Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 250922
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
322 AM MDT TUE AUG 25 2015

.SHORT TERM...TODAY THROUGH THURSDAY NIGHT

WV IMAGERY SHOWING LARGE BROAD AMPLIFIED RIDGE ACROSS THE WRN CONUS
WITH THE MAJOR AXIS STRETCHING FROM UT UP THROUGH CNTRL WY AND ERN
MT. BEST MOISTURE STILL HELD WELL SOUTH OVR NRN MEXICO...AZ AND SRN
CALIFORNIA. MINOR SW CURRENTLY MOVING THRU CNTRL TO ERN WY THIS
MORNING WITH THE ISOLATED SHRA/TSRA THAT WERE SPRINKLED AROUND THE
SRN AND WRN ZONES EARLIER NOW GONE. UPSTREAM OVR ERN NV INTO CNTRL
UT...THE NEXT MINOR WAVE SITS...WITH ARRIVAL OVR THE FA BY
AFTERNOON. GENERALLY WEAK HIGH P ACROSS THE REGION.

TODAY...H5 HEIGHTS WILL CONTINUE TO RISE...BRINGING 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 FROM THE 4 CORNERS
REGION AND CENTERS 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...KEEPING HAINES VALUES INTO THE 5 - 6
RANGE...ESPECIALLY ACROSS THE CNTRL AND NRN FA. ANY MOSTLY DRY
STORMS THAT DO FORM TODAY WILL BE ISOLATED AND CONFINED MAINLY TO
THE HIGHER MOUNTAINS...WITH LESSER CHCS OVR DEVELOPING WEAK LOW P
AND ATTENDING BOUNDARY FROM SWEETWATER COUNTY N INTO ERN FREMONT AND
WRN NATRONA COUNTIES.

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 OVR SRN AZ. AT THE SFC...A LEE SIDE LOW AND N/S
ORIENTED FRONT WILL FORM...AIDING IN ADDITIONAL LL FORCING AND SOME
DECENT LL MOISTURE RETURN INTO THE SRN FA. OVERALL MOISTURE FLUX
CONTINUES TO LOOK GOOD. CONVECTION WILL BE SCATTERED...WITH BEST
CHCS ACROSS THE SRN AND WRN FA. STORMS WEST OF THE DIVIDE WILL BE
CHARACTERIZED BY INCREASING MOISTURE...WITH LOW CAPE...AND MODEST
SHEAR OF 20 TO 30 KTS...PRODUCING MAINLY FAST MOVING HEAVY RAIN
STORMS. E OF THE DIVIDE...BETTER CAPE AND SOMEWHAT LOWER SHEAR
GIVING ELEVATED STORMS...PRODUCING BRIEF HEAVY RAIN AND
STRONG...MARGINALLY SEVERE GUSTY WINDS AT TIMES ON AN ISOLATED
BASIS. EVENING/OVERNIGHT THRU THU MORNING...UPR WAVE AND LEE LOW
WILL MOVE EWD INTO THE PLAINS...TAKING SOME PRECIP WITH IT.
HOWEVER...A BACK DOOR FRONT SHOULD REMAIN...STRETCHING FROM THE
FRONT RANGE OF CO INTO SRN WY...CONTINUING ENOUGH LL FORCING TO KEEP
SHRA/ISOLATED TSRA LINGERING...WHILE WRAP AROUND MOISTURE INCREASES
CHANCES ACROSS THE NERN ZONES. BY THU AFTERNOON...BACKSIDE OF THE
UPR WAVE WILL FINALLY BEGIN TO CLEAR THE AREA...DRAGGING THE SFC
FRONT WITH IT. DIURNAL/TERRAIN FORCED SHRA/TSRA WILL INCREASE AGAIN
ACROSS THE WEST IN THE AFTERNOON UNDER RELATIVELY COOL UPR
LVLS...BUT ON A MORE ISOLATED TO WIDELY SCATTERED BASIS...ENDING
IN THE THU EVENING/OVERNIGHT PERIOD.

.LONG TERM...FRIDAY THROUGH TUESDAY

MEDIUM RANGE MODELS CONTINUE TO SHOW THE LONGWAVE TROUGH/UPPER LOW ACROSS
THE GULF OF ALASKA TO OFF THE WEST COAST IN THE SHORT TERM...GRADUALLY
SHIFTING ACROSS THE WEST COAST/ROCKIES BY EARLY TO MIDDLE OF NEXT
WEEK. HOWEVER GLOBAL MODELS HAVE SIGNIFICANT DIFFERENCES ON HOW
THIS PATTERN SHIFT WILL OCCUR THIS WEEKEND INTO NEXT WEEK.
OVERALL...THE WEATHER THROUGH THE WEEKEND WILL BE WARM TO VERY
WARM AND MAINLY DRY...WITH THE MAIN CONCERN BEING FIRE WEATHER
BEHAVIOR SATURDAY AND SUNDAY.

