COULD SEE OVER 10 INCHES.

ODDS OF ACCUMULATING SNOW DECREASE CONSIDERABLY BY WEDNESDAY NIGHT
IN THE LOWER ELEVATIONS WITH SOME LIGHT ACCUMULATIONS CONTINUING AT
TIMES IN THE MOUNTAINS WITH CYCLONIC NW FLOW AND SEVERAL EMBEDDED
DISTURBANCES. A MORE SIGNIFICANT SHORTWAVE AROUND FRIDAY COULD
PROVIDE A LITTLE MORE WIDESPREAD VERY LIGHT TO LIGHT SNOW IN THE
UPSLOPE AREAS OF THE EAST BUT EITHER WAY...IT TURN BITTER TO
DANGEROUSLY COLD BEHIND THIS ARCTIC FRONT TUESDAY INTO WEDNESDAY AND
CONTINUE THROUGH THE MOST IF NOT ALL OF THIS PERIOD. THE BASINS AND
VALLEYS WILL REMAIN TRAPPED WITH FRESH SNOW AND MODERATE IF NOT
STRONG INVERSIONS. THE BIGGER QUESTION IS HOW COLD CAN IT GET? THE
PATTERN HAS SOME SIMILARITIES TO THE DECEMBER 1972 OUTBREAK WHEN
AREAS DROPPED TO BETWEEN -25 AND -40F FOR SEVERAL MORNINGS.
HOWEVER...THERE WERE MANY AREAS THAT HAS AROUND 5 OR 6 INCHES OF
SNOW FROM DAY 1 OF THAT SYSTEM WHICH CAN MAKE A HUGE DIFFERENCE. IF
SOME AREAS GET THAT MUCH...THEN IT WILL CERTAINLY BE FRIGID COLD. IF
MOST AREAS ONLY GET AN INCH OR TWO...THEN IT WILL BE BITTER COLD BUT
PROBABLY NOT RECORD COLD. WITH THE EXPECTED 2 TO 4 INCHES IN MOST
AREAS...WE MAY BE SOMEWHERE IN BETWEEN. MAYBE 15-25 BELOW ZERO FOR A
COUPLE NIGHTS WITH YELLOWSTONE NATIONAL PARK PROBABLY GETTING A
COUPLE COLDER NIGHTS BETWEEN WEDNESDAY AND FRIDAY. THE CYCLONIC FLOW
AND EMBEDDED DISTURBANCES MAY HELP SOME WITH CLOUD COVER AND OCNL
FLURRIES BUT THE DECEMBER 1972 PATTERN ALSO HAD THIS SIMILAR LOOK.
WILL CONTINUE TO WATCH THIS CLOSELY AND SIDE TOWARD COLDER GUIDANCE.
HIGHS WILL STRUGGLE WEDNESDAY THROUGH FRIDAY WITH GENERALLY SINGLE
DIGIT BELOW TO SINGLE DIGIT ABOVE ZERO WITH LOWS WELL BELOW
ZERO...MAINLY MINUS 10 TO MINUS 25. LARGE DIFFERENCES BETWEEN THE
ECMWF AND GFS OVER THE WEEKEND WITH THE ECMWF SHOWING THE TROUGH
DEVELOPING TO THE WEST AGAIN JUST LIKE YESTERDAY WITH A WIDESPREAD
AT LEAST LIGHT SNOW ACROSS THE AREA. THE GFS HAS COLD NW FLOW BUT NO
TROUGH. THE SYSTEM THAT THE ECMWF DEVELOPS INTO NEXT WEEKENDS UPPER
LOW TO OUR WEST IS JUST COMING OFF OF ASIA. THE GFS HAS IT TOO
INITIALLY BUT THEN LOSES IT OVER CENTRAL CANADA. BOTH MODELS KEEP
THE HIGH LATITUDE RIDGE JUST OFF THE WEST COAST SO I LEAN TOWARDS
MORE TOWARDS THE ECMWF ATTM. WILL STICK WITH AT LEAST SOME SLGT CHC
TO CHC POPS AND QUITE COLD FOR NOW AS THIS PATTERN MAY VERY WELL
HOLD LONGER THAN JUST THE EARLY PART OF THE WEEKEND.

&&

.AVIATION.../00Z ISSUANCE/

EAST OF THE CONTINENTAL DIVIDE...

VFR CONDITIONS WILL OCCUR TONIGHT AT THE TERMINAL SITES. SNOW
WILL OCCUR OVER THE ABSAROKA AND WIND RIVER MOUNTAINS TONIGHT
WITH MVFR TO IFR CONDITIONS. WINDY CONDITIONS AT THE KCPR TERMINAL
SITE DURING MUCH OF THE PERIOD. SOUTHWEST WINDS OF 35 TO 45 MPH
ARE LIKELY BETWEEN THE RED DESERT AND THE GREEN MOUNTAINS
BEGINNING AROUND 06Z TONIGHT. THESE WINDS WILL EXTEND ALL THE WAY
TO KCPR BY 12Z MONDAY. MVFR CONDITIONS DUE TO LIGHT SNOW AND
LOWERING CIGS ARE EXPECTED AT KWRL AND KCOD AFTER 22Z MONDAY AFTER
A COLD FRONTAL PASSAGE.

WEST OF THE CONTINENTAL DIVIDE...

SNOW WILL OCCUR OVER MOST OF THE WESTERN MOUNTAINS TONIGHT. MVFR
TO IFR CONDITIONS WILL PREVAIL WITH FREQUENT MOUNTAIN OBSCURATION.
THE KJAC AND KPNA TERMINAL SITES WILL BE IMPACTED BY MVFR AND IFR
CONDITIONS AS WELL WITH THE KJAC SITE EXPERIENCING THE LOWEST CIGS
AND VSBY. KBPI WILL ENCOUNTER MVFR CONDITIONS. AT KRKS TERMINAL
SITE WIND WILL BE THE MAIN IMPACT TONIGHT AS THE WIND SPEEDS WILL
BE ON THE INCREASE. VFR CONDITIONS ARE FORECAST TO PREVAIL AT KRKS
TERMINAL SITE. SOUTHWEST WINDS OF 35 TO 45 MPH ARE LIKELY BETWEEN
KRKS AND THE RED DESERT AFTER 06Z TONIGHT.

&&

.FIRE WEATHER...

HEAVY SNOW IS EXPECTED IN WESTERN WYOMING TONIGHT AND MONDAY IN
WESTERN WYOMING. VERY STRONG WINDS ARE EXPECTED IN AREAS EAST OF
THE DIVIDE AS WELL AS IN SWEETWATER COUNTY TONIGHT AND MONDAY.
ARCTIC AIR WILL PLUNGE SOUTH MONDAY NIGHT AND TUESDAY. BITTER
COLD WEATHER AND LIGHT SNOW IS EXPECTED FOR THE REMAINDER OF THE
WEEK.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...

HIGH WIND WARNING FROM 11 PM THIS EVENING TO 5 PM MST MONDAY
WYZ019-030.

HIGH WIND WATCH FROM MONDAY MORNING THROUGH MONDAY AFTERNOON
WYZ018.

WINTER STORM WARNING UNTIL 5 AM MST TUESDAY
WYZ001-002-012>014-023>025.

HIGH WIND WARNING UNTIL 11 AM MST MONDAY WYZ003.

HIGH WIND WARNING FROM 5 AM TO 5 PM MST MONDAY WYZ020.

&&

$$

SHORT TERM...LIPSON
LONG TERM...SKRBAC
AVIATION...LIPSON
FIRE WEATHER...LIPSON