Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 251700
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
1000 AM MST WED FEB 25 2015

.SHORT TERM...TODAY THROUGH FRIDAY NIGHT

THE COLD FRONT APPEARS TO HAVE SLIPPED SOUTH ACROSS NORTHERN JOHNSON
COUNTY WHERE NORTH WINDS ARE NOW GUSTING TO 30 KNOTS AT BYG.
BILLINGS RADAR IS SHOWING STRATIFORM PRECIP FALLING BEHIND THE
FRONT...MOSTLY VIRGA ATTM. THIS PRECIP...WHICH WILL START AS RAIN AT
THE LOWER ELEVATIONS BEFORE TURING TO SNOW AND WILL SPREAD SOUTH
ACROSS JOHNSON AND NATRONA COUNTIES DURING THE REMAINDER OF THE PRE
DAWN HOURS AND WILL LIKELY REACH THE DIVIDE BY 12Z THIS MORNING. THE
TROUGH RESPONSIBLE FOR PUSHING THE FRONTAL BOUNDARY SOUTH WILL BE
FAIRLY CONVOLUTED AND WILL COME THROUGH IN ABOUT 4 SEPARATE PIECES
OF ENERGY TODAY THROUGH TONIGHT FROM THE NW. DYNAMICS WITH THIS
SYSTEM ARE NOT VERY IMPRESSIVE BUT MOST AREAS OF THE CWA WILL SEE
SNOWFALL IN ONE FORM OR ANOTHER WITH THE BEST CHANCES EAST OF THE
DIVIDE AND THE LEAST LIKELY AREAS OVER SOUTHWEST WY. LIFTED INDICES
HAVE BEEN PROGGED TO RANGE FROM ZERO TO -2 OVER THE WESTERN CWA FOR
THIS AFTERNOON WITH THE LAST FEW RUNS SO A LIGHTNING STRIKE OF TWO
COULD OCCUR RIGHT ALONG THE DIVIDE WHERE THE BEST MOISTURE AND
INSTABILITY COULD OVERLAP JUST AHEAD OF THE FRONT AS COLD AIR POURS
IN ALOFT BUT WILL LEAVE OUT OF THE GRIDS. KEPT SUB ADVISORY OR LOW
END ADVISORY FOR SNOW AMOUNTS MOST AREAS ALONG AND EAST OF THE
DIVIDE TODAY THROUGH TONIGHT ALTHOUGH THERE WILL LIKELY BE LOCALLY
HIGHER AMOUNTS UNDERNEATH HEAVIER SNOW BANDS. DECIDED TO ISSUE A
WINTER WEATHER ADVISORY VALID FROM 09Z THIS MORNING THROUGH 07Z
THU AS INCREASING NORTH WIND WILL COMBINE WITH THE SNOWFALL TO
PRODUCE MORE OF AN IMPACT IN THOSE AREAS. THIS WILL BLEND IN WELL
WITH BYZ ADVISORIES. THE LOCAL WRF IS ADVERTISING FAIRLY GOOD
SNOWBANDING OFF THE BIG HORN MTNS WITH 6 HOUR PRECIP AMOUNTS OF MORE
THAN ONE HALF INCH FOR WHAT ITS WORTH. IN FACT...MUCH OF THE
SNOWFALL THAT OCCURS TODAY WILL BE IN THE FORM OF BANDING WITH THE
COLD AIR ADVECTING IN AT H7 FROM THE NNW. SNOWFALL RATES SHOULD
GRADUALLY DECLINE TONIGHT AS H7 TEMPS BECOME COLD ENOUGH FOR LESS
FAVORABLE DENDRITIC GROWTH. BY 12Z THURSDAY THE H7 TEMPS WILL DROP
TO -23C OVER NE JOHNSON COUNTY. LIGHT SNOW WILL CONTINUE IN
WESTERN WYOMING THU AS THE ENERGY WITHIN THE MAIN TROUGH
PROPAGATES SOUTHWEST TOWARD THE GREAT BASIN. THEN A TROUGH
TRACKING FROM THE PACIFIC NORTHWEST TO THE GREAT BASIN WITHIN THE
MAIN TROUGH WILL AMPLIFY THE MAIN TROUGH AND WILL KEEP THE LIGHT
SNOW GOING IN PARTS OF THE WEST THURSDAY NIGHT...FRIDAY AND FRIDAY
NIGHT. AFTER THE COLDEST OF THE 700MB TEMPERATURES PASS
BY...FRIDAY MORNING SHOULD BE THE COLDEST PERIOD OF THE WEEK ALONG
WITH SOME BELOW ZERO READINGS AS SKIES CLEAR EAST OF THE DIVIDE
AND FRESH SNOW COVER REFRIGERATES THE AIR.

.LONG TERM...SATURDAY THROUGH WEDNESDAY

SYNOPSIS...A STORM SYSTEM DIGGING INTO THE SOUTHWEST U.S. WILL BRING
CHANCES OF SNOW ACROSS THE AREA SATURDAY WITH THE MOST WIDESPREAD
COVERAGE EXPECTED ACROSS THE SOUTH AND WEST.  COLD CANADIAN SURFACE
HIGH PRESSURE AND A DRIER AIRMASS WILL BUILD IN FROM THE NORTH
SATURDAY NIGHT AND SUNDAY.  MAINLY DRY WEATHER WITH MORE SEASONAL
TEMPERATURES ARE EXPECTED ON MONDAY.   ANOTHER PACIFIC STORM SYSTEM
AND ASSOCIATED CANADIAN COLD FRONT WILL SPREAD CHANCES OF SNOW
ACROSS THE AREA ON TUESDAY.

