FXUS65 KRIW 102204
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
304 PM MST THU DEC 10 2009

.SHORT TERM...TONIGHT THROUGH SUNDAY
CYCLONIC NORTHWEST FLOW WILL WEAKEN AND SHIFT EAST TONIGHT AND
FRIDAY WITH THE FLOW ALOFT BECOMING MORE WESTERLY AND WEAKENING. 
WARMER TEMPERATURES WILL RESULT AS THE FLOW BECOMES MORE WESTERLY.
STRONG DOWNSLOPE WINDS ALONG THE EAST SLOPES OF THE MOUNTAINS WILL
CAUSE MUCH OF THE WARMING WITH BETTER MIXING OF THE COLD AIR IN
THE VALLEYS AND BASINS. A PACIFIC TROUGH MOVING INTO THE CENTRAL
CALIFORNIA COAST THIS AFTERNOON WILL MOVE EAST AND SPREAD
MOISTURE EAST ACROSS THE GREAT BASIN. THE LEADING EDGE OF THIS
MOISTURE WILL BRING INCREASED HIGH AND MID CLOUDS TO THE WEST ON
FRIDAY. LATE FRIDAY NIGHT THE MOISTURE WILL DEEPEN COMBINED WITH
OROGRAPHIC LIFT...ISENTROPIC LIFT AND DYNAMICS WILL PRODUCE
SNOWFALL IN THE WESTERN MOUNTAINS...WITH SOME SNOW REACHING THE
VALLEY FLOORS. PERIODS OF SNOW WILL INCREASE ACROSS THE WEST AND
MOVE INTO THE SOUTHWEST SATURDAY AND SATURDAY NIGHT. MUCH OF THE
WEST AND SOUTHWEST WILL RECEIVE SNOWFALL THROUGH SUNDAY WITH SOME
SNOW SPREADING INTO THE NORTH AS WELL ON SUNDAY. SNOWFALL MAY BE
SIGNIFICANT IN THE WESTERN MOUNTAINS WITH OVER A FOOT OF SNOW
EXPECTED IN THE TETON MOUNTAINS. SNOWFALL ESTIMATES WILL LIKELY
CHANGE OVER THE NEXT FEW FORECAST CYCLES AS THE FEATURES OF THE POTENTIAL
STORM BECOME CLEARER WITH TIME...PRECIPITATION ESTIMATES WILL
LIKELY INCREASE WITH TIME AND SOME SHIFTING OF THE LOCATIONS OF THE
HEAVIER SNOWFALL IS LIKELY TO OCCUR AS WELL. OVERALL THE GFS IS
FASTEST WITH THIS INCOMING SYSTEM WITH THE NAM BEING ABOUT 6 HOURS
SLOWER WITH THE LEADING EDGE OF THE MOISTURE. TIMING ISSUES WILL
ALSO LIKELY SHIFT THE TIMING OF THE PRECIPITATION OVER THE NEXT 36
HOURS AS WELL.  

.LONG TERM...SUNDAY NIGHT THROUGH THURSDAY
CHANCES OF SNOW CONTINUE ACROSS THE WEST AND SOUTH INTO SUNDAY AND
ACROSS THE WEST THROUGH THE REST OF THE PERIOD. OTHERWISE WARMING
TREND FORECAST TUESDAY THROUGH THURSDAY WITH DRY CONDITIONS ACROSS
THE CENTRAL AND SOUTH.

FORECAST CONTINUES TO REFLECT ECMWF OVERALL WITH BETTER SEPARATION
IN SPLIT FLOW EARLY IN THE PERIOD...KEEPING ARCTIC FRONT NORTH AND
EAST OF AREA BEHIND TROUGH PASSAGE SUNDAY NIGHT AND MONDAY.
LATEST GFS AND GEM SEEM TO BE TRENDING TOWARD ECMWF TONIGHT WITH
BETTER OVERALL AGREEMENT EARLY IN THE PERIOD. SECOND PACIFIC
TROUGH MOVES INTO THE GREAT BASIN SUNDAY WITH NORTHERN STREAM
STREAM SHORTWAVE ROUNDING BASE OF CENTRAL CANADIAN UPPER LOW
ACROSS MONTANA. WYOMING LARGELY CAUGHT IN BETWEEN THE TWO WITH
UNSTABLE NW FLOW KEEPING CHANCES OF SNOW SHOWERS ACROSS THE WEST
AND SOUTHERN WYOMING ON THE FRINGE OF PRECIPITATION FROM SOUTHERN
STREAM ENERGY. SPLIT TROUGHS MOVE INTO PLAINS MONDAY WITH FLAT
RIDGE TAKING SHAPE OVER WESTERN U.S. TUESDAY THROUGH THURSDAY.
PER ECMWF SOLUTION...GRADUAL AMPLIFICATION OF RIDGE THROUGH PERIOD
WILL RESULT IN OROGRAPHIC PRECIPITATION IN THE WEST LIFTING
NORTHWARD TO PRIMARILY YNP AND THE NW MOUNTAINS BY THURSDAY WITH
H7 TEMPERATURES NEAR 0C ACROSS THE AREA BY THURSDAY.
THUS...TEMPERATURES SHOULD RETURN TO ABOVE NORMAL TUESDAY THROUGH
THURSDAY WITH INVERSION PRONE WESTERN VALLEYS AND CENTRAL BASINS
FORECAST TO LAG 5F-10F BEHIND BETTER MIXED AREAS. 

&&

.AVIATION...

SOUTHWEST WINDS OF 15 TO 25 MPH WILL BLOW TONIGHT FROM THE GREEN 
MOUNTAINS TO KCPR. THESE SOUTHWEST WINDS WILL INCREASE AGAIN FRIDAY 
AT SPEEDS OF 25 TO 35 MPH ALONG WITH SOME BLOWING SNOW. ALONG THE EAST 
SLOPE OF THE WIND RIVER...ABSAROKA AND THE BIG HORN MOUNTAINS EXPECT 
WEST WINDS OF 40 TO 50 MPH ALONG WITH GUSTS OF 60 TO 70 MPH TONIGHT 
THROUGH FRIDAY. OTHERWISE ISOLATED SNOW SHOWERS CAN BE EXPECTED 
ACROSS THE NORTHERN WYOMING MOUNTAINS TONIGHT RESULTING IN LOCAL 
MVFR CONDITIONS FOR LOWERED CIGS AND VSBY WHERE SOME HIGHER MOUNTAIN 
OBSCURATIONS CAN BE EXPECTED. THESE ISOLATED SNOW SHOWERS WILL MOVE NORTH 
OF WYOMING FRIDAY AS THE JET STREAM SHIFTS NORTH. VFR CONDITIONS
CAN BE EXPECTED ELSEWHERE TONIGHT. VFR CONDITIONS WILL PREVAIL
FRIDAY.

&&

.FIRE WEATHER...
THE COLD AIR MASS IN THE VALLEYS AND BASINS WILL CONTINUE TO
MODIFY TONIGHT AND FRIDAY AS STRONGER WINDS ALOFT MIX DOWN TO THE
SURFACE CAUSING WARMING. CONDITIONS WILL BE DRY WITH BREEZY TO WINDY
AREAS ALONG AND NEAR THE EAST SLOPES OF THE MOUNTAINS AND THROUGH
THE WIND CORRIDOR FROM ROCK SPRINGS TO CASPER. MIXING HEIGHTS WILL
INCREASE TO AROUND 2500 FEET FRIDAY WITH STRONG MIXING.

&&

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

$$

SHORT TERM...C.BAKER
LONG TERM...AEM
AVIATION...LIPSON
FIRE WEATHER...C.BAKER