Area Forecast Discussion
Issued by NWS Riverton, WY

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FXUS65 KRIW 280839
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
239 AM MDT SUN SEP 28 2014

.SHORT TERM...TODAY THROUGH TUESDAY NIGHT

A LARGE CUTOFF LOW WILL CONTINUE TO TREK NORTHEAST TODAY AS ITS
PARENT TROUGH TAKES ON EVEN MORE OF A NEGATIVE TILT. AS IT
NEARS...RAINFALL WILL CONTINUE AND WILL BECOME MORE WIDESPREAD AS
STRONG UPPER LEVEL DIVERGENCE ASSOCIATED WITH A 100KT JET STREAK
ROUNDING THE BASE OF THE TROUGH APPROACHES. PRECIPITABLE WATER
VALUES WILL AGAIN RANGE FROM 0.75 INCHES TO A LITTLE OVER AN INCH
TODAY...NEARING 200 PERCENT OF NORMAL FOR THIS TIME OF YEAR...THIS
MOIST AIR MOVING BENEATH THE GENEROUS UPPER LEVEL FORCING WILL YIELD
PERIODS OF LIGHT TO MODERATE RAINFALL FOR MOST OF THE AREA...A MORE
ORGANIZED SWATH OF RAIN WILL MOVE ACROSS THE AREA FROM SOUTHWEST TO
NORTHEAST TODAY AS A PRONOUNCED AREA OF UL DIVERGENCE AND QG FORCING
ASSOCIATED WITH A VORT LOBE ROTATING AROUND THE BASE OF THE TROUGH
MOVES THROUGH...ARRIVING FIRST IN SW WYOMING BEFORE NOON...MOVING TO
CENTRAL BASINS INCLUDING RIVERTON AND CASPER AROUND SUNSET AND THEN
CONTINUING NORTH AND EAST TO JOHNSON COUNTY AND THE BIGHORNS BY
MIDNIGHT. EXPECT AROUND ONE HALF INCH OF RAIN WITH THIS WAVE WITH
SOME MORE FAVORABLE OROGRAPHICALLY ENHANCED SPOTS ACCUMULATING OVER
AN INCH.

GIVEN THE ABUNDANT CLOUD COVER...IT WILL BE DIFFICULT TO REALIZE
MUCH SURFACE BASED INSTABILITY...BUT THERE COULD BE SOME BREAKS
LATER ON ACROSS THE SOUTH AND WEST AS THE DRY SLOT BEGINS TO POKE
IN...AND GIVEN THE AMOUNT OF SHEAR PRESENT AROUND THE LOW...SOME
STORMS IN THIS AREA COULD BE STRONG IF THEY CAN DEVELOP.
ELSEWHERE...WITH ANY SYSTEM THIS DYNAMIC...THERE COULD BE SOME
EMBEDDED THUNDER IN ANY OF THE SHOWERS TODAY BUT THE LIKELIHOOD OF
ANYTHING STRONG IS MINUSCULE.

WHILE THIS IS A COOL SYSTEM IT IS NOT COLD ENOUGH TO BRING SNOW LEVELS
DOWN TO AN IMPACTFUL LEVEL...AT THIS TIME IT LOOKS AS THOUGH
ACCUMULATING SNOW WILL REMAIN ABOVE 9500 FEET IN MOST AREAS...HIGHER
IN THE BIGHORNS.

ON MONDAY THE UPPER LOW WILL BE RIGHT ON TOP OF US...WORKING ON ITS
TRANSITION TO AN OPEN WAVE WITH A DISTINCTLY NEGATIVE TILT. THERE
WILL BE MORE INSTABILITY ON MONDAY AND THERE COULD STILL BE SOME
DECENT RAINFALL AMOUNTS...ESPECIALLY EAST OF THE DIVIDE AND ALONG
THE I25 CORRIDOR. COOLER AIR WILL ALSO BE MOVING INTO THE
WEST...DROPPING SNOW LEVELS TO ABOUT 8500FT LATE MONDAY NIGHT BUT
THE MORE ABUNDANT MOISTURE WILL BE MOVING EAST OF THE AREA BY THIS
TIME SO ANY SNOW FALL WOULD NOT ACCUMULATE MUCH.

ON TUESDAY...MOST OF THE ACTIVITY WILL BE EAST OF THE AREA AS WE
TRANSITION TO MOIST WESTERLY FLOW. OROGRAPHIC LIFT ACROSS THE WEST
AND A DEFORMATION AXIS POKING INTO OUR EASTERN ZONES COULD KEEP SOME
MORE PERSISTENT RAIN GOING IN THESE AREAS...WITH SNOW ABOVE 8500FT IN
THE WESTERN MOUNTAINS AND YELLOWSTONE. IT WILL BE VERY WEAKLY
UNSTABLE ELSEWHERE...AND THAT COMBINED WITH DECENT RESIDUAL MOISTURE
COULD ALLOW FOR SOME ISOLATED AFTERNOON SHOWERS IN BETWEEN.
OTHERWISE...WESTERLY WINDS WILL PICK UP AS THE LOW EXITS TO THE EAST
WITH 30 TO 45 MPH WINDS POSSIBLE ACROSS THE SOUTH AND 15 TO 25 MPH
WEST TO NORTHWEST WINDS EXPECTED ELSEWHERE. IT WILL DEFINITELY STILL
BE A COOL...MOIST AND RAW DAY FOR MUCH OF THE AREA. AS TEMPERATURES
DIP OVERNIGHT AND WINDS CALM THERE COULD BE MORE FROSTY AREAS LATE
TUESDAY NIGHT ACROSS THE WEST.

