Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 230917
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
317 AM MDT SUN AUG 23 2015

.SHORT TERM...TODAY THROUGH TUESDAY NIGHT

WV IMAGERY SHOWING MODEST RIDGING OVR BOTH COASTS WITH SHARPLY
AMPLIFIED TROF DIPPING DOWN INTO THE CNTRL CONUS AS FAR SOUTH AS THE
CNTRL PLAINS. THIS FEATURE...ALONG WITH AN ASSOCIATED COLD FRONT...
IS DEPRESSING MONSOONAL MOISTURE WELL TO THE SOUTH INTO SRN CO...SRN
UT AND ACROSS NM. SFC HAS COLD FRONT WELL SOUTH MOVING INTO THE SRN
PLAINS THEN BACKED UP ALONG THE FRONT RANGE TO AROUND EXTREME SRN
WY. POST FRONTAL SEASONLY STRONG HIGH P ACROSS THE REST OF WY AND
MT. EXCEPT FOR SMOKE OVERHEAD...SKIES ARE CLEAR ACROSS THE FA
TONIGHT...WITH GOOD RADIATIONAL COOLING EXPECTED WITH A NICE PRE-
FALL CHILL INT HE AIR. A FEW RECORD LOWS POSSIBLE THIS MORNING.

A WARM-UP BEGINS TODAY...WHICH WILL HOPEFULLY GET JUST UNSTABLE
ENOUGH TO GET RID OF ALL INVERSIONS...INCLUDING THE MID-LEVEL
INVERSION THAT KEPT A PALL OF SMOKE OVER THE CWA SATURDAY.
ATTM...LAPSE RATES DON`T LOOK GREAT ACROSS THE NRN HALF OF THE
FA...MAXING OUT THIS AFTERNOON IN THE 7 TO 8 DEG/KM RANGE...PERHAPS
JUST ENOUGH. THE SRN PORTION OF THE CWA HOWEVER WILL SEE THEM
SOAR/RANGE FROM 9 TO 11 DEG/KM WITH MUCH BETTER MIXING THROUGH
DEPTH. OTHERWISE...LITTLE TO NO CHANCE FOR PRECIP ANYWHERE TODAY AS
MOISTURE CONTINUES SUPPRESSED TOT HE S...AND HIGH P AT THE SFC AND
ALOFT KEEP A LID ON THINGS. IT WILL BE QUITE DRY AGAIN TODAY...BUT
WITH SOMEWHAT LIGHTER WINDS. FIRE DANGER WILL BE ELEVATED FOR SURE
NEARLY EVERYWHERE...BUT JUST BELOW CRITICAL LEVELS ATTM.

MONDAY AND TUESDAY...UPR RIDGE AXIS CONTINUES TO MIGRATE FROM W TO
E...CENTERING OVR WY BY MON NIGHT. MONSOONAL MOISTURE REMAINS
LARGELY WELL SOUTH THROUGH THE FCST PERIOD...WITH ONLY VERY SLIGHT
INCREASES IN MOISTURE INTO THE FA FROM THE SOUTH AND MOST
SIGNIFICANT MOISTURE NOT REALLY MAKING ITS WAY BACK INTO THE FA
UNTIL THE VERY END OF THE FCST PERIOD. RIDGE TOPPING FORCING TOO
WILL VERY LIMITED...WITH MOST ISOLATED CONVECTION TAKING PLACE
ACROSS THE MOUNTAINS. MINOR EXCEPTION TO THIS SCENARIO WILL FOLLOW
THE FCST DEVELOPMENT OF SFC LOW PRESSURE AND A WEAK BOUNDARY LATER
ON MONDAY IN PARTIAL RESPONSE TO A WEAK RIDGE TOPPING SW SCHEDULED
TO MOVE THROUGH THE UPR LVLS MON AFTERNOON. SOME MARGINAL INCREASE
IN WRLY WINDS W OF THE DIVIDE. TUE...H5 HEIGHTS WILL RISE EVEN MORE
BRINGING THE WARMEST TEMPS OF THE FCST PERIOD. ISOLATED PRECIP CHCS
WILL BE HIGHLY CONFINED TO ONLY THE MOUNTAIN AREAS. WINDS WILL BE
QUITE LIGHT FA WIDE. FIRE DANGER ELEVATED ACROSS THE FA...BUT
HOPEFULLY REMAINING JUST BELOW CRITICAL. `HOPEFULLY` BECAUSE AS
WARMING OCCURS WITHOUT MUCH INTRUSION OF MOISTURE OF
CLOUDS...INSTABILITY WILL INCREASE...PERHAPS TO THE POINT OF PUSHING
HAINES VALUES ABOVE 5...AS THEY ARE NOW...PARTICULARLY ACROSS THE
CNTRL AND NRN FA. ADDITIONALLY...ALTHOUGH ANY THUNDERSTORMS THAT DO
FORM MON OR TUE...ESPECIALLY MON...WILL BE OF LITTLE
RAINFALL....ESPECIALLY OVR THE LOWLANDS...THEY SHOULD ONLY BE
DISTRIBUTED IN ISOLATED FASHION. CURRENTLY...WHILE LOCALIZED
MARGINAL RED FLAG CONDITIONS COULD EXIST BOTH DAYS...IT WILL TAKE
ANOTHER FCST PERIOD OR TWO TO SEE IF MORE WIDESPREAD INSTABILITY CAN
MANIFEST AND RAISE HAINES VALUES TO 6 ACROSS A WIDER AREA.

