Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 151808
AFDRIW

AREA FORECAST DISCUSSION...UPDATE
NATIONAL WEATHER SERVICE RIVERTON WY
1107 AM MST SAT NOV 15 2014

.UPDATE...
CANCELLED REMAINING WINTER WEATHER ADVISORIES AND WARNINGS WITH
DRIER AIR AND SUBSIDENCE PREVAILING FROM NW-SE BEHIND SHORTWAVE
WITH SOME LINGERING AREAS OF FLURRIES OR VERY LIGHT SNOW.

&&

.PREV DISCUSSION... /ISSUED 329 AM MST SAT NOV 15 2014/

SHORT TERM...TODAY THROUGH MONDAY NIGHT

AN ARCTIC FRONT WILL CONTINUE TO MARCH SOUTH THIS MORNING...MOVING
ACROSS THE COLORADO BORDER BEFORE NOON. THE VERY COLD AIR HAS VERY
LITTLE ADDITIONAL MOISTURE TO OFFER...BUT THERE WILL BE A FEW HOURS
WORTH OF VERY LIGHT CRYSTALLINE SNOW...AS MOST OF THE PACIFIC
MOISTURE THAT WAS IN PLACE ON FRIDAY HAS MOVED OUT OF THE AREA.
EXPECT CLEARING FROM NORTH TO SOUTH THIS MORNING...WITH ANOTHER VERY
COLD NIGHT WITH MORE BELOW ZERO TEMPERATURES EXPECTED TONIGHT. THE
AIRMASS IS NOT AS COLD AS THE SYSTEM EARLIER THIS WEEK BUT IT WILL
CERTAINLY BE COLD ENOUGH TO THREATEN SOME RECORD DAILY LOW
TEMPERATURES.

FOR SUNDAY...A SHORT WAVE WILL ZIP BY IN THE NORTHWEST FLOW...THE
BULK OF THE ENERGY AND MOISTURE FROM THIS SYSTEM WILL REMAIN
NORTHEAST OF THE AREA BUT THERE COULD BE SOME IMPACT ON JOHNSON
COUNTY AND THE BIGHORNS...AND PERHAPS FURTHER WEST INTO THE
BIGHORN BASIN AND CODY FOOTHILLS...THIS SYSTEM HAS BEEN TRENDING
STRONGER SO IT WILL NEED TO BE WATCHED AS THERE DOES APPEAR TO BE
SOME DECENT QG FORCING AND JET ENERGY WITH THIS WAVE...THE MODELS
RARELY HANDLE MODERATE TO STRONGLY FORCED SYSTEMS IN NORTHWEST
FLOW WELL SO I WAS GENEROUS WITH THE ISOLATED SNOW SHOWER COVERAGE
ACROSS THE NORTHEASTERN THIRD OF OUR FORECAST AREA.

BY MONDAY THE RIDGE BUILDING TO OUR WEST WILL BEGIN TO PUSH
IN...BRINGING MORE MILD TEMPERATURES AT LEAST TO THE MORE BREEZY
AREAS AND TO THE MOUNTAINS...AT LEAST BY LATE IN THE AFTERNOON. MOST
VALLEYS AND BASINS WILL LIKELY REMAIN TRAPPED ON MONDAY.
TEMPERATURES WERE BUMPED DOWN A LITTLE IN THESE AREAS BUT MORE
ADJUSTMENT MAY BE NEEDED. A LEE SIDE TROUGH BUILDS INTO THE EVENING
HOURS...TURNING ON THE WIND MACHINE FROM ROCK SPRINGS TO CASPER...SO
THESE AREAS MAY ACTUALLY SEE THEIR HIGH TEMPERATURES FOR THE DAY
AFTER SUNSET.

LONG TERM...TUESDAY THROUGH SATURDAY

SYNOPSIS...A TRANSITION TO A MILDER PACIFIC FLOW OCCURS WEDNESDAY
THROUGH SATURDAY WITH A SERIES OF PACIFIC SYSTEMS BRINGING CHANCES
OF LIGHT SNOW MAINLY TO THE WEST.

DISCUSSION...HIGH AMPLITUDE RIDGE REMAINS ALONG THE WEST COAST ON
TUESDAY WITH ONE LAST RIPPLE DIVING SSE DOWN FRONT SIDE OF RIDGE
ACROSS NORTHERN AND EASTERN WYOMING ON TUESDAY.  THE ECMWF IS
FURTHER WEST THAN GFS WITH THIS FEATURE...WITH THE ECMWF GENERATING
SOME LIGHT QPF ACROSS THE NORTH.  ECMWF/S SLOWER PROGRESSION
PREFERRED WITH SOME CHANCES OF SNOW MAINLY ACROSS THE BIGHORN
RANGE.

