Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 120905
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
305 AM MDT SAT APR 12 2014

.SHORT TERM...SATURDAY THROUGH MONDAY NIGHT

REMEMBER ALL THAT NICE...MILD WEATHER WE HAVE HAD FOR THE PREVIOUS
FEW DAYS? WELL...WE HOPE YOU DID NOT GET USED TO IT. A COLD FRONT
NOW OVER MONTANA WILL DROP SOUTHWARD THROUGH THE DAY AND MOVE
THROUGH WYOMING STARTING LATER THIS MORNING AND CONTINUING INTO THE
AFTERNOON. BEFORE THE PASSAGE...WE SHOULD HAVE ONE MORE MILD DAY
FOR MOST AREAS. NORTHERN PORTIONS OF THE STATE WILL BE A BIT TRICKIER
HOWEVER WITH THE FRONT PASSING SOONER. CONTINUITY LOOKED GOOD
HOWEVER SO WE MADE FEW CHANGES. AHEAD OF THE FRONT...SOME SHOWERS
WILL BREAK OUT BUT WILL LIKELY BE MAINLY OF THE RAIN VARIETY WITH
THE WARM TEMPERATURES WITH THE EXCEPTION OF THE HIGHEST ELEVATIONS.
HOWEVER...THE SOUTHERN TWO THIRDS OF THE STATE STILL LOOK TO BE
MAINLY DRY THROUGH THE AFTERNOON WITH ONLY LATE DAY SHOWERS. THERE
IS SUFFICIENT INSTABILITY FOR A FEW THUNDERSTORMS AS WELL SO WE KEPT
THOSE IN THE FORECAST AS WELL. THERE COULD BE SOME MIXING WITH SNOW
LATER IN THE DAY ACROSS THE NORTH ALTHOUGH THERE WILL BE LITTLE
ACCUMULATION WITH THE HIGH SUN ANGLE AND WARM GROUND TEMPERATURES
EXCEPT IN THE MOUNTAINS.

THE MAIN EVENT GETS GOING TONIGHT...ESPECIALLY FROM LATER THIS
EVENING THROUGH SUNDAY MORNING. STRONG COLD ADVECTION WILL OCCUR
TONIGHT AFTER THE FRONTAL PASSAGE. IN ADDITION...WITH LEE
CYCLOGENESIS EXPECTED TO OCCUR OVER COLORADO THIS WILL TURN 700
MILLIBAR WINDS MORE TO THE NORTHEAST. THIS WILL SET UP THE FAVORABLE
UPSLOPE FLOW FROM THE EASTERN SLOPES OF THE WIND RIVER RANGE THROUGH
NATRONA COUNTY AS WELL AS THE CODY FOOTHILLS. AS A RESULT...WE HAVE
UPGRADED THE WINTER STORM WATCHES TO WARNINGS THIS MORNING FOR THOSE
AREAS. STILL A BIT OF DOUBT THOUGH...SINCE THIS EVENT IS DEPENDENT
MAINLY ON UPSLOPE FLOW WITH THE BEST UPPER LEVEL JET SUPPORT
EXPECTED TO BE EASTERN WYOMING AND WESTERN NEBRASKA. ELSEWHERE...WE
HAVE DECIDED TO GO WITH WINTER WEATHER ADVISORIES.PLACES LIKE THE
WIND RIVER BASIN MAY SEE SOME DOWNSLOPING FLOW OFF OF THE OWL CREEK
RANGE THAT MAY LIMIT ACCUMULATIONS. ALSO...PLACES LIKE THE EASTERN
BIG HORN BASIN WILL SEE DOWNSLOPING FLOW. IN JOHNSON COUNTY...DRIER
AIR MOVING IN MAY SHUT THE SNOW OFF EARLIER THEN EXPECTED. THE MAIN
THREAT WILL LIKELY BE A FLASH FREEZE ON ROADS WITH INITIAL MELTING
WITH THE WARM GROUND TEMPERATURES THEN FREEZING WITH A RAPID DROP OF
TEMPERATURES LATER AT NIGHT. AND...WEST OF THE DIVIDE WITH
DOWNSLOPING FLOW FOR MOST AREAS PRECIPITATION SHOULD BE LIMITED
EXCEPT FOR THE FAVORED UPSLOPE AREAS OF THE SALT AND WYOMING RANGE
WHERE THERE COULD BE A FEW INCHES OF NEW SNOW. THE HEAVIEST SNOW
LOOKS TO FALL FROM AROUND 3 AM THROUGH NOON ON SUNDAY. AFTER
THAT...WE SHOULD SEE GRADUALLY IMPROVING CONDITIONS WITH SNOW
TAPERING OFF AND ENDING BY THE SUNDAY EVENING. WE AGREE WITH THE
PREVIOUS SHIFT WITH POSSIBLE FOG FORMATION SO WE LEFT THAT IN THE
FORECAST.

AS WE HEAD INTO MONDAY...IT LOOKS TO BE A QUIET WEATHER DAY WITH
HIGH PRESSURE AND RIDGING OVER THE STATE. TEMPERATURES WILL BE
COOLER HOWEVER WITH SNOW COVER AND MUCH OF THE RADIATION BEING USED
TO MELT THE SNOW.