DISCUSSION...UPPER RIDGE IS EXPECTED TO REMAIN ACROSS THE EASTERN
PACIFIC THROUGH EARLY NEXT WEEK WITH A SERIES OF PACIFIC STORM
SYSTEMS CONTINUING TO RIDE OVER THE TOP OF THE RIDGE...ACROSS
ALASKA...BEFORE PLUNGING SOUTH ACROSS THE WESTERN U.S.   THE NEXT IN
THIS SERIES WILL DIVE ACROSS THE NW U.S. ON FRIDAY...WITH AN H5 LOW
CLOSING OFF OVER NEVADA ON SATURDAY.  LIKE THE LAST FEW SYSTEMS...A
SEPARATE CLIPPER IN THE NORTHERN STREAM WILL DRIVE A CANADIAN COLD
FRONT SOUTH INTO THE NORTHERN PLAINS WITH THE FRONT BACKING INTO THE
EAST SLOPES OF THE DIVIDE ON SATURDAY.  THIS WILL RESULT IN ANOTHER
SHOT OF VERY COLD AIR WITH AREAS OF UPSLOPE LIGHT `DRY` SNOW ACROSS
CENTRAL WYOMING.  THE SOUTHWEST PORTION OF THE STATE WILL STAND THE
BETTER CHANCE OF RECEIVING ANYTHING CLOSE TO ADVISORY AMOUNTS.

UPPER TROUGH IS EXPECTED TO SHEAR OUT ON SUNDAY WITH DEFORMATION
AREA NEAR OUR SOUTH OF INTERSTATE 80 SUNDAY AFTERNOON...WITH ANY
PRECIPITATION PUSHING NORTH FROM REMAIN SW LOW NOT MAKING MUCH
FURTHER NORTH THAN INTERSTATE 80.

SW LOW IS EXPECTED TO GET KICKED OUT BY NEXT SYSTEM DIVING DOWN INTO
THE NW U.S. ON MONDAY.  WYOMING LOOKS TO BE MAINLY DRY BETWEEN THESE
TWO SYSTEMS ON MONDAY.  THIS NEXT SYSTEM TAKE A SIMILAR TRACK INTO
THE SW U.S. ON TUESDAY AND IS LIKEWISE EXPECTED TO PULL A CANADIAN
COLD FRONT AND CHANCES OF SNOW SOUTH ACROSS THE AREA.

&&

.AVIATION.../18Z ISSUANCE/

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

A COLD FRONT CONTINUES TO DROP SOUTHWARD ACROSS THE AREA. PERIODS OF
MVFR/IFR CONDITIONS IN LOW CLOUDS...AS WELL AS A COMBO OF SNOW
SHOWERS AND UPSLOPE STRATIFORM SNOW...DEVELOPING MAINLY ALONG AND
BEHIND THE COLD FRONT. THESE LOW CLOUDS AND BANDS OF SNOW SHOWERS
WILL CONTINUE THROUGH THE EVENING...BECOMING LESS WIDESPREAD AND
INTENSE AS THE NIGHT PROGRESSES. HOWEVER...AREAS OF MVFR CEILINGS
AND SOME LIGHT SNOW IS EXPECTED THROUGHOUT THE TAF PERIOD. BREEZY TO
WINDY NORTHERLY WINDS WILL OCCUR TODAY IN WAKE OF THE COLD
FRONT...WITH WINDS WEAKENING TONIGHT.

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

A DISTURBANCE SLIDING SOUTH ACROSS THE AREA WILL RESULT IN SOME SNOW
SHOWER ACTIVITY...BUT NOT ENOUGH COVERAGE TO PREVAIL ANY LOW CLOUDS
OR VISIBILITIES AT THIS TIME. THE ONLY EXCEPTION WILL BE OVER THE
SOUTH...WHERE A COLD FRONT WILL SLIDE WEST ACROSS SWEETWATER COUNTY
LATE THIS AFTERNOON INTO EARLY EVENING WITH A DECENT CHANCE OF SNOW
ALONG AND BEHIND THE COLD FRONT. MVFR AND IFR CONDITIONS ARE LIKELY
TO DEVELOP AT KRKS BETWEEN 22Z AND 01Z AND LAST A FEW HOURS.
ALSO...VERY WINDY NORTHEAST/EAST WINDS WILL DEVELOP IN THE EVENING
OVER SWEETWATER COUNTY.

&&

.FIRE WEATHER...

A COLD FRONT AND A COUPLE OF ASSOCIATED UPPER LEVEL DISTURBANCES
WILL CROSS THE AREA TODAY AND BRING INCREASED CLOUD COVER ALONG
WITH A GOOD CHANCE OF SNOW TODAY...ESPECIALLY EAST OF THE DIVIDE.
GUSTY NORTH WINDS WILL ACCOMPANY THE SNOW...ESPECIALLY THIS
AFTERNOON. BELOW NORMAL TEMPERATURES WILL CONTINUE INTO THE END OF
THE WEEK ALONG WITH POOR SMOKE DISPERSAL AND LOW MIXING HEIGHTS.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
WINTER WEATHER ADVISORY UNTIL MIDNIGHT MST TONIGHT WYZ008>010.

&&

$$

SHORT TERM...LIPSON
LONG TERM...AEM
AVIATION...HATTINGS
FIRE WEATHER...LIPSON










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