000
FXUS65 KRIW 171731
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
1130 AM MDT FRI MAY 17 2013

.SHORT TERM...TODAY THROUGH SUNDAY NIGHT

SHOWERS CONTINUE TO MOVE ACROSS PORTIONS OF THE FORECAST AREA THIS
MORNING. LINGERING SHOWER ACTIVITY IS EXPECTED THROUGH THE MORNING
HOURS.

AN APPROACHING TROUGH OF LOW PRESSURE WILL MOVE INTO THE
INTERMOUNTAIN WEST BY THE AFTERNOON HOURS. THIS WILL STRENGTHEN
SOUTHWEST FLOW OVER THE AREA...INCREASE INSTABILITY...AND INTRODUCE
ANOTHER ROUND OF MOISTURE TO THE AREA. THIS COMBINATION WILL RESULT
IN SCATTERED SHOWERS AND THUNDERSTORMS ACROSS MUCH OF WESTERN
WYOMING TODAY. COVERAGE OF THUNDERSTORMS WILL NOT BE AS MUCH AS WHAT
WAS EXPERIENCED ON THURSDAY...BUT MODELS DID PUSH MOISTURE A BIT
FURTHER EAST. MODIFICATIONS WERE MADE TO CENTRAL WYOMING INITIATE
SHOWER ACTIVITY A BIT EARLIER IN THE AFTERNOON THAN WAS PREVIOUSLY
FORECAST. THUNDERSTORMS MAY AGAIN CONTAIN GUSTY WIND...AND SOME
STORMS MAY HAVE PERIODS OF HEAVY RAINFALL. THUNDERSTORM ACTIVITY IS
EXPECTED TO WEAKEN WITH THE LOSS OF DAYLIGHT.

THE AFOREMENTIONED TROUGH WILL CONTINUE ITS EASTWARD PROGRESSION ON
SATURDAY...WITH ANOTHER ROUND OF SHOWERS AND THUNDERSTORMS. COOLER
TEMPERATURES ASSOCIATED WITH THIS TROUGH WILL BEGIN TO IMPACT THE
AREA LATE SATURDAY MORNING...ESPECIALLY ACROSS THE WEST AND SOUTH.
EVEN WITH THESE COOLER TEMPERATURES...SCATTERED SHOWERS AND
THUNDERSTORMS WILL BE FOUND ACROSS MUCH OF THE AREA.

MODELS VARY SLIGHTLY AT THE PROGRESSION OF THE TROUGH ONCE IT MOVES
FURTHER INTO WYOMING ON SUNDAY. THE NAM CREATES A CLOSED LOW OVER
NORTHEASTERN WYOMING BY SUNDAY AFTERNOON...WITH MOST OTHER MODELS
KEEPING THE SYSTEM AS AN OPEN WAVE. REGARDLESS...TEMPERATURES WILL
COOL FURTHER ON SUNDAY AS THE TROUGH AXIS MOVES THROUGH THE STATE.
THUNDERSTORMS WILL AGAIN BE POSSIBLE ON SUNDAY AFTERNOON...THOUGH
PERIODS OF RAINFALL ARE ANTICIPATED THROUGH MUCH OF THE WEEKEND.

.LONG TERM...MONDAY THROUGH FRIDAY

A NEARLY STATIONARY UPPER LOW ACROSS EASTERN SOUTH DAKOTA/NEBRASKA
AND A SHORTWAVE DIVING SOUTHEAST ACROSS UTAH INTO THE FOUR CORNERS
AREA WILL COMBINE FOR A COOL AND INCLEMENT DAY MONDAY INTO MONDAY
NIGHT. THIS WILL BE ESPECIALLY TRUE IN FAVORED UPSLOPE AREAS EAST OF
THE DIVIDE...WITH MODELS SHOWING NORTHERLY FLOW IN THE LOW AND MID
LEVELS OF THE ATMOSPHERE. HAVE COOLED TEMPERATURES DOWN
MONDAY...ESPECIALLY IN FAVORED UPSLOPE REGIONS WHERE PRECIPITATION
COULD OCCUR FOR MUCH OF THE DAY. 700MB TEMPERATURES ARE EXPECTED TO
BE -1C TO -3C EAST...AND -1C TO +1C WEST MONDAY. THESE TEMPERATURES
TRANSLATE ROUGHLY TO SNOW DOWN TO AROUND 7500 FT EAST...AND 8500
FT WEST. FAVORED UPSLOPE AREAS EAST OF THE DIVIDE COULD SEE SNOW
DOWN TO AROUND 6500 FEET. COULD BE CLOSE TO WINTER WEATHER
ADVISORY SNOW AMOUNTS FOR EASTERN WIND RIVER MTS AND/OR CASPER
MOUNTAIN.

GLOBAL MODELS THEN DIVERGE SIGNIFICANTLY ON HOW QUICKLY TO END THE
PRECIPITATION ACROSS THE REGION. THE 00Z GFS SHUTS OF THE PRECIP
EARLY MONDAY NIGHT...WHILE THE ECMWF CONTINUES TO KEEP PRECIPITATION
ONGOING ACROSS THE EXTREME EAST THROUGH THE DAY TUESDAY...WITH THE
GEM IN BETWEEN THE TWO...BUT LEANING MORE TOWARD THE ECMWF. WILL
HAVE SOME POPS TUESDAY...ESPECIALLY EAST TO RESPECT THE SLOWER ECMWF
SOLUTION.

