Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 050559
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
1156 PM MDT FRI SEP 4 2015

UPDATE... AVIATION.../06Z ISSUANCE/

ISOLATED SHOWERS AND THUNDERSTORMS ACROSS THE AREA SHOULD DISSIPATE
BY AROUND 08Z.  THE MAIN HAZARD WITH THESE STORMS WILL BE GUSTY
OUTFLOW WINDS OF 30-40KTS.

ON SATURDAY...A STRONG COLD FRONT WILL PUSH INTO WESTERN WYOMING IN
THE AFTERNOON AND EAST OF THE DIVIDE SATURDAY EVENING.  SFC
SOUTHWEST WINDS WILL INCREASE AHEAD OF THE FRONT SATURDAY MORNING.
THE COLD FRONT WILL PUSH SCATTERED SHOWERS AND THUNDERSTORMS VCNTY
AND WEST OF THE CONTINENTAL DIVIDE BTWN 18Z-20Z.  A FEW STORMS WILL
BE STRONG WITH HAIL AND WIND GUSTS OVER 40KTS POSSIBLE. AREAS OF
MVFR...LOCAL IFR CONDITIONS ARE LIKELY TO DEVELOP VCNTY AND NORTH OF
A KJAC-KDUB LINE THROUGH THE AFTERNOON WITH INCREASING MTN
OBSCURATIONS...SNOW LEVELS LOWERING TO NEAR 9KFT BY 00Z SUNDAY.

THE COLD FRONT WILL SPREAD SHOWERS AND THUNDERSTORMS EAST OF THE
CONTINENTAL DIVIDE 20Z SAT-00Z SUNDAY WITH THE STRONGEST STORMS
EXPECTED VCNTY AND NORTH OF A KCOD-KBYG LINE WHERE AN ISOLATED
SEVERE STORM OR TWO MAY DEVELOP WITH LARGE HAIL.  FURTHER
SOUTH...THE MAIN HAZARD WILL BE WIND GUSTS 35-50KTS WITH
THUNDERSTORMS.

RAPID DRYING AND DIMINISHING WINDS WILL FOLLOW THE COLD FRONT ACROSS
WESTERN WYOMING SATURDAY EVENING AND INTO CENTRAL AREAS OVERNIGHT.

&&

.UPDATE...  /ISSUED AT 914 PM MDT FRI SEP 4 2015/

ALLOWED RED FLAG WARNINGS TO EXPIRE THIS EVENING ACROSS CENTRAL AND
SOUTHWEST WYOMING...AND UPGRADED FIRE WEATHER WATCHES ACROSS
PORTIONS OF CENTRAL WYOMING SATURDAY AND SUNDAY TO RED FLAG
WARNINGS.

ONGOING THUNDERSTORM ACTIVITY STILL ON TRACK TO MOSTLY DIMINISH BY
MIDNIGHT.

&&

.PREV DISCUSSION... /ISSUED 324 PM MDT FRI SEP 4 2015/

SHORT TERM...TONIGHT THROUGH SATURDAY

WV IMAGERY SHOWING BROAD RIDGING PATTERN ACROSS MOST OF THE CONUS
EXCEPT FOR THE WCST WHERE A DEEP TROF EXISTS...WITH AXIS CENTERED
FROM WRN WA SWD INTO NRN CA. WY CURRENTLY UNDER INCREASING SW
FLOW...WITH NRN BRANCH OF THE JET JUST TO THE W OF WY WITH AN
EMBEDDED JET STREAK OVR ERN NV AND UT. SRN BRANCH OF JET ASSOCIATED
WITH SUB-TROPICAL MOISTURE BUT N OF TROPICAL STORM KEVIN OFF THE
BAJA COAST...WILL ALSO PUSH ANOTHER DISTURBANCE ACROSS SRN/SERN/ERN
WY AGAIN THIS EVENING...SIMILAR TO THURSDAY. SFC OBS SHOWS LOW P AND
ASSOCIATED BOUNDARY/STATIONARY LOW EXTENDING FROM THE DAKOTAS...W
THRU THE CENTER OF WY...THEN SW THRU UT AND INTO ERN NV. HIGH P
LOCATED UNDER DISTURBANCE OVR CO. CURRENT SHRA/ ISO TSRA CONFINED TO
DISTURBANCE AND MOISTURE OVR WRN CO.

