FXUS65 KREV 261104
AFDREV

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RENO NV
304 AM PST THU NOV 26 2009

.SHORT TERM...
MODELS HAVE CHANGED SIGNIFICANTLY OVER THE LAST 24 HOURS REGARDING
THE SHORTWAVE THAT AFFECTS THE WEST COAST LATE TONIGHT AND FRIDAY.
SOUTHWEST FLOW ALOFT IS STILL PROGGED TO SLOWLY INCREASE TODAY
WITH ENOUGH LOW LEVEL MIXING FOR MANY VALLEY LOCATIONS TO MIX OUT
AND GET CLOSE TO 60 DEGREES. SURFACE WINDS WILL CONTINUE TO BE
VERY LIGHT HOWEVER RIDGES AND FOOTHILLS WILL SEE INCREASING
SOUTHWEST WINDS LATE IN THE DAY AHEAD OF THE APPROACHING UPPER
TROUGH.

THE EURO AND NAM STARTED GOING TOWARDS A SLOWER AND FURTHER SOUTH
TRACK TO THE UPPER TROUGH 24 HOURS AGO AND NOW ALL MODELS ARE
CONTINUING WITH THIS SOLUTION. THE SLOWER SOLUTION MEANS THAT THE
PRECIP BAND WILL NOT MOVE INTO THE NRN SIERRA UNTIL AFTER 12Z
FRIDAY. BY THIS TIME...H7 WINDS INCREASE TO ROUGHLY 40 KTS
BRINGING BREEZY CONDITIONS TO RIDGES AND OTHER WIND PRONE AREAS
WHILE VALLEYS SHOULD REMAIN MOSTLY DECOUPLED.

MODELS NOW CUT THE UPPER LOW OFF OVER CENTRAL CALIFORNIA BY
FRIDAY AFTERNOON WITH A SURFACE COLD FRONT STILL PROGGED TO MOVE
INTO WRN NEVADA DURING THE DAY FRIDAY. THIS SOLUTION IS MUCH MORE
FAVORABLE FOR PRECIP WITH BETTER UPPER DYNAMICS INCLUDING GOOD
DEFORMATION AHEAD OF THE LOW AND ALSO THE SLOWER MOVEMENT MAKING
FOR A LONGER PRECIP EVENT. THIS IS ALSO AN EVENT WITH VERY LITTLE
RAIN SHADOW WHERE WRN NEVADA COULD GET NEARLY AS MUCH PRECIP AS
THE SIERRA. THE LOW SLOWLY MOVES SOUTH ALONG THE SIERRA FRIDAY
NIGHT UNTIL SHOWERS FINALLY MOVE SOUTH OF THE CWA ON SATURDAY.
TOTAL MODEL QPF HAS BEEN STEADILY INCREASING WITH ROUGHLY 0.75"
NOW ALONG THE SIERRA CREST NORTH OF LAKE TAHOE...AND NEARLY 0.5"
EXTENDING JUST EAST OF RENO ALONG INTERSTATE 80. SNOW LEVELS DROP
THROUGHOUT THE DAY FRIDAY WITH SNOW EXPECTED ON MOST VALLEY FLOORS
BY FRIDAY EVENING. CONTINUED TO RAISE POPS AND QPF ALTHOUGH
CONFIDENCE IS STILL NOT HIGH SINCE THE MODELS CONTINUE TO CHANGE.
THERE WAS ALSO A RECENT NAM RUN THAT BROUGHT THE CLOSED LOW MUCH
FURTHER WEST OFF THE COAST WHICH WOULD BE A SIGNIFICANTLY DRIER
SOLUTION. LATER SHIFTS MAY HAVE TO ISSUE WINTER WEATHER ADVISORIES
FRIDAY...ESPECIALLY CONSIDERING THE BUSY HOLIDAY WEEKEND.

