Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 231915
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
115 PM MDT SAT MAY 23 2015

.SHORT TERM...TODAY THROUGH MONDAY NIGHT

ISSUING FLASH FLOOD WATCH FOR JOHNSON...NATRONA AND PORTIONS OF
FREMONT COUNTIES FROM 9 PM MDT THIS EVENING THROUGH 6 PM MDT
SUNDAY. SATURATED TO NEAR SATURATED SOILS OVER THE AREA WITH LOCAL
HEAVY RAIN THROUGH THE PERIOD WILL COMBINE TO SIGNIFICANTLY INCREASE
THE CHANCE OF FLASH FLOODING OVER AND NEAR THE FOOTHILLS OF THE
WIND RIVER...BIGHORN AND NORTH LARAMIE MOUNTAINS INCLUDING CASPER...
BUFFALO AND LANDER.

WV IMAGERY SHOWING LARGE UPR TROF ACROSS THE WRN CONUS WITH CLOSED
LOW OVR WRN AZ...EMBEDDED SW WITHIN CLOSED PORTION ROTATING
ANTI-CLOCKWISE THRU CNTRL WY AS WE SPEAK. SFC HAS GENERAL LOW P ACROSS
THE WEST...HIGH P TO THE EAST AND WEAK TROF/BOUNDARY STRETCHING FROM
CNTRL NE INTO NERN CO.

THE FCST FOR THE MEMORIAL DAY WEEKEND...UPR LOW MIGRATES FROM WEST
OF THE 4 CORNERS REGION SLOWLY TO THE N...CENTERING ACROSS WRN CO
BUT SPREADING OVR TOP OF WY BY DAY`S END TODAY. MID TO UPR LVL
VORTICITY WILL INCREASE WITH THE APPROACH OF THE UPR LOW. THE DAY
WILL BE MARKED BY SUFFICIENT BUOYANCY BUT LOW SHEAR THRU MUCH DEPTH.
DISORGANIZED SHOWERS WITH SOME EMBEDDED THUNDER BY AFTERNOON WILL BE
RELATIVELY SLOW MOVERS AGAIN...FROM S TO N AND 10 MPH +/- E OF THE
DIVIDE...SE TO NW/E TO W WEST OF THE DIVIDE. ML CAPE SHOULD TOP OUT
ACROSS THE CWA AT <=500 J/KG...BUT AVG 250 J/KG FOR MOST LOCATIONS.
PWS LOOK TO BE A BIT LESS THAN TODAY...BUT STILL AOA 0.60 E OF THE
DIVIDE...AOB 0.5 W OF THE DIVIDE...WITH HIGHEST VALUES SETTLING IN
OVR THE BIGHORN BASIN AND THE EXTREME ERN PORTIONS OF JOHNSON AND
NATRONA COUNTIES. EXPECTING SOME MID LVL DRYING THRU THE MORNING HRS
WITH BITS OF SUNSHINE POSSIBLE ACROSS THE SRN...CNTRL AND ERN FA.
THIS WILL INCREASE LL LAPSE RATES AND INSTABILITY OVR THESE
AREA...EMBEDDING A FEW THUNDERSTORMS AMIDST THE AMORPHOUS MASS OF
SHOWERS EXPECTED BY MID TO LATE AFTERNOON. ADDITIONALLY...WITH A
MODEST SFC LOW MOVING INTO WRN CO AND SRN WY...ERLY RETURN FLOW AND
SMALL WARM FRONT SETS UP ACROSS ERN AND CNTRL WY...ALLOWING FOR
TERRAIN INDUCED CONVERGENCE TO SET UP FROM THE ERN WIND RIVER
MOUNTAINS...EWD ALONG THE GREEN MOUNTAINS AND SOUTH ACROSS ERN
SWEETWATER COUNTY. THIS AREA WOULD LIKELY SEE A COUPLE OF THE
STRONGER STORMS OF THE DAY.

TONIGHT THROUGH SUNDAY...EARLY MORNING FOG WILL ALSO BE POSSIBLE OVR
MANY AREAS THROUGHOUT THE FCST. OTHERWISE...SFC CYCLOGENESIS TAKES
PLACE FROM ERN/NERN CO INTO SERN WY AND WRN NE. BETWEEN LOW TO MID
LVL MOISTURE RETURN AROUND THE SFC LOW...AND MID UPR MOISTURE FROM
ACCOMPANYING THE UPR WAVE/CLOSED LOW...RELATIVELY PLENTIFUL RAINFALL
WILL BE HAD OVER THE FAR ERN FA THRU THE DAY SUNDAY. THIS SYSTEM
CONTINUES TO LOOK COMPARATIVELY WARM...WITH H7 TEMPS REMAINING ABOVE
-1 DEG C THRU THE ENTIRE FCST PERIOD...AND REALLY ABOVE ZERO FOR
MOST OF THAT TIME.  GENERALLY...A TRACE TO AND INCH OR TWO WILL BE
POSSIBLE DURING THE COLDEST PORTION OF THE NIGHTS ABOVE 9500
FEET...WITH ONLY THE HIGHEST ELEVATIONS RECEIVING AN ISOLATED 3 OR
SO INCHES. STORM MOVEMENT IMPROVES SLIGHTLY...NOW REVERSING AND
BECOMING N TO S AT 15 TO 20 MPH. SEVERE WX WILL REMAIN MOSTLY EAST
TO SOUTHEAST OF FA THROUGH THE PERIOD WITH CONTINUED LOW SHEAR
CONDITIONS ONLY ABLE TO PROVIDE OCCASIONAL ISOLATED SMALL
HAIL...ESPECIALLY W OF THE DIVIDE.

