Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 201649
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
1049 AM MDT WED AUG 20 2014

.SHORT TERM...TODAY THROUGH FRIDAY NIGHT

THE MID LEVEL CIRCULATION ASCT WITH THE SHORTWAVE TROUGH...WHICH
IS TRACKING EAST ACROSS WESTERN WY...APPEARS TO BE OVER THE RED
DESERT ALONG THE BACK EDGE OF THE WIDESPREAD AREA OF PATCHY LIGHT
RAIN EXTENDING FROM CODY TO RIW TO RKS THAT USED TO BE CONVECTIVE
ACTIVITY OVER IDAHO BEFORE THE PRECIP MOVED INTO MORE STABLE AIR
OVER WY. THE DRY SLOT WILL MOVE NORTHEAST INTO CENTRAL WY AND INTO
JOHNSON COUNTY ALONG WITH THE MID LEVEL CIRCULATION BY 12Z THIS
MORNING. ADDITIONAL RAIN SHOWERS WILL LINGER IN THE MOUNTAINS
LATER INTO THE MORNING AS THE BACK EDGE OF THE SHORTWAVE TROUGH
CLEARS THROUGH. THEN BEHIND THIS AREA OF EXITING
CLOUDINESS...CONVECTION WILL FIRE RAPIDLY ACROSS WESTERN WY WITHIN
THE DIFLUENT WEST SOUTHWESTERLY FLOW AHEAD OF THE NEXT APPROACHING
VORT LOBE ROTATING AROUND THE BASE OF THE MAIN TROUGH OVER
SOUTHWEST CANADA WHERE THE BEST CAPES WILL BE. NUMEROUS TO SCT
SHOWERS AND THUNDERSTORMS ARE EXPECTED IN WESTERN WY WITH MAINLY
SCT EAST. TOTAL TOTALS WILL BE FAVORABLE FOR WIDESPREAD
CONVECTION. THEN ON THURSDAY ANOTHER VORT LOBE WILL ROTATE AROUND
THIS TROUGH FOR MORE CONVECTION MAINLY IN THE NORTH AND WEST. THEN
ON FRIDAY MORNING...THE MAIN TROUGH WILL CLOSE OFF OVER WASHINGTON
AND TRACK TO IDAHO BY FRIDAY NIGHT. THE GFS HAS COME MORE INTO
LINE WITH THE FASTER ECMWF WITH THIS SOLN. AS THE
DYNAMICS...INCLUDING LEFT FRONT QUAD JET DYNAMICS...AND INCREASING
SOUTHWEST DIFLUENT FLOW INCREASE WITH THIS APPROACHING LOW...WITH
ITS ASCT WEAK H7 CIRCULATION APPROACHING NW WY...SHOWERS AND
THUNDERSTORMS WILL BECOME MORE WIDESPREAD FRIDAY AND FRIDAY NIGHT
ALONG WITH COOLER TEMPS AS A RESULT OF THE ADDED CLOUDS AND
PRECIP. A SFC LOW WILL TRACK ACROSS CASPER ASCT WITH THE SOUTHERN
STREAM H5 LOW THAT THE GFS HAS OVER CO ADDING TO THE DYNAMICS AND
PULLING SOME COOLER AIR BEHIND IT FRIDAY EVE. THE ECMWF HAS THE H5
LOW OVER AZ DURING THIS TIME. COOLER AIR WILL FILTER INTO THE
NORTHWEST PORTION OF WY FRI NT AS THE MAIN PAC NW WEATHER SYSTEM
NEARS WITH H7 TEMPS EXPECTING TO DROP TO 2.2C BY 12Z SAT OVER NW
WY WHERE SNOW LEVELS COULD DROP TO 9500 FEET.

