Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 301747 AAA
AFDRIW

AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE RIVERTON WY
1147 AM MDT SUN AUG 30 2015

.PREV DISCUSSION... /ISSUED 250 AM MDT SUN AUG 30 2015/

.SHORT TERM...TODAY THROUGH TUESDAY NIGHT

A STRONG TROUGH OF LOW PRESSURE JUST OFF THE PACIFIC NORTHWEST COAST
HAS EJECTED A SHEARED UPPER LEVEL SHORTWAVE EASTWARD THAT APPEARS TO
BE LOCATED OVER NORTHERN IDAHO AND WESTERN MT. ACTIVE CONVECTION
CONTINUES OUT AHEAD OF THIS FEATURE ACROSS EASTERN IDAHO NEAR THE
WYOMING BORDER AND INTO YNP AT THE TIME OF THIS WRITING. IT WILL BE
AN EARLY SHOW FOR CONVECTION IN WESTERN WY TODAY WITH THESE STORMS
ABOUT TO SWEEP ACROSS FAR WEST WYOMING DURING THE REMAINDER OF THE
NIGHT. THEN THE MAIN TROUGH WILL APPROACH THE NORTHERN ROCKIES TODAY
FOR ANOTHER ROUND OF CONVECTION FIRING UP IN FAR WESTERN WY BY MID
DAY AHEAD OF THE MAIN TROUGH AND ASCT COLD FRONT. IN THE
MEANTIME...EAST OF THE DIVIDE...SOUTHWEST WINDS WILL INCREASE OVER
WHAT THEY WERE SATURDAY AS THEY DRAW IN SLIGHTLY HIGHER LOWER LEVEL
MOISTURE. INSTABILITY WILL BE IN PLACE OVER THE ENTIRE CWA BUT ONLY
SCANT MOISTURE TO WORK WITH TO PRODUCE THUNDERSTORMS EAST OF THE
DIVIDE...AT LEAST INITIALLY IN THE EARLY TO MID AFTERNOON. THERE
WILL BE A SEPARATE FINGER OF WEAK HIGH AND MID LEVEL MONSOON
MOISTURE THAT WILL STREAM IN ACROSS RKS AND CPR. HOWEVER THE BEST
INSTABILITY WILL BE IN PLACE ACROSS NORTHERN WY WHERE THE BEST CAPES
WILL BE. IT APPEARS THE BEST CHANCE OF ANY DRY LIGHTNING WILL OCCUR
ON THE PERIPHERY OF ANY HIGH BASED THUNDERSTORMS ALONG THE
AFOREMENTIONED WEAK MONSOON MOISTURE PLUME AS WELL AS ACROSS
NORTHERN WY EAST OF THE DIVIDE WHERE RELATIVELY HIGH BASED STORMS
WILL CORRELATE WITH THE BEST CAPES. DRY THUNDERSTORMS WILL MORPH
INTO MORE WET STORMS AS THE FRONT DRAWS NEAR. THE TROUGHPA WILL
SWEEP ACROSS THE CWA TONIGHT. THEN ON MONDAY...ANOTHER SURGE OF WEAK
MID/UPPER LEVEL MOISTURE WILL OVERLAP SOME LINGERING INSTABILITY
OVER EXTREME SOUTHERN WY WHERE SOME ISOLD T STORMS ARE EXPECTED.
THEN BY TUESDAY...ANOTHER CLOSED LOW WILL ESTABLISH ITSELF ALONG THE
SOUTHERN BC COAST. THIS LOW WILL EJECT A WEAK VORT LOBE EAST
NORTHEAST ACROSS THE NORTHERN ROCKIES THAT WILL RESULT IN ISOLD
CONVECTION...MAINLY OVER THE NW MTNS TUE AFTERNOON. TODAY WILL BE A
COUPLE OF DEGREES COOLER THAN SATURDAY AS CLOUDS AND SHOWER ACTIVITY
DECREASE THE SOLAR INSULATION. AFTERWARD...TEMPS WILL COOL A FEW
DEGREES MONDAY BEFORE WARMING BACK UP TUESDAY.

