Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 241000
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
400 AM MDT MON AUG 24 2015

.SHORT TERM...TODAY THROUGH WEDNESDAY NIGHT

WV IMAGERY SHOWING LARGE BROAD AMPLIFIED RIDGE ACROSS THE WRN CONUS
WITH THE MAJOR AXIS STRETCHING FROM UT UP THROUGH WRN WY AND CNTRL
MT. BEST MOISTURE ONCE AGAIN HELD WELL SOUTH OVR NRN MEXICO...ERN AZ
AND NM. MINOR SW CURRENTLY MOVING THRU SWRN WY THIS MORNING WITH
NOTHING MOISTURE-WISE TO WORK ON...WITH ANOTHER SMALL UPR WAVE
LOCATED OVR CNTRL NV...DUE TO ARRIVE LATER THIS AFTERNOON WITH
PERHAPS SLIGHTLY BETTER RESULTS. SFC HAS GENERAL HIGH P ACROSS THE
REGION WITH WEAK LOW P JUST STARTING T DEVELOP OVR CNTRL/ERN UT.

UPR RIDGE AXIS CONTINUES TO MIGRATE FROM W TO E...CUTTING WY I TWO
BY LATER TONIGHT. MONSOONAL MOISTURE STILL REMAINS LARGELY WELL
SOUTH...WITH ONLY SLIGHT INCREASES IN MOISTURE INCHING INTO THE FA
FROM THE SOUTH. WEAK RIDGE TOPPING SW FORCING WILL AID IN
PRODUCING ISOLATED RELATIVELY DRY CONVECTION MAINLY ACROSS THE
MOUNTAINS THIS AFTERNOON...AND LATER OVR THE DEVELOPING SFC LOW AND
ATTENDING BOUNDARY BUILDING INTO SWRN WY OUT OF UT. WINDS
WILL INCREASE SLIGHTLY FROM THE WEST TODAY...PARTICULARLY W OF THE
DIVIDE...TO 10 TO 15 MPH ACROSS PORTIONS OF SWEETWATER COUNTY. WITH
RH LEVELS ALSO DIPPING AGAIN BETWEEN 10 AND 15 PERCENT FOR MOST OF
THE LOWER ELEVATIONS...AND HAINES INDEX VALUES AROUND 5...FIRE
DANGER WILL REMAIN ELEVATED...CONTINUING THROUGH TUESDAY.

TUE...H5 HEIGHTS WILL RISE EVEN MORE...ALONG WITH THE WARMEST HIGH
TEMPS OF THE FCST PERIOD. ISOLATED PRECIP CHCS WILL BE HIGHLY
CONFINED TO ONLY/MAINLY THE MOUNTAIN AREAS. WINDS WILL BE QUITE
LIGHT FA WIDE AS THE CENTER OF THE RIDGE DRIFTS NWD OVR CO. FIRE
DANGER ELEVATED AGAIN...BUT HOPEFULLY REMAINING JUST BELOW
CRITICAL. DRY INSTABILITY WILL INCREASE THROUGH A DEEP ATMOSPHERE
WITH LAPSE RATES RANGING FROM 12 TO 9 FROM SFC TO 300 MB
...PUSHING HAINES VALUES INTO THE 5 - 6 RANGE...ESPECIALLY ACROSS
THE CNTRL FA. HOWEVER...DRY STORMS THAT FORM THIS DAY WILL BE VERY
ISOLATED AND CONFINED MAINLY TO THE HIGHER MOUNTAINS.

WED...RIDGE AXIS SHIFTS EWD INTO THE HIGH/CNTRL PLAINS ALLOWING
MID/UPR MONSOONAL MOISTURE TO RETURN OUT OF THE SWRN CONUS ALONG
WITH THE ARRIVAL OF A STRONGER UPR LVL WAVE/JET STREAK THAT
CURRENTLY RESIDES NEAR THE BAJA REGION OF MEXICO. AT THE SFC...A LEE
SIDE LOW AND N/S ORIENTED FRONT WILL FORM...AIDING IN ADDITIONAL LL
FORCING AND SOME MODEST LL MOISTURE RETURN INTO THE SRN FA. MOISTURE
FLUX LOOKS BETTER THAN A DAY AGO...HOWEVER STILL BEING SOMEWHAT
CAUTIOUS ON THE AREAL EXTENT OF THE PRECIPITATION AT THIS POINT AS
AN EXTREMELY DRY ATMOSPHERE JUST STARTS TAKING ON WHAT SEEM TO BE
GOOD MOISTURE. WILL KEEP CONVECTION SCATTERED OR LESS FOR THE
MOMENT...WITH BEST CHCS ACROSS THE SRN AND WRN FA. STORMS WEST OF
THE DIVIDE WILL BE CHARACTERIZED BY INCREASING MOISTURE...LOW
CAPE...AND MODEST SHEAR OF 20 TO 30 KTS...FAST MOVING GIVING HEAVY
PRODUCERS. E OF THE DIVIDE...BETTER CAPE AND SOMEWHAT LOWER SHEAR
ELEVATED STORMS...PRODUCING BRIEF HEAVY RAIN AND STRONG...MARGINALLY
SEVERE GUSTY WINDS AT TIMES ON AN ISOLATED BASIS.

