FXUS65 KREV 060338
AFDREV

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RENO NV
738 PM PST SAT DEC 5 2009

.UPDATE...
CLOUDS HAVE THINNED FASTER THAN ORIGINALLY FORECAST ACROSS THE SRN
PART OF THE CWA THIS EVENING AND ISOLATED SNOW SHOWERS HAVE ENDED.
IR SATELLITE IMAGERY DOES INDICATE MORE CLOUDS TRYING TO SLIDE
SOUTH INTO ERN OREGON BUT THESE ARE HAVING A DIFFICULT TIME. MOST
RECENT MODEL FORECASTS INCREASE MSTR BY EARLY SUNDAY AHEAD OF THE
NEXT SYSTEM. SO THINK IDEA OF INCREASING CLOUDS EARLY SUNDAY FROM
NORTH TO SOUTH IS STILL VALID AND WILL CONTINUE TO SHOW THIS IN
THE FORECAST. HAVE UPDATED EVENING FORECAST TO REMOVE ANY MENTION
OF POPS AND TO DECREASE SKY COVER OVER THE FORECAST AREA. ALSO
INCREASED WINDS AND WIND GUSTS JUST A BIT OVER LAKE TAHOE SUNDAY
INTO SUNDAY EVENING AS WINDS SHOULD INCREASE AHEAD OF THE
APPROACHING LOW AND BE ABLE TO MAKE IT DOWN TO THE LAKE SFC.

WILL HOLD OFF ON UPGRADING WINTER STORM WATCHES TO WARNINGS FOR A
FEW HOURS TO REEVALUATE AMOUNTS IN NEW FORECAST MODELS. BUT FEEL
CONFIDENT THAT AREAS CURRENTLY UNDER WATCHES WILL NEED WARNINGS
AND SURPRISE VALLEY AND NORTHERN WASHOE COUNTY WILL LIKELY NEED
ADVISORIES FOR SNOW SUNDAY NIGHT INTO MONDAY. MLF

&&

.PREVIOUS DISCUSSION... /ISSUED 446 PM PST SAT DEC 5 2009/ 

SHORT TERM...
THE COLD FRONT THAT MOVED THROUGH WESTERN NV THIS MORNING HAS
MOVED SOUTH OF THE CWA THIS AFTERNOON. A NARROW POST FRONTAL BAND
OF CLOUDS WITH LIGHT SNOW SHOWERS MOVED THROUGH THE RENO AREA
DURING THE MORNING. THIS BAND IS SOUTH OF CARSON CITY THIS
AFTERNOON AND COULD PRODUCE ISOLATED SNOW SHOWERS BEFORE
DISSIPATING LATER THIS EVENING.

THE FRONT IS ASSOCIATED WITH A SHORT WAVE DROPPING SOUTH ON THE
EAST SIDE OF A HIGHLY AMPLIFIED RIDGE IN THE GULF OF ALASKA. A
SECOND SHORT WAVE WILL FOLLOW ON SUNDAY AND WILL DIG WESTWARD...
PINCHING OFF THE RIDGE WHILE DEVELOPING AN UPPER LOW ALONG THE
CA/OREGON BORDER SUNDAY AFTERNOON. 

THERE IS AN OVER WATER TRAJECTORY WITH THIS SYSTEM THAT WILL ADD
TO THE AVAILABLE MOISTURE IS IT APPROACHES THE CWA. IN
ADDITION...UNDERCUTTING ENERGY WORKS THROUGH THE PINCHED-OFF RIDGE
AND PHASES WITH THE SHORT WAVE AS IT DROPS SOUTH ACROSS NORTHERN
CA LATE SUNDAY NIGHT AND MONDAY. WINTER STORM WATCHES HAVE BEEN
CONTINUED FOR THE EASTERN CA ZONES AND INTO EXTREME WESTERN NV.
THE START TIMES HAVE BEEN PUSHED BACK A FEW HOURS FOR LASSEN
COUNTY TO LAKE TAHOE SO ALL WATCHES NOW BEGIN 10PM SUNDAY EVENING
AND RUN THROUGH 10 PM MONDAY EVENING.

