FXUS65 KREV 101207 CCA
AFDREV

AREA FORECAST DISCUSSION...CORRECTED
NATIONAL WEATHER SERVICE RENO NV
404 AM PST THU DEC 10 2009

.SHORT TERM...
AN ACTIVE PATTERN IS ABOUT TO BEGIN AND PERSIST THROUGH THE WEEKEND.
THE FIRST DISTURBANCE OF INTEREST IS CURRENTLY CENTERED AT ABOUT
135W LONGITUDE OFF THE NORTHERN CALIFORNIA COAST. A SECOND...MUCH
MORE POTENT SYSTEM IS LOCATED AT AROUND 42N/160W. 

THE FIRST SYSTEM CURRENTLY OFF THE NORTHERN CALIFORNIA COAST WILL
SPLIT INTO TWO PIECES...WITH ONE TRYING TO GO NORTH AROUND THE
ELONGATED RIDGE OVER THE PACIFIC NORTHWEST COAST AND THE OTHER
SLIDING INTO OUR REGION TONIGHT AND FRIDAY. FOR SUCH A WEAK-
LOOKING SYSTEM...AMSU IMAGERY INDICATES AROUND ONE INCH OF PWAT AT
THIS TIME. BY THE TIME THE SYSTEM REACHES THE SIERRA...IT IS
PROGGED TO STILL HAVE AS MUCH AS 0.75" PER THE GFS PROGS. GIVEN
CURRENT MODEL PRECIP PROGS THURSDAY NIGHT AND FRIDAY...WINTER
WEATHER ADVISORIES ARE POSSIBLE IN THE SIERRA WITH NEAR ADVISORY
AMOUNTS POSSIBLE INTO WESTERN NEVADA BY FRIDAY MORNING AS A PERIOD
OF WARM ADVECTION MOVES OVERHEAD. SPILLOVER DOES NOT LOOK TO BE A
PROBLEM WITH SOUNDINGS INDICATING A CLASSIC WARM ADVECTION PATTERN
WITH A STABLE LAYER BELOW MOUNTAIN TOPS TO INHIBIT DOWNSLOPING.
STAY TUNED TO THE DAYSHIFT PACKAGE TODAY FOR MORE INFORMATION ON
THIS FIRST SYSTEM AND ITS POSSIBLE IMPACTS INTO WESTERN NEVADA.

THE SECOND...MUCH MORE POTENT SYSTEM IS DUE IN FRIDAY NIGHT AND
SATURDAY. UNLIKE THE FIRST SYSTEM...THIS ONE WILL BECOME QUITE
COMPLICATED AS TO SNOW LEVELS AND PRECIP AMOUNTS. THE 06Z GFS AND
THE NAM TO SOME DEGREE HAVE COME IN MUCH WETTER THAN PREVIOUS
RUNS AND THE ECMWF. THIS IS DUE TO THEIR FURTHER NORTH (ESPECIALLY
THE GFS) POSITION OF THE JET STREAK DUE IN WITH THE SYSTEM. THE
GFS HAS THE JET STREAK MOVING INTO THE NORTHERN SIERRA WHILE THE
ECMWF HAS THE STREAK MOVING INTO SOUTHEAST CALIFORNIA. THE PROBLEM
IS THAT BOTH MODELS HAVE STUCK TO THEIR GUNS RECENTLY AND SREF 300
MB WIND PROGS SHOW A LARGE STANDARD DEVIATION OVER THE NORTHERN
AND CENTRAL SIERRA. THIS HINTS AT THE POSSIBILITY THAT BOTH THE GFS
AND THE ECMWF ARE POSSIBLE OUTCOMES FOR SATURDAY'S JET POSITION.

BACK TO PRECIP PROGS...THE GFS WOULD INDICATE A LARGE SNOWFALL
IN THE NORTHERN SIERRA AND UP INTO PLUMAS COUNTY WHILE THE ECMWF
WOULD TARGET MONO COUNTY SOUTHWARD FOR THE BEST SNOWFALL. FOR
NOW...WE WILL ISSUE A WINTER STORM WATCH FOR MONO COUNTY CLEAR UP
INTO LASSEN COUNTY WITH POTENTIAL FOR A BIG SNOWFALL TO HIT
THE SIERRA ANYWHERE FROM SUSANVILLE SOUTHWARD.

