Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 290831 CCA
AFDRIW

AREA FORECAST DISCUSSION...CORRECTED
NATIONAL WEATHER SERVICE RIVERTON WY
231 AM MDT SAT AUG 29 2015

.SHORT TERM...TODAY THROUGH MONDAY NIGHT

A WELL DEFINED CIRCULATION OFF THE SOUTHERN OREGON COAST WILL MOVE
NORTH TODAY OFF THE PACIFIC COAST. THIS CIRCULATION WILL GET CAUGHT
UP WITHIN THE ASCT MAIN TROUGH AS IT MAKES LANDFALL TODAY. AS THIS
TROUGH MOVES EAST...OUR GRADIENT WILL INCREASE FROM THE SOUTHWEST
OUT AHEAD OF THIS APPROACHING TROUGH WITH THE FLOW BECOMING
INCREASINGLY DIFLUENT. THIS WILL RESULT IN ISOLATED TO SCT
THUNDERSTORM ACTIVITY IN THE FAR WEST AND NORTHWEST THIS AFTERNOON
AND EVENING WHERE THE BEST CAPES WILL BE. MEANWHILE A DRY SOUTHWEST
FLOW WILL BE IN PLACE ACROSS SOUTHERN WY AND AREAS EAST OF THE
DIVIDE. TODAY WILL BE THE WARMEST DAY OF THE FORECAST PERIOD IN MOST
AREAS. THEN ON SUNDAY THE TROUGH WILL CLOSE IN ON WY WITH SOUTHWEST
WINDS INCREASING ALONG WITH AN INCREASING CHANCE OF THUNDERSTORMS
ALONG WITH INCREASING CAPES AND LOWERING LIFTED INDICES. THE BEST
CHANCE OF ANY STORMS WILL LIKELY OCCUR OVER THE BIG HORN MTNS SUNDAY
WHERE THE BEST CAPES WILL BE BEFORE THEY DRIFT EAST INTO THE DRIER
AIR OVER THE LOWER ELEVATIONS OF JOHNSON COUNTY WHERE HIGHER BASED
STORMS AND LESS RAINFALL WILL PROBABLY OCCUR AS WILL BE THE CASE
OVER THE REST OF THE LOWER ELEVATIONS EAST OF THE DIVIDE SUNDAY. QG
FORCING MORE DIRECTLY ASSOCIATED WITH THE TROUGH WILL REMAIN WELL TO
THE WEST AND NORTH OVER IDAHO AND MONTANA AS THE TROUGH IS FORCED UP
AND OVER THE RIDGE WHICH WILL THEN BE OVER THE EASTERN US. ON MONDAY
ONLY EXPECT SOME SPOTTY CONVECTION OVER FAR SOUTHERN WY WHERE
LINGERING INSTABILITY WILL BE IN PLACE. OTHERWISE EXPECT DRY WX AND
LIGHT WINDS MONDAY.

.LONG TERM...TUESDAY THROUGH SATURDAY

OVERALL PATTERN IS DEPICTED BY A DEEPENING PACNW TROUGH WHICH SOME
SLIGHT PROGRESSION TOWARDS THE END OF THE PERIOD. DECENT AMPLITUDE
TO THE PATTERN AS AT LEAST TWO PREVIOUS TROPICAL SYSTEMS ARE
CURRENTLY GETTING INTO THE FLOW ACROSS THE WRN PACIFIC. GFS SHOWS A
DEEPER TROUGH AT THE END OF THIS PERIOD THAN THE ECMWF. THE GEM IS
FURTHER NORTH YET. OVERALL FORECAST WILL BE WARM AND GENERALLY
DRY...WARM AND BREEZY WITH ISOLATED MAINLY LATE DAY MTN TSTMS. COLD
FRONT WILL LIKELY COME INTO PLAY LATER FRIDAY INTO SATURDAY WITH OUR
SLOWLY PROGRESSING PACNW TROUGH.

&&

.AVIATION.../12Z ISSUANCE/

VFR CONDITIONS AND MAINLY LIGHT WIND...LESS THAN 12KTS...WILL
PREVAIL THROUGH MID TO LATE MORNING.   A SOUTHERLY LOW LEVEL FLOW
WILL RESULT IN AN OVERALL CONTINUED DECREASE IN AREAS OF SMOKE
THOUGH LCL MVFR CONDITIONS IN FU MAY STILL BE ENCOUNTERED VCNTY AND
EAST OF THE DIVIDE THROUGH THIS MORNING.

SOUTH TO SOUTHWEST SFC WINDS WILL INCREASE TO 10-20KTS ACROSS MOST
AREAS THIS AFTERNOON.  ISOLD-WDLY SCT -TSRA ARE EXPECTED TO PUSH
INTO FAR WEST WYOMING...VCNTY KJAC-KAFO...BY AROUND 21Z AND SLOWLY
TO A VCNTY KEVW-KPNA-KCOD LINE 3Z-6Z SUNDAY.  A FEW THUNDERSTORMS
WILL LIKELY LINGER INTO SUNDAY MORNING VCNTY AND WEST OF THE DIVIDE
WITH AREAS OF GUSTY S-SW WINDS CONTINUING AHEAD OF AN APPROACHING
COLD FRONT IN MOST AREAS.