.LONG TERM...THURSDAY THROUGH MONDAY

GLOBAL MODELS ARE IN GOOD AGREEMENT WITH A DISTINCT SHORTWAVE
EXITING THE CWA INTO THE PLAINS THURSDAY MORNING...WITH UPPER
LEVEL RIDGING BUILDING IN ITS WAKE ACROSS THE COWBOY STATE
THURSDAY. THERE COULD INITIALLY BE SOME SHOWER ACTIVITY OVER THE
EAST ASSOCIATED WITH THIS DISTURBANCE. DESPITE RISING HEIGHTS
THURSDAY AFTERNOON...MODELS SHOW ENOUGH MOISTURE AND INSTABILITY
FOR SOME CONVECTION MAINLY OVER THE HIGHER TERRAIN.

UPPER LEVEL RIDGING IS EXPECTED FRIDAY WITH A WARMING TREND WITH
MAINLY DRY CONDITIONS...WITH SLIGHT POPS CONFINED MAINLY TO THE
MOUNTAINS AND NORTHWEST SECTIONS OF FORECAST AREA.

GLOBAL MODELS AND THEIR ENSEMBLES DIVERGE CONSIDERABLY ON THE
EVOLUTION OF A STRONG UPPER LOW/TROUGH(S) OFF THE PACIFIC
NORTHWEST COAST THIS WEEKEND INTO EARLY NEXT WEEK. THE 00Z GFS
ENDS UP HAVING TWO SYSTEMS IMPACTING THE AREA /SATURDAY AND
MONDAY/ WHILE THE 00Z ECMWF SHOWS JUST ONE SYSTEM AROUND SUNDAY.

THE 00Z GFS SHOWS THE INITIAL UPPER LOW OPENING UP AND EJECTING
NORTHEAST ACROSS THE PACIFIC NORTHWEST/EXTREME NORTHERN ROCKIES IN
THE FRIDAY AND SATURDAY TIME FRAME...AS ANOTHER STRONG UPPER LOW
SLIDES SOUTH OUT OF THE GULF OF ALASKA. THE 00Z GFS SHOWS THE
SECOND UPPER LOW HAVING THE SAME FATE AS THE FIRST...AS IT GETS
EJECTED INTO THE NORTHWEST...BY ANOTHER UPPER LOW DROPS SOUTH
FROM THE GULF OF ALASKA. ON THE OTHER HAND THE GEM AND ECMWF ARE
SHOWING THE INITIAL UPPER LOW OFF THE PACIFIC NORTHWEST COAST
BEING PICKED UP A BIT LATER THAN THE GFS...AND BY A TROUGH
PUSHING INTO THE BRITISH COLUMBIA INSTEAD OF ANOTHER UPPER LOW
DROPPING ALONG THE COAST. THIS RESULTS IN ONLY ONE SYSTEM
IMPACTING THE AREA IN THE SATURDAY NIGHT/SUNDAY TIME FRAME. THIS
FORECAST PACKAGE LEANS TOWARD THE 00Z ECMWF SOLUTION...BUT OVERALL
CONFIDENCE IS LOW WITH LITTLE RUN TO RUN CONSISTENCY. THUS HAVE
BROAD BRUSHED SLIGHT POPS WITH AN EMPHASIS ON SUNDAY. ALSO THIS
PATTERN FAVORS EXTREME FIRE WEATHER CONDITIONS ESPECIALLY AHEAD OF
THE FRONT ON SATURDAY.

&&

.AVIATION.../12Z ISSUANCE/

VFR CONDITIONS TO PREVAIL THROUGH THE PERIOD. ISOLATED SHOWERS AND
STORMS WILL OCCUR OVER WESTERN WY AFTER 19Z UNTIL 03Z. OUTFLOW WIND
GUSTS TO 30 KNOTS WILL OCCUR FROM SOME OF THE STORMS AND SHOWERS.

&&

.FIRE WEATHER...

ELEVATED FIRE DANGER ACROSS THE ENTIRE FORECAST AREA TODAY THROUGH
TUESDAY AS WARM AND VERY DRY CONDITIONS RETURN.

WARMING CONTINUES ALONG WITH MINIMUM RH VALUES DROPPING INTO THE
LOWER TO MID TEENS WITH ISOLATED MAINLY MOUNTAIN STORMS TODAY AND
TUESDAY. WINDS WILL REMAIN BELOW 15 MPH FOR NEARLY ALL ZONES
TODAY...BECOMING EVEN LIGHTER ON TUESDAY. INCREASING DEEP DRY
INSTABILITY ACROSS THE CENTRAL AND NORTHERN FORECAST AREA TODAY AND
TUESDAY...MAY PUSH HAINES VALUES TO EXTREME VALUES BETWEEN 5 AND
6...ESPECIALLY TUESDAY. ADDITIONALLY...THERE WILL BE LOW CHANCES FOR
ISOLATED DRY TYPE THUNDERSTORMS WEST OF THE DIVIDE...MORE ISOLATED
AND CONFINED TO THE MOUNTAINS TUESDAY. INCREASING MOISTURE AND
PRECIPITATION CHANCES WILL ARRIVE ON WEDNESDAY...BRINGING A REPRIEVE
FROM THE HOT AND DRY CONDITIONS.

&&

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

&&

$$

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



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