FXUS65 KRIW 022259
AFDRIW

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RIVERTON WY
359 PM MST WED DEC 2 2009

.SHORT TERM...TONIGHT THROUGH SATURDAY
A LARGE UPPER LEVEL LOW MOVING SOUTHWARD THROUGH THE DAKOTAS
CONTINUES TO BRING COLD ARCTIC AIR SOUTHWARD INTO THE CWA. HIGHS
TODAY AROUND THE REGION REMAINED 15 TO 20 F BELOW 30 YEAR NORMS. A
WEAK...MOISTURE STARVED WAVE WILL BRING LITTLE IN THE WAY OF
SNOWFALL TONIGHT SO HAVE CONFINED CHANCE POPS TO THE BIG HORN
MOUNTAINS WITH SLIGHT CHANCE IN THE NORTHWEST MOUNTAINS. ONLY
MINIMAL...SUB ADVISORY ACCUMULATION IS EXPECTED. THE CWA WILL
REMAIN UNDER THE WESTERN PORTION OF THE COLD DOME ASSOCIATED WITH
THE PLAINS LOW THROUGH THURSDAY. H7 TEMPS WILL RANGE FROM -16 TO
-23 C ON THURSDAY.  AS A RESULT...LOWS TONIGHT AND HIGHS THURSDAY
WILL REMAIN 15 TO 20 DEGREES BELOW NORMAL...WITH THE CENTRAL
BASINS APPROACHING ZERO WHILE THE WEST WILL SEE LOWS IN THE -5 TO
-15 F RANGE TONIGHT. A WEAK TRANSITORY RIDGE WILL MOVE THROUGH
ON FRIDAY. DUE TO A POCKET OF WARMER AIR...H7 TEMPS -6 TO -9
C...HAVE INCREASED HIGHS ABOVE GUIDANCE ACROSS THE SOUTHERN WIND
CORRIDOR AS BREEZY CONDITIONS SHOULD MIX BL. NEXT SYSTEM MOVES IN
WITH A COLD FRONT LATE FRIDAY/EARLY SATURDAY. GFS SLIGHTLY MORE
PROGRESSIVE THAN NAM AND EURO...BRINGING SNOW INTO THE NORTH BY
12Z SATURDAY. SNOW WILL SPREAD SOUTH AND EAST THROUGH THE DAY.
WILL LEAVE FOR NEXT SHIFT TO FURTHER NAIL DOWN TIMING AND
INTENSITY OF THE FRONT.

.LONG TERM...SATURDAY NIGHT THROUGH WEDNESDAY
A COMPLEX...POSSIBLY ACTIVE...WEATHER PATTERN WILL OCCUR DURING THE 
EXTENDED.  ALTHOUGH ONE THING IS CERTAIN...OLD MAN WINTER WILL STICK 
AROUND WITH COLD TEMPERATURES EXPECTED. THE ONLY QUESTION IS HOW 
COLD WILL IT BE.  THERE IS A DECENT CHANCE A LARGE AREA OF THE 
FORECAST AREA...INCLUDING THE LOWER ELEVATIONS WILL OBSERVE LOWS 
BELOW ZERO OR VERY CLOSE.  TEMPERATURES ARE EXPECTED TO GENERALLY BE 
10 TO 20 DEGREES BELOW AVERAGE THROUGH MUCH OF THE LONG TERM.

THE FORECAST IS WEIGHTED TOWARD A BLEND OF THE UKMET/ECMWF AND
CANADIAN MODELS WITH THEIR FARTHER WEST SOLUTION AND A ELONGATED
WEST TO EAST TROUGH DEVELOPING SUNDAY...WITH THE CENTER OVER
IDAHO/OREGON. MODELS ARE ALSO INDICATING A SHORTWAVE DISTURBANCE
PUSHING EAST ACROSS THE CENTRAL ROCKIES SATURDAY NIGHT/SUNDAY.
THIS WEST TO EAST ORIENTED TROUGH IS EXPECTED TO SWING ACROSS THE
WESTERN STATS MONDAY AND TUESDAY. THIS WILL KEEP PRECIPITATION
CHANCES IN THE FORECAST THROUGH THE EXTENDED...WITH AN EMPHASIS
SATURDAY AND SUNDAY...WITH THE COLD FRONT...AND MULTIPLE SYSTEMS
IN CLOSE PROXIMITY TO THE AREA. A LITTLE EARLY TO SEE HOW THE
DETAILS IN THIS VERY COMPLEX PATTERN WILL PLAY OUT THIS WEEKEND
INTO EARLY NEXT WEEK. ALTHOUGH A ZONAL FLOW...AND A WARMING TREND
COULD OCCUR ON WEDNESDAY...THIS ZONAL FLOW COULD RESULT IN
PRECIPITATION OVER THE WEST. SNOWFALL AMOUNTS CURRENTLY APPEAR TO
BE ON THE LIGHT SIDE...AS DENDRITIC GROWTH WILL BE NEARLY NON
EXISTENT DUE TO THE VERY COLD CLOUD LAYER EXPECTED.

&&

.AVIATION...
THE AREA OF LOW PRESSURE WHICH BROUGHT SNOW SHOWERS ACROSS MUCH OF 
CENTRAL AND WESTERN WYOMING ON TUESDAY WILL CONTINUE TO MOVE EAST 
TODAY. AS THIS SYSTEM MOVES EAST...WRAPAROUND MOISTURE WILL MOVE 
OVER PORTIONS OF WYOMING... BRINGING ADDITIONAL ISOLATED SNOW 
SHOWERS THIS EVENING. EXPECT SHOWERS TO REACH MANY AREAS ACROSS THE 
EASTERN TWO THIRDS OF THE FORECAST AREA. THESE ISOLATED SHOWERS WILL 
BRING PERIODS OF MVFR TO IFR CONDITIONS... ESPECIALLY WITH LOW 
CEILINGS. CONDITIONS ARE EXPECTED TO STEADILY IMPROVE...ESPECIALLY 
AFTER 12Z THURSDAY. WIDESPREAD VFR CONDITIONS ARE EXPECTED THURSDAY 
AFTERNOON...ESPECIALLY AFTER 22Z.

&&

.FIRE WEATHER...
LIGHT SNOW IS EXPECTED IN THE BIG HORN MOUNTAINS TONIGHT THROUGH
THURSDAY IN NORTHERLY FLOW AS AN UPPER LEVEL LOW PRESSURE SYSTEM
MOVES OFF INTO THE CENTRAL PLAINS. THE REST OF THE AREA WILL BE
DRY AND COLD THURSDAY. WINDS INCREASE ON FRIDAY IN AREAS PRONE TO
HIGHER WIND IN RESPONSE TO AN APPROACHING SYSTEM FROM THE NORTH.
LATE FRIDAY NIGHT SNOW WILL AGAIN MOVE INTO THE NORTHWEST
MOUNTAINS.

&&

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

$$

SHORT TERM...BERC
LONG TERM...WM
AVIATION...BRANHAM
FIRE WEATHER...BERC