Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 310346
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
946 PM MDT SUN AUG 30 2015

.UPDATE...CANADIAN COLD FRONT WAS JUST NORTH AND WEST OF BILLINGS
AT 3Z...AND STILL ON TRACK TO PLOW ACROSS THE NORTHERN BORDER
AROUND 06Z...UP TO THE OWL CREEK MOUNTAINS 2Z-3Z. DRY AIR HAS
PUNCHED IN ACROSS WESTERN AND CENTRAL WYOMING WITH A FEW STORMS
CONTINUING TO TRACK MAINLY ALONG AND SOUTH OF INTERSTATE 80 AND
AHEAD OF THE FRONT ACROSS THE NORTHERN BIGHORN RANGE AND EASTERN
MONTANA. MAIN CONCERN CONTINUES TO BE RAPID SPREAD OF ANY FIRES
ACROSS NORTH CENTRAL AREAS WITH GUSTY NORTH WIND BEHIND THE
FRONT. RED FLAG WARNINGS REMAIN IN EFFECT FOR THESE AREAS UNTIL 3
AM. ALL OTHER RED FLAG WARNINGS HAVE BEEN ALLOWED TO EXPIRE.

&&

.PREV DISCUSSION... /ISSUED 237 PM MDT SUN AUG 30 2015/

SHORT TERM...TONIGHT THROUGH MONDAY

A TROUGH AND COLD FRONT CONTINUE TO TRACK EASTWARD ACROSS THE
INTERMOUNTAIN WEST. THE COLD FRONT IS LYING NEAR THE MONTANA BORDER
AND IS EXPECTED TO SURGE SOUTHWARD AGAIN AROUND SUNSET WITH THE
MAIN PUSH ARRIVING TOWARD MIDNIGHT...THOUGH THE TIMING ON COLD
FRONTS CAN BE TRICKY AS THEY OFTEN MOVE FASTER THAN EXPECTED. WE
CAN CONTINUE TO EXPECT RED FLAG CONDITIONS SOUTH OF THE FRONT AND
MUCH COOLER AND MOIST CONDITIONS TO THE NORTH THROUGH THE EARLY
EVENING HOURS.

THERE IS ALSO SOME DECENT LIFT ON THE FRONT SIDE OF THIS TROUGH
WITH JET SUPPORT ALOFT...A THETA E RIDGE LYING APPROXIMATELY FROM
FLAMING GORGE TO KAYCEE...AND QG FORCING ARRIVING LATER IN THE
EVENING. AREAS WITH DECENT LOW LEVEL MOISTURE IN PLACE...ACROSS
THE WEST AND THE SOUTH...AND ON THE OTHER SIDE OF THE FRONT WILL
SEE WET THUNDERSTORMS AND AREAS LIKE THE SOUTHERN BIGHORN
BASIN...NATRONA COUNTY AND FREMONT COUNTY WILL SEE MAINLY DRY
THUNDERSTORMS THROUGH SUNSET...SHOWERS AND STORMS HAVE A BETTER
CHANCE OF PRODUCING A WETTING RAIN THEREAFTER.

MONDAY WILL BE DRY AGAIN BUT LESS WINDY AS WE LOSE THE GRADIENT AT
THE SURFACE AND ALOFT. TEMPERATURES WILL BE CLOSER TO SEASONAL
NORMS WITH HIGHS IN THE LOW TO MID 80S EAST...MID TO UPPER 70S
WEST AND SOUTH. A PLUME OF MOISTURE ACROSS COLORADO COULD BEGIN TO
POKE NORTH INTO SOUTHERN SWEETWATER COUNTY WITH A FEW STRAY
AFTERNOON STORMS POSSIBLE OTHERWISE IT WILL BE DRY AND RAIN FREE.

