Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 212006
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
206 PM MDT SUN SEP 21 2014

.SHORT TERM...TONIGHT THROUGH MONDAY

AN UPPER LEVEL LOW MOVING QUICKLY OUT OF THE PACIFIC WILL REACH
WESTERN WYOMING BY EARLY MONDAY MORNING BUT IT IS ALREADY BEGINNING
TO AFFECT THE REGION. THE PACIFIC ORIGINS OF THIS SYSTEM COMBINED
WITH THE MOIST SOUTHERLY UPSLOPE FROM THE PLAINS WILL PROVIDE A
ROBUST SUPPLY OF MOISTURE TO THE REGION WITH PRECIPITABLE WATER
VALUES EXPECTED TO EXCEED AN INCH TONIGHT ACROSS THE BOARD...PUTTING
US INTO THE 99TH PERCENTILE FOR THIS TIME OF YEAR...AT THE SAME
TIME...A THETA E RIDGE WILL ROTATE IN FROM THE SOUTHWEST ENCOURAGING
THUNDERSTORMS TO SPREAD FROM SOUTHERN WYOMING THIS AFTERNOON INTO
CENTRAL AND THEN NORTHEASTERN PORTIONS OF THE STATE OVERNIGHT.
EXPECT THE MAIN THREAT WITH THESE STORMS TO BE TORRENTIAL DOWNPOURS
POTENTIALLY SLOWING TRAVEL ALONG INTERSTATE 80 AND PERHAPS CAUSING
LOCAL FLOODING ISSUES. WHILE THE LONGER TERM PRECIPITATION AVERAGES
HAVE BEEN WELL ABOVE NORMAL ACROSS THE STATE...THE PAST TWO WEEKS
HAVE BEEN DRIER THAN NORMAL WITH SOIL MOISTURES BEGINNING TO DRY
TOWARD A MORE NORMAL RANGE...MEANING THAT THE SOIL IS ACTUALLY IN A
POSITION TO ACCEPT MORE MOISTURE FROM THIS SYSTEM...ESPECIALLY
ACROSS CENTRAL AND NORTHERN SWEETWATER COUNTY NORTH TO THE OWL
CREEKS INCLUDING FREMONT...NATRONA...SUBLETTE AND TETON COUNTIES
WHERE THE RECENT DEFICIT IS THE LARGEST.

ON MONDAY...THE UPPER LOW TURNS INTO MORE OF AN OPEN WAVE AS IT
REACHES OUR WEST. IT WILL STILL PROVIDE ENOUGH BROAD SCALE
LIFT...AND WITH THE RESIDUAL MOISTURE LEFTOVER FROM TONIGHT...PLENTY
OF MOISTURE TO WORK WITH TOO TO PRODUCE SCATTERED SHOWERS AND
ISOLATED THUNDERSTORMS ACROSS THE FORECAST AREA. THE THETA E RIDGE
SHIFTS TO THE EAST BUT THERE WILL BE MORE CAPE TO WORK WITH ACROSS
THE STATE...AND SOME WEAK TO MODEST SHEAR REMAINING ACROSS THE
SOUTH...SO EXPECT MORE ORGANIZED STORMS TO FORM THERE.
OTHERWISE...IT WILL BE CLOUDY FOR MOST OF THE DAY WITH OCCASIONAL
SHOWERS AND THUNDERSTORMS CAPABLE OF PRODUCING SOME MORE HEAVY RAIN
WITH MINOR FLOOD POTENTIAL AND OCCASIONAL LIGHTNING BUT THAT IS
ABOUT IT AS FAR AS HAZARDS GO.