TODAY/TONIGHT...WITH THE STATIONARY FRONT STRETCHED FROM E TO W
ACROSS CNTRL WY AND A RELATIVELY STRONG LOCAL HIGH P MASS ONCE AGAIN
OVR CNTRL CO...PRESSURE DIFFERENCES BETWEEN THE TWO ALONG WITH
INCREASING FLOW ALOFT FROM AS THE TROF SLOWLY APPROACHES...WILL MAKE
FOR A BREEZY TO WINDY DAY ACROSS THE SRN/CNTRL FORECAST ZONES. RED
FLAG WARNING THROUGH THIS EVENING FOR SWRN AND CNTRL WY.
FRONTOGENESIS CURRENTLY TAKING PLACE ACROSS MT...EXPECTING TO JUST
PUSH S OF THE BORDER THIS EVENING. ADDITIONALLY...AN ISOLATED
THUNDERSTORM OR TWO MAY ALSO BE POSSIBLE IN THE AREA LATE IN THE
DAY/EVENING...WITH OCCASIONAL LIGHTNING A POSSIBLITY AS
BOUNDARY/STATIONARY FRONT AND FLEETING UPR DISTURBANCE OUT OF THE
GREAT BASIN COMBINE TO ALLOW CONVECTION TO OCCUR AGAIN THIS EVENING.
ELSEWHERE...ISO SHRA/TSRA WILL AGAIN BE POSSIBLE ACROSS THE
FA...ESPECIALLY OVR THE HIGHER TERRAIN OF THE WRN FA... NRN
ABSAROKAS...AND POSSIBLY BIG HORN MOUNTAINS AS TERRAIN FORCED
ELEVATED CONVERGENCE ALONG WITH THESE DISTURBANCES COMBINE TO ERODE
THE CAP BY EVENING. IN ADDITION TO THE UPR LVL DISTURBANCES...SFC
BOUNDARIES...MESOSCALE/FRONTAL...OR OUTFLOW FROM EARLIER CONVECTION
WILL BE NECESSARY TO TRIGGER STORM ACTIVITY TODAY/TONIGHT.

SATURDAY...EARLY IN THE DAY...SIMILAR ATMOSPHERIC SET-UP TO THAT OF
TODAY WITH LOW P CENTERED ACROSS THE DAKOTAS WITH TRAILING
STATIONARY FRONT AGAIN CUTTING ACROSS CNTRL WY FROM E TO W. FIRE
WEATHER WATCH FROM SAT AFTERNOON THROUGH MONDAY AS EARLY WINTER WX
SYSTEM HEADS THIS WAY. BIGGER CHANGES ON THE WAY AS THE LARGE UPR
TROF FINALLY MAKES A PUSH THIS DIRECTION...SPAWNING CYCLO/FRONTO-
GENESIS FROM THE NRN GREAT BASIN INTO THE NRN ROCKIES THRU THE
DAY. WHAT THIS MEANS FOR FOR CNTRL WY IS THAT THE DEVELOPMENT OF
THE CYCLONE WILL TAKE PLACE ACROSS FROM NRN WY INTO SERN MT...WITH
MOST OF THE FA IN THE WARM SECTOR THRU THE DAY...FRONT NOT COMING
INTO THE WEST UNTIL EVENING. FLOW ALOFT WILL INCREASE AND BECOME
QUITE DIFFLUENT AS TROF DIGS THIS DIRECTION. BY
AFTERNOON...ISOLATED SHOWERS AND STORMS SHOULD BE ONGOING. TERRAIN
FORCED ACROSS THE WEST...WITH SFC TROF OR REMNANTS OF
TODAY`S/TONIGHT`S STATIONARY FRONT...AHEAD OF DEVELOPING MAIN COLD
FRONT ACROSS THE SRN FA...ANOTHER AREA TO WATCH FOR CI EARLY. ONLY
MODEST MOISTURE ASSOCIATED WITH THIS ACTIVITY AT BEST...SO NO
HYDRO CONCERNS OTHER THAN BRIEF HEAVY RAIN. GUSTY OUTFLOW WINDS 35
TO 50 MPH POSSIBLE. BEST AREA FOR SUSTAINED AND SLIGHTLY STRONGER
STORMS WILL BE LOCATED ACROSS THE NRN BIG HORN BASIN...TO THE LEE
OF THE BIG HORN MOUNTAINS ACROSS NRN JOHNSON COUNTY. THIS IS DUE
TO THE POSITION OF THE DEVELOPING SFC LOW AND WARM FRONT...WITH
SHEAR MAXIMIZED OVR THIS REGION. MOSTLY SMALL (NEAR SVR) HAIL AND
STRONG(ER) BORDERLINE SVR WINDS WOULD BE POSSIBLE WITH STRONGER
STORMS IN THIS AREA. BY THE VERY END OF THIS FCST
PERIOD...ACTIVITY RIGHT AT THE COLD FRONT NEAR THE NW BORDER
REGION COULD START TO PRODUCE A FEW LIGHT SNOW SHOWERS...MOSTLY
ABOVE 9K FT.