NORTHEAST FLOW ON THE NORTH SIDE OF THE DEPARTING LOW INCREASES
SIGNIFICANTLY SATURDAY AND SATURDAY NIGHT. H7 WINDS INCREASE TO
GREATER THAN 50 KTS WHICH IS RATHER RARE FROM THE NORTHEAST
DIRECTION. VALLEY WINDS WILL BE QUITE BRISK WITH TEMPERATURES ONLY
IN THE LOW 40S...BUT NORTHEAST WINDS OVER THE SIERRA COULD GUST TO
80 MPH MAKING FOR VERY CHILLY CONDITIONS. THE NORTHEAST FLOW WILL
ALSO BRING REVERSE UPSLOPE FLOW ALONG THE SIERRA FRONT OF MONO
COUNTY WHICH COULD ENHANCE SNOW AMOUNTS THERE. JORDAN

.LONG TERM...SUNDAY THROUGH WEDNESDAY...

RIDGE ALONG THE WEST COAST SUNDAY...FLATTENS MONDAY AS PACIFIC 
ENERGY PUSHES INTO BRITISH COLUMBIA.  MODELS AGREE HIGH AMPLITUDE 
RIDGE WILL REBUILD TUESDAY AND WEDNESDAY...WITH THE ECMWF HAVING A 
SHARPER RIDGE AXIS THAN THE GFS.

LIMITED WARMING ON SUNDAY AND MONDAY IN THE LOWER VALLEYS DUE TO THE 
SUBSIDENCE INVERSION OVER THE AREA.  HIGHS IN THE SIERRA AT NEAR TO 
ABOVE NORMAL VALUES...WITH HIGHS NEAR TO BELOW NORMAL FOR THE NEVADA 
VALLEYS.  WINDS TO REMAIN LIGHT...EXCEPT FOR MODERATE TO STRONG 
NORTHEAST-EAST WINDS OVER THE SIERRA CREST.

TUESDAY AND WEDNESDAY SHOULD REMAIN DRY...BUT MODELS DISAGREE ON THE 
FLOW ALOFT.  ECMWF WITH ITS SHARP RIDGE AXIS NEAR 130W TRIES TO DROP 
A DRY TROUGH INTO THE PACIFIC NORTHWEST...REINFORCING THE LOW LEVEL 
COLD AIR AFTER HELPING TO MIX OUT THE VALLEYS WEDNESDAY.  THE GFS 
AND ITS BROADER RIDGE ALONG THE WEST COAST WOULD MAINTAIN LIGHT 
WINDS AND PREVENT MUCH MIXING.  UNCERTAINTY ALSO INCREASES AS MODELS 
FIGURE OUT HOW TO ABSORB THE TROPICAL SYSTEM IN THE WESTERN PACIFIC 
TUESDAY-THURSDAY.  BRONG

&&

.AVIATION...

HIGH PRESSURE OVERHEAD WILL MOVE EAST AS A SYSTEM ENTERS THE PACIFIC 
NORTHWEST LATER TODAY.  SOUTH-SOUTHWEST WINDS ALOFT WILL INCREASE 
WITH LIGHT WINDS AT THE TERMINALS TODAY.  WINDS MAY INCREASE AT THE 
SIERRA TERMINALS LATE TONIGHT AS RIDGE TOP FLOW REACHES 30-40KTS.  

SYSTEM WILL DROP SOUTH AND PUSH A COLD FRONT THROUGH THE SIERRA AND 
WESTERN NEVADA ON FRIDAY.  MODELS ARE MUCH WETTER WITH THIS 
SYSTEM...INCREASING CONFIDENCE IN PERIOD OF MVFR/IFR CONDITIONS FOR 
THE SIERRA DUE TO SNOW FRIDAY AFTERNOON THROUGH SATURDAY MORNING.

FOR KRNO...RAIN/SNOW POSSIBLE FRIDAY NIGHT AND SATURDAY MORNING WITH 
FREEZING LEVELS LOWERING TO 4000-4500 FEET.  WINDS AHEAD OF THE 
FRONT SHOULD GUST FOR A FEW HOURS FRIDAY AFTERNOON/EVENING...WITH 
SPEEDS LIKELY TO REMAIN BELOW 40KTS.  BRONG

&&

.REV WATCHES/WARNINGS/ADVISORIES...
NV...NONE.
CA...NONE.
&&

$$

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