Snow Advisory 12, NWS Reno, NV

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RENO NV
332 AM PST THU FEB 21 2008

.SHORT TERM...
PROGRESSIVE SHORTWAVE RIDGE IS QUICKLY PUSHING EAST OF THE AREA WITH
THE NEXT PAC STORM SYSTEM JUST OFF SHORE OF NORTHERN CA EXPECTED TO
MOVE IN THROUGH THE DAY. HIGH CLOUDS HAVE SPREAD ACROSS THE WESTERN
HALF OF THE CWA...AND WILL CONTINUE TO INCREASE THROUGH THE DAY FROM
WEST TO EAST AHEAD OF THE APPROACHING SHORTWAVE TROUGH.

WITH MODEL QPF TRENDING HIGHER THAN PREVIOUS RUNS...DECIDED TO GO
WITH A SNOW ADVISORY FOR THE SIERRA ZONES FROM NOON TODAY THROUGH 4
AM FRIDAY MORNING. SEE /WSWREV/ FOR DETAILS.

THE STRONG ZONAL JET STREAM WILL PUSH INTO CENTRAL CA THIS
AFTERNOON...WITH GOOD DIFFLUENCE ALOFT SETTING UP OVER THE NORTHERN
AND CENTRAL SIERRA THIS AFTERNOON AS THE SHORTWAVE TROUGH BECOMES
NEGATIVELY TILTED. ZONAL JET WILL BRING MUCH BETTER MOISTURE TAP
ONSHORE...WITH 0.75 TO 1.0 INCHES PRECIPITABLE WATER OFF THE CENTRAL
CALIFORNIA COAST. LATEST GFS IS SHOWING EXCELLENT INSTABILITY AHEAD
OF THE SHORTWAVE...WITH STRONG SIGNAL OF ELEVATED CAPE VALUES WITHIN
THE DENDRITIC SNOW GROWTH ZONE OF OVER 100 JOULES/KG...STARTING OVER
THE LAKE TAHOE AREA AND THEN DROPPING INTO MONO COUNTY BY EARLY
EVENING. LOW LEVEL FRONTOGENESIS IS ALSO SHOWING A STRONG SIGNAL
ALONG THE CREST. BASICALLY... THIS WILL ALL TRANSLATE INTO A PERIOD
OF HEAVY SNOW SOMETIME IN THE LATE AFTERNOON AND EARLY EVENING IN
THE NORTHERN AND CENTRAL SIERRA. EAST OF THE SIERRA...LOOKS LIKE A
MIX OF RAIN AND SNOW FOR TODAY. BUMPED UP SNOW LEVELS ACROSS MOST OF
THE CWA AS MODELS ARE TRENDING WARMER. MOST DESERT VALLEYS WILL SEE
RAIN TODAY AND TONIGHT FOR THE MOST PART...WITH UP TO ONE INCH OF
SNOW ACCUMULATION ABOVE 5000 FEET. SNOW LEVELS WILL COME DOWN TO
BELOW 4500 FEET BY EARLY FRIDAY MORNING.

FRIDAY AFTERNOON...ANOTHER QUICK SHOT OF SNOW AND RAIN AS A
SECONDARY LITTLE SHORTWAVE MOVES THROUGH THE AREA. SNOW LEVELS WILL
BE BE AROUND 4500 FEET RISING THROUGH THE DAY. WE WILL RIDGE UP
FRIDAY NIGHT WITH A LITTLE BREAK IN THE ACTION BEFORE THE MONSTER
MOVES IN FOR THE WEEKEND.  HOON

LONG TERM...SATURDAY THROUGH WEDNESDAY...
MODELS REMAIN IN GOOD AGREEMENT FOR THE MAIN SYSTEM THIS WEEKEND. SO
CONFIDENCE CONTINUES TO INCREASE IN THIS STORM...BUT MODELS ARE ALSO
CONTINUING TO INDICATE SLIGHTLY WARMER AIRMASS AND AREAS EAST OF THE
SIERRA STAYING IN THE WARM SECTOR LONGER INTO THE EVNG HOURS SAT
BEFORE CHANGING TO MOSTLY SNOW BY SUN MRNG. OVERALL BEST QPF WILL BE
IN THE SIERRA SAT NIGHT AND SUN MRNG WITH SIGNIFICANT SNOWFALL
LIKELY. WILL PROBABLY NEED TO ISSUE WATCHES BY EARLY FRI MRNG FOR
THE SAT NIGHT/SUN TIME FRAME.