&&

.FIRE WEATHER...

...RED FLAG WARNING TODAY AND SUNDAY FOR PORTIONS OF SOUTHERN AND
CENTRAL WYOMING...

A CRITICAL FIRE WEATHER PATTERN IS SETTING UP FOR THE WEEKEND AS
SOUTHWEST FLOW AHEAD OF A TROUGH DIGGING OVER THE PACNW WILL DRAW IN
DRY...SWIFT MOVING AND UNSTABLE AIR FROM THE FOUR CORNERS REGION.
SOUTHWEST WINDS SHOULD KICK IN FAR EARLIER IN THE MORE EXPOSED AREAS
LIKE RKS TO CPR BEGINNING AROUND 1000 LATE THIS MORNING BEFORE THE
RH HAS A CHANCE TO LOWER AND THEN BY THE TIME THE SOUTHWEST WINDS
KICK IN IN THE MORE PROTECTED BASIN AREAS AROUND 1400...THE RH WOULD
HAVE LOWERED TO MORE CRITICAL LEVELS. RELATIVE HUMIDITY WILL DIP
WELL IN THE THE TEENS WITH SOME SINGLE DIGIT RHS EXPECTED IN CENTRAL
AND EASTERN ZONES. IN ADDITION...IT WILL BE QUITE WARM DURING THE
DAY WITH HIGHS IN THE 90S...WHICH WILL INCREASE LAPSE RATES AT THE
SURFACE WITH A HAINES INDEX OF 6 EXPECTED AS WELL. SO EVEN THOUGH
THE WINDS AND MINIMUM RELATIVE HUMIDITIES MAY NOT SYNC
PERFECTLY...THERE WILL BE SEVERAL ELEMENTS OVERLAPPING TO INCREASE
THE FIRE DANGER TO CRITICAL LEVELS.

ON SUNDAY...OVERALL IT WILL BE WINDIER BUT WITH SLIGHTLY HIGHER
RELATIVE HUMIDITY. THE COLD FRONT ASSOCIATED WITH THE TROUGH DIGGING
TO THE WEST ADVANCES...IT WILL LIKELY SETTLE INTO WESTERN...NORTHERN
AND SOUTHERN WYOMING FAIRLY EARLY IN THE DAY...WHICH WOULD END THE
FIRE WEATHER THREAT IN THOSE LOCATIONS ALONG WITH INCREASING CLOUDS
AS WELL AS A SLIGHT CHANCE FOR A WETTING RAIN. HOWEVER...LATEST
GUIDANCE SUGGESTS THAT THE FRONT COULD RETREAT BACK INTO
MONTANA...CLEARING THE WAY FOR MORE STRONG SOUTHWEST FLOW AND LOW
HUMIDITY ACROSS FREMONT AND NATRONA COUNTIES. SO...THESE ZONES WILL
CONTINUE THE RED FLAG FOR SUNDAY. IN ADDITION...A SWATH OF MID LEVEL
MOISTURE AND CONTINUED INSTABILITY WILL KICK OF A FEW MIXED WET AND
DRY THUNDERSTORMS...SOME OF WHICH COULD CONTRIBUTE TO NEW FIRE
STARTS WITH THE MOST ACTIVITY EXPECTED ACROSS NORTHERN AND WESTERN
WYOMING...WITH A SECONDARY SWATH POSSIBLE OVER NATRONA AND EASTERN
FREMONT COUNTIES LATE SUNDAY AFTERNOON...THE CHANCE OF A WETTING
RAIN OUT OF THESE STORMS IS NEXT TO ZERO. THE BEST CHANCE OF ANY DRY
LIGHTNING STARTS WILL LIKELY BE ON THE PERIPHERY OF ANY
THUNDERSTORMS OVER NATRONA AND JOHNSON COUNTY WHERE THE COMBINATION
OF DRIEST BOUNDARY LEVEL MOISTURE...WIDEST TEMP/DEW POINT SPREADS
AND INSTABILITY WILL LIE ON SUNDAY AFTERNOON.

A REPEAT OF THESE CONDITIONS WILL BE POSSIBLE ON TUESDAY AND
WEDNESDAY OF NEXT WEEK.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
RED FLAG WARNING FROM 11 AM THIS MORNING TO 9 PM MDT THIS
EVENING FOR WYZ275>277-279-281-282-285-287.

RED FLAG WARNING FROM 11 AM THIS MORNING TO 9 PM MDT SUNDAY FOR
WYZ280-283-289-300.

&&

$$

SHORT TERM...LIPSON
LONG TERM...PS
AVIATION...AEM
FIRE WEATHER...LIPSON



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