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000
FXUS65 KRIW 121726
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
1126 AM MDT SUN MAY 12 2013

.SHORT TERM...TODAY THROUGH TUESDAY NIGHT (ISSUED AT 311 AM MDT)

A HIGH PRESSURE RIDGE AXIS EXTENDING NORTH FROM A HIGH PRESSURE
CENTER OVER THE SOUTHWEST UNITED STATES WILL BUILD IN FROM THE WEST
TODAY AND MONDAY. AS A RESULT...A WARMING TREND WILL CONTINUE
THROUGH MONDAY. THE H7 TEMPS WILL CLIMB TO 9C TODAY AND TO 13C
MONDAY. TODAY WE WILL SEE HIGH TEMPERATURES REACH THE LOW TO MID 80S
IN MANY AREAS FOR THE FIRST TIME THIS SEASON IN THE WIND RIVER AND
BIG HORN BASINS. HIGHS WILL BE NEAR 80S AT SOME LOCATIONS AT THE
LOWER ELEVATIONS OF JOHNSON AND NATRONA COUNTIES. THE OTHER LOWER
ELEVATIONS WILL SEE 70S TODAY. BY MONDAY EVEN HOTTER TEMPERATURES
ALONG WITH RECORD BREAKING READINGS CAN BE EXPECTED WITH EVEN SOME
90 DEGREE TEMPERATURES EXPECTED IN PORTIONS OF THE BIG HORN BASIN.
ISOLATED TO WIDELY LATE DAY SCATTERED THUNDERSTORMS ARE EXPECTED
MAINLY IN THE HIGHER ELEVATIONS OF NORTHWEST WYOMING WHERE THE BEST
CAPES ARE PROGGED. THESE STORMS SHOULD BE WEAK SINCE WARMING ALOFT
WILL PRODUCE A CAP. EVEN THOUGH NEGATIVED LIFTED INDICES ARE
EXPECTED IN MUCH OF THE CWA TODAY...LOW LEVEL MOISTURE WILL BE
LACKING SO MAINLY VIRGA SHOWERS ARE EXPECTED ELSEWHERE. ON MONDAY
ONLY ISOLATED STORMS ARE EXPECTED IN THE NORTHWEST MTNS AND BIG HORN
MTNS. A PROMINENT CIRCULATION AT 45N/146W WILL TRACK EAST ACROSS
MONTANA IN THE FORM OF A VIGOROUS VORT MAX ON TUESDAY WITH A
TRAILING SHORTWAVE TROUGH SWEEPING ACROSS WYOMING. THIS FEATURE WILL
DRIVE A DRY SURFACE COLD FRONT SOUTH ACROSS WYOMING WHICH WILL KNOCK
TEMPERATURES DOWN 15 TO 20 DEGREES IN NORTHERN WYOMING...10 DEGREES
CENTRAL AND 5 IN THE SOUTH INCLUDING RKS. THIS FROPA WILL BE
FOLLOWED BY 7MB SURFACE 3 HOURLY PRESSURE RISES TUESDAY IN NORTHERN
JOHNSON COUNTY RESULTING IN BRISK NORTH WINDS IN BYG.

.LONG TERM...WEDNESDAY THROUGH SUNDAY

WARM WLY FLOW TO START THE MEDIUM RANGE PERIOD WITH A COUPLE WEAK
RIPPLES IN THE FLOW. NOT MUCH INSTABILITY SHOWING UP ON WEDNESDAY SO
ONLY WENT WITH ISOLATED MOUNTAIN SHOWERS AND NO THUNDER. LITTLE MORE
INSTABILITY AND MOISTURE ON THURSDAY FOR A FEW MORE MAINLY MOUNTAIN
SHOWERS AND THUNDERSTORMS. BIGGER CHANGES STILL TAKING SHAPE FOR
FRIDAY THROUGH THE WEEKEND AS BOTH THE ECMWF AND GFS ARE TAKING A
STRONG MID MAY TROUGH WEST TO EAST ACROSS THE AREA. THE GFS IS MUCH
FASTER WITH THE TROUGH WHILE THE ECMWF IS CONSIDERABLY SLOWER BY
ALMOST A DAY. BOTH MODELS PICK UP THE DISTURBANCE CURRENTLY NEAR
KAMCHATKA BUT THE ECMWF WITH TIME SHOWS A STRONGER DISTURBANCE OVER
THE CENTRAL PACIFIC WHILE THE GFS IS WEAKER AND THUS FASTER. HARD TO
RESOLVE THIS BASED ON THE ANALYSIS SO WILL STICK WITH A BLEND OF THE
TWO. THE CHANCE OF SHOWERS AND THUNDERSTORMS WILL INCREASE ON FRIDAY
MAINLY WEST OF THE DIVIDE WITH A SLIGHT CHANCE EAST ALONG WITH
INCREASING SW WINDS IN THE SOUTH HALF. COLD FRONT MAY SLIDE INTO THE
NRN HALF DEPENDING ON THE TROUGH TIMING. QUITE A FEW SHOWERS AND
SOME THUNDERSTORMS AROUND ON SATURDAY IN BOTH SOLUTIONS. COULD BE
QUITE ACTIVE IN THE WEST AND NORTH WITH FRONTAL BOUNDARY AND JET
STREAK DYNAMICS. TROUGH AXIS SHOULD BE BY SATURDAY NIGHT OR SUNDAY
WITH A COOLER...UNSTABLE NW FLOW AND PLENTY OF SHOWERS BUT A MORE
STABLE NW FLOW AND POTENTIALLY EVEN SOME UPSLOPE.