THE AIRMASS IS VERY COLD AND SNOW RATIOS ARE EXPECTED TO BE IN THE
20-30 TO 1 RANGE WITH THIS STORM. H7 TEMPERATURES WILL BE -14 TO
-18 WITH SNOW AT EVEN THE LOWEST VALLEY FLOORS. BUFKIT SOUNDINGS
SHOW POTENTIAL FOR LAKE ENHANCEMENT OFF OF LAKE TAHOE ON MONDAY AS
LAKE-INDUCED CAPE EXCEEDS 1000 J/KG. THIS WOULD INCREASE SNOWFALL
EAST OF THE LAKE AND INTO EXTREME WESTERN NV SO WILL NEED TO BE
MONITORED. SNOW TOTALS DURING THE TIME OF THE WATCH WILL BE UP TO
9 INCHES IN FOOTHILLS OF WRN NV WITH 24-30 INCHES IN THE HIGHER
ELEVATIONS OF THE SIERRA. THE SHORT WAVE WILL ROTATE AROUND THE
NORTHERN CA LOW AND EJECT EASTWARD TAKING THE SNOW EAST OF THE CWA
MONDAY NIGHT.

LOW TEMPERATURES WILL DIP INTO SINGLE DIGITS IN WESTERN NV VALLEYS
MONDAY NIGHT AND AGAIN TUESDAY NIGHT. HIGHS MONDAY WILL STRUGGLE
TO GET OUT OF THE 20S WITH SIMILAR COLD TEMPS TUESDAY. RC

LONG TERM...WEDNESDAY THROUGH SATURDAY...
NEXT PUSH OF PACIFIC MOISTURE IS STILL EXPECTED TO ARRIVE LATER THIS 
WEEK...ALTHOUGH MOST OF THE GUIDANCE DATA IS TRENDING TOWARD A LATER 
TIME OF PRECIP ONSET...FAVORING WED NIGHT INSTEAD OF DURING THE DAY. 
THE ECMWF AND MOST OF THE GFS ENSEMBLE MEMBERS ARE IN AGREEMENT WITH 
THIS LATER START TIME...WHILE THE CANADIAN IS 6-12 HOURS FASTER. THE 
INVERSION IS LESS LIKELY TO BE BROKEN IN THE VALLEYS ON WED WITH 
LIGHT NE FLOW NEAR THE SFC...RESULTING IN TEMPS POSSIBLY STAYING 
BELOW FREEZING ALL DAY AS WIDESPREAD MID AND HIGH LEVEL CLOUDS 
SPREAD OVER THE REGION.  

THE BULK OF PRECIP WITH THIS LOW PRESSURE SHOULD OCCUR FROM WED 
NIGHT THRU THURSDAY MORNING...THEN TAPER TO SHOWERS THURS AFTN OR 
EVE.  THERE ARE SOME TRACK DIFFERENCES WITH THIS SYSTEM WITH SOME OF 
THE ENSEMBLE MEMBERS AND ECMWF FAVORING A MORE SOUTHERLY TRACK WHICH 
WOULD HIT MONO AND INYO COUNTIES HARDER THAN THE TAHOE BASIN. THE 
CURRENT FORECAST ALREADY HITS MONO HARDER BUT THE MOISTURE BAND WILL 
PROBABLY BE BROAD ENOUGH TO AFFECT NEARLY ALL OF THE REGION.   

THE MODEL SOUNDINGS FROM THE 12Z GFS ARE INDICATING NO SIGNIFICANT 
WARM AIR INTRUSION FOR THE LOWER ELEVATIONS WHICH COULD KEEP MOST 
VALLEYS OF WRN NV ALL SNOW FOR THIS EVENT. SNOW LEVELS WILL CONTINUE 
TO BE TRENDED DOWNWARD DUE TO THIS DATA. DESPITE 700 MB TEMPS 
RANGING FROM -4 TO -7 DEGREES C...THE TEMPERATURE PROFILE BELOW 700 
MB IS LARGELY ISOTHERMAL AND HELD BELOW FREEZING WITH SATURATION 
DOWN TO THE SURFACE THRU 18Z THURSDAY. TEMPS IN WRN NV MAY NUDGE 
INTO THE MID OR UPR 30S FOR A FEW HOURS IN THE AFTN IF THE STEADY 
PRECIP ENDS SOON ENOUGH...BUT THE TIMING OF SHORTWAVES WITHIN THE 
MOIST PACIFIC FLOW MAY NOT ALLOW THIS RISE IN TEMPS TO OCCUR. 