.LONG TERM...MONDAY NIGHT THROUGH SUNDAY

UPPER LOW CURRENTLY MOVG INTO NRN NV IS EXPECTED TO BE OVER NCNTRL
WYOMING TO START THIS PERIOD WITH A GRADUAL SE DRIFT MONDAY NIGHT.
STILL A GOOD CHANCE OF SHOWERS AND SOME THUNDERSTORMS ACROSS THE
MOUNTAINS OF THE NORTH HALF WITH LESSER CHANCES IN THE SOUTH AND
LOWER ELEVATIONS. GRADUAL IMPROVEMENT LATER MONDAY NIGHT AS THE LOW
MOVES OFF INTO THE BLACK HILLS REGION BY TUESDAY MORNING. RAPIDLY
BUILDING RIDGE BEHIND THIS SYSTEM WILL LEAD TO A QUICK WARM UP WITH
TEMPERATURES BECOMING QUITE WARM WEDNESDAY INTO THURSDAY AND
CONTINUING INTO FRIDAY EAST OF THE DIVIDE. BIG CHANGES STILL IN THE
OFFING AT SOME POINT NEXT WEEKEND WITH LARGE UPPER TROUGH CARVING OUT
OVER THE WEST. EVOLUTION BEGINS WITH ENERGY MOVING EAST AROUND 40N
BETWEEN 150-160W WHICH HELPS REDEVELOP OUR ERN PAC/GULF OF ALASKA
TROUGH FURTHER SOUTH. AT THE SAME TIME...A SYSTEM OVER THE WESTERN
PACIFIC AND ANOTHER NEAR KAMCHATKA AID IN DEVELOPING A DECENT
UPSTREAM RIDGE. ALL THESE FEATURES TRANSLATE EWD WITH TIME AND CARVE
OUT A RATHER ANOMALOUS LATE SEP TROUGH OVER THE WEST BEGINNING
FRIDAY AND LASTING POTENTIALLY ALL THE WAY INTO NEXT MONDAY. OP
MODELS AND ENSEMBLE MODELS VARY CONSIDERABLY IN TIMING AND AMOUNT OF
SEPARATION WITH THIS TROUGH. MORE MEMBERS TODAY SHOW MORE DIGGING
AND EVENTUAL CLOSING OFF OF THIS LOW THAN YESTERDAY. IT`S VERY EARLY
IN THE SEASON FOR THIS TYPE OF LOW TO FORM BUT THEN AGAIN SO WAS
THIS EXTREMELY ANOMALOUS PATTERN THAT BROUGHT SNOW TO MUCH OF OUR
AREA ON SEPTEMBER 11TH. 12Z GFS CONTINUES ITS IDEA OF A CLOSED UPPER
LOW FORMING OVER UTAH FRIDAY NIGHT AND SLOWLY CHUGGING EAST INTO NE
COLORADO THROUGH SUNDAY NIGHT. FASTER THAN LAST NIGHT AND A LITTLE
FURTHER SOUTH. THE ECMWF IS SLOWER AND MUCH FURTHER WEST WITH ITS
DEVELOPING CUTOFF...MORE OVER NEVADA...OVER THE WEEKEND BEFORE
HAVING IT KICKED NE ACROSS US EARLY NEXT WEEK AHEAD OF A COLDER
SYSTEM FROM THE NW. THE ECMWF SHOWED THIS IDEA SEVERAL RUNS
BACK...THE COLDER KICKER THAT IS...AND NOW ITS TRENDING BACK TOWARDS
THIS. THIS KICKER SYSTEM IS CURRENTLY THE COLD SHORTWAVE NEAR
KAMCHATKA. HARD TO RESOLVE THESE POSSIBILITIES YET BUT AM LEANING
TOWARDS A COMPROMISE IN TIMING BETWEEN THE ECMWF AND GFS BUT WITH
NOT QUITE THE CLOSED LOW ENERGY OF THE GFS. WE`LL HAVE TO WATCH THIS
CLOSELY BECAUSE SOME OF THE MODEL SOLUTIONS ARE QUITE BULLISH ON
PRECIPITATION POTENTIAL. NOT A LOT OF COLD AIR WITH THIS SYSTEM SO
NO SNOW IN THE LOWER ELEVATIONS BUT THE HIGHER MOUNTAINS WOULD
OBVIOUSLY SEE SNOW. SO WITH ALL THAT SAID...WILL KEEP IT WARM TO
VERY WARM IN MOST AREAS INTO FRIDAY BEFORE THE COOL DOWN BEGINS AND
RAIN CHANCES WITH INITIAL THUNDERSTORMS CHANCES INCREASE AS EARLY AS
FRIDAY AFTERNOON OR EVENING IN THE FAR WEST AND EAST OF THE DIVIDE
SATURDAY INTO SUNDAY.

&&

.AVIATION.../00Z ISSUANCE/

SHRA AND -TSRA WILL CONTINUE TO DEVELOP ACROSS SOUTHERN WY AND
SPREAD NORTHWARD INTO NORTHWEST AND CENTRAL WY BETWEEN 03Z AND 09Z
MONDAY...OCCNL SHRA AND TSRA WILL BE POSSIBLE THROUGH THE TAF
PERIOD WITH A BIT OF A LULL ACROSS THE SOUTH BETWEEN 09Z AND 18Z
MONDAY...WITH REDEVLOPMENT EVERYWHERE AFTER 18Z...CONTINUING
THROUGH THE END OF THE TAF PERIOD. WIND GUSTS TO 35 KNOTS WILL BE
POSSIBLE WITH STORMS ALONG WITH LOCAL MVFR CONDITIONS IN AREAS OF
HEAVY RAIN AND LOWER CEILINGS. MOUNTAINS WILL BE OBSCURED AT
TIMES. ACROSS THE NORTHEAST...VFR CONDITIONS WILL PREVAIL OVER THE
REGION FOR MOST OF THE FORECAST PERIOD WITH SOME -TSRA AND -SHRA
DEVELOPING AFTER 18Z MON.

&&

.FIRE WEATHER...

SHOWERS AND WET THUNDERSTORMS WITH A GOOD CHANCE OF WETTING RAIN
WILL SPREAD SOUTH TO NORTH INTO MONDAY. SHOWERS AND STORMS WILL
REDEVELOP TOMORROW AFTERNOON ACROSS THE STATE WITH RIDGING
BUILDING FROM THE WEST OVERNIGHT. THE RIDGE WILL BRING DRIER AND
WARMER WEATHER FOR THE MIDDLE OF NEXT WEEK. ALL IN ALL...FIRE
WEATHER CONCERNS SHOULD REMAIN FAIRLY MINIMAL.

&&

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

$$

SHORT TERM...ALLEN
LONG TERM...SKRBAC
AVIATION...ALLEN
FIRE WEATHER...ALLEN





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