THE FIRST IN A SERIES OF PACIFIC STORMS BEGINS TO UNDERCUT THE RIDGE
ON WEDNESDAY...MOVING INTO NORTHERN CALIFORNIA WHILE DEEP UPPER
TROUGH EAST OF THE ROCKIES BEGINS TO PROGRESS EASTWARD AND
DEAMPLIFY.  GFS IS ALSO A BIT QUICKER ADVANCING THE MILDER PACIFIC
AIRMASS INTO THE AREA MIDWEEK...THE ECMWF BRINGS ONE FINAL ARCTIC
FRONT INTO THE NORTHERN PLAINS WEDNESDAY NIGHT AND
THURSDAY...BRINGING A SHALLOW BACKDOOR COLD FRONT INTO CENTRAL AND
EASTERN WYOMING.  FOR SNOW-COVERED CENTRAL BASINS...THE ECMWF
SCENARIO MAY ACTUALLY PROVIDE BETTER MIXING POTENTIAL AND MORE
UNIFORM TEMPERATURES....WHILE MUCH WARMER AIR ADVECTING IN ALOFT PER
GFS WILL JUST RESULT IN STEEP SHALLOW INVERSIONS WITH THE LOWER
BIGHORN BASIN AND WIND RIVER BASIN STUCK IN THE 20S WHILE OTHER
AREAS BASK IN THE 40S.  FORECAST CONTINUES TO LEAN TOWARD THE COLDER
ECMWF GUIDANCE OVERALL.  BOTH ECMWF AND GFS HAVE YET TO SHOW ANY
RUN-TO-RUN CONSISTENCY IN THE HANDLING OF INCOMING PACIFIC ENERGY
SPLITTING ALONG THE COAST AS THE LARGE SCALE PATTERN TRANSITIONS
FROM A HIGH AMPLITUDE FLOW OVER NORTH AMERICA TO MORE SPLIT FLOW AS
A STRONG 160+ KT ZONAL JET STREAM CROSSES THE PACIFIC...EXTENDING
FROM THE DATELINE TO THE WEST COAST BY 12Z SATURDAY.  EXPECTING A
WEAK NORTHERN STREAM IN THIS SPLIT FLOW WITH MOST OF THE ENERGY
TAKING THE SOUTHERN ROUTE ACROSS THE FOUR CORNERS AREA CLOSER TO
ECMWF/S OVERALL DEPICTION OF THE TRANSITION...THOUGH CONFIDENCE IN
THE DETAILS REMAINS LOW.

AVIATION.../18Z ISSUANCE/

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

CONDITIONS WILL CONTINUE TO IMPROVE FROM NORTHWEST TO SOUTHEAST THIS
AFTERNOON. THE LIGHT SNOWFALL AND MVFR CONDITIONS WILL BE EXITING
KBYG...KCPR AND KRKS IN THE NEXT COUPLE OF HOURS. BY 20Z MAINLY VFR
CONDITIONS WILL PREVAIL AT THE TERMINAL SITES THROUGH 18Z SUNDAY.
ONE EXCEPTION WILL BE KCPR WHERE SOME BLOWING SNOW MAY CAUSE SOME
MVFR AND LOCAL IFR CONDITIONS AFTER 08Z SUNDAY. NW WINDS WILL
INCREASE TONIGHT AND SUNDAY IN RESPONSE TO A CLIPPER TYPE SYSTEM
GRAZING WYOMING TO THE NORTHEAST. SEE INDIVIDUAL TERMINAL FORECASTS
FOR MORE DETAILS.

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

LIGHT SNOW WILL CONTINUE FOR A COUPLE OF MORE HOURS IN SOUTHWEST
WYOMING INCLUDING THE VCNTY OF KRKS ALONG WITH MVFR CONDITIONS
BEFORE SLIDING OFF TO THE SOUTH BY 20Z THIS AFTERNOON. OTHERWISE
EXPECT STRATOCUMULUS CLOUDS AT 5K TO OBSCURE THE HIGHER RIDGES AND
PRODUCE SOME WDLY SCT LIGHT SNOW SHOWERS THIS AFTERNOON. EXPECT
MAINLY VFR CONDITIONS TONIGHT AND SUNDAY. NW WINDS WILL INCREASE
TONIGHT AND SUNDAY IN RESPONSE TO A CLIPPER TYPE SYSTEM GRAZING
WYOMING TO THE NORTHEAST. SEE INDIVIDUAL TERMINAL FORECASTS FOR MORE
DETAILS.

FIRE WEATHER...

THE HEAVY SNOW IS OVER AND DONE WITH BUT THERE WILL BE SOME AREAS OF
VERY LIGHT AND FLUFFY SNOW CLEARING FROM NORTH TO SOUTH THIS
MORNING...WITH ONLY A FEW PATCHES REMAINING IN THE AFTERNOON ACROSS
PORTIONS OF RAWLINS DISPATCH. GENERALLY LOW MIXING HEIGHTS WILL BE
IN PLACE WITH SOME WINDY AREAS THIS MORNING THEN THE LID WILL GO
BACK ON TONIGHT WITH VERY POOR SMOKE DISPERSAL FOR TONIGHT AND
TOMORROW FOR MANY BASINS. WINDS WILL PICK UP IN SOME AREAS ON
SUNDAY...ESPECIALLY IN MOUNTAIN ZONES...MOST OF CASPER DISPATCH AND
THE UPPER GREEN RIVER BASIN...AND ALL OF RAWLINS DISPATCH. THIS IS
IN RESPONSE TO ANOTHER SYSTEM THAT WILL GRAZE THE AREA...WITH MOST
OF THE SNOW REMAINING OUT OF OUR FORECAST AREA BUT IT WILL AT LEAST
ALLOW FOR SOME MORE MIXING SUNDAY AND INTO MONDAY.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...NONE.

&&

$$

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








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