THE MAGNITUDE OF THE PRECIP EVENT FROM WED-THURS CONTINUES TO TREND 
DOWN FOR THE SIERRA WITH MAX QPF ALONG THE CREST AROUND 2 INCHES AND 
A SHORTER DURATION OF ENHANCED LIFT AS THE LOW MOVES ONSHORE. THIS 
WOULD STILL YIELD A DECENT AMOUNT OF SNOW FOR THE SIERRA AND INTO 
PARTS OF WRN NV...BUT THIS IS UNLIKELY TO BE AN EXTREME EVENT. 

FOR FRI-SAT...ANOTHER SURGE OF PACIFIC MOISTURE IS EXPECTED TO REACH 
THE SIERRA AND WRN NV. TIMING AND INTENSITY REMAIN UNCERTAIN AT THIS 
TIME...ALTHOUGH OVERALL GUIDANCE APPEARS TO INDICATE THAT THIS 
SYSTEM WILL PRODUCE HIGHER PRECIP AMOUNTS THAN THE WED-THURS SYSTEM. 
SNOW LEVELS ALSO MAY RISE ABOVE THE VALLEYS WITH THIS SYSTEM. 
HOWEVER...POPS AND SNOW LEVELS WERE ONLY NUDGED UPWARD MODESTLY FOR 
FRI-FRI NIGHT SINCE THE MODELS COULD ALSO BE OVERESTIMATING QPF AND 
MID LEVEL TEMPS...AS THEY ORIGINALLY PROJECTED WITH THE WED-THURS 
SYSTEM A FEW DAYS AGO. MJD  
 

AVIATION...
BAND OF STRATOCUMULUS MOVING SOUTH ACROSS THE REGION HAS PRODUCED 
CIGS BTWN 3500-5500 FT AGL TODAY...STILL IN THE VFR RANGE. FOR 
KTRK-KLOL-KRNO THIS CLOUD DECK WILL DEPART BEFORE 00Z...WHILE AT 
KTVL THIS CLOUD DECK SHOULD BREAK UP BY 02Z. LIGHT WINDS AND VFR 
VSBY EXPECTED TONIGHT THRU SUN AM. 

SW WINDS TO INCREASE BTWN 18-20Z AT KTVL-KTRK AND BTWN 20-22Z AT 
KRNO WITH GUSTS 30-35 KT AT TIMES. AREAS OF -SN REACHING KTRK-KTVL 
AROUND 00Z SUN WITH MVFR VSBY/CIGS...THEN INTENSIFYING TO IFR/LIFR 
CONDS BY 06Z AND CONTINUING THRU MOST OF MONDAY.

VFR CIGS TO CONTINUE AT KRNO THRU 06Z SUN EVE...THEN -SN TO BEGIN 
AFT 06Z WITH MVFR CIGS/VSBY AND INTENSIFYING TO IFR AND OCNL LIFR 
CONDS BY 12Z MONDAY AND CONTINUING THRU MOST OF THE DAY. MJD  

&&

.REV WATCHES/WARNINGS/ADVISORIES...
NV...WINTER STORM WATCH FROM SUNDAY EVENING THROUGH MONDAY EVENING 
     FOR NVZ002-003.

     LAKE WIND ADVISORY FROM 10 AM TO 10 PM PST SUNDAY FOR NVZ002.

CA...WINTER STORM WATCH FROM SUNDAY EVENING THROUGH MONDAY EVENING 
     FOR CAZ071>073.

     LAKE WIND ADVISORY FROM 10 AM TO 10 PM PST SUNDAY FOR CAZ072.

&&

$$

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