ON FRIDAY...AN UPPER RIDGE WILL BE FIRMLY IN PLACE OVER THE ROCKIES...WITH
AN UPPER LOW OFF THE WEST COAST NEAR 43N 135W. DESPITE UPPER LEVEL
RIDGING...ENOUGH MOISTURE AND INSTABILITY WILL BE PRESENT FOR
ISOLATED AFTERNOON/EVENING THUNDERSTORMS MAINLY OVER THE MOUNTAINS
POSSIBLY SPILLING INTO THE ADJACENT FOOTHILLS. TEMPERATURES WILL
CONTINUE TO REBOUND FROM THE BRIEF COOLING THAT OCCURRED FROM THE
SYSTEM WEDNESDAY/EARLY THURSDAY.

THE AFOREMENTIONED UPPER LOW IS EXPECTED TO OPEN UP AND EJECT
NORTHEAST ACROSS THE PACIFIC NORTHWEST LATE FRIDAY NIGHT AND
ACROSS THE NORTHERN INTERMOUNTAIN WEST SATURDAY...AS ANOTHER UPPER
LOW/TROUGH DIVES SOUTHWARD OFF THE BRITISH COLUMBIA COAST. THIS
SYSTEM TRACKING ACROSS NORTHERN IDAHO/NW MONTANA SATURDAY WILL
RESULT IN A LEE TROUGH DEVELOPING WHICH WILL RESULT IN BREEZY
SOUTHWESTERLY SURFACE FLOW ESPECIALLY IN THE RKS-CPR CORRIDOR
SATURDAY AFTERNOON. THIS COMBINED WITH VERY WARM TEMPERATURES AND
LOW RH WILL RESULT IN ELEVATED TO POSSIBLY EXTREME FIRE WEATHER
BEHAVIOR SATURDAY. THE COLD FRONT ASSOCIATED WITH THIS FIRST
SYSTEM IS TRENDING TO STAY WEST AND NORTH OF THE AREA SAT/SAT
NIGHT.

THE ECWMF OPENS UP THE SECOND UPPER LOW AND SWINGS IT INTO THE
PACIFIC NORTHWEST SUNDAY...AND THEN ACROSS THE NORTHERN ROCKIES
BY LATE SUNDAY NIGHT/MONDAY. ON THE OTHER HAND...THE GFS KEEPS THE
UPPER LOW OFF THE PACIFIC NORTHWEST THROUGH THE DAY
MONDAY...BEFORE SWINGING IT EAST NORTHEAST ACROSS THE PACIFIC
NORTHWEST/NORTHERN INTERMOUNTAIN WEST TUESDAY. THE ECMWF SOLUTION
WOULD BRING A DECENT COLD FRONT ACROSS THE AREA LATE SUNDAY
NIGHT/MONDAY WHILE THE GFS WOULD NOT BRING THE MAIN FRONT THROUGH
UNTIL TUESDAY/TUESDAY NIGHT. BOTH SOLUTIONS SHOW THE BEST DYNAMICS
REMAINING NORTHWEST OF THE REGION...WITH THE AREA REMAINING
MAINLY DRY. MAIN CONCERNS WITH THIS SYSTEM AND FOR MUCH OF THE
EXTENDED WILL BE WITH FIRE WEATHER.

&&

.AVIATION.../12Z ISSUANCE/

VFR CONDITIONS TO PREVAIL THROUGH THE PERIOD. ISOLATED SHOWERS AND
STORMS WILL FORM OVER WESTERN WY AFTER 20Z AND SPREAD TO THE EAST
THE REST OF THE AFTERNOON INTO THE EVENING HOURS. MOST OF THE
ACTIVITY WILL END BY 03Z. OUTFLOW WIND GUSTS TO 35 KNOTS WILL OCCUR
WITH SOME OF THE STORMS AND SHOWERS.

&&

.FIRE WEATHER...

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

TODAY...WARM/HOT TEMPERATURES WITH MINIMUM RH VALUES DROPPING INTO
THE LOWER TO MID TEENS AND ISOLATED...MAINLY MOUNTAIN...STORMS. WITH
LITTLE LARGE SCALE FORCING...WINDS WILL REMAIN RELATIVELY LIGHT AND
TERRAIN DRIVEN FOR NEARLY ALL ZONES EXCEPT THE FAR SOUTHWEST WHERE
STORMS OVER THE UINTAS WILL PRODUCE SOME GUSTY OUTFLOW WINDS LATER
THIS AFTERNOON. OTHERWISE...DEEP DRY INSTABILITY ACROSS THE CENTRAL
AND NORTHERN FORECAST AREA TODAY...WILL PUSH/KEEP HAINES VALUES
BETWEEN 5 AND 6. ADDITIONALLY...THERE WILL BE LOW CHANCES FOR A FEW
ISOLATED DRY TYPE THUNDERSTORMS WEST OF THE DIVIDE....WITH A COUPLE
OF STORMS ALSO DEVELOPING OVER A WEAK SURFACE BOUNDARY EXTENDING
FROM SWEETWATER COUNTY INTO PORTIONS OF THE CENTRAL BASIN. THEN...A
RATHER STRONG UPPER LEVEL DISTURBANCE AND DEVELOPING LEE SIDE LOW
PRESSURE SYSTEM...WITH INCREASING MOISTURE...WILL RAISE
PRECIPITATION CHANCES ACROSS THE FORECAST AREA 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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