000
FXUS65 KRIW 212129
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
329 PM MDT TUE MAY 21 2013

.SHORT TERM...TONIGHT THROUGH WEDNESDAY

TODAY...CAUGHT BETWEEN TWO UPPER LOWS TODAY WITH MODERATE RIDGING
BEGINNING TO SLOWLY INCH ITS WAY INTO THE STATE FROM LEFT TO RIGHT.
WARMER TODAY WITH HIGH TEMPS RISING SOME 6 TO 8 DEG OVER YESTERDAY
GIVING US WIDESPREAD 60S ACROSS THE CWA...WITH NEAR 70S BIG HORN
BASIN...AND 50S MOUNTAINS. OVERALL...LOW INSTABILITY TODAY...BUT A
FEW SHOWERS OR ISOLATED THUNDER WILL BE POSSIBLE OVER THE HIGHER
MOUNTAINOUS TERRAIN...PARTICULARLY THE WIND RIVER AND ABSOROKA
MOUNTAINS PLUS NEARBY ADJACENT AREAS.

TONIGHT AND WEDNESDAY...OVERNIGHT LOWS FALLING INTO THE 40S MOST
AREAS WITH 30S HIGH MOUNTAINS. PERHAPS SOME LOCALIZED LIGHT PATCHY
FOG IN THE SOAP HILL BASIN AREA...AS SOME LIGHT E TO SE FLOW WORKS
ITS WAY IN OVERNIGHT AND INTO WED MORNING. DURING THE DAY
TOMORROW...WE RETURN TO SOME STORMINESS ONCE AGAIN AS SFC LOW
PRESSURE DEVELOPS OVER SW WY WITH LL FLOW TURNING SOUTH TO SOUTHEAST
ACROSS MUCH OF CWA AND LOW TO MID LVL MOISTURE
INCREASES...PARTICULARLY IN THE NWRN PORTION THE CWA. WARMER DAYTIME
TEMPS...INCREASED MOISTURE AND SOME GOOD UPPER LVL DIVERGENCE WITH
ASSOCIATED SHEAR VALUES IN THIS AREA INCREASING THROUGH THE
AFTERNOON...GIVING EFFECTIVE SHEAR OF 35 TO 40 KTS. THIS WILL ALL
COME TOGETHER TO YIELD TERRAIN BASED CAPE FROM 400 TO 800 J/KG...AND
ML CAPE FROM 300 TO 700 OVER THE HIGHER TERRAIN. FORECAST CLOCKWISE
TURNING HODOGRAPHS FROM OVER THE NORTHERN HALF OF THE CWA GIVE STORM
MOTION FROM THE SSW TO SW WITH THE POSSIBILITY OF THE STRONGER
STORMS BEING RIGHT TURNERS. STRONGEST CONVECTION SHOULD OCCUR
NORTHWEST OF RIVERTON TOWARDS YELLOWSTONE.

.LONG TERM...WEDNESDAY NIGHT THROUGH FRIDAY

A COUPLE FEATURES WILL KEEP THE THREAT OF THUNDERSTORMS INTO THE
EVENING WEDNESDAY. THE MOIST E TO SE FLOW ACROSS NRN
FREMONT...NATRONA AND NWD WILL AID IN THUNDERSTORMS ALONG AND IN THE
MOUNTAINS AND ALSO ALONG THE DRIER PUNCH OF AIR MOVING NWD OUT OF
THE RED DESERT. GENERAL NEWD STEERING FLOW WILL LIKELY ALLOW SOME OF
THESE TO MOVE OFF THE MOUNTAINS INTO THE EVENING...ESPECIALLY IN THE
BIG HORN BASIN AND ALONG THE CONVERGENCE BOUNDARY BETWEEN THE DRY
AND MOIST AIR OVER NERN FREMONT AND NATRONA. ALL THESE AREAS WILL
HAVE A CHANCE OF THUNDERSTORMS THROUGH WEDNESDAY EVENING. OUT WEST...
THE SHORTWAVE MOVING UP FROM THE SW MAY PROVIDE ENOUGH FORCING TO
KEEP SHOWERS AND A FEW THUNDERSTORMS GOING WELL INTO THE NIGHT. A
FEW OF THESE ARE EVEN POSSIBLE IN THE CENTRAL AREAS. ON THURSDAY...A
DRIER PUSH WILL ADVANCE NWD INTO THE BIG HORN BASIN AND AT LEAST WRN
NATRONA COUNTIES WHILE A MOIST SELY FLOW CONTINUES IN ERN NATRONA
AND MOST OF JOHNSON COUNTY AND ACROSS THE NRN BIG HORN BASIN. OUT
WEST...THE NEXT SHORTWAVE AND ASSOCIATED COLD FRONT WILL MOVE INTO
THE WEST. THE AIRMASS IS NOT AS UNSTABLE AS WEDNESDAY BUT STILL
ENOUGH TO FIRE SHOWERS AND SOME THUNDERSTORMS OVER THE
NW...ESPECIALLY DURING THE AFTERNOON AND EVENING. A DRIER SW FLOW IS
STILL EXPECTED TO OVERTAKE THE AREA ON FRIDAY WITH LITTLE OR NO
CONVECTION EXPECTED. LIMITED THE ISOLATED STORMS TO THE NW THIRD.
WARMER THURSDAY AND THEN A LITTLE COOLER MOST AREAS FRIDAY BEHIND
THE WEAK COLD FRONT...ESPECIALLY WEST HALF.

