FXUS65 KRIW 111047
AFDRIW
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
347 AM MST FRI DEC 11 2009
.SHORT TERM...TODAY THROUGH SUNDAY NIGHT
A FEW CLOUDS CONTINUE TO MOVE THROUGH NORTH CENTRAL WYOMING THIS
MORNING...WITH OTHER AREAS REMAINING GENERALLY CLEAR. TEMPERATURES
IN THE SOUTHWESTERN VALLEYS ARE VERY COLD UNDER THE GENERALLY CLEAR
SKIES...WITH BIG PINEY SEEING A CURRENT TEMPERATURE OF NINETEEN
BELOW ZERO. WIND OVER THE CORRIDOR BETWEEN ROCK SPRINGS AND CASPER
HAS INCREASED THIS MORNING. GENERALLY DRY CONDITIONS EXPECTED
THROUGH THE AFTERNOON...THOUGH INCREASED WIND OVER THE CORRIDOR WILL
REMAIN UNDER THE CURRENT NORTHWEST FLOW AND TIGHTENED PRESSURE
GRADIENT BETWEEN ROCK SPRINGS AND GILLETTE. MODELS CONTINUE TO
INDICATE INCREASED WIND DEVELOPING OVER THE ABSAROKA AND WIND RIVER
MOUNTAINS THIS MORNING. WHILE THIS IS NOT EXPECTED TO DEVELOP INTO A
STRONG MOUNTAIN WAVE...SOME LOCAL AREAS MAY SEE WIND GUSTS AS STRONG
AS 50 MPH THROUGH THE EARLY AFTERNOON HOURS.
ZONAL FLOW WILL DEVELOP THROUGH THE LATE AFTERNOON AND EARLY EVENING
HOURS. AS THIS FLOW DEVELOPS...MOISTURE ASSOCIATED WITH A SHORTWAVE
WILL BEGIN TO IMPACT SOUTHWESTERN WYOMING...BRINGING ISOLATED SNOW
SHOWERS THIS AFTERNOON. SNOWFALL WILL BECOME MORE WIDESPREAD
OVERNIGHT ACROSS THE WESTERN VALLEYS AND MOUNTAINS. SNOWFALL OVER
THE WESTERN MOUNTAINS AND VALLEYS WILL CONTINUE THROUGH THE DAY ON
SATURDAY WITH WARMING TEMPERATURES EXPECTED. WESTERLY FLOW WILL
BRING WARM DOWN-SLOPE WINDS OVER THE MOUNTAINS...ALLOWING DAYTIME
TEMPERATURES TO POTENTIALLY WARM ABOVE FREEZING IN THE VALLEYS EAST
OF THE DIVIDE...AND IN JOHNSON COUNTY. MODELS INDICATE ANOTHER PUSH
OF INCREASED WIND OVER THE WIND CORRIDOR SATURDAY AFTERNOON.
A PIECE OF JET ENERGY WILL MOVE THROUGH WESTERN WYOMING LATE
SATURDAY NIGHT AND INTO SUNDAY MORNING. THIS ADDED ENERGY WILL
INCREASE THE ISENTROPIC LIFT OVER WESTERN WYOMING TO BRING
ADDITIONAL...WIDESPREAD SNOWFALL. THIS INCREASE IN SNOWFALL ACROSS
SOUTHERN AND WESTERN WYOMING IS SUPPORTED IN BOTH THE GFS AND NAM
MODELS...WHICH HAVE COME INTO SLIGHTLY BETTER AGREEMENT WITH THE
MOST RECENT RUN. SNOWFALL WILL CONTINUE THROUGH SUNDAY EVENING.
TEMPERATURES WILL CONTINUE THEIR SLIGHT WARMING TREND SUNDAY...WITH
ADDITIONAL AREAS EAST OF THE DIVIDE APPROACHING THE LOW TO MID 30S.
SUNDAY THROUGH SUNDAY EVENING MAY POTENTIALLY SEE INCREASED SNOW
RATES. FUTURE SHIFTS WILL LIKELY CONSIDER WINTER WEATHER HIGHLIGHTS
FOR SATURDAY EVENING THROUGH SUNDAY EVENING...WHEN 5 TO 10 INCHES OF
SNOWFALL WILL FALL OVER THE WESTERN MOUNTAINS. THE VALLEYS WILL SEE
THE POTENTIAL OF 2 TO 4 INCHES. THIS SCENARIO WILL CONTINUE TO BE
MONITORED.
.LONG TERM...MONDAY THROUGH FRIDAY
MAIN CHANGE TO MONDAY AND TUESDAY FORECAST WAS TO NUDGE ARCTIC
FRONT INTO NORTH CENTRAL WYOMING MONDAY ALONG WITH INCREASING
CHANCES FOR LIGHT SNOW BEHIND THE FRONT. AFTERWARDS...FEW CHANGES
TO FORECAST WITH FLAT UPPER RIDGE BUILDING OVER THE WESTERN U.S.
WITH CHANCES OF SNOW GRADUALLY DECREASING OUT WEST AND AN OVERALL
WARMING TREND TUESDAY THROUGH FRIDAY.
