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000
FXUS65 KRIW 092251
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
351 PM MST TUE FEB 9 2010

.SHORT TERM...TODAY THROUGH FRIDAY
PESKY LOW AND MID-LEVEL STRATUS/FLURRIES CONTINUE TO DEVELOP OVER
EASTERN IDAHO AND WESTERN WYOMING DUE TO SHALLOW ISENTROPIC
UPGLIDE. LATEST NAM12 AND GFS40 ARE SLIGHTLY DEEPER WITH THE UPGLIDE
AND STRENGTH OF NEAR H7 OVER FAR WESTERN WYOMING TONIGHT. ADDED
SLIGHT CHANCE TO CHANCE SNOW PROBABILITIES AS A RESULT OF CURRENT
SATELLITE...RADAR OBSERVATIONS FROM KSFX (POCATELLO)...AND SFC
OBS. UPGLIDE EXPECTED TO DEEPEN SOMEWHAT WEDNESDAY AFTERNOON WITH
SNOW BECOMING SLIGHTLY MORE WIDESPREAD AND LIGHT TO MODERATE IN
INTENSITY. NORTHWEST FLOW AT H5 WITH WEST-SOUTHWEST FLOW AT H7 IS
EXPECTED TO CONTINUE THROUGH THE DAY THURSDAY OVER THE FAR
NORTHWEST...AS A HIGH AMPLITUDE RIDGE REMAINS OVER THE NORTHERN
GREAT BASIN AND INLAND NORTHWEST. CONTINUED TREND OF INCREASING
SNOW CHANCES OVER THE FAR WEST FOR THURSDAY WITH THE LATEST MODEL
GUIDANCE TRENDING STRONGER WITH THE MID-LEVEL FLOW AND THE
RESULTING UPGLIDE/WAA SNOW. FOR THE CENTRAL BASINS...WAA ADVECTION
WILL RESULT IN STRONGER TEMPERATURE INVERSIONS THURSDAY AND FRIDAY
AFTERNOON. SURFACE HIGH PRESSURE ANCHORED OVER SOUTHWEST-WESTERN
WYOMING AND EVENTUALLY THE EASTER GREAT BASIN...WILL STRENGTHEN IN
RESPONSE TO STRONG SUBSIDENCE AT AND BELOW H5 IN THE MODERATE
NORTHWEST FLOW. SOUTHWEST TO NORTHEAST SURFACE PRESSURE GRADIENT
PROGGED TO STRENGTHEN WEDNESDAY NIGHT...PRODUCING MODERATE TO
STRONG SOUTHWEST WIND IN THE CODY FOOTHILLS AND WIND CORRIDOR FROM
JUST NORTHEAST OF ROCK SPRINGS TO CASPER BOTH WEDNESDAY NIGHT AND
THURSDAY MORNING AND AGAIN THURSDAY NIGHT AND FRIDAY FORENOON.
STRONG GAP FLOW A POSSIBILITY IN THE CLARK AREA WITH THIS
FAVORABLE FLOW PATTERN BOTH WEDNESDAY AND THURSDAY NIGHTS.

.LONG TERM...FRIDAY NIGHT THROUGH TUESDAY
MEDIUM RANGE DETAILS CONTINUE TO FLUCTUATE WITH EACH MODEL RUN
BUT THE OVERALL PATTERN IS STILL DOMINATED BY A RIDGE AXIS JUST TO
THE WEST WITH TEMPORARY FLATTENING AS SHORTWAVES RIDE OVER THE RIDGE.
ISENTROPIC UPGLIDE ACCOMPANIES EACH OF THESE SHORTWAVES WITH A
PERIOD OF LIGHT SNOW EXPECTED IN THE FAR WEST WITH EACH ONE. THE
MODELS ARE STILL TRYING TO INDICATE THAT THIS SATURDAY SHORTWAVE
MAY HAVE A MORE FAVORABLE NW TO SE TRAJECTORY AND BRING A LITTLE
LIGHT SNOW EAST OF THE DIVIDE. WILL PUT IN SOME 20 POPS FOR ALL
AREAS EAST OF THE DIVIDE FOR NOW AND LET THE NEXT CREW LOOK AT
THIS AGAIN TONIGHT. BASIN AND VALLEY TEMPERATURES WILL BE A LITTLE
TRICKY AS DEPENDING ON THE STRENGTH OF THESE WAVES SOME OF THESE
AREAS COULD MIX...AT LEAST PARTIALLY. DETAILS WILL CONTINUE TO
CHANGE AS THEY HAVE FOR THE PAST 4 NIGHTS BUT THE BIGGER PICTURE
REMAINS FAIRLY STABLE AND CONDUCIVE TO SOME PERIODS OF AT LEAST
LIGHT SNOW OR FLURRIES OUT WEST AT TIMES IN LIGHT TO MODERATE
ISENTROPIC UPGLIDE PATTERN AT TIMES.

&&

.AVIATION...
A FLAT HIGH PRESSURE RIDGE WILL PROVIDE VFR CONDITIONS ACROSS MUCH
OF THE AREA THROUGH WEDNESDAY. EXCEPTIONS WILL BE THE WESTERN
WYOMING VALLEYS AND BASINS WHERE TRAPPED LOW LEVEL MOISTURE WILL
BECOME REESTABLISHED WITH A CLOUD DECK OF AROUND 3K EXPECTED TO
DEVELOP AROUND 00Z THIS EVENING THERE WITH THE HIGHER MOUNTAINS
EXPERIENCING VFR CONDITIONS. SOUTHWEST WINDS OF 20 TO 30 MPH ARE
EXPECTED FROM THE GREEN MOUNTAINS TO KCPR WEDNESDAY AS THE PRESSURE
GRADIENT TIGHTENS IN RESPONSE TO AN APPROACHING WEAKENING TROUGH OF
LOW PRESSURE NOW ALONG THE PACIFIC COAST. AS THE TROUGH APPROACHES...LIGHT
SNOWFALL WILL BREAK OUT IN FAR WESTERN WYOMING AFTER 18Z WEDNESDAY
RESULTING IN MVFR CONDITIONS AND MOUNTAIN OBSCURATIONS.

&&

.FIRE WEATHER...
VALLEY AND BASIN TEMPERATURE INVERSIONS WILL CONTINUE TONIGHT AND
WEDNESDAY AS A MOSTLY DRY NORTHWEST FLOW ALOFT CONTINUES. AREAS OF
FLURRIES/VERY LIGHT SNOW WILL CONTINUE OVER FAR WESTERN WYOMING
TONIGHT AND WEDNESDAY. POOR SMOKE DISPERSAL WILL REMAIN THE RULE
THROUGHOUT THE DISTRICT THROUGH WEDNESDAY NIGHT.

&&

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

$$

SHORT TERM...CHURCH
LONG TERM...SKRBAC
AVIATION...LIPSON
FIRE WEATHER...CHURCH