Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 210515
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
1115 PM MDT SAT SEP 20 2014

.SHORT TERM...TONIGHT THROUGH SUNDAY (ISSUED AT 227 PM MDT)

POST FRONTAL NORTH WINDS IN JOHNSON COUNTY WILL DECREASE BY
SUNSET. ONE MORE BONUS FALL DAY SUNDAY IS EXPECTED BEFORE THE NEXT
CLOSED LOW PRESSURE SYSTEM NOW SPINNING OVER SOUTH CENTRAL
CALIFORNIA AFFECTS OUR WEATHER BY LATE SUNDAY AFTERNOON...ESPECIALLY
IN SOUTHWEST WY. THE HIGH PRESSURE RIDGE WILL CONTINUE TO BUILD IN
FROM THE WEST WITH THE RIDGE AXIS OVER CENTRAL WY BY 18Z SUNDAY.
HIGHS TEMPS WILL BE ROUGHLY 5 DEGREES WARMER SUNDAY THAN WHAT THEY
WERE TODAY. THE CLOSED LOW WILL MOVE NORTHEAST TO NEVADA BY SUNDAY
AFTERNOON. AS THE LOW APPROACHES...BOUNDARY LAYER MOISTURE WILL
FILTER INTO SOUTHWEST WY ALONG WITH LOWERING LIFTED INDICES...
RISING CAPES AND MORE FAVORABLE TOTAL TOTALS. THIS WILL RESULT IN
SCATTERED THUNDERSTORMS IN SOUTHWEST AND WEST CENTRAL WYOMING BY
LATE SUNDAY AFTERNOON. AN AREA OF WEAK QG FORCING WILL MOVE INTO
SOUTHWEST WY FROM THE SOUTHWEST AS WELL.

.LONG TERM...SUNDAY NIGHT THROUGH SATURDAY
UPPER LOW MOVING INTO SOCAL IS EXPECTED TO LIFT NE AS IT GETS KICKED
OUT AHEAD OF ERN PAC TROUGH. MODELS ARE ALL SIMILAR IN LIFTING IT
NEWD INTO CENTRL WYO MONDAY AFTERNOON AND THEN OFF TO THE NE MONDAY
NIGHT. GOOD MOISTURE SURGE EXPECTED AHEAD OF IT SUNDAY NIGHT
STARTING IN THE SW AND LIFTING INTO THE CENTRAL OVERNIGHT INTO
MONDAY MORNING. ENOUGH INSTABILITY FOR SOME DECENT STORMS INTO THE
EVENING SUNDAY ACROSS THE SW WITH AT LEAST ISOLATED THUNDER ALL
NIGHT SPREADING NWD OVERNIGHT. GOOD COVERAGE AROUND THE CIRCULATION
MONDAY AND IN MOST OF THE MOUNTAINS WITH SOME DRIFT SEWD OFF THE
MOUNTAINS IN DEVELOPING NW FLOW ALOFT. RESIDUAL MOISTURE AND
POTENTIAL WEAK SHORTWAVE MAY WARRANT KEEPING THE ISOLD IN THE WEST
AND NRN MOUNTAINS TUESDAY.

SHARP RIDGE WILL DEVELOP AND MOVE OVERHEAD WED/THU AHEAD OF OUR
POTENTIAL SIGNIFICANT UPPER LOW FOR NEXT WEEKEND. LARGE
DISCREPANCIES IN THE OP AND ENSEMBLE MEMBERS. QUITE A FEW ARE SLOWER
BY A DAY OR TWO AND SOME DON`T HAVE A TROUGH AT ALL OR AT LEAST MUCH
FURTHER NORTH. WILL GIVE SOME WEIGHT TO THE SLOWER OP ECMWF AS THE
GFS HAS A SIGNIFICANT STORM UNDERWAY FRIDAY NIGHT AND SATURDAY
ALREADY WHEREAS THE ECMWF HOLDS OFF UNTIL BEYOND THE MEDIUM RANGE
PERIOD...IE SUNDAY. WITH THAT SAID...IT MAKES TEMPERATURES TRICKY AS
SLOWER SOLUTION WOULD RESULT IN MUCH WARMER CONDITIONS FOR AN EXTRA
DAY OR TWO. NUDGED TEMPS UP SOME FRIDAY AND SATURDAY BUT NOWHERE
NEAR AS MUCH AS THE ECMWF WOULD PRODUCE. WILL LOOK AT THE TRENDS AND
HEMISPHERIC PATTERN AGAIN TOMORROW.

&&

.AVIATION.../06Z ISSUANCE/

SHOWERS AND THUNDERSTORMS WILL DEVELOP OVER SOUTHWEST WY AFTER 19Z
SUNDAY. THIS ACTIVITY WILL SPREAD NORTHWARD THROUGH 06Z MONDAY.
WIND GUSTS TO 35 KNOTS WILL BE POSSIBLE WITH STORMS. LOCAL MVFR
CONDITIONS WILL OCCUR IN SOUTHWEST WY AFTER 00Z THROUGH 06Z IN AREAS
OF HEAVY RAIN AND LOWER CEILINGS.

&&

.FIRE WEATHER...

DRY CONDITIONS EXPECTED THROUGH SUNDAY MORNING AS THE HIGH
PRESSURE RIDGE AXIS MOVES INTO WYOMING. SUNDAY AFTERNOON WILL SEE THE
ARRIVAL OF MOISTURE INFLUENCED BY AN APPROACHING AREA OF LOW
PRESSURE FROM CALIFORNIA. SHOWERS AND THUNDERSTORMS WILL MOVE INTO
SOUTHWESTERN WYOMING SUNDAY EVENING AND SPREAD TO COVER THE VAST
MAJORITY OF THE FORECAST AREA BY MONDAY MORNING. SHOWER ACTIVITY
WILL WEAKEN MONDAY EVENING ACROSS SOUTHWESTERN WYOMING...THOUGH
SHOWERS WILL CONTINUE ACROSS THE NORTHERN HALF OF THE AREA THROUGH
MONDAY NIGHT.

&&

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

&&

$$

SHORT TERM...LIPSON
LONG TERM...SKRBAC
AVIATION...AR
FIRE WEATHER...LIPSON






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