FRIDAY NIGHT THROUGH TUESDAY

LARGE UPPER LOW BETWEEN THE PACNW AND NRN GREAT BASIN WILL BE THE
MAIN WEATHER FEATURE THROUGH THE MEDIUM RANGE PERIOD. THE WEEKEND
WILL GENERALLY BE WARM AND DRY EXCEPT FOR POTENTIAL STORMS IN THE
NRN MTNS...FAR NORTH AND FAR EAST WHERE THE DRIER SW WINDS MEET THE
MORE MOIST SE FLOW. ON MEMORIAL DAY...A STRONGER SHORTWAVE IN THE SW
FLOW MAY PROVIDE AN INCREASED CHANCE OF THUNDERSTORMS. ON
TUESDAY...ANOTHER SHORTWAVE IN THE SW FLOW WILL LIKELY KEEP AN
INCREASED CHANCE OF CONVECTION WITH THE PEAK IN THE AFTERNOON AND
EARLY EVENING BUT THERE WILL LIKELY BE SOME SHOWERS AND POTENTIALLY
ISOLATED THUNDERSTORMS MONDAY NIGHT.


&&

.AVIATION.../00Z ISSUANCE/

VFR CONDITIONS WILL PREVAIL ACROSS THE
FORECAST AREA TONIGHT. ONE AREA TO WATCH COULD BE IN THE SOAP HOLE
BASIN AFTER 09Z...INCLUDING EITHER KBPI OR KPNA...AS THERE ARE
INDICATIONS THAT PATCHY FOG COULD FOR NEAR SUNRISE.
OTHERWISE...ISOLATED SHOWERS OR THUNDERSTORMS ARE POSSIBLE THIS
AFTERNOON FROM THE SOUTHERN BIG HORNS TO NEAR CASPER MOUNTAIN AND
ALSO ALONG AND NEAR THE GREEN...SALT...WIND RIVER AND ABSOROKA
MOUNTAINS TIL AROUND 03Z. LOCAL MVFR CONDITIONS WILL BE POSSIBLE
WITH THESE STORMS ALONG WITH GUSTY WINDS TO 25 KNOTS.

&&

.FIRE WEATHER...

UPPER LEVEL RIDGING HAS DRIED OUT AND WARMED UP THE FORECAST AREA
TODAY. THERE COULD STILL BE AN ISOLATED SHOWERS OR A THUNDERSTORM
OVER THE WESTERN HIGHER MOUNTAINS. THIS CONDITION WILL BE SHORT
LIVED HOWEVER AS ANOTHER STORM SYSTEM BRINGS A BETTER CHANCE FOR
THUNDERSTORMS IN THE NORTHWEST FIRE ZONES ON WEDNESDAY. THESE STORMS
WILL MOVE FROM SOUTH TO NORTH AND BE CAPABLE OF VERY GUSTY AND
ERRATIC WINDS. AMBIENT WINDS WILL LARGELY BE FROM THE SOUTH TO
SOUTHEAST ACROSS THE AREA.

&&

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

$$

SHORT TERM...BRAUN
LONG TERM...SKRBAC
AVIATION...BRAUN
FIRE WEATHER...BRAUN