Area Forecast Discussion
Issued by NWS Riverton, WY

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000
FXUS65 KRIW 261830
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
1230 PM MDT WED AUG 26 2015

.SHORT TERM...TODAY THROUGH FRIDAY NIGHT

WV IMAGERY SHOWING LARGE BROAD AMPLIFIED RIDGE ACROSS THE WRN CONUS
WITH THE MAJOR AXIS STRETCHING FROM NM UP THROUGH ECNTRL WY AND ERN
MT. MODEST MOISTURE JUST STARTING TO MAKE ITS WAY UP THE WRN BORDER
REGION...WITH BETTER MOISTURE STILL TO THE SOUTH AND OVR THE 4
CORNERS AREA. MINOR SW EMBEDDED IN RIDGE CURRENTLY MOVING THRU
ECNTRL WY THIS MORNING WITH THE ISOLATED SHRA. UPSTREAM OVR ERN NV
INTO CNTRL/NRN UT...THE NEXT SW WAVE IS POISED FOR ARRIVAL LATER IN
THE DAY...WITH ANOTHER EMBEDDED WAVE OVR NERN AZ ALONG WITH SOME OF
THE BETTER MOISTURE ARRIVING THIS EVENING ACROSS THE SRN FA. THE SFC
STILL HAS GENERALLY WEAK HIGH P ACROSS THE REGION...WITH A WEAK SFC
TROF/FRONT OVR ERN WY.

TODAY...RIDGE AXIS SHIFTS EWD TOWARD THE HIGH/CNTRL PLAINS ALLOWING
MID/UPR MONSOONAL MOISTURE TO BEGIN TO RETURN OUT OF THE SWRN CONUS
ALONG WITH THE ARRIVAL OF A STRONGER UPR LVL WAVE/JET STREAK THAT
CURRENTLY RESIDES OVR NERN AZ. AT THE SFC...A LEE SIDE LOW ALONG
WITH THE ABOVE MENTIONED N/S ORIENTED FRONT WILL DEVELOP
FURTHER...AIDING IN ADDITIONAL LL FORCING AND SOME DECENT LL
MOISTURE RETURN TOWARD THE LOW AND INTO THE SRN FA.
OVERALL...MOISTURE FLUX LOOKS DECENT BUT NOT QUITE AS GOOD AS IT HAS
THE PAST FEW DAYS. CONVECTION WILL BE SCATTERED AROUND...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 35 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 MORE OF AN
ISOLATED BASIS. AS MOISTURE FEEDS INTO THE FA FROM THE SOUTH
TODAY...MIN RH VALUES WILL RISE ACCORDINGLY...WITH DRIER LOCATIONS
NOW MORE CONFINED TO THE ERN COUNTIES AND BIG HORN BASIN. BRIEF NEAR
CRITICAL FIRE DANGER WILL BE POSSIBLE...ESPECIALLY OVER JOHNSON
COUNTY BY LATE AFTERNOON. FREQUENCY AND AREAL LIGHTNING COVERAGE
TODAY MAY ALSO POSE A PROBLEM FOR AN HOUR OR TWO ACROSS THE SOUTHERN
ZONES EARLY THIS AFTERNOON.

OVERNIGHT WED THRU THU NIGHT...UPR WAVE AND LEE LOW WILL MOVE EWD
INTO THE PLAINS...EVENTUALLY 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 SCATTERED
AREAS OF 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.

FRI...UPR RIDGE BUILDS BACK OVR THE FA WITH SOME WEAK WARMING
BEGINNING AGAIN. LITTLE IF ANY PRECIP IS EXPECTED ACROSS THE
FA...WITH ONLY AN ISOLATED STORM POSSIBLE ACROSS THE HIGHER PEAKS OF
CNTRL/NRN WY. DRYING...BUT GENERALLY LIGHT WINDS ACROSS MOST OF THE
REGION.

.LONG TERM...SATURDAY THROUGH WEDNESDAY

...ELEVATED TO EXTREME FIRE WEATHER CONDITIONS POSSIBLE AT TIMES
THROUGH THE EXTENDED...

A SHORTWAVE TROUGH WILL EJECT ACROSS THE PACIFIC NORTHWEST INTO
SOUTHERN ALBERTA SATURDAY...AHEAD OF A TROUGH/UPPER LOW DIGGING
OFF THE BRITISH COLUMBIA/PAC NW COAST. A LEE SIDE TROUGH IS
EXPECTED TO DEVELOP SATURDAY IN RESPONSE TO THE FORMER
SYSTEM...WITH THE BREEZY SOUTHWEST SURFACE WINDS EXPECTED
ESPECIALLY IN THE TYPICAL WIND CORRIDORS. IT WILL ALSO BE VERY
WARM WITH 700 MB TEMPS PROGGED TO BE BETWEEN 14C AND 18C. THIS
HEAT COMBINED WITH A BREEZY AND LOW HUMIDITY COULD RESULT IN
EXTREME FIRE WEATHER CONDITIONS. THE COLD FRONT ASSOCIATED WITH
THE INITIAL SYSTEM IS EXPECTED TO BARELY REACH THE FORECAST AREA
SATURDAY NIGHT BEFORE WASHING OUT OR RETREATING BACK NORTH.

