Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 292043
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
243 PM MDT SAT AUG 29 2015

.SHORT TERM...TONIGHT THROUGH SUNDAY NIGHT

HEIGHTS CONTINUE TO LOWER AS AN UPPER LEVEL RIDGE CONTINUES TO
SHIFT SOUTHEAST INTO THE CENTRAL PLAINS AND A DYNAMIC TROUGH
CONTINUES TO APPROACH FROM THE WEST. SOUTHWEST FLOW WILL CONTINUE
TO INCREASE INTO THE EVENING AND WILL REMAIN SPEEDY THROUGH
SUNDAY. AT THE SURFACE...A LEE TROUGH WILL DIG THROUGH THE DAY ON
SUNDAY FURTHER INCREASING THE GRADIENT IN PORTIONS OF CENTRAL AND
EASTERN WYOMING. AT THE SAME TIME...THE COLD FRONT ASSOCIATED
WITH THE PREVIOUSLY MENTIONED TROUGH WILL ADVANCE INTO WESTERN
WYOMING EARLY...DECREASING FIRE DANGER OVER WESTERN ZONES...THEN
IT WILL STALL FOR MUCH OF THE DAY ALLOWING DRY SOUTHWEST FLOW TO
RETURN TO AREAS EAST OF THE DIVIDE...SO THE RED FLAG WARNINGS WERE
EXTENDED IN JOHNSON FREMONT AND NATRONA COUNTIES INTO SUNDAY
EVENING. ALOFT...THE RIGHT ENTRANCE REGION OF A JET STREAK WILL
ENTER THE AREA INCREASING FORCING FOR ASCENT ACROSS MUCH OF THE
STATE...THE LIMITING FACTOR FOR THUNDERSTORMS OF COURSE WILL BE
MOISTURE. THERE ARE INDICATIONS OF SOME MID LEVEL MOISTURE NOSING
NORTH INTO EASTERN SWEETWATER...EASTERN FREMONT...NATRONA AND
SOUTHERN JOHNSON COUNTIES THAT COULD PROVIDE ENOUGH FUEL FOR
SOME VERY DRY AND HIGH BASED STORMS IN THE AFTERNOON. ACROSS
CENTRAL WYOMING...THERE WILL BE A LITTLE LESS MOISTURE AVAILABLE
FOR THUNDERSTORMS BUT THERE COULD BE A STRAY COME OFF OF THE
HIGHER TERRAIN...SO EXPANDED POPS TO ALL AREAS IN THE LATE
AFTERNOON. IN THE WESTERN MOUNTAINS...THERE WILL BE PLENTY OF
MOISTURE AND FORCING ALONG THE STALLED FRONT FOR SCATTERED
THUNDERSTORMS...THOUGH PRECIPITABLE WATER IS A BIT LIMITED SO
EXPECTING A MIX OF WET AND DRY STORMS THERE. RELATIVE HUMIDITY
WILL BE QUITE A BIT HIGHER SO THAT WILL LIMIT FIRE POTENTIAL. THE
COLD FRONT WILL FINALLY GAIN TRACTION AGAIN SUNDAY EVENING AND
WILL FINISH ITS PROGRESSION EASTWARD BY SUNRISE ON MONDAY.

.LONG TERM...MONDAY THROUGH SATURDAY

AGREEMENT AMONG MEDIUM-RANGE FORECAST MODELS IS VERY GOOD THROUGH
THE MIDDLE OF NEXT WEEK. SMALL DIFFERENCES FOR FRIDAY AND SATURDAY
HAVE BECOME BETTER ALIGNED WITH MOST RECENT MODELS RUNS WITH REGARDS
TO THE TRACK AND EVOLUTION OF A PACIFIC NORTHWEST TROUGH THAT WILL
MOVE THROUGH THE INTERMOUNTAIN WEST AND ROCKIES NEXT FRIDAY AND
SATURDAY.

