Heavy Snow and Winter Weather in California, December 19 to 20, 2007

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RENO NV
324 PM PST WED DEC 19 2007

.SHORT TERM...
PRECIP SHIELD HAS ALREADY PUSHED INTO NORTHERN CA AND WILL GRADUALLY
MOVE SOUTH DOWN THE NORTHERN SIERRA AND INTO FAR NORTHWESTERN
NEVADA DURING THE EVENING HOURS. PHASING OF MOISTURE AND FORCING
LOOKS TO HOLD TOGETHER AS THE LOW DROPS SOUTH THROUGH THE FORECAST
AREA TONIGHT AND THURSDAY PRIOR TO SPLITTING. WITH GOOD JET AND
FRONTAL DYNAMICS ALONG WITH DECENT MOISTURE...EXPECT THE NORTHERN
SIERRA...INCLUDING THE TAHOE BASIN...TO RECEIVE 1 TO 2 FEET OF
SNOW ABOVE 6500-7000 FEET WITH LOCALLY HIGHER AMOUNTS. LIQUID
PRECIP AMOUNTS FOR MONO COUNTY ARENNT QUITE AS IMPRESSIVE DUE TO
THE MOISTURE PLUME THINNING AND A SECONDARY WAVE SPLITTING THE
SYSTEM AS IT DROPS SOUTH OF TAHOE. THIS SHOULD STILL RESULT IN A
GOOD 10-18 INCHES OF SNOW NEAR THE SIERRA CREST IN MONO COUNTY.

THE BIGGEST CHALLENGES OF THE FORECAST ARE WINDS AND SNOW POTENTIAL
IN THE LEE OF THE SIERRA/WARNERS...NOTABLY NORTHERN WASHOE AND THE
395 CORRIDOR BETWEEN COLD SPRINGS AND GARDNERVILLE. THE NAM SHOWS
THE STRONGEST WINDS AND HAS IMPRESSIVE MOS WIND VALUES.
HOWEVER...THE MODEL IS WAY OVERDOING THE AMOUNT OF WARMING NEAR
700 MB...WHICH IS CRITICAL FOR DOWNSLOPE WINDS. NAM FORECASTED TEMPS
TO WARM TO NEAR FREEZING AT THIS LEVEL TODAY...YET SLIDE MTN REMAINS
NEAR 20 DEGREES. THIS IS MUCH CLOSER TO WHAT THE LIGHTER GFS WIND
SOLUTION PREDICTED. WITH COOLER RIDGE TEMPS...THE CRITICAL LEVEL
DOESNT LOOK TO BE AS PRONOUNCED. HOWEVER THERE ARE SOME NICE
LENTICULAR CLOUDS THAT HAVE DEVELOPED AND A SPECIAL 21Z HMT
PROJECT SOUNDING SHOWING SOME CRITICAL LEVEL DEVELOPMENT.
THEREFORE...WILL NOT TOTALLY DISCOUNT WIND THREAT AND CONVERT HIGH
WIND WATCH TO A WIND ADVISORY FOR RENO/CC ZONE. THE STRONGEST
WINDS SHOULD BE SEVERAL HOURS PRECEDING FRONTAL PASSAGE...ROUGHLY
AROUND 1 TO 4 AM. WITH ISENTROPIC PRECIP FALLING INTO NORTHERN
WASHOE/CEDARVILLE BY EARLY THIS EVENING ALONG WITH THE COLD
SATURATED PROFILES AHEAD OF THE FRONT...DO NOT SEE GUSTS GETTING
OUT OF THE 30-40 MPH RANGE AND THUS NO WIND STATEMENTS FOR ZONES
70 AND 5. DID HOWEVER EXPAND THE WIND ADVISORY SOUTH TO INCLUDE
MINERAL AND SOUTHERN LYON COUNTIES AS WINDS AHEAD OF THE FRONT DO
LOOK STRONG (30 AGL OVER 40 KTS). WINDS GUSTING TO OVER 80 MPH ARE
EXPECTED OVER THE SIERRA RIDGES AS 700 MB FLOW OF OVER 60 KTS
MOVES IN OVERNIGHT.

HAVE ISSUED A SNOW ADVISORY FOR ZONE 5 (NORTHERN WASHOE/SURPRISE
VALLEY) AS OVERRUNNING PRECIP THIS EVENING...PRECIP ALONG THE
FRONT...AND THEN SOME CONVECTIVE PRECIP IN THE COLD POOL SHOULD ADD
UP TO 2-4 INCHES...ESPECIALLY WITH THE GOOD LIQUID/SNOW RATIOS
EXPECTED. THE TRICKIER PART OF THE FORECAST IS SNOW POTENTIAL FOR
RENO/CC. FRONTEGENESIS ALONG WITH STRONG JET DYNAMICS LOOK TO MOVE
THROUGH THE AREA BEGINNING AROUND 4 AM. WITH THE STRONG WEST TO
SOUTHWEST FLOW FROM RIDGETOP TO 300 MB (BEFORE SPLITTING OCCURS)
BELIEVE ADVECTION OF HYDROMETEORS AND DYNAMICS SHOULD BE ENOUGH TO
LET UP TO A COUPLE INCHES OF SNOW ACCUMULATE ON VALLEY FLOORS WITH
2-4 IN THE FOOTHILLS. IDEALLY WOULD LIKE TO SEE THE FRONT AND JET
STALL OR REMAIN OVER US SLIGHTLY LONGER TO IMPROVE CONFIDENCE.
HOWEVER...TIMING OF FRONT AND PRECIP IS IDEAL FOR SETTING UP FOR
A POTENTIALLY REAL MESSY MORNING COMMUTE...SO HAVE PULLED THE TRIGGER
AND ISSUED A SNOW ADVISORY.