.LONG TERM...TUESDAY THROUGH SATURDAY

TUESDAY WILL BE DRY FOR MOST PLACES. ISOLATED SHOWERS TO DEVELOP
OVER THE NW WY MOUNTAINS TUE AFTERNOON. THE NEXT WEATHER SYSTEM
IMPACTS NW WY TUE NIGHT AND THEN MOST OF THE REGION WEDNESDAY. THE
STRENGTH AND THE TRACK OF MAIN MOISTURE REMAINS UNCLEAR AS THE
GFS AND GEM MODELS ARE MORE BULLISH THAN THE ECMWF FOR NOW. WILL
NEED TO WATCH MODEL TRENDS IN COMING DAYS. THE SYSTEM MOVES OUT
WED NIGHT TAKING THE PRECIP WITH IT FOR THE MOST PART. LINGERING
MOUNTAIN SNOW SHOWERS ARE POSSIBLE AND ISOLATED SHOWERS EAST OF
THE DIVIDE WED NIGHT. THURSDAY LOOKS DRY FOR MOST OF THE REGION.
SOME MOISTURE MAY CREEP BACK INTO THE NW WY MOUNTAINS THURS
AFTERNOON FOR ISOLATED SHOWERS. AS FOR NEXT FRIDAY THE GFS MODEL
IS THE MOST UNSETTLED WITH ANOTHER WEATHER SYSTEM MOVING THROUGH
THE REGION. THE ECMWF IS WEAKER. THE GEM IS SLOWER AND WAITS UNTIL
FRIDAY NIGHT INTO SATURDAY. FOR NOW WILL BRING IN LOW PRECIP CHANCE
TO THE WEST THURS NIGHT. THEN A BETTER CHANCE OF PRECIP FRIDAY AND
FRI NIGHT OVER THE WEST. WILL HAVE A LOW CHANCE OF PRECIP EAST OF
THE DIVIDE FOR FRIDAY AFTERNOON AND FRI NIGHT. NEXT SATURDAY LOOKS
DRY FOR MOST AREAS...ISOLATED SHOWERS IN THE WESTERN MOUNTAINS
MAINLY FOR THE AFTERNOON AND EVENING HOURS. HIGH TEMPS ON TUESDAY
LOOK TO BE MILDER...THEN COOLER ON WED DUE TO THE WEATHER SYSTEM.
THURS LOOKS TO WARM UP A LITTLE. FRI WILL BE A TAD MILDER EAST OF
THE DIVIDE AHEAD OF THE WEATHER SYSTEM WHILE THE WEST IS NEAR OR
SLIGHTLY COOLER TO THE THURS HIGHS. NEXT SATURDAY SHOULD BE WARMER
PROVIDED THE PATTERN HOLDS.

&&

.AVIATION.../12Z ISSUANCE/

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

VFR CONDITIONS WILL PREVAIL THROUGH 20Z SATURDAY.  INCREASING
MOISTURE AND INSTABILITY WILL INITIALLY BRING SCT SHRA AND ISOLD
TSRA VCNTY AND SOUTH OF INTERSTATE 80 AND INTO THE NW CORNER OF
WYOMING AFTER 20Z SATURDAY.  A COLD FRONT WILL SPREAD NMRS SHSN AND
WIDESPREAD MTN OBSCURATIONS SOUTH FROM MONTANA TO A VCNTY KJAC-KDUB
LINE BTWN 0Z-6Z SUNDAY.  HOWEVER INCREASING NE DOWNSLOPE FLOW WILL
RESULT IN SOME IMPROVEMENT ACROSS THE NW AFTER 06Z.  RAIN AND SNOW
...WIDESPREAD IFR/LIFR CONDITIONS...WILL DEVELOP PRIMARILY FROM KRKS
TO THE SOUTH PASS AREA 6Z-12Z SUNDAY AS THE COLD FRONT BACKS INTO SW
WYOMING.

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

VFR CONDITIONS WILL PREVAIL ACROSS THE AREA THROUGH 21Z SATURDAY
WITH A PREVAILING N-NE SFC FLOW DEVELOPING THROUGH THE MORNING.
WIDESPREAD RAIN AND SNOW WILL RAPIDLY DEVELOP ACROSS THE NORTH AFTER
21Z SATURDAY AND SPREAD SOUTH THROUGH THE EVENING.  THE RAIN OR
RAIN/SNOW MIX WITH MVFR/IFR CONDITIONS WILL LAST A COUPLE OF HOURS
WITH THE INITIAL FRONTAL BAND WITH SNOW LEVELS RAPIDLY LOWERING
BEHIND THE FRONT.  EXPECT WIDESPREAD IFR/LIFR CONDITIONS VCNTY AND
NORTH OF A KCOD-KBYG LINE BY 00Z SUNDAY SPREADING TO A VCNTY
KLND-KCPR LINE BETWEEN 05Z-08Z SUNDAY.

&&

.FIRE WEATHER...

A COLD FRONT WILL DROP SOUTHWARD ACROSS THE AREA TODAY AND BRING
SOME SHOWERS AND ISOLATED THUNDERSTORMS...ESPECIALLY OVER THE
HIGHER TERRAIN ACROSS THE NORTH AND ALONG RAWLINS DISPATCH AREA.
THIS COLD FRONT WILL BRING WETTING RAIN AND SNOW TO MUCH OF THE
FORECAST AREA ALONG AND EAST OF THE DIVIDE TONIGHT AND SUNDAY.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
WINTER STORM WARNING FROM 9 PM THIS EVENING TO 3 PM MDT SUNDAY
WYZ015-018>020-022.

WINTER WEATHER ADVISORY FROM 9 PM THIS EVENING TO 3 PM MDT
SUNDAY WYZ004>007-010-011-016-017.

WINTER WEATHER ADVISORY FROM 6 PM THIS EVENING TO NOON MDT
SUNDAY WYZ002.

WINTER WEATHER ADVISORY FROM 6 PM THIS EVENING TO 3 PM MDT
SUNDAY WYZ008-009.

WINTER STORM WARNING FROM 6 PM THIS EVENING TO NOON MDT SUNDAY
WYZ003.

&&

$$

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





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