SUNDAY NIGHT THRU MONDAY...SYSTEM PUSHES EWD INTO THE PLAINS WITH
TRANSITIONAL RIDGING TAKING PLACE ACROSS THE WRN FA WHILE THE ERN FA
IS FCST TO BE THE RECIPIENT OF A BACKSIDE UPR SW ROTATING AROUND THE
EXITING LOW...WITH BETTER SHOWER/THUNDER ACTIVITY/COVERAGE ACROSS
THE ERN CWA. OTHERWISE...A BIT WARMER AND MORE UNSTABLE WITH PERHAPS
AN BETTER SPRINKLING OF STRONGER THUNDERSTORMS MIXED IN WITH
SHOWERS. LOW SHEAR CONDITIONS ALSO PERSIST HOWEVER...KEEPING STORM
ORGANIZATION AT A MINIMUM...WHILE STORM MOTION IMPROVES EVEN
MORE...FROM THE NW TO SE AT 20 TO 25 MPH.

.LONG TERM...TUESDAY THROUGH SATURDAY

GFS MODEL CONTINUES ITS CONSISTENT FORECAST FOR TUESDAY THROUGH
FRIDAY. IT HAS A WEATHER SYSTEM SPINNING OVER THE PACIFIC NW STATES
WHICH CREATES SW FLOW ALOFT OVER WY TUE AND WED. THE SYSTEM MOVES
INTO IDAHO THURSDAY AND INTO NW WY FRIDAY...THEN ACROSS NORTHERN WY
FRIDAY NIGHT AND INTO WESTERN SOUTH DAKOTA NEXT SATURDAY AFTERNOON.
A DAILY CHANCE FOR SHOWERS AND STORMS IN THE AFTERNOON AND EVENING
HOURS WITH LINGERING SHOWERS LATE NIGHT AND MORNING HOURS TUESDAY
THROUGH FRIDAY. NEXT SATURDAY THE SHOWERS AND STORMS LOOK TO BE
MAINLY OVER THE NORTH WITH LESSER COVERAGE CENTRAL AND SOUTH. THE
ECMWF MODEL IS SIMILAR TO THE GFS MODEL WITH THE WEATHER PATTERN
THROUGH THURSDAY NIGHT. IT HAS A DIFFERENT TRACK ON FRIDAY BUT THE
TROUGH IS STILL MOVING THROUGH FRIDAY INTO FRI NIGHT. THEN THE ECMWF
MODEL IS ONLY SHOWING ISOLATED SHOWERS OR STORMS. THE FORECAST WILL
REFLECT A CHANCE OF SHOWERS AND STORMS EACH AFTERNOON AND EVENING
WITH LESSER COVERAGE OF SHOWERS LATE NIGHT AND DURING THE MORNING.
NEXT SATURDAY WILL BE MOSTLY DRY IN THE SOUTHWEST WITH ISOLATED
COVERAGE FOR THE REST OF THE AREA. WILL HOWEVER KEEP A LITTLE CHANCE
OF PRECIP IN THE NORTHEAST ZONES NEXT SATURDAY. AS FOR HIGH TEMPS...
THERE WILL BE LITTLE CHANGE FROM DAY TO DAY WITH 60S TO LOWER 70S
FOR THE LOWER ELEVATIONS AND 40S AND 50S FOR THE MOUNTAINS. THESE
HIGHS ASSUME SOME SUNSHINE EACH DAY AND NOT A LOW OVERCAST DAY(S)
LIKE WE HAVE SEEN DURING THE PAST WEEK.

.AVIATION.../18Z ISSUANCE/

AREAS OF MVFR TO IFR CEILINGS AND VISIBILITY REDUCED IN RAIN AND
FOG WILL INCREASE ACROSS THE AREA THIS AFTERNOON AND EVENING WITH
MOUNTAIN OBSCURATIONS BECOMING MORE WIDESPREAD.  THE HEAVIEST
AND MOST WIDESPREAD SHOWERS...WITH ISOLATED EMBEDDED THUNDERSTORMS
WITH SMALL HAIL...WILL BE ALONG AND EAST OF THE CONTINENTAL DIVIDE.
ALL OF THE AIRPORTS WILL HAVE A THREAT OF MVFR TO IFR CONDITIONS...
ESPECAILLY FROM 06Z TO 15Z SUNDAY.

.FIRE WEATHER...

FIRE DANGER LOW AND WILL REMAIN LOW THROUGH THE HOLIDAY WEEKEND
AS COOL AND RELATIVELY WET WEATHER REMAINS IN PLACE AS YET ANOTHER
VIGOROUS UPPER LEVEL SYSTEM ROTATES ACROSS THE REGION. WIDESPREAD
PRECIPITATION WITH LIGHT MOUNTAIN SNOW IS EXPECTED EACH DAY...WITH
EMBEDDED THUNDERSTORMS POSSIBLE EACH AFTERNOON AND EARLY EVENING.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
FLASH FLOOD WATCH FROM 9 PM MDT THIS EVENING THROUGH SUNDAY
AFTERNOON WYZ009>011-015-018>020-022.

&&

$$

SHORT TERM...BRAUN
LONG TERM...AR
AVIATION...C.BAKER
FIRE WEATHER...BRAUN







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