&&

.AVIATION.../18Z ISSUANCE/

AN UPPER LEVEL RIDGE OF HIGH PRESSURE WILL BUILD OVER THE REGION
THROUGH TONIGHT. SLIGHT INSTABILITY AND TRAPPED MOISTURE WITHIN THIS
RIDGE WILL RESULT IN ISOLD/WDLY SCT HIGH-BASED -SHRA FORMING IN THE
AFTERNOON AND EARLY EVENING...CLOUD BASES MOSTLY 140-160 MSL.
THUNDERSTORM ACTIVITY IS EXPECTED TO REMAIN VERY ISOLATED AND WAS
NOT MENTIONED IN THE VCNTY OF ANY TERMINALS.  THE MAIN HAZARD WITH
THESE HIGH-BASED SHOWERS/VIRGA WILL ERRATIC OUTFLOW WIND GUSTS.
OTHERWISE A NW SFC WIND AT 10-20KTS WILL PREVAIL ACROSS MOST AREAS
IN THE AFTERNOON AND EARLY EVENING. QUIET WEATHER FROM 06Z TONIGHT
THROUGH 18Z MONDAY AND VFR CONDITIONS.

&&

.FIRE WEATHER...

UNSEASONABLY WARM TEMPERATURES ARE EXPECTED THROUGH AT LEAST MONDAY AS
HIGH PRESSURE BUILDS IN FROM THE WEST. TODAY WE WILL SEE HIGH
TEMPERATURES REACH THE LOW TO MID 80S IN MANY AREAS FOR THE FIRST
TIME THIS SEASON IN THE WIND RIVER AND BIG HORN BASINS. HIGHS WILL
BE NEAR 80S AT SOME LOCATIONS AT THE LOWER ELEVATIONS OF JOHNSON AND
NATRONA COUNTIES. THE OTHER LOWER ELEVATIONS WILL SEE 70S TODAY. BY
MONDAY EVEN HOTTER TEMPERATURES ALONG WITH RECORD BREAKING
TEMPERATURES CAN BE EXPECTED WITH EVEN SOME 90 DEGREE TEMPERATURES
EXPECTED IN THE BIG HORN BASIN. A FEW THUNDERSTORMS WILL FORM IN
NORTHWEST WYOMING TODAY THAT WILL PRODUCE A FEW LIGHTNING STRIKES
AND GUSTY WINDS. ONLY ISOLATED THUNDERSTORMS ARE EXPECTED IN THE
NORTHWEST MOUNTAINS AND BIG HORN MOUNTAINS MONDAY. A DRY COLD FRONT
WILL MOVE SOUTH ACROSS WYOMING TUESDAY WHICH WILL KNOCK TEMPERATURES
DOWN 15 TO 20 DEGREES IN NORTHERN WYOMING...10 DEGREES CENTRAL AND 5
IN THE SOUTH INCLUDING RKS. THE MINIMUM RH AT THE LOWER ELEVATIONS
EAST OF THE DIVIDE AS WELL AS SWEETWATER COUNTY WILL BE IN THE
LOWER TEENS.

&&

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

&&

$$

SHORT TERM...LIPSON
LONG TERM...SKRBAC
AVIATION...AEM/AR
FIRE WEATHER...LIPSON