.LONG TERM...WEDNESDAY THROUGH SUNDAY

WEDNESDAY CONTINUES TO LOOK UNSETTLED AS A WEATHER SYSTEM MOVES
THROUGH THE ROCKIES. SCATTERED SHOWERS WITH ISOLATED AFTERNOON
THUNDERSTORMS LOOKS GOOD FOR NOW. SNOW SHOWERS FOR THE HIGHER
MOUNTAINS. SHOWERS LINGER INTO WED NIGHT AND END AFTER MIDNIGHT. THE
GFS MODEL BRINGS ANOTHER FAST MOVING WEATHER SYSTEM THROUGH MONTANA
THURSDAY AND CUTS IT ACROSS NORTHEAST WY THURS EVE AND NIGHT. THE
00Z ECMWF MODEL HAS BACKED AWAY FROM THIS AND KEEPS THE SYSTEM TO
OUR NORTHEAST. ALWAYS HAVE TO WATCH NW FLOW AND FOR NOW WILL NOT
TOTALLY DISCOUNT THIS WEATHER SYSTEM. WILL HAVE LOW CHANCE OF PRECIP
EAST OF THE DIVIDE AND KEEP WEST OF THE DIVIDE MOSTLY DRY. THE
SYSTEM DEPARTS AFTER MIDNIGHT THURS NIGHT. FRIDAY THROUGH SUNDAY
LOOK TO BE DRY AT THIS TIME IN A NW FLOW ALOFT WITH THE JET STREAM
TO OUR NORTH KEEPING WEATHER SYSTEMS AWAY. MAY SEE HIGH CLOUDS FROM
TIME TO TIME. AS FOR TEMPS...COOL FALL HIGHS WED AND THURS...THEN A
GRADUAL WARMING FRI THROUGH SUNDAY.

&

.AVIATION.../12Z ISSUANCE/

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

WIDESPREAD MVFR/SOME IFR CEILINGS WILL OCCUR ACROSS MUCH OF THE AREA
THROUGH AT LEAST 18Z...WITH WIDELY SCATTERED SHOWERS. IN THE
AFTERNOON...CEILINGS WILL TRY TO LIFT TO LOW VFR...BUT A LARGE AREA
OF CONVECTION ACROSS SOUTHWEST WYOMING IN THE MORNING WILL BEGIN TO
OVERSPREAD AREAS EAST OF THE DIVIDE IN THE AFTERNOON. THIS LARGE
AREA OF CONVECTION SHOULD GENERALLY IMPACT CENTRAL WYOMING IN THE
MID AFTERNOON AND SPREAD INTO THE NORTHEAST/NORTH WYOMING IN THE
EARLY TO MID EVENING. AREAS TO THE NORTH AND EAST OF THIS CONVECTION
IN THE AFTERNOON COULD SEE SOME LOW VFR CEILINGS DEVELOP...BEFORE
RETURNING TO MVFR/IFR CONDITIONS WITH THE SHRA/VCTS. THIS ACTIVITY
SHOULD LAST BETWEEN 2 AND 4 HOURS...BEFORE LIFTING AWAY. EVEN AFTER
THIS ACTIVITY DEPARTS...BOUNDARY LAYER MOISTURE WILL REMAIN
HIGH...KEEPING GOOD CHANCES OF LOW CEILINGS AND POSSIBLY SOME FOG IN
THE MIX THROUGH 12Z MONDAY. MOUNTAINS WILL BE OBSCURED THROUGH THE
PERIOD...AND ICING WILL BE A MAJOR CONCERN GOING OVER HIGH MOUNTAIN
PASSES.


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

NUMEROUS SHRA AND POSSIBLY ISOLATED TSRA ONGOING ACROSS MUCH OF THE
AREA EARLY THIS MORNING...WILL CONTINUE THROUGH THE MORNING HOURS.
THE MOST ACTIVE AREA THIS MORNING WILL BE SOUTH OF A KJAC-KPNA-30NE
KRKS WHERE PERIODS OF MVFR/IFR CONDITIONS WILL OCCUR. NOT MUCH
CHANGE THROUGH THE DAY UNTIL AFTER 21Z WHEN THE LARGE AREA OF
CONVECTION BEGINS TO LIFT NORTH AND NORTHEAST AWAY FROM THE AREA.
ALTHOUGH THE PERSISTENT SHRA SHOULD GRADUALLY COME TO AN END LATE IN
THE AFTERNOON/EARLY EVENING...ISOLATED TO SCATTERED CONVECTION WITH
THE UPPER LOW SHOULD PERSIST FOR MUCH OF TONIGHT. MOUNTAINS WILL BE
OBSCURED THROUGH THE PERIOD...AND ICING WILL BE A MAJOR CONCERN
GOING OVER HIGH MOUNTAIN PASSES.

&

.FIRE WEATHER...

WETTING RAIN LIKELY ACROSS WESTERN AND CENTRAL WYOMING TODAY AND
AGAIN ON MONDAY. SHOWERS AND STORMS BECOME MORE ISOLATED TO
SCATTERED ON TUESDAY BUT IT WILL REMAIN COOL AND CLOUDY. ANOTHER
REENFORCING SHOT OF COLD AIR POSSIBLE ON WEDNESDAY WITH MORE WETTING
RAIN POSSIBLE.

&&

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

&&

$$

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






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