Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 291153 AAA
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
456 AM MST MON DEC 29 2014

.SHORT TERM...TODAY THROUGH WEDNESDAY NIGHT

SNOW WILL CONTINUE TO TAPER OFF WITH FLURRIES STICKING AROUND INTO
TUESDAY. THERE WILL BE A 1058MB HIGH SETTLING IN BEHIND THIS SYSTEM
TONIGHT WITH 700MB TEMPERATURES IN THE 25 BELOW RANGE. GIVEN HOW
COLD THE TEMPERATURES WILL BE...IT WILL NOT TAKE MUCH WIND TO CREATE
WIND CHILL ISSUES. IT LOOKS LIKE THERE WILL BE A STEADY NORTH TO
NORTHEAST WIND BETWEEN KEMMERER AND CASPER TONIGHT THAT WILL BE
ENOUGH TO WARRANT A WIND CHILL WATCH FOR THESE AREAS AFTER MIDNIGHT.
SINCE IT IS FAIRLY BORDERLINE AND THERE ARE ALREADY VARIOUS WINTER
WEATHER ADVISORIES OUT FOR THE MORNING...STUCK WITH JUST A WATCH FOR
NOW.

TUESDAY NIGHT TEMPERATURES ARE INTERESTING AS A FOLD OVER RIDGE OF
SORTS MOVES IN. AIR LOOKS TO BE QUITE DRY WITH THE RIDGE SO SKIES
WILL BE CLEAR ENOUGH TO ALLOW SIGNIFICANT RADIATIVE COOLING.
HOWEVER...THE RIDGE WILL ALSO BRING A MUCH MORE MILD AIRMASS TO THE
MOUNTAINS...PICKING OUT HOW SHARP THE INVERSIONS WILL BE LATE
TUESDAY NIGHT WILL BE AN INTERESTING CHALLENGE. THIS SAME SETUP
CREATES A CHALLENGE FOR HIGHS AND LOWS ON WEDNESDAY. IF THE BASINS
AND VALLEYS DO MIX...THEY COULD BE QUITE A BIT WARMER THAN FORECAST.
HOWEVER...THE TYPICAL FORCES FOR MIXING ARE NOT EVIDENT...
SIGNIFICANT WIND AND SOME ELEMENT OF DOWNWARD FORCING OR
PRECIPITATION...SO TEMPERATURES WERE DROPPED SIGNIFICANTLY IN THE
VALLEYS AND BASINS AND MAY NEED TO BE DROPPED EVEN FURTHER IF MIXING
STILL APPEARS UNLIKELY.

.LONG TERM...THURSDAY THROUGH MONDAY

SYNOPSIS...A RIDGE OF HIGH PRESSURE AND LIGHT WIND WILL BRING MUCH
WARMER CONDITIONS TO MIXED AREAS THURSDAY AND FRIDAY WHILE LOWER
CENTRAL BASINS AND WESTERN VALLEYS REMAIN TRAPPED UNDER STEEP
INVERSIONS WITH TEMPERATURES 10 TO 20 DEGREES COLDER THAN
SURROUNDING FOOTHILLS.  THE STEEP INVERSIONS WILL ALSO LIKELY RESULT
IN FOG FORMATION.  THE POLAR FRONT MAY DIP SOUTH ACROSS THE HIGH
PLAINS OVER THE WEEKEND BUT THE COLDEST AIR IS EXPECTED TO STAY
NORTH AND EAST OF WYOMING.  NORTHWEST FLOW WILL BRING SOME CHANCES
OF SNOW TO THE NORTHERN AND WESTERN MOUNTAINS OVER THE WEEKEND BUT
AMOUNTS NOW APPEAR TO BE ON THE LIGHT SIDE.

DISCUSSION...MEDIUM-RANGE GUIDANCE HAS TRENDED FLATTER WITH THE
UPPER FLOW OVER NORTH AMERICA LATE THIS WEEK INTO THE WEEKEND.  THIS
CHANGE IS LARGELY THE RESULT OF HIGH AMPLITUDE HIGH OVER THE EASTERN
PACIFIC INTO THE ARCTIC CIRCLE GETTING SPLIT INTO BY PACIFIC STORMS
PUSHING INTO THE GULF OF ALASKA AND PINCHING OFF A CLOSED UPPER HIGH
OVER THE ARCTIC OCEAN/BEAUFORT SEA.  HEIGHTS OVER THE NORTHERN
ROCKIES CONTINUE TO BUILD THURSDAY AND FRIDAY AS SW UPPER LOW EJECTS
INTO THE SOUTHERN PLAINS AND RIDGE ALONG THE WEST COAST EXPANDS
INLAND.  GULF OF AK IS EXPECTED TO SHEAR SE INTO BACKSIDE OF HUDSON
BAY LOW.  THE 00Z GFS BRINGS THIS ENERGY FURTHER SOUTH ALONG WITH
THE POLAR FRONT SAGGING INTO NORTHERN WYOMING NEXT WEEKEND.  ECMWF
KEEPS THIS COLDER AIR MUCH FURTHER NORTH.  06Z GFS IS TRENDING
TOWARD THE FLATTER ECMWF...SO OVERALL FORECAST IS TRENDED TOWARD A
MILDER AND DRIER PATTERN OVER THE WEEKEND.

&&

.AVIATION.../12Z ISSUANCE/

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

MVFR TO IFR CONDITIONS WILL OCCUR THROUGH 19Z. GRADUAL IMPROVEMENT
WILL OCCUR BY 00Z WITH VFR TO MVFR CONDITIONS PREVAILING. CONDITIONS
WILL CONTINUE TO IMPROVE AFTER 00Z WITH VFR CONDITIONS PREVAILING
AND ONLY LOCAL MVFR CONDITIONS OCCURRING AT 06Z TUE.

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

AREAS OF SNOW AND LOW CLOUDS WILL PRODUCE MVFR TO IFR CEILINGS AND
VISIBILITIES THROUGH 19Z. AT 00Z VFR CONDITIONS WILL PREVAIL. THE
KRKS TERMINAL SITE WILL SEE A GUSTY WIND ALONG WITH SOME SNOW AND
BLOWING SNOW AT TIMES TODAY INTO THIS EVENING.

&&

.FIRE WEATHER...

CLOUDY WITH FLURRIES TODAY WITH NORTHEAST BREEZES DEVELOPING IN
RAWLINS AND CASPER DISPATCH AREAS. MIXING WILL BE FAIR IN THESE
AREAS. OTHERWISE...SMOKE DISPERSION WILL BE VERY POOR FOR THE NEXT
SEVERAL DAYS AS SHALLOW INVERSIONS SET UP IN THE BASINS AND VALLEYS.
THE NEXT CHANCE FOR SIGNIFICANT SNOW AND MIXING WILL BE LATE NEXT
WEEKEND.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
WINTER WEATHER ADVISORY UNTIL NOON MST TODAY
WYZ003>011-015>020-022-026>030.

WIND CHILL WATCH FROM LATE TONIGHT THROUGH TUESDAY MORNING
WYZ014-019-020-022-027>030.

&&

$$

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










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