.LONG TERM...WEDNESDAY THROUGH SUNDAY

SYNOPSIS...AN UPPER LEVEL DISTURBANCE WILL SPREAD CHANCES OF
SHOWERS AND THUNDERSTORMS ACROSS THE AREA ON WEDNESDAY AND
WEDNESDAY NIGHT. AN UPPER RIDGE OF HIGH PRESSURE WILL PREVAIL
THURSDAY AND WITH SOME EMBEDDED MONSOONAL MOISTURE BRINGING SLIGHT
CHANCES OF THUNDERSTORMS...MAINLY ACROSS THE WESTERN MOUNTAINS.
SOUTHWEST FLOW IS EXPECTED SATURDAY AND SUNDAY WITH THUNDERSTORM
CHANCES RETREATING TO THE NORTHWEST MOUNTAINS AND FAR NORTH...DRY
AND BREEZY CONDITIONS ACROSS SOUTH AND CENTRAL AREAS PREVAIL
ACROSS THE SOUTH AND CENTRAL WITH ELEVATED FIRE DANGER.

DISCUSSION...LARGE-SCALE PATTERN CONSISTS OF HIGHLY AMPLIFIED PATTER
WITH UPPER RIDGE ACROSS THE CENTRAL PACIFIC...TROUGH OFF THE WEST
COAST AND BROAD UPPER RIDGE ACROSS THE ROCKIES AND HIGH PLAINS TO
BEGIN THE PERIOD.  A SUB-TROPICAL DISTURBANCE CURRENTLY OVER
NORTHERN MEXICO IS FORECAST TO LIFT NORTH ACROSS THE GREAT BASIN
LATE TUESDAY AND TURN EAST ACROSS WYOMING ON WEDNESDAY.  GFS HAS
SHOWN BETTER RUN-TO-RUN CONSISTENCY WITH THIS FEATURE OVER THE LAST
FEW DAYS...WITH ECMWF TRENDING FASTER THIS MORNING MORE IN LINE WITH
GFS.  FORECAST CONTINUES TO LEAN TOWARD THE GFS TRACKING THIS SYSTEM
TO VCNTY KRKS BY 00Z THURSDAY AND TRACKING EAST ALONG THE INTERSTATE
80 CORRIDOR INTO SE WYOMING THURSDAY MORNING.   THIS TRACK WILL
BRING INCREASING CHANCES OF THUNDERSTORMS ACROSS THE SW WEDNESDAY
MORNING WITH SCATTERED THUNDERSTORMS VCNTY AND SOUTH OF A BIG PINEY-
LANDER-CASPER LINE WEDNESDAY AFTERNOON AND EVENING WITH LESS
COVERAGE TO THE NORTH.  SUB-TROPICAL ORIGINS OF THIS SYSTEM COMBINED
WITH SLOW MOVEMENT MAY BRING LOCALLY HEAVY RAINFALL ACROSS SOUTHERN
WYOMING. SOME MOISTURE AND INSTABILITY MAY LINGER WITH THIS SYSTEM
ACROSS EAST CENTRAL WYOMING INTO THURSDAY AFTERNOON WHERE SLIGHT
CHANCES OF THUNDERSTORMS WERE ADDED.  OTHERWISE SOME MID AND HIGH
LEVEL MONSOONAL MOISTURE WILL RESULT IN A FEW LATE DAY THUNDERSTORMS
BLOSSOMING MAINLY OVER THE WESTERN MOUNTAINS THURSDAY AND FRIDAY.
TEMPERATURES WILL TREND COOLER ACROSS THE SOUTH ON WEDNESDAY WITH
INCREASING CLOUDS AND SHOWERS WITH COOLER...CLOSER TO
NORMAL...TEMPERATURES ACROSS THE AREA ON THURSDAY.

ECMWF AND GFS HAVE TRENDED TOWARD DEAMPLIFICATION AND SOME
PROGRESSION OF LONGWAVE PATTERN FRIDAY THROUGH SUNDAY COMPARED TO
PREVIOUS RUNS.  UPPER TROUGH OFF THE WEST COAST IS EXPECTED TO BEGIN
TO SHEAR INLAND ON SATURDAY...ACROSS THE INTERMOUNTAIN WEST INTO
WESTERN MONTANA.  THIS WILL RESULT IN INCREASING SW FLOW ALOFT OVER
WYOMING WITH STRENGTHENING OF LEE SIDE SURFACE TROUGH ACROSS EASTERN
WYOMING.  THE MAIN CONCERN IN THIS PATTERN OVER THE WEEKEND WILL BE
ELEVATED FIRE DANGER WITH POSSIBLE DRY...WARM AND WINDY CONDITIONS
DEVELOPING ACROSS THE SOUTH AND CENTRAL.  THERE IS STILL A MODERATE
DEGREE OF UNCERTAINTY WITH THIS PATTERN NEXT WEEKEND AS MODELS HAVE
YET TO SHOW RUN-TO-RUN CONSISTENCY WITH PROGRESSION (OR
RETROGRESSION) OF EASTERN PACIFIC TROUGH...WESTERN U.S. RIDGE.

&&

.AVIATION.../12Z ISSUANCE/

VFR CONDITIONS WILL PREVAIL THROUGH THE PERIOD. THERE WILL BE AREAS
OF SMOKE THAT WILL REDUCE VISIBILITY TO MVFR VALUES THROUGH 18Z.
MOUNTAINS MAY BE PARTIALLY OBSCURED BY SMOKE AS WELL. THE SMOKE WILL
DISPERSE FOR THE MOST PART THIS AFTERNOON WITH VFR VISIBILITY
PREVAILING.

&&

.FIRE WEATHER...

ELEVATED FIRE DANGER ACROSS THE ENTIRE FORECAST AREA TODAY THROUGH
TUESDAY...BUT REMAINING BELOW CRITICAL RED FLAG CONDITIONS FOR THE
MOST PART.

WARMING OVER THE NEXT FEW DAYS WITH MINIMUM RH VALUES DROPPING INTO
THE LOWER TO MID TEENS AND LITTLE CHANCE FOR ANY PRECIPITATION...
SANS ISOLATED MAINLY MOUNTAIN STORMS MONDAY AND TUESDAY...WILL KEEP
FIRE DANGER ON THE HIGH SIDE. HOWEVER...WINDS SHOULD BE MUCH MORE
TAME THAN THE PAST COUPLE OF DAYS...AND THIS WILL HELP IN KEEPING
FIRE WEATHER CONDITIONS IN CHECK AROUND THE REGION.

TWO CAVEATS EXIST WITH THIS PREMISS HOWEVER. ONE HAS TO DO WITH
INCREASING INSTABILITY AND CONTINUED DRY CONDITIONS ACROSS THE
CENTRAL AND NORTHERN FORECAST AREA MONDAY AND TUESDAY...PERHAPS
PUSHING HAINES VALUES...WHILE CURRENTLY FORECAST AT AROUND A VALUE
OF 5...INTO THE CATEGORY OF 6 IF MOISTURE CONTINUES LACKING THROUGH
A DEEP ATMOSPHERE. ADDITIONALLY...THERE WILL BE SMALL CHANCES FOR
ISOLATED DRY TYPE THUNDERSTORMS...MAINLY MONDAY...WEST OF THE
DIVIDE. WILL HAVE TO CONTINUE TO MONITOR THE AREA AND FORECAST OVER
THE NEXT DAY OR SO TO SEE IF ANY OF THESE CIRCUMSTANCES CHANGE IN
ANY FASHION...WARRANTING AN INCREASE IN FIRE DANGER.

&&

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

$$

SHORT TERM...BRAUN
LONG TERM...AEM
AVIATION...AR
FIRE WEATHER...BRAUN



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