Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 251740
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
1140 AM MDT SAT APR 25 2015

.SHORT TERM...TODAY THROUGH MONDAY NIGHT

ONE MORE SHOWERY/STORMY DAY BEFORE A COOLER MORE STRATIFORM
PRECIPITATION EVENT MOVES IN ON SUNDAY.

THE FIRST OF A COUPLE OF FRONTS IS ILL DEFINED AT THIS POINT IN
THE MORNING BUT IT IS EXPECTED TO RESUME ITS PUSH SOUTHWARD TOWARD
SUNRISE EAST OF THE DIVIDE...AS A TROUGH SWINGS THROUGH AND TILTS
UPPER/MID LEVEL FLOW MORE SOUTHWESTERLY WITH A FETCH OF MODERATE
MOISTURE COMING ALONG WITH IT. COLD POOLING AT THE MID LEVELS
WASHES OVER THE REGION BY THE AFTERNOON...WHICH SHOULD KICK OFF A
FEW MORE THUNDERSTORMS...ESPECIALLY ALONG THE SURFACE COLD FRONT
AS IT RESTS ALONG THE DIVIDE...IN THE MOUNTAINS...AND ACROSS THE
SOUTH WHERE THE LAPSE RATES WILL BE STEEPER.

THE MAIN EVENT BEGINS TONIGHT AS THE MAIN TROUGH DEEPENS AND
CLOSES OFF INTO AN UPPER LOW TO OUR SOUTH. WHILE MOISTURE WILL BE
GOOD OVERALL...THE SYSTEM WILL BE COMING INTO A RATHER MILD
AIRMASS AND THERE IS STILL A LOT OF UNCERTAINTY REGARDING MINIMUM
TEMPERATURES...ESPECIALLY SINCE MOSTLY CLOUDY TO OVERCAST SKIES
WILL LIKELY PREVENT TEMPERATURES FROM BOTTOMING OUT TONIGHT. AS A
RESULT...KEPT SNOW LEVELS AND SNOW AMOUNTS PESSIMISTIC CONSIDERING
THE FAVORABLE DYNAMICS...DEVELOPING UPSLOPE FLOW FUNNELING INTO
THE WIND RIVER BASIN ESPECIALLY. THIS COULD REALLY BE A SOAKER FOR
THE UPPER WIND RIVER BASIN INCLUDING DUBOIS AND FOR THE LANDER
FOOTHILLS AND FOR FORT WASHAKIE. THERE COULD BE A PERIOD OF QUITE
HEAVY SNOW EARLY SUNDAY MORNING IN THESE AREAS BUT IT WILL
PROBABLY CHANGE BACK OVER TO RAIN AT SOME POINT DURING THE DAY ON
SUNDAY...THAT COMBINED WITH THE WARM ROAD TEMPERATURES...THE
IMPACT IS NOT EXPECTED TO BE ENOUGH TO WARRANT A HIGHLIGHT AT
THIS TIME BUT SNOW COULD BE INTENSE ENOUGH FOR SOME SLUSHY AND
SLIPPERY ROADS EARLY SUNDAY IN LANDER...DUBOIS AND CROWHEART.
OTHERWISE...LOOKS LIKE THE BEST CHANCE FOR ACCUMULATION AND TRAVEL
IMPACT WILL BE IN THE MOUNTAINS... ESPECIALLY OVER SOUTH PASS SO
WE HOISTED A WINTER WEATHER ADVISORY FOR THE WIND RIVER MOUNTAINS
SO FAR. THERE WAS ALSO DISCUSSION ABOUT THE MEETEETSE AREA...UPPER
WIND RIVER BASIN AND THE WYOMING AND SALT RANGES FOR POSSIBLE
INCLUSION BUT HELD OFF AT THIS TIME DUE TO THE UNCERTAINTY
REGARDING TEMPERATURES.

THERE COULD BE ONE LAST HURRAH OVER THE I25 CORRIDOR SUNDAY NIGHT
BEFORE THE SYSTEM EXITS LATE BUT AGAIN TEMPERATURES WILL BE
BORDERLINE AND THE ACTIVITY TOO SHORT LIVED TO ENTERTAIN AN
ADVISORY FOR THE CASPER AREA AT THIS POINT. HOWEVER...THERE COULD
BE A QUICK 4 TO 6 INCHES ON CASPER MOUNTAIN THAT MAY COMPLICATE
ANY TRAVEL ATTEMPTED AT THAT ELEVATION. OTHERWISE...MOST AREAS
WILL BE FREE OF PRECIPITATION BY MONDAY MORNING AS A WARMING AND
DRYING TREND BEGINS WITH THE RISING HEIGHTS BEHIND THE TROUGH.

.LONG TERM...TUESDAY THROUGH SATURDAY

SYNOPSIS...A RIDGE OF HIGH PRESSURE WILL BRING DRY WEATHER ON
TUESDAY.  SOUTHWEST FLOW WILL THEN BRING AN UNSEASONABLY MILD
AIRMASS ACROSS THE REGION WEDNESDAY THROUGH FRIDAY.  CHANCES OF
SHOWERS AND THUNDERSTORMS WILL BE CONFINED TO THE NW CORNER ON
WEDNESDAY...ACROSS THE WEST ON THURSDAY...AND THEN A CHANCE OF AT
LEAST AN ISOLATED THUNDERSTORM THE ENTIRE AREA FRIDAY AND SATURDAY.