LONG TERM...SATURDAY NIGHT THROUGH FRIDAY

MODELS ARE NOW DELIVERING MORE OF A GLANCING BLOW FOR NORTHWEST
WYOMING FOR SATURDAY NIGHT AND SUNDAY. THE COLDEST AIR AND THE
HIGHEST AMOUNT OF PRECIPITATION EXPECTED WITH THE FRONT ARE NOT
LINING UP TOO WELL...BUT THE TIMING OF THE FRONT WOULD STILL
BRING IT INTO THE WEST SATURDAY EVENING...AND THROUGH THE REST OF
THE FORECAST AREA BY SUNRISE ON SUNDAY. HOWEVER...THERE IS STILL
A DECENT AMOUNT OF QG FORCING AT THE FRONT END OF THE TROUGH
SATURDAY NIGHT ALONG WITH SOME JET SUPPORT AS THE LEFT EXIT REGION
OF THE JET APPEARS TO AFFECT MUCH OF THE AREA EARLY SUNDAY
MORNING. MOISTURE...OR LACK THEREOF WILL BE THE KEY DECIDER AS TO
WHETHER OR NOT ACCUMULATING SNOWFALL WILL APPEAR IN YELLOWSTONE
AND THE ABSAROKAS...PERHAPS AS FAR SOUTH AS THE JACKSON
VALLEY/MOOSE AREA. GIVEN THE AMOUNT OF UNCERTAINTY THAT STILL
EXISTS...AND SOME LOOSE SUPPORT FOR SOME PRECIPITATION SATURDAY
NIGHT INTO SUNDAY...KEPT A BROAD AREA OF POPS.

AHEAD OF THE FRONT...IT WILL BE QUITE DRY AND WINDY WITH HIGH FIRE
DANGER. IN ADDITION...LAPSE RATES WILL BE ON THE RISE AS THE COLD
POOL ALOFT SETTLES OVER THE REGION LEADING TO A BROAD AREA OF HIGH
HAINES INDEX EAST OF THE DIVIDE FOR SUNDAY.

SUBSIDENCE BEHIND THE EXITING TROUGH WILL ENCOURAGE SOME VERY DRY
AND A BIT BREEZY AIR TO SURFACE FOR MONDAY AND TUESDAY WITH STEEP
LAPSE RATES AND A HIGH HAINES INDEX REMAINING ACROSS MUCH OF THE
AREA...CONTINUING THROUGH TUESDAY AS FLOW TURNS MORE WESTERLY AND
POSSIBLY BRINGS IN SOME VERY LOW HUMIDITIES FOR TUESDAY
AFTERNOON.