UPSTREAM...A STRONG UPPER LOW WILL BE MOVING INTO THE PACIFIC
NORTHWEST TUESDAY. GLOBAL MODELS SHOW THIS STRONG UPPER LOW
REMAINING NEARLY STATIONARY ACROSS THE PACIFIC NORTHWEST THROUGH AT
THURSDAY. THIS WILL RESULT IN INCREASING SOUTH/SOUTHWESTERLY FLOW
AND WARMER TEMPERATURES WEDNESDAY AND THURSDAY. THE ECMWF AND GFS
SHOW A DISTINCT SHORTWAVE TROUGH PUSHING NORTHWARD ACROSS IDAHO AND
WESTERN MONTANA WEDNESDAY/WEDNESDAY NIGHT. THE ENERGY ASSOCIATED
WITH THIS SYSTEM COULD ALLOW FOR ISOLATED TO SCATTERED CONVECTION
ACROSS MUCH OF THE AREA...BUT ESPECIALLY OVER THE NORTHWEST. A COLD
FRONT ASSOCIATED WITH THIS STRONG SHORTWAVE IS EXPECTED TO PUSH INTO
THE WEST/NORTHWEST LATE WEDNESDAY/WEDNESDAY NIGHT...BUT QUICKLY
RETREAT BACK NORTHWEST DURING THE DAY THURSDAY.

THE MAIN UPPER LOW/TROUGH IS EXPECTED TO REMAIN WEST OF THE FORECAST
AREA THURSDAY AND FRIDAY WITH CONTINUED SOUTHERLY FLOW...AND A CHANCE
OF MAINLY AFTERNOON/EVENING CONVECTION...BUT WILL KEEP CONVECTION
MAINLY CONFINED TO THE MOUNTAINS FOR NOW.

&&

.AVIATION.../18Z ISSUANCE/

SHOWERS CONTINUE ACROSS THE WEST TERMINAL AREA AS ANOTHER WAVE MOVES
OVERHEAD IN SOUTHWEST FLOW...KJAC HAS BEEN IN AND OUT OF MVFR TO IFR
CONDITIONS THROUGH THE MORNING WITH WIDESPREAD MOUNTAIN OBSCURATIONS
ONGOING. SOME CLEARING HAS TAKEN PLACE THIS MORNING AROUND KBPI AND
KPNA AS A RIDGE OF HIGH PRESSURE BATTLES THE TROF TO THE WEST.
TRICKY FORECAST AT KBPI AND KPNA AS THEY WILL LAY JUST ON THE
INFLECTION POINT BETWEEN THE TWO DIFFERING AIR MASSES.
HOWEVER...SHOWERS AND SOME STORMS WILL INCREASE IN COVERAGE AFTER
22Z...SO WILL KEEP VCTS AND VCSH IN FORECAST FOR THOSE TWO
TERMINALS. OTHERWISE...SHOWERS WILL CONTINUE ON AND OCCASIONALLY
OFF THROUGH THE TAF PERIOD AT KJAC AS A COLD FRONT PUSHES IN AND
STALLS IN ADDITION TO AN UPPER LEVEL SHORTWAVE MOVING OVERHEAD FOR
MUCH OF THE DAY.

AT ALL OTHER TERMINALS...SCATTERED CONVECTION IS EXPECTED TO RETURN
FROM NOW THROUGH THE EVENING TODAY EXCEPT KCPR AND KRKS WHERE MOSTLY
DRY SOUTHWEST WINDS COULD GUST AS HIGH AS 35 KTS IN THE AFTERNOON.
AN ISOLATED SHOWER OR TWO CANNOT BE RULED OUT NEAR KRKS IF THEY ARE
INITIATED OFF THE UNITAH MOUNTAINS. HOWEVER...OVERALL CONDITIONS ARE
NOT CONDUCIVE TO MAINTAINING THUNDERSTORMS FROM KRKS TO KCPR. STORMS
TODAY WILL FAVOR THE BIG HORN VALLEY AND ADJACENT MOUNTAINS...AND
THE WIND RIVER MOUNTAINS AND VALLEY SECONDLY. STORMS THAT DO FORM
WILL HAVE VERY GUSTY AND ERRATIC OUTFLOWS...ESPECIALLY FROM 18-00Z.
A COLD FRONT MOVES THROUGH FROM WEST TO EAST THROUGH THE FORECAST
PERIOD.

&&

.FIRE WEATHER...

AN APPROACHING AREA OF LOW PRESSURE WILL MOVE INTO THE INTERMOUNTAIN
WEST TODAY. THIS SYSTEM WILL SPREAD SHOWER AND THUNDERSTORM ACTIVITY
ACROSS THE WESTERN HALF OF THE STATE BY THE AFTERNOON HOURS...WITH
COVERAGE INCREASING TO INCLUDE MUCH OF THE FORECAST AREA BY THIS
EVENING. AS THE AREA OF LOW PRESSURE APPROACHES...EXPECT INCREASES
IN DAYTIME HUMIDITY...AND COOLER TEMPERATURES ACROSS THE AREA.
PERIODS OF GUSTY WIND WILL ALSO BE POSSIBLE...ESPECIALLY THIS
AFTERNOON. ANY THUNDERSTORM ACTIVITY MAY RESULT IN GUSTY AND ERRATIC
WIND...ALONG WITH BRIEF PERIODS OF HEAVY RAIN. THE AREA OF LOW
PRESSURE WILL HAVE A FIRM GRIP ON THE FORECAST AREA BY SUNDAY
AFTERNOON...WITH WIDESPREAD SHOWER ACTIVITY AND CONTINUED COOLING
TEMPERATURES.

&&

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

&&

$$

SHORT TERM...BRANHAM
LONG TERM...ALLEN
AVIATION...BRAUN
FIRE WEATHER...BRANHAM