GFS DETERMINISTIC REMAINS THE MOST AGGRESSIVE BRINGING ARCTIC FRONT
BACK DOWN ACROSS NORTH CENTRAL WYOMING MONDAY AS LOBE ON WEST END
OF CANADIAN VORTEX ROTATES SOUTHWARD INTO THE NORTHERN PLAINS AS
PATTERN OVER NORTHWEST AMPLIFIES WITH RIDGE BUILDING OFF NW COAST.
ECMWF REMAINS FLATTER WITH THE FLOW WITH MORE MODIFIED ARCTIC
FRONT NUDGING INTO NORTHEAST WYOMING BEFORE QUICKLY WASHING OUT
MONDAY NIGHT. IN COLLABORATION WITH OFFICES TO THE NORTH AND
EAST...A COMPROMISE NOT-SO-GFS-COLD...NOT-SO-MILD-ECWMF SOLUTION
WORKED OUT WITH HIGHS THE NORTHERN BIG HORN BASIN AND JOHNSON CTY
IN THE TEENS MONDAY ALONG WITH HIGH CHANCE/LIKELY -SN. SURFACE
PRESSURES FALL RAPIDLY MONDAY NIGHT ACROSS NORTHEAST WYOMING AS
ARCTIC HIGH SLIDES EASTWARD...AS RIDGE BUILDS IN ALOFT. FLAT DIRTY
RIDGE TO CONTINUE TO BRING CHANCES OF SNOW TO THE WEST THROUGH THE
PERIOD IN WESTERLY FLOW WITH SEVERAL EMBEDDED WEAK SHORTWAVES.
SPREAD BETWEEN GFS AND ECMWF INCREASE GREATLY BY FRIDAY WITH ECMWF
BUILDING SHARP RIDGE OVER INTERMOUNTAIN WEST WHILE GFS KEEPS STRONG
ONSHORE FLOW INTO NORTHWEST AND FAVORABLE OROGRAPHIC PRECIPITATION
PATTERN INTO WESTERN MOUNTAINS. PERSISTENCE FAVORED IN THIS LATTER
PERIOD...LESS AMPLIFICATION WITH NEAR CLIMO-POPS IN THE WEST. H7
TEMPERATURES UP TO -2C TO -4C ACROSS THE AREA BY WEDNESDAY SO
SIGNIFICANT WARMING IN STORE FOR AREAS ABLE TO MIX OUT. FOR
WIND RIVER BASIN AND LOWER ELEVATIONS OF BIG HORN BASIN THIS
WARMING ALOFT MAY ONLY ACT TO STEEPEN INVERSIONS. FORECAST HIGHS
THUS LAG AROUND 8-10F IN THESE AREAS WITH NIGHTTIME LOWS ALSO
REFLECTING MUCH COLDER TEMPERATURES ALONG LOWER RIVER VALLEYS.
&&
.AVIATION...
VFR IS EXPECTED THROUGH THIS EVENING. HOWEVER...SOME SNOW SHOWERS
COULD OCCUR ACROSS THE FAR WEST AFTER 06Z SATURDAY...INCLUDING AT
KJAC...WHICH COULD RESULT IN MVFR CONDITIONS. OTHERWISE...EXPECT
INCREASING MID AND HIGH LEVEL CLOUDINESS TODAY AND TONIGHT. EXPECT
WINDY SOUTHWEST WINDS OVER SOUTHEAST FREMONT INTO NATRONA COUNTY
INCLUDING AT KCPR. WINDY WEST/SOUTHWEST WINDS ARE ALSO EXPECTED
OVER THE ABSAROKA AND EASTERN WIND RIVER MTS TODAY...AND ADJACENT
LOWER ELEVATIONS.
&&
.FIRE WEATHER...
GENERALLY DRY CONDITIONS EXPECTED TODAY. THE WIND CORRIDOR BETWEEN
ROCK SPRINGS AND CASPER IS EXPECTED TO SEE GUSTY SOUTHWEST WIND
TODAY. WIND OF 30G40KT WILL BE POSSIBLE...ESPECIALLY IN THE GREEN
MOUNTAIN AND RATTLESNAKE RANGE AND NATRONA COUNTY.
ADDITIONALLY...BREEZY WIND IS EXPECTED OVER THE EASTERN SLOPES OF
THE ABSAROKA AND WIND RIVER MOUNTAINS TODAY. WIND OF 40G50KT WILL BE
POSSIBLE OVER THE HIGHER ELEVATIONS...WITH WIND OF 30G40KT POSSIBLE
IN THE IMMEDIATE VALLEYS. LIGHT SNOW SHOWERS WILL DEVELOP THIS
EVENING OVER THE WESTERN MOUNTAINS AND VALLEYS. THIS SNOWFALL WILL
INCREASE IN INTENSITY AND COVERAGE THROUGH SATURDAY AND SUNDAY. THE
HEAVIER SNOWFALL RATES ARE EXPECTED TO OCCUR BETWEEN SATURDAY NIGHT
AND SUNDAY NIGHT...THOUGH THIS STORM WILL CONTINUE TO BE MONITORED
FOR UPDATES.
&&
.RIW WATCHES/WARNINGS/ADVISORIES...
NONE.
&&
$$
SHORT TERM...BRANHAM
LONG TERM...AEM
AVIATION...WM
FIRE WEATHER...BRANHAM