WESTERLY FLOW REIGNS THROUGH THURSDAY WHEN HIGH PRESSURE BEGINS TO
BUILD OVER NEVADA...AMPLIFYING THE FLOW A BIT AND SHIFTING US INTO
MORE OF A NORTHWESTERLY FLOW PATTERN FOR FRIDAY.

AVIATION.../00Z ISSUANCE/

VFR CONDITIONS WILL PREVAIL FOR ALL TERMINALS THROUGH THE END OF THE
FORECAST PERIOD.

SOUTHWEST FLOW ALOFT WILL REMAIN OVER THE REGION AHEAD OF A LARGE
UPPER TROUGH OVER THE PAC NW THROUGH THE FORECAST PERIOD. GUSTY
S-SW SFC WINDS 10-25KTS WILL ONCE AGAIN PREVAIL OVER MUCH OF THE
AREA THIS AFTERNOON AND TOMORROW AFTERNOON. THEN...A SUB-
TROPICAL DISTURBANCE WILL LIFT ACROSS THE CENTRAL
ROCKIES...CLIPPING EASTERN WYOMING THIS AFTERNOON AND
EVENING...WITH ISOLATED THUNDERSTORMS MAINLY VCNTY AND EAST
INTERSTATE 25...KCPR- KBYG FROM 00Z-07Z. ADDITIONALLY...ANOTHER
UPPER LEVEL DISTURBANCE WILL RIDE OUT OF THE BASE OF THE
TROUGH...ALLOWING ISOLATED TO WIDELY SCATTERED THUNDERSTORMS TO
SPREAD ACROSS WESTERN WYOMING...VCNTY AND WEST OF THE CONTINENTAL
DIVIDE THRU 06Z WITH A FEW STORMS PUSHING INTO THE CENTRAL
BASINS...VCNTY KLND-KRIW-KCOD-KWRL 01Z- 06Z. THE MAIN HAZARD WITH
THESE STORMS WILL BE ERRATIC OUTFLOW WIND GUSTS 35-50KTS. MOST OF
THE STORM ACTIVITY SHOULD DISSIPATE 06Z-09Z...WITH A FEW SHOWERS
LINGERING OVER THE HIGH WESTERN/NORTHWESTERN MOUNTAINS UNTIL
AROUND SUNRISE. WINDS WILL PICK UP ONCE AGAIN ACROSS THE SOUTHERN
AND CENTRAL FORECAST AREA AFTER 19Z.

FIRE WEATHER...

FIRE DANGER ELEVATED MOST LOCATIONS TODAY AND SATURDAY...EXTREME
ACROSS PORTIONS OF CENTRAL AND SOUTHWESTERN WYOMING...WHERE A RED
FLAG WARNING HAS BEEN POSTED. STRONG SOUTH TO SOUTHWEST WINDS
GUSTING TO 25 TO 35 MPH AHEAD OF A DEVELOPING EARLY WINTER
SYSTEM...AND RH VALUES DECREASING TO 15 TO 20 PERCENT ARE CAUSING
THE PROBLEM.  ADDITIONALLY...AN ISOLATED THUNDERSTORM OR TWO MAY
ALSO BE POSSIBLE ACROSS THE FORECAST AREA LATER THIS AFTERNOON AND
EVENING...WITH OCCASIONAL LIGHTNING A POSSIBLITY. SUNDAY AND
MONDAY...A STRONG COLD FRONT WILL RACE THROUGH THE FORECAST
AREA...WITH COOL...DRY...AND QUITE WINDY AIR BEHIND IT...ENOUGH SO
THAT IT HAS WARRANTED A FIRE WEATHER WATCH THROUGH MONDAY EVENING.

&&

.RIW WATCHES/WARNINGS/ADVISORIES...
RED FLAG WARNING FROM 1 PM SATURDAY TO 9 PM MDT SUNDAY FOR
WYZ280-283-289.

&&

$$

UPDATE...AEM
SHORT TERM...BRAUN
LONG TERM...ALLEN
AVIATION...BRAUN/AEM
FIRE WEATHER...BRAUN


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