DRIER WESTERLY FLOW WILL COVER THE FORECAST AREA MONDAY IN THE WAKE
OF THE DEPARTING SHORTWAVE TROUGH. SOME WEAK INSTABILITY AND
LINGERING MID-LEVEL MOISTURE NEAR THE COLORADO BORDER IN SWEETWATER
COUNTY DURING THE AFTERNOON AND EVENING. CONVECTION WOULD BE SHORT-
LIVED AND WEAK IF IT CAN MAKE MUCH PROGRESS OFF THE HIGHER TERRAIN
OF NORTHEAST UTAH. OTHERWISE...A DRY MONDAY WITH SEASONAL
TEMPERATURES AND LESS WIND THAT WHAT WILL BE SEEN THIS WEEKEND.
UPPER LEVEL FLOW BEGINS TO BACK TO THE SOUTHWEST ON TUESDAY IN
RESPONSE TO DEEPENING UPPER LEVEL LOW DIGGING INTO THE PACIFIC
NORTHWEST. THERE COULD BE ENOUGH MID-LEVEL MOISTURE TO INITIATE WEAK
MOUNTAIN CONVECTION TUESDAY THROUGH THURSDAY. THEREFORE...HAVE LEFT
LOW PRECIP CHANCES OVER MAINLY THE NORTHWEST MOUNTAINS. BELIEVE THAT
VERY LITTLE RAIN WOULD FALL WITH ANY CONVECTION THAT DEVELOPS AS DEW
POINTS WILL BE LESS THAN 40F AND PRECIPITABLE WATER VALUES LESS THAN
IMPRESSIVE. 700MB TEMPERATURES TUESDAY AND WEDNESDAY WILL AGAIN
APPROACH +15C WHICH WILL HAVE US STARING AT 90F SURFACE
TEMPERATURES. WOULD NOT BE SURPRISED TO SEE MAXIMUM TEMPERATURES
EXCEED 90F...ESPECIALLY ON WEDNESDAY AND MAYBE THURSDAY...DUE TO
INCREASING SOUTHWEST WIND AT THE SURFACE. THE MIDWEEK PATTERN...
BETWEEN TUESDAY AND THURSDAY...IS SHAPING UP TO BE ONE THAT COULD
HAVE CRITICAL FIRE WEATHER IMPLICATIONS. UNSEASONABLY WARM
TEMPERATURES...RELATIVE HUMIDITY IN THE TEENS...AND INCREASING
SOUTHWEST WIND AHEAD OF THE PACIFIC NORTHWEST TROUGH ALL LOOK TO BE
PRESENT ON THESE DAYS. IN FACT...DRIER SURFACE AIR WILL BE ADVECTED
INTO THE STATE FROM THE GREAT BASIN WITH AFTERNOON DEW POINTS
POSSIBLE FALLING TO LESS THAN 30F. THOSE WITH FIRE WEATHER INTERESTS
SHOULD CONTINUE TO MONITOR THE FORECAST FOR POSSIBLE HIGHLIGHTS.
APPROACH OF THE TROUGH WILL LEAD TO A BETTER OPPORTUNITY FOR
CONVECTION ACROSS THE NORTHWEST FRIDAY AFTERNOON AND EVENING. STILL
SOME DISCREPANCIES REGARDING HOW FAR SOUTH THE PACIFIC NORTHWEST
TROUGH WILL DIG AND HOW FAST IT WILL MOVE ACROSS THE FORECAST AREA.
BOTH ISSUES WILL HAVE AN IMPACT ON TEMPERATURES AND SENSIBLE WEATHER
HEADING INTO NEXT WEEKEND. COOLER AIR WILL SLIP SOUTH BEHIND A COLD
FRONT AND COOL THINGS DOWN FOR SATURDAY. FRIDAY TEMPERATURES WILL BE
MORE TRICKY AND DEPENDENT UPON THE TIMING OF THE COLD FRONTAL
PASSAGE. FORECAST FRIDAY TEMPERATURES EAST OF THE CONTINENTAL DIVIDE
COULD END UP BEING TOO COOL...BUT AT THIS RANGE HAVE NOT STRAYED
FROM KEEPING CONSISTENT REGIONAL FORECAST.

&&

.AVIATION.../00Z ISSUANCE/

SOUTH TO SOUTHWEST SFC WINDS 15-25KTS ACROSS MOST AREAS THROUGH
THE TAF PERIOD. ISOLD-WDLY SCT -TSRA PUSHING INTO FAR WEST
WYOMING...SLOWLY EXPECTED TO REACH 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.
HIGH BASED -TSRA WITH GUSTY OUTFLOW WINDS WILL BE MORE LIKELY ON
SUNDAY AFTER 19Z EAST OF THE DIVIDE VCNTY KRKS KCPR...STORMS WILL
BE A BIT LOWER BASED ACROSS WESTERN WYOMING VCNTY KJAC KP60 KPNA
KBPI. EXPECT MOUNTAIN OBSCURATIONS...SOME MOUNTAIN WAVE
ACTIVITY...AND TURBULENCE THROUGH THE PERIOD AS WINDS INCREASE.

FIRE WEATHER...

.RED FLAG WARNING THROUGH SUNDAY FOR PORTIONS OF CENTRAL
WYOMING...

ON SUNDAY...OVERALL IT WILL BE WINDIER THAN IT WAS TODAY 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. LATEST GUIDANCE CONTINUES TO SUGGEST THAT THE FRONT
WILL RETREAT BACK INTO MONTANA DURING THE LATE MORNING...CLEARING
THE WAY FOR MORE STRONG SOUTHWEST FLOW AND LOW HUMIDITY ACROSS
FREMONT...JOHNSON AND NATRONA COUNTIES. SO...THESE AREAS WILL
CONTINUE TO BE UNDER THE RED FLAG WARNING 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 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 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 MID WEEK.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
RED FLAG WARNING UNTIL 9 PM MDT THIS EVENING FOR WYZ275>277-279-
282-287-414>416.

RED FLAG WARNING UNTIL 9 PM MDT SUNDAY FOR WYZ280-281-283-285-
289-300.

&&

$$

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


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