THE OTHER COMPLICATION WILL BE SNOW LEVELS. THE GFS WOULD NOW
INDICATE A SWITCHOVER FROM SNOW TO RAIN BELOW 5000-5500 FEET
SATURDAY MORNING/AFTERNOON FOLLOWED BY A RETURN TO SNOW BY
SATURDAY EVENING. WITH HEAVY PRECIP POSSIBLE IN THE GFS...IT COULD
BE A SIGNIFICANT CONCERN WHERE THE SNOW LEVEL WINDS UP BEING FOR
PORTIONS OF WESTERN NEVADA. AS FAR AS THE EC...IT WOULD BE ALL
SNOW TO THE LOWER VALLEYS BUT WITH MUCH LESS QPF. HOWEVER...IN
BOTH THE GFS AND THE EC THERE COULD BE SIGNIFICANT SNOWFALL IN
PORTIONS OF THE SIERRA AND AT LEAST A LIGHT SNOWFALL OUT INTO
WESTERN NEVADA. 

.LONG TERM...SUNDAY THROUGH WEDNESDAY...
UPPER LEVEL TROF AXIS TO PASS THROUGH SUN WITH SOME SNOW SHOWERS 
ACROSS THE AREA IN WAKE OF SAT STORM. TEMPS TO REMAIN BELOW NORMAL 
AND IN THE 30S MOST AREAS. CLEARING EXPECTED SUN NIGHT FOR SOME GOOD 
RADIATIONAL COOLING AND LIGHT WINDS. SOME RIDGING FOR MON WITH 
INVERSIONS PRESENT ACROSS LOWER VALLEYS...THAT COULD BE STRONGER 
THAN IN CURRENT GRIDS DEPENDING ON HOW MUCH/IF SNOW LEVELS RISE WITH 
SAT STORM.

MODELS THEN DIVERGE FOR TUE/WED. GFS SHOWS MORE RIDGING WITH LITTLE 
IN THE WAY OF ANY PRECIP ACROSS THE AREA WHILE EC HAS RIDGE AXIS 
FURTHER EAST AND OPENS THE DOOR FOR SOME MOISTURE IN THE MOIST SW 
FLOW ALOFT. TELECONNECTIONS FROM BLOCK NEAR ICELAND AND WEAKER 
RIDGING IN BERING SEA SUPPORT EC MORE. HOWEVER...WILL JUST LEAN THAT 
WAY A BIT AS EC ENSEMBLE IS NOT AS WET AND HIGHER WITH HEIGHTS. LEFT 
IN CHC POPS FOR THE NW AREAS TUE-WED. WITH THIS SOMEWHAT WETTER 
IDEA...HAVE WARMED TEMPS TO ABOVE NORMAL AS INVERSIONS SHOULD BE 
MINIMIZED WITH STRONGER WINDS ALOFT AND CLOUD COVER WHICH SHOULD 
MELT OFF THE VALLEY SNOW.

NOTE HOWEVER...THAT IF THE GFS IS CORRECT...THERE WILL BE MORE 
RIDGING AND LOWER VALLEY INVERSIONS WOULD BE STG. TEMPS WOULD REMAIN 
BELOW NORMAL IN THAT PATTERN WITH ABOVE NORMAL TEMPS FOR MTN AREAS. 
FOG COULD ALSO BE AN ISSUE AS SNOW COVER WOULD BE LESS LIKELY TO 
MELT. WALLMANN
&&

.AVIATION...
PATCHY FZFG CONTINUES NEAR KLOL AND KTRK THIS MORNING WITH OCNL IFR 
VIS. KRNO LESS AFFECTED WITH OCNL MVFR VIS THERE. EXPECT FZG TO 
DISSIPATE BY 17Z. VFR CONDS AFTERWARD THRU 12Z FRI WITH LIGHT WINDS.

TWO SYSTEMS TO AFFECT THE AREA FRI-SAT. FRI IS WEAKER WITH SOME IFR 
CIGS/VIS AT KTRK/KTVL FOR 6-8 HRS AND PERHAPS 2-3 HRS KRNO/KLOL. 
BIGGER STORM SAT WITH LARGER IMPACTS DUE WIND/SNOW. WALLMANN
&&

.REV WATCHES/WARNINGS/ADVISORIES...
NV...WINTER STORM WATCH FROM FRIDAY EVENING THROUGH LATE SATURDAY 
     NIGHT FOR NVZ002.

     FREEZING FOG ADVISORY UNTIL 10 AM PST THIS MORNING FOR NVZ001-
     004.

CA...WINTER STORM WATCH FROM FRIDAY EVENING THROUGH LATE SATURDAY 
     NIGHT FOR CAZ071>073.

&&

$$

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