Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 240438
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
1038 PM MDT THU APR 23 2015

.SHORT TERM...TONIGHT THROUGH FRIDAY

REMNANT CIRCULATION THAT WAS ON TOP OF OUR DEFORMATION AXIS TO OUR
SOUTH YESTERDAY IS NOW OVER SWRN WYO WITH CONVECTION FORMING AROUND
IT AS IT MOVES NE. IT LOOKS LIKE IT WILL MOVE SLOWLY NE AND END UP
IN NRN NATRONA OR SRN JOHNSON COUNTY EARLY THIS EVENING AND HANG OUT
THE REMAINDER OF THE NIGHT IN THIS GENERAL AREA. SURFACE LOW
STRENGTHENING OVER NRN NATRONA COUNTY WILL AIDING IN FAVORABLE
UPSLOPE FLOW ALONG THE SRN BIG HORNS WHERE A TSTM DEVELOPED BEFORE
NOON. THIS AREA WILL BE FAVORABLE DUE TO THE AVAILABLE CAPES OF
400-600 J/KG AND CONVERGENCE/UPSLOPE SUPPORT ALONG THE BIGHORNS.
ANOTHER AREA OF BUILDING CAPE IS ALONG THE CODY FOOTHILLS WHERE
MORNING SOUNDING ANALYSIS PRODUCED UP TO 600 J/KG WITH TEMPS IN THE
UPPER 50S AND DP IN THE MID TO UPPER 30S. OTHERWISE...GENERAL
CONVECTION WITH ENOUGH T/DP SPREAD FOR GUSTY WIND. SMALL HAIL IN THE
FAVORABLE AREAS ARE CERTAINLY POSSIBLE. EXCEPT FOR THE NERN
ZONES...SHOWERS ALL BUT END OR RETREAT TO THE PICKS BY LATER THIS
EVENING. NEXT TROUGH CURRENTLY MOVING INTO THE PACNW WILL MOVE
TOWARDS THE WEST/SW LATE FRIDAY AFTERNOON WITH AN INCREASING CHANCE
OF SHOWERS AND ISOLD TSTMS. UPPER CIRCULATION/CONVERGENCE ZONE MAY
LINGER CLOSE ENOUGH TO THE ERN BORDER AREAS TO KEEP SOME SHOWERS
AROUND THERE INTO THE MORNING HOURS BEFORE SOME AFTERNOON CONVECTION
FORMS. CENTRAL PARTS LOOK DRY UNTIL A FEW SHOWERS/ISOLD TSTMS TRY TO
MOVE OFF THE MTNS AFTER 21Z.

.LONG TERM...FRIDAY NIGHT THROUGH THURSDAY

BEGINNING OF FCST...FLAT RIDGING EAST W/ TROF APPROACHING AND
KNOCKING ON THE WRN FA`S DOORSTEP. BETTER MOISTURE AND INSTABILITY
W/ FALLING TEMPS ALOFT MAKES IT BACK INTO SWRN/WRN FA WITH INCREASED
TERRAIN FORCED PRECIP COUPLED WITH CONVECTIVE SHOWERS OVR THE WRN
MOUNTAINS AND ALONG/NEAR STATIONARY BOUNDARY OVR SWRN/SCNTRL WY. THE
CONVECTIVE NATURE OF THIS PRECIPITATION WILL ALLOW FOR LOCALIZED
AREAS...ESPECIALLY OVR THE HIGHER MOUNTAINS...TO RECEIVE A FEW
TENTHS OF AN INCH LIQUID...EQUATING TO 1 TO 3 INCHES OF SNOW (HIGH
MOUNTAINS/OVERNIGHT) IN THE SALTS/WYOMING/TETON AND WRN WIND RIVER
RANGES.