COLD POOL MOVES OVER THE AREA DURING THE AFTERNOON FOR SOME
CONVECTIVELY DRIVEN SNOW SHOWERS.  THESE SHOULD DISSIPATE AROUND
SUNSET AND MAKE FOR SOME COLD NIGHTS AHEAD. THERE IS SOME POTENTIAL
FOR LAKE EFFECT SNOW OVER PYRAMID THURSDAY EVENING WITH NORTHERLY
700 FLOW AROUND 15-20 KTS AND 700 TEMPS OF -14 TO -16C. HAVE
ADDED A COUPLE OF INCHES OF SNOW ON SOUTH END OF PYRAMID FETCH NEAR
NIXON AND PAH RAH RANGE FOR THURS EVENING. MILNE

.LONG TERM...SUNDAY THROUGH WEDNESDAY...
THERE IS STILL A LOT OF DISAGREEMENT AMONG THE MODELS IN THE
EXTENDED PERIOD...LIKELY DUE TO THE PROGRESSIVE FAST FLOW. A WEAK
SYSTEM PASSING BY TO THE NORTH OF THE AREA ON SUNDAY MAY GENERATE
SOME LIGHT SHOWERS OVER MAINLY NRN ZONES BEFORE A WEAK RIDGE MOVES
ACROSS THE AREA AHEAD OF THE NEXT SYSTEM. MODELS DO SEEM TO AGREE A
SYSTEM WILL MOVE THROUGH THE AREA SOMETIME MONDAY INTO TUESDAY...BUT
DISAGREE GREATLY ON THE DETAILS. THE GFS AND MOST OF ITS ENSEMBLE
MEMBERS HINT AT A SYSTEM ONLY BRUSHING NRN NV MONDAY AND MONDAY
NIGHT. THE 12Z EUROPEAN HAS COME MORE IN LINE WITH THE GFS AS
OPPOSED TO THE 00Z RUN WHERE IT BROUGHT A DEEPER TROUGH OVER THE
AREA. THE CANADIAN STILL INDICATES A DEEPER TROUGH...BUT THIS IS
NOW CONSIDERED THE LONE OUTLIER. THEREFORE THE CURRENT FORECAST
WILL STICK WITH THE WEAKER GFS SOLUTION FOR NOW. THIS WILL LIKELY
PRODUCE SOME LIGHT SNOW OR SHOWERS MAINLY NORTH OF INTERSTATE 80
ON CHRISTMAS EVE INTO EARLY CHRISTMAS DAY...AND CHANCE POPS IN THE
SIERRA AND WRN NV WILL CONTINUE. UNCERTAINTY CONTINUES AS THE NEXT
SYSTEM MOVES INTO THE AREA ON WEDNESDAY. AGAIN THE EUROPEAN IS
MUCH STRONGER WITH A 539 DAM LOW DROPPING SOUTH ALONG THE SIERRA
WEDNESDAY AND WEDNESDAY NIGHT...WHILE THE GFS AND ITS ENSEMBLE
MEMBERS FAVOR A WEAKER LOW DROPPING SOUTH...BUT KEEP THE TROUGH
FURTHER EAST. THIS SYSTEM WILL BE MONITORED AND HAS THE POTENTIAL
TO BRING MORE SNOW TO THE SIERRA AND WRN NV DEPENDING ON THE
TRACK. TEMPERATURES WILL DEPEND GREATLY ON STORM TRACK...CLOUDS
AND PRECIP...BUT LOOK TO REMAIN NEAR OR SLIGHTLY BELOW NORMAL
THROUGH THE PERIOD. HIGHS GENERALLY WILL BE IN THE LOW TO MID 40S
OVER WRN NV AND IN THE 30S IN THE SIERRA. MCGUIRE

&&

.AVIATION...
WIND GUSTS UP TO 25 KTS THIS AFTERNOON WILL INCREASE AHEAD OF A
STRONG COLD FRONT DURING THE EVENING. WIDESPREAD WIND GUSTS UP TO
30-40 KTS ARE EXPECTED MAINLY BETWEEN 04Z-15Z IN WRN NV. IN THE
SIERRA WINDS WILL REMAIN GUSTY 20-30 KTS THROUGH THE NIGHT...BUT MAY
LIGHTEN UP SOMEWHAT AS SNOW BEGINS AFTER 01Z. MDT-HVY SNOW EXPECTED
06Z-22Z IN THE KTRK/KTVL TERMINAL AREAS...WITH SNOW SHOWERS
CONTINUING THROUGH THROUGH 06Z. LIFR TO VLIFR EXPECTED WITH SNOW
THROUGH 22Z. MOUNTAIN OBSCUREMENT EXPECTED