ECMWF IS A LITTLE SLOWER BRINGING PCPN INTO THE REGION SAT THAN GFS
AND HAVE OPTED TO HAVE SLIGHTLY LOWER POPS FOR THE DAYTIME HOURS
SAT. AND AS STATED ANY PCPN THAT FALLS EAST OF THE SIERRA SHOULD
START AS RAIN. SNOW LEVELS SHOULD START AROUND 5500 FEET OR HIGHER
SAT EVNG THEN FALL BY SUNDAY TO NEAR THE VALLEY FLOORS. RECENT
MODELS RUNS HAVE BEEN GRADUALLY RAISING SNOW LEVELS BUT THINK LEVELS
COULD FALL RATHER QUICKLY AS PCPN BEGINS. SOME QUESTION REMAINS
ABOUT TIMING OF SPILLOVER AND WILL NOT ADDRESS THAT THIS FAR OUT.
ANOTHER CONCERN FOR SATURDAY AND SAT EVNG IS WIND POTENTIAL FOR THE
LEE SIDE. MODELS...ESPECIALLY THE GFS...INDICATE H7 WINDS OF AROUND
50 KTS BY LATE SAT AFTN. FORECAST SOUNDINGS DO NOT SEEM TO INDICATE
MUCH OF A CRITICAL LEVEL DEVELOPING ON SAT AND WITH WAA ALOFT
THROUGH MUCH OF THE DAY...NOT SURE HOW WELL WINDS WILL MIX DOWN.
SOUNDINGS SHOW AIRMASS MOISTENING BY LATE EVENING SO THINK PCPN
WOULD BE INTO WRN NV BY THEN WITH LESS CHANCE FOR WIND. THIS WILL
HAVE TO BE WATCHED CLOSELY NEXT FEW RUNS. QPF AMOUNTS IN THE SIERRA
MAY APPROACH 1 1/2 INCHES IN THE HIGHEST ELEVATIONS SO SNOW FALL
COULD EASILY REACH AROUND 2 FEET OR MORE IN THE HIGHEST ELEVATIONS.
WILL COVER THIS AGAIN WITH SPS.

PCPN SHOULD LINGER INTO EARLY SUN AFTN ACROSS MOST OF THE CWA WITH
SNOW LEVELS FALLING TO THE VALLEY FLOORS BRIEFLY BEFORE RISING AGAIN
LATE IN THE AFTN. MODELS HAVE BEEN CONSISTENT IN SHOWING HIGHS FOR
SUN INTO THE 40S EAST OF THE MOUNTAINS...SO AM NOT CONVINCED
ACCUMULATIONS EAST OF THE MTNS WILL BE THAT GREAT. THIS SYSTEM
SHOULD LIFT OUT OF THE REGION BY LATE SUNDAY WITH LONG WAVE RIDGE
STARTING TO DEVELOP ALONG THE WEST COAST BY EARLY MON.

OVERNIGHT LOWS SUN NIGHT/MON MRNG COULD BE RATHER COOL WITH LOW LVL
NORTHERLY FLOW DEVELOPING...BUT THEN RIDGE BEGINS TO BUILD OVER THE
AREA AND TEMPS SHOULD START TO RISE. MODELS CONTINUE TO SHOW LOW LVL
WINDS MORE E TO SE BY LATE MON INTO TUE INDICATING LESS CHANCE OF
TRAPPING COLD AIR IN THE VALLEYS UNDER A SIG INVERSION. SO WILL
BEGIN TO INDICATE RISING TEMPS FOR TUE AND WED...BUT STILL NOT
QUITE AS WARM AS GUIDANCE. MLF
&&

AVIATION...
FOG IN THE KTRK VICINITY THIS MRNG WITH MAINLY VFR CONDITIONS
ELSEWHERE. THE FOG AROUND KTRK WILL LIFT SHORTLY AFTER SUNRISE.
CONDITIONS IN THE SIERRA WILL BEGIN TO DETERIORATE LATER TODAY WITH
SNOW PUSHING INTO THE RGN. CIGS/VSBYS SHOULD FALL TO AROUND 1000
FT/1 MILE OR LESS EARLY THIS AFTN. PCPN WILL BE SLOW TO PUSH EAST OF
THE MTNS SO BOTH KRNO AND KLOL WILL REMAIN VFR INTO THE EARLY EVNG
BEFORE CONDITIONS DETERIORATE. EXPECT MVFR CONDITIONS BY AROUND 06
UTC FOR KRNO THEN OCCASIONAL IFR AFTER THAT. KLOL WILL SEE MVFR A
COUPLE OF HOURS LATER. MLF
&&

.REV WATCHES/WARNINGS/ADVISORIES...
NV...SNOW ADVISORY FROM NOON TODAY TO 4 AM PST FRIDAY FOR NVZ002.

CA...SNOW ADVISORY FROM NOON TODAY TO 4 AM PST FRIDAY FOR CAZ071>073.

&&

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