DISCUSSION...UPPER RIDGE BUILDS OVER THE AREA ON TUESDAY AS NEXT
PACIFIC TROUGH...OVER THE CENTRAL PACIFIC EARLY THIS MORNING...
MOVES TO THE WEST COAST.   THE UPPER RIDGE WILL SHIFT EAST INTO THE
PLAINS WEDNESDAY AND THURSDAY WITH A SERIES OF SHORTWAVES EJECTING
UP THE BACK SIDE OF THE RIDGE ACROSS THE NW U.S. INTO THE NORTHERN
ROCKIES. MAIN UPPER TROUGH WILL THEN PUSH INTO THE WESTERN U.S. NEXT
WEEKEND.

DETERMINISTIC ECMWF REMAINS STRONGER WITH THE FIRST COUPLE OF
SHORTWAVES EJECTING ACROSS THE NORTHERN ROCKIES TUESDAY NIGHT AND
WEDNESDAY NIGHT RESPECTIVELY...RESULTING IN MORE FLATTENING OF THE
DOWNSTREAM RIDGE.  GFS AND EC/GFS ENSEMBLE MEANS KEEP RIDGE MOSTLY
INTACT WITH WEAKER SHORTWAVES RIDING UP BACKSIDE OF RIDGE INTO
CENTRAL CANADA.  FORECAST REFLECTS THIS LATTER SOLUTION THAT OFFERS
THE BEST CONSISTENCY...RESULTING IN INTRODUCING SLIGHT CHANCE POPS
OVER THE NW CORNER WEDNESDAY WITH TEMPERATURES CONTINUING TO WARM TO
10F TO 20F ABOVE SEASONAL NORMALS.  DETERMINISTIC MODELS ARE IN POOR
AGREEMENT IN TIMING AND DETAILS BRINGING MAIN UPPER TROUGH INTO
WESTERN U.S. LATE IN THE WEEK.  ENSEMBLE MEANS SHOW SPLIT FLOW WITH
TROUGH AXIS NEAR 120W BY 00Z SUNDAY WITH BROAD RIDGE OVER THE
CENTRAL U.S.   THIS OVERALL SLOW PROGRESSION SHOULD ALLOW SOME WEAK
SHORTWAVE ENERGY TO INTRUDE FURTHER SOUTH ACROSS THE NORTHERN GREAT
BASIN AND WYOMING BY FRIDAY...BRINGING A CHANCE OF AT LEAST AN
ISOLATED SHOWER OR THUNDERSTORM TO MOST AREAS FRIDAY AND
SATURDAY...WITH A SLIGHT COOLING TREND ON SATURDAY.


&&

.AVIATION.../18Z ISSUANCE/

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

VFR CONDITIONS WILL PREVAIL THROUGH THE FORECAST PERIOD FOR MOST
LOCATIONS EXCEPT KCOD WHICH WILL REMAIN MVFR THROUGH MID AFTERNOON
BEFORE BECOMING VFR FOR A PERIOD BEFORE RETURN TO MVFR CONDITIONS
EARLY SUNDAY MORNING. OTHER TERMINALS WILL SEE PRECIPITATION AND
LOWER CIGS SPREADING INTO THE REGION FROM THE SOUTH THIS
EVENING...WITH MVFR CONDITIONS IN LOW CIGS AND VIS RESTRICTIONS DUE
TO PRECIPITATION...RAIN CHANGING TO RAIN/SNOW MIX OR ALL SNOW...
BECOMING PREVALENT MOST LOCATIONS BY 06Z. MOUNTAIN OBSCURATIONS WILL
OBVIOUSLY BECOME PREVALENT DURING THE SAME PERIOD OF TIME.
OCCASIONAL IFR CONDITIONS EXPECTED MOST TERMINALS AFTER
09Z...LASTING THROUGH THE END OF THE FORECAST.

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

VFR TO  NEAR MVFR IN LOW CIGS AS SHOWERS AND ISOLATED THUNDERSTORMS
MOVE ACROSS THE AREA...LASTING INTO THE EARLY EVENING. THEN CIGS AND
VIS WILL DROP BECOMING PRIMARILY MVFR...TO OCCASIONAL IFR IN LOW
CIGS AND VIS AS RAIN AND/OR SNOW SHOWERS BECOME INCREASINGLY LIKELY
OVERNIGHT. AFTER 07Z AT THE TERMINALS AND OVR THE MOUNTAINS...MVFR
TO OCCASIONAL IFR CONDITIONS IN LOW CIGS AND VIS RESTRICTIONS DUE TO
PRECIPITATION...RAIN CHANGING TO RAIN/SNOW MIX OR ALL SNOW... WILL
BE PREVALENT...LASTING THROUGH THE END OF THE FORECAST
PERIOD.

&&

.FIRE WEATHER...

COOL AND WET THROUGH THE WEEKEND AS SEVERAL STORM SYSTEMS MOVE
THROUGH THE REGION. THE BEST CHANCE FOR A WETTING RAIN OR SNOW WILL
BE ON SUNDAY FOR MOST AREAS. SHOWERS AND WET THUNDERSTORMS WILL BE
THE PRIMARY DELIVERY METHOD FOR WETTING RAINS TODAY WITH MORE OF A
STRATIFORM RAIN OR SNOW TAKING OVER ON SUNDAY. A WARMING AND
DRYING TREND BEGINS ON MONDAY AND COULD LAST THE WEEK. NO
WIDESPREAD HOT/DRY/WINDY DAYS ARE STANDING OUT QUITE YET BUT
THURSDAY AND FRIDAY HAVE POTENTIAL TO BE BREEZY...WITH 15 TO 25
MPH SOUTHWESTERLY WINDS POSSIBLE ACROSS MUCH OF THE AREA.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
WINTER WEATHER ADVISORY FROM 9 PM THIS EVENING TO MIDNIGHT MDT
SUNDAY NIGHT WYZ014-015.

&&

$$

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











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