MODELS THEN DIVERGE ON THE EVOLUTION OF THE SECOND TROUGH/UPPER
LOW PUSHING INTO THE NORTHWEST. THE 00Z ECMWF IS NOT AS AMPLIFIED
WITH A MUCH WEAKER SYSTEM PUSHING EAST ACROSS WASHINGTON AND THE
EXTREME NORTHERN ROCKIES SUNDAY. ON THE OTHER HAND...THE 00Z GFS
HAS A WELL DEFINED UPPER LOW THAT REMAINS OFF THE PACIFIC
NORTHWEST COAST SUNDAY...BEFORE OPENING UP AND PUSHING EAST INTO
THE PACIFIC NORTHWEST SUNDAY NIGHT AND THEN ACROSS THE NORTHERN
ROCKIES MONDAY. THE ECMWF SOLUTION WOULD HAVE A COLD FRONT
PUSHING ACROSS THE AREA SUNDAY/SUNDAY NIGHT...WHILE THE GFS WOULD
HAVE A STRONGER COLD FRONT SWEEPING ACROSS THE AREA ABOUT 24
HOURS LATER. OVERALL PRECIPITATION CHANCES WILL REMAIN SLIM
REGARDLESS OF SOLUTION...WITH FIRE WEATHER CONCERNS BEING THE MAIN
CONCERN.

ON TUESDAY AND WEDNESDAY...MODELS ARE SHOWING THE AREA IN A
MAINLY DRY WESTERLY FLOW AT THE BASE OF A LOW AMPLITUDE LONGWAVE
TROUGH OVER WESTERN CANADA.

&&

.AVIATION.../18Z ISSUANCE/

WIDESPREAD SHOWERS AND A FEW THUNDERSTORMS WILL CONTINUE ACROSS
THE WEST THROUGH THIS EVENING WITH MORE SCATTERED COVERAGE EAST
OF THE DIVIDE...ESPECIALLY LATER THIS AFTERNOON THROUGH THE NIGHT.
LOCAL MVFR CONDITIONS MAY OCCUR AT TIMES DUE TO LOWERED CIGS OR
VSBYS. WIND GUSTS TO 30 KNOTS MAY OCCUR NEAR STORMS. MOUNTAINS MAY
BE PARTIALLY OBSCURED. A FEW SHOWERS WILL LINGER INTO THE MORNING
THURSDAY BEFORE ANOTHER ROUND OF SHOWERS AND THUNDERSTORMS FIRE
THURSDAY AFTERNOON...ESPECIALLY ALONG AND WEST OF THE DIVIDE.

&&

.FIRE WEATHER...

ELEVATED FIRE DANGER ACROSS THE CENTRAL...NORTHERN AND EASTERN
FORECAST ZONES EARLY TODAY BEFORE DEEPER MOISTURE INVADES THE
REGION FROM THE SOUTH.

TODAY...COOLING HIGH TEMPERATURES...BUT STILL WARM WITH MINIMUM RH
VALUES DROPPING INTO THE MID TEENS TO MID 20S (PERCENT) OVER THE LOW
ELEVATIONS EAST OF THE DIVIDE. SCATTERED SHOWERS AND THUNDERSTORMS
WEST AND SOUTH...ISOLATED STORMS CENTRAL...NORTH AND EAST. DEEP DRY
INSTABILITY WILL BE REPLACED BY A MORE MOIST AND SOMEWHAT MORE
STABLE ATMOSPHERE...WITH HAINES INDEX VALUES OF 5 BEING REPLACED
THROUGH THE DAY BY 3 FROM SOUTHWEST TO NORTHEAST. HOWEVER...BRIEF
NEAR CRITICAL FIRE DANGER WILL BE POSSIBLE...ESPECIALLY OVER JOHNSON
COUNTY BY LATE AFTERNOON. LIGHTNING COVERAGE WILL ALSO HAVE TO BE
MONITORED THIS AFTERNOON FOR A COUPLE OF NEW LIGHTNING STARTS.
THURSDAY...COMBINATION OF LOWER TEMPERATURES AND LINGERING MOISTURE
WILL KEEP FIRE DANGER RELATIVELY LOW BEFORE BEGINNING TO INCREASE
ONCE AGAIN ON FRIDAY.

&&

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

$$

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


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