LONG TERM...MONDAY NIGHT THROUGH SUNDAY

MEDIUM-RANGE FORECAST MODELS ARE CONSISTENT AND AGAIN IN FAIRLY GOOD
AGREEMENT THROUGH THE FORECAST PERIOD. WEST-SOUTHWEST FLOW ALOFT ON
TUESDAY WILL BACK TO THE SOUTHWEST WEDNESDAY AND THURSDAY AS A
TROUGH DEEPENS OVER THE PACIFIC NORTHWEST. LIMITED MOISTURE IN
SOUTHWEST FLOW FOR TUESDAY...BUT SOME PRECIPITABLE WATER VALUES
AROUND 0.70 INCHES COULD LEAD TO WEAK CONVECTION IN JOHNSON AND
NATRONA COUNTIES. HOWEVER...WITH DEW POINTS IN THE 30S HAVE
CONTINUED A GENERALLY DRY FORECAST FOR NOW. SOUTHWEST FLOW ADVECTS
IN DRIER AIR FROM THE GREAT BASIN WEDNESDAY AND THURSDAY AND DO NOT
SEE MUCH IN THE WAY OF AVAILABLE MOISTURE FOR ANY CONVECTION.
SURFACE DEW POINTS WILL GENERALLY RANGE FROM 25-35F. ALSO...
WEDNESDAY LOOKS TO BE THE WARMEST DAY OF THE WEEK WITH TEMPERATURES
RUNNING ABOUT 5-10F ABOVE NORMAL AND THE NORTHERN BASINS PUSHING
INTO THE 90S AGAIN. THE COMBINATION OF THE UNSEASONABLY HOT
TEMPERATURES...LOW HUMIDITY...AND INCREASING SOUTHWEST WIND OF 15-30
MPH WILL VERY LIKELY LEAD TO CRITICAL FIRE WEATHER CONDITIONS FOR
WEDNESDAY AND MOST PROBABLY THURSDAY. MOST INTENSE WINDS WILL BE IN
THE CORRIDOR FROM ROCK SPRINGS TO CASPER. THOSE WITH FIRE WEATHER
INTERESTS SHOULD BE AWARE OF THE LIKELIHOOD OF THESE CONDITIONS FOR
WEDNESDAY AND THURSDAY AND BE ON THE ALERT FOR POSSIBLE HIGHLIGHTS ISSUED
EARLY THIS WEEK. FIRST UPPER LOW IN THE PACIFIC NORTHWEST TROUGH
WILL EJECT INTO ALBERTA THURSDAY. THIS UPPER LOW WILL QUICKLY BE
REPLACED BY ANOTHER SHORTWAVE THAT WILL DROP INTO THE BASE OF THE
TROUGH OVER THE INTERMOUNTAIN WEST FOR FRIDAY. UPPER FLOW WILL
PARALLEL SURFACE FRONT THAT WILL BE ORIENTED NORTHEAST-SOUTHWEST
JUST TO THE WEST OF WYOMING FOR FRIDAY. HAVE NUDGED TEMPERATURES
UP A TAD FOR FRIDAY AS IT DOES LOOK LIKE THE COOLER AIR WILL HANG
BACK ONE MORE DAY. AM STILL CONCERNED THAT FRIDAY MAY INDEED BE
WARMER THAN CURRENTLY FORECAST WITH COMPRESSIONAL WARMING AHEAD OF
THE APPROACHING FRONT/TROUGH. MOISTURE WILL RIDE ALONG OUR
NORTHWESTERN ZONES FOR FRIDAY AFTERNOON AND SATURDAY.
THEREFORE...HAVE INCREASED PRECIP CHANCES OVER YELLOWSTONE AND THE
SURROUNDING MOUNTAINS. ENERGY IN THE BASE OF THE TROUGH WILL BEGIN
TO EJECT SATURDAY WHILE AT THE SAME TIME A SURFACE LOW TAKES SHAPE
TO OUR SOUTHEAST. THIS WILL BRING THE COLD FRONT ACROSS THE
FORECAST AREA SATURDAY. BOTH THE ECMWF AND GFS HAVE THE FORECAST
AREA DRY SLOTTED SATURDAY AS THE UPPER LOW TRACKS TO OUR NORTH.
BEST CHANCE FOR MEASURABLE PRECIP OVER THE WEEKEND WILL BE ACROSS
THE NORTHERN THIRD OF THE FORECAST AREA CLOSEST TO THE UPPER LOW.
SURFACE HIGH PUSHES INTO WYOMING SUNDAY AND 700MB TEMPERATURES
FALL INTO THE ZERO TO +4C RANGE ACROSS THE NORTH. THIS WILL LEAD
TO COOLER THAN AVERAGE TEMPERATURES FOR SUNDAY IN ALL BUT
SOUTHWEST AND FAR WEST WYOMING.

AVIATION.../00Z ISSUANCE/

A COLD FRONT WILL CONTINUE TO PUSH EAST ACROSS THE INTERMOUNTAIN
WEST WITH SFC WINDS GENERALLY FROM THE SOUTHWEST TO WEST ACROSS MUCH
OF THE STATE AHEAD OF THE COLD FRONT...WINDS WILL BRIEFLY SWITCH TO
THE NW BEHIND THE FRONT WITH SOME AREAS OF 30-40KT GUSTS BEHIND THE
FRONT VCNTY KBYG KPOY KU68. THE MAIN AREA OF ISOLD-SCT -TSRA 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 LATE 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
THROUGH THE EVENING HOURS...

A COLD FRONT ASSOCIATED WITH THE TROUGH DIGGING TO THE WEST REMAINS
IN THE AREA NEAR THE MONTANA BORDER THIS AFTERNOON WITH COOLER
TEMPERATURES AND MODERATE HUMIDITY TO THE NORTH OF IT AND RED FLAG
TO NEAR RED FLAG CONDITIONS JUST TO THE SOUTH OF IT. MODELS ARE
CALLING FOR SOUTHWEST FLOW TO OVERTAKE THE FRONT AND CAUSE IT TO
RETREAT BACK NORTH FOR THE LATE AFTERNOON HOURS. THE MAIN FRONTAL
SURGE WILL COME LATER TONIGHT WITH STEADY 25 TO 35 MPH WINDS IN
JOHNSON COUNTY AND NORTHERN BIGHORN COUNTY BEHIND THE
FRONT...EXTENDED THE NORTHERN RED FLAG WARNINGS TO 3AM TO COVER THE
FRONTAL PASSAGE THOUGH IT MAY HAPPEN SOONER.

IN ADDITION...ISOLATED TO WIDELY SCATTERED MIXED WET AND DRY
THUNDERSTORMS ARE EXPECTED TO CONTINUE TO DEVELOP INTO THE EVENING
HOURS...WITH A FEW PERSISTING INTO THE NIGHT...ESPECIALLY EAST OF
THE DIVIDE.

IT WILL BE DRY BEHIND THE FRONT FOR MONDAY BUT A BIT COOLER. THERE
WILL BE SOME 20MPH BREEZES IN THE AFTERNOON BUT RED FLAG CONDITIONS
ARE NOT EXPECTED.

IT SILL LOOKS LIKE THERE COULD BE WIDESPREAD RED FLAG CONDITIONS MID
WEEK...STAY TUNED FOR UPDATES ON THE NEXT SYSTEM.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
RED FLAG WARNING UNTIL 3 AM MDT MONDAY FOR WYZ275-276-281-282-
287.

&&

$$

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



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