.LONG TERM...SATURDAY THROUGH WEDNESDAY

SIGNIFICANT COOL LATE AUGUST TROUGH IS EXPECTED TO SWING ACROSS
THE AREA ON SATURDAY WITH A GOOD CHANCE OF SHOWERS AND SOME
THUNDERSTORMS ALONG WITH SKIFF OF SNOW TO THE HIGHER MOUNTAINS.
SNOW LEVELS WHICH SHOULD BE ABOVE 10-11K FEET DURING THE DAY
SATURDAY WILL LOWER SATURDAY EVENING TO AS LOW AS 8.5-9K FEET
IN THE FAR NRN MOUNTAINS. SUNDAY WILL BE DRIER AND MORE STABLE
EXCEPT IN THE NW WHERE NEXT APPROACHING TROUGH IS ALREADY
SPREADING SOME MOISTURE AND FORCING INTO THE AREA. NEXT COOL
TROUGH WILL BRING IN ANOTHER ROUND OF SHOWERS AND SOME
THUNDERSTORMS MONDAY INTO TUESDAY FOR MANY AREAS...ESPECIALLY
THE MOUNTAINS...NORTH HALF AND THE SERN CORNER. ANOTHER COUPLE
UNSEASONABLY COOL DAYS WITH THIS TROUGH AS H7 TEMPERATURES DROP
WELL DOWN INTO THE SINGLE DIGITS. ANOTHER SKIFF OR MORE OF
MOUNTAIN SNOW IS VERY POSSIBLE AGAIN ABOVE 10-11K MONDAY THROUGH
TUESDAY AT TIMES. ECMWF HAS MORE RIDGING BY WEDNESDAY WHILE THE
GFS IS STILL SHOWING A MORE MOIST NW FLOW. WILL KEEP IT FAIRLY DRY
FOR NOW WITH ONLY A HINT OF HIGHER MOUNTAIN SHOWERS IN THE NRN
MOUNTAINS.

.AVIATION.../18Z ISSUANCE/

AN UPPER LEVEL LOW PRESSURE TROUGH OVER THE NORTHERN ROCKIES
EXTENDING SOUTH INTO THE NORTHERN GREAT BASIN WILL PARTIALLY MOVE EAST
ACROSS MONTANA AND WYOMING WITH REFORMATION OCCURRING TO THE WEST
THROUGH 18Z THURSDAY. SUFFICIENT MOISTURE...SHEAR AND INSTABILITY
WILL ACCOMPANY THE TROUGH AS IT MOVES EAST FOR SCATTERED SHOWERS WITH
ISOLATED THUNDERSTORMS WITH STRONG WIND GUSTS. LOCAL IFR
CONDITIONS MAY PERSIST IN THE FAR WESTERN VALLEYS WHERE STRATUS
REMAINS TRAPPED IN JACKSON HOLE...YELLOWSTONE NATIONAL PARK AND THE
STAR VALLEY. THE SHOWERS AND THUNDERSTORMS WILL BE MOST NUMEROUS
THROUGH 06Z THURSDAY WITH LOCAL MOUNTAIN OBSCURATIONS EXPECTED.

SOME SHOWERS AND POSSIBLE THUNDERSTORMS WILL LINGER THROUGH THE
NIGHT AND THURSDAY MORNING...ESPECIALLY OVER THE WESTERN
MOUNTAINS.


.FIRE WEATHER...

WAVES OF ENERGY COMING OFF THE PACIFIC BISECTING MONSOON ENERGY
WILL KEEP A GOOD CHANCE OF AFTERNOON AND EVENING THUNDERSTORMS
GOING ALL WEEK. MIN RH WILL GENERALLY RUN IN THE 20S AT THE LOWER
ELEVATIONS
..EXCEPT 30S IN THE WESTERN VALLEYS...THROUGH THURSDAY BEFORE INCREASING.
A COOLER PACIFIC WEATHER SYSTEM WILL BRING MORE WIDESPREAD
SHOWERS AND THUNDERSTORMS FRIDAY AND SATURDAY WITH COOLER AIR
FILTERING IN. THE SNOW LEVEL SHOULD LOWER TO 9500 FEET OR SO BY
SATURDAY MORNING IN ZONE 140.

&&

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

$$

SHORT TERM...LIPSON
LONG TERM...SKRBAC
AVIATION...C.BAKER
FIRE WEATHER...LIPSON






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