SAT THRU SUNDAY NIGHT...DIFFERENCES AGAIN SHOWING WITH THE APPROACH
OF THE UPSTREAM TROF...STILL ASSOCIATED WITH GULF OF ALASKA LOW
CURRENTLY MOVING DOWN THE COAST...AS IT IS FCST TO PLUNGE MUCH
FURTHER S BECOMING NEGATIVELY TILTED BEFORE CLOSING OFF AND SLOWING
DOWN OVR THE FOUR CORNERS. THIS AGAIN WILL KEEP THE JET WEST AND
FORCED WELL SOUTH UNDER THE DEVELOPING LOW...BUT WITH MORE MOISTURE
AND INSTABILITY ROTATING IN TO WY FROM THE SE/E AND STICKING AROUND
LONGER. PRECIP CHCS AGAIN HAVE BEEN RAISED AND EXTEND A BIT BEFORE
THE TROF AXIS FINALLY BURSTS THRU THE CWA MONDAY
AFTERNOON/EVE...SCOURING THINGS (MOSTLY) OUT BY MON NIGHT. WHILE H7
TEMPS RANGE FROM -2 TO -6 FROM SE TO NW...SNOW ELEVATIONS WILL FALL
FROM AROUND 9.5K FT SAT NIGHT TO START WITH...TO AROUND 7K FT
SUNDAY...THEN MIXING WITH RAIN AND EVENTUALLY TURNING TO ALL SNOW
AT/NEAR BASIN/VALLEY FLOOR LATER SUN AFTERNOON ACROSS THE
NORTH...TRACKING SWD OVERNIGHT REACHING POSSIBLY INTO SWEETWATER
COUNTY AND THE I80 CORRIDOR THRU MON MORNING. WHILE QPF VALUES LOOK
DECENT ENOUGH...HEAVIER SNOWFALL SHOULD BE CONFINED TO THE MOSTLY TO
THE HIGHER MOUNTAINS OF CENTRAL AND NORTHERN WY WHERE THE MOISTURE
WILL BE MAXIMIZED AND PROLONGED. HIGHER MOUNTAIN TWO DAY SNOWFALL
FROM SATURDAY THRU SUNDAY COULD RANGE FROM 4 TO 6 INCHES EACH
DAY...LOCALLY/ISOLATED TO 8 INCHES EACH DAY OVR THE HIGHEST
PEAKS/RIDGES OF CENTRAL AND NORTHERN WY. GRANITE PASS AND TOGWOTEE
PASS LOOK TO BE THE MOST IMPACTED ATTM. ADDITIONALLY...AS AN
ASSOCIATED COLD FRONT MOVES S AND THRU THE FA SUNDAY...GUSTY NORTH
WIND MAY COMBINE WITH WET SLUSHY SNOW TO CAUSE SOME PERIODS OF
HAZARDOUS TRAVEL EAST OF THE DIVIDE AT LOWER ELEVATIONS...REGARDLESS
OF THE AMOUNT OF SNOW THAT STICKS TO THE ROADWAY. WILL MONITOR FOR
CHANGES IN ORIENTATION AND QPF WITH THIS SYSTEM.

FOR MONDAY NIGHT THRU THURSDAY...POST UPR TROF/SFC FRONT WITH
RIDGING RETURNING FROM THE WEST WILL KEEP THE FA MOSTLY DRY THROUGH
AT LEAST MID-DAY THURSDAY AS THE MOISTURE/STORM TRACK WILL PUSH WELL
NORTH. LR MDLS DIVERGE WIDELY BY THE END OF THE FCST...WITH THE GFS
KEEPING A GOOD RIDGE OVR THE FA WHILE THE EURO FLATTENS AND ALLOWS A
DISTURBANCE TO ROLL THROUGH. AS A DEFINITE SOLUTION IS FAR FROM
BEING DETERMINED...AM LEANING TOWARD THE GFS SOLUTION W/ WARM DRY
RIDGE PERSISTING LONGER BEFORE BREAKING DOWN BY THE END OF THE WEEK.
HAVE THEREFORE TAKEN A 70/30 GFS/EURO APPROACH FOR LATE THURSDAY.

&&

.AVIATION.../06Z ISSUANCE/

EAST OF THE DIVIDE...KCOD/KCPR/KLND/KRIW/KWRL ROUTES

A WEAK SURFACE BOUNDARY OVER EASTERN WYOMING...AN UPPER LEVEL
DISTURBANCE SKIRTING THE AREA...COMBINED WITH DAYTIME HEATING AND A
LITTLE INSTABILITY WILL BE ENOUGH FOR ISOLATED TO LOCALLY SCATTERED
MAINLY AFTERNOON/EVENING SHOWERS AND THUNDERSTORMS FRIDAY. THE BEST
CHANCE FOR CONVECTION APPEARS TO BE ACROSS JOHNSON COUNTY...AS WELL
AS OVER THE MOUNTAINS AND ADJACENT FOOTHILLS. WILL ONLY MENTION VCSH
AT TERMINALS NEAR MTS...KEEPING INTERIOR SITES DRY. FUTURE UPDATES
CAN EXPAND SHRA AND EVEN MENTION TS AT AREA TERMINALS WHEN THE TIME
DRAWS CLOSER. SURFACE WINDS FRIDAY WILL BE MAINLY LIGHT OUTSIDE OF
CONVECTION...BUT WILL BE GUSTY AND ERRATIC NEAR ANY CONVECTION.
THROUGH 03Z...VFR CONDITIONS ARE EXPECTED...EXCEPT BRIEF MVFR/IFR
CONDITIONS DUE TO CONVECTION. THERE COULD BE ICING ISSUES GOING OVER
MOUNTAIN PASSES...ESPECIALLY ROUTES TRAVELING OVER THE DIVIDE WHERE
PRECIPITATION WILL BE MORE ABUNDANT.

PLEASE SEE THE AVIATION WEATHER CENTER FOR THE LATEST INFORMATION ON
ICING FORECASTS.

DURING LATE FRIDAY NIGHT...A WEAK COLD FRONT IS EXPECTED TO PUSH
SOUTH EAST OF THE DIVIDE WHICH COULD RESULT IN AREAS OF MVFR
CEILINGS DEVELOPING IN WAKE OF THE FRONT WITH A CONTINUED CHANCE OF
SHOWER ACTIVITY ESPECIALLY OVER THE NORTH.

WEST OF THE DIVIDE...KBPI/KJAC/KPNA/KRKS ROUTES

SOME FOG COULD DEVELOP LATER TONIGHT AND LAST INTO FRIDAY MORNING
OVER THE WESTERN VALLEYS ESPECIALLY WHERE SOME RAINFALL OCCURRED
THURSDAY AFTERNOON. OTHERWISE...A DISTURBANCE PUSHING NORTHEAST
ACROSS IDAHO INTO SW MONTANA WILL RESULT IN SCATTERED SHOWERS AND
ISOLATED THUNDERSTORMS FRIDAY AFTERNOON AND EVENING. A FEW SHOWERS
COULD BEGIN TO BREAK OUT LATER THIS MORNING...AND SOME WILL LAST
WELL PAST MIDNIGHT FRIDAY NIGHT. BREEZY WESTERLY WINDS WILL OCCUR
OVER SWEETWATER COUNTY FRIDAY AFTERNOON...WHILE THE REST OF THE AREA
SHOULD SEE MAINLY LIGHT WINDS. HOWEVER...WINDS ARE EXPECTED TO
BECOME INCREASINGLY DOMINATED BY GUSTY AND ERRATIC CONVECTIVE
OUTFLOW WINDS AS THE AFTERNOON PROGRESSES. WEAKENING WIND FIELDS
AFTER SUNSET. DESPITE THE WIDESPREAD CONVECTION...ONLY BRIEF
MVFR/IFR CONDITIONS ARE EXPECTED ESPECIALLY IN THE LOWER ELEVATIONS
AND WILL NOT PREVAIL MVFR/IFR CONDITIONS AT THIS TIME. MOUNTAINS
WILL BE OBSCURED AT TIMES...WITH SNOW LEVELS ABOVE 8500+ FEET. THIS
RAIN/SNOW COULD CAUSE ICING ISSUES GOING OVER MOUNTAIN PASSES.

PLEASE SEE THE AVIATION WEATHER CENTER FOR THE LATEST INFORMATION ON
ICING FORECASTS.

&&

.FIRE WEATHER...

AN UPPER LEVEL DISTURBANCE WILL CONTINUE MOVING NE ACROSS THE AREA
TODAY WHILE ENOUGH MOISTURE EXISTS TO PRODUCE SCATTERED SHOWERS AND
ISOLATED THUNDERSTORMS THROUGH THE EVENING. EXPECT ANOTHER CHANCE
FOR FRIDAY...ESPECIALLY WEST OF THE DIVIDE. A SLOW MOVING LOW
PRESSURE AREA COULD BRING A WETTING RAIN FOR SOME AREAS FOR SATURDAY
NIGHT AND SUNDAY BUT PINPOINTING THE AREA IS DIFFICULT AT THIS POINT.

&&

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

&&

$$

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










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