.LONG TERM...WEDNESDAY THROUGH SUNDAY

HIGH LATITUDE ENERGY CURRENTLY OVER AK AND THE BERING SEA AREA WILL
DIVE SWD INTO OUR PACNW TROUGH AS UPSTREAM ENERGY...TYPHOON
REMNANTS...PUMPS UP THE UPSTREAM RIDGE OVER THE ERN PACIFIC. THE
END RESULT IS A DEEPENING COLD EARLY SEPTEMBER TROUGH OVER THE
PACNW AND INTERMOUNTAIN WEST THE SECOND HALF OF NEXT WEEK THAT
GRADUALLY TRANSLATES EWD AS THE FLOW DEAMPLIFYS TOWARDS THE END OF
THE PERIOD AND POSSIBLE SPLIT DEVELOPS OVER THE WEST EARLY NEXT
WEEK. THE MIDWEEK PERIOD WILL GENERALLY BE DRY AND QUITE WARM WITH
BREEZY TO LOCALLY WINDY PERIODS AND VERY LIMITED MAINLY MTN TSTMS
CHANCES AT BEST. BY FRIDAY...THE FIRST SIGNIFICANT SHORTWAVE OUT
AHEAD OF THE TROUGH IS ALREADY IMPACTING THE WEST AND NORTH AS A
COLD FRONT TRIES TO SAG SWD INTO THE NRN ZONES. THIS FRONTAL
BOUNDARY WILL BE A SIGNIFICANT PLAYER THROUGH THE WEEKEND WITH
CONSIDERABLY COOLER IF NOT COLDER WEATHER BEHIND IT AS THE MAIN
TROUGH OPENS UP AND PASSES ACROSS NRN WYO AND MONTANA OVER THE
LABOR DAY WEEKEND. A GOOD CHANCE OF SHOWERS AND TSTMS SHOULD OCCUR
IN AT LEAST THE NWRN SECTIONS WITH THIS FIRST SHORTWAVE AND THEN
MORE OF THE WEST AND NORTH AS THE DYNAMICS INTERACT/OVERRUN THE
FRONTAL BOUNDARY OVER THE WEEKEND. DETAILS ARE STILL UNCERTAIN BUT
THE GFS AND ECMWF ARE IN PRETTY GOOD AGREEMENT WITH THE BIG
PICTURE. THE SOURCE REGION OF THIS TROUGH IS QUITE COLD AND MODEL
GUIDANCE IS DROPPING H7 TEMPS DOWN TO AROUND ZERO TO EVEN BELOW
WITH THE FIRST SHORTWAVE FRIDAY NIGHT AND THEN WITH THE MAIN
TROUGH SATURDAY NIGHT SO MOUNTAINS WOULD GET A DECENT SHOT OF SOME
SNOW WITH EACH OF THESE. IT IS LABOR DAY WEEKEND AFTER ALL.

.AVIATION.../18Z ISSUANCE/

A COLD FRONT WILL CONTINUE TO PUSH EAST ACROSS THE INTERMOUNTAIN
WEST TODAY WITH SFC SW FLOW INCREASING ACROSS WEST AND CENTRAL
WYOMING AHEAD OF THE FRONT...ALONG WITH AREAS OF LLWS.  SFC SW WINDS
AT 20-30KTS WILL PREVAIL ACROSS THE AREA INTO THIS AFTERNOON. THE
MAIN AREA ISOLD-SCT -TSRA ARE DEVELOPING VCNTY AND SOUTH OF A
KEVW- KLND- KCPR LINE WITH HIGH BASED WINDY STORMS EXPECTED INTO
THE EARLY EVENING. THE COLD FRONT WILL BRING DRIER AIR AND GUSTY
WEST WIND INTO NW WYOMING MID TO LATE THIS AFTERNOON...PUSHING
ACROSS NORTH AND CENTRAL WYOMING THIS EVENING. CLEAR SKIES AND
DIMINISHING WIND WILL PREVAIL ACROSS THE AREA BEHIND THE FRONT
EARLY MONDAY MORNING.

.FIRE WEATHER...

...RED FLAG WARNING FOR PORTIONS OF CENTRAL AND NORTHERN WYOMING...

OVERALL IT WILL BE WINDIER THAN IT WAS SATURDAY BUT WITH SLIGHTLY
HIGHER RELATIVE HUMIDITY. AS 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. STRONG SOUTHWEST FLOW BUT STILL LOW HUMIDITY ACROSS
FREMONT...JOHNSON AND NATRONA COUNTIES WILL OCCUR SO THESE AREAS
WILL CONTINUE TO BE UNDER THE RED FLAG WARNING FOR TODAY. 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 EXPECTED 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 THAT
FORM ACROSS NORTHERN WYOMING EAST OF THE DIVIDE THROUGH MID DAY
WHERE THE BEST INSTABILITY WILL BE...THEN FROM EASTERN SWEETWATER
COUNTY TO CASPER AND THE REST OF NATRONA COUNTY LATER IN THE
AFTERNOON WHERE THE COMBINATION OF DRIEST BOUNDARY LEVEL MOISTURE
AND WIDEST TEMP/DEW POINT SPREADS WILL LIE.

A REPEAT OF THESE CONDITIONS WILL BE POSSIBLE MID WEEK.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
RED FLAG WARNING UNTIL 9 PM MDT THIS EVENING FOR WYZ280-283-285-
289-300.

RED FLAG WARNING UNTIL 3 AM MDT MONDAY FOR WYZ275-276-281-282-
287.

&&

$$

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


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