Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 230917 CCA
AFDRIW

AREA FORECAST DISCUSSION...CORRECTED
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 TO THE 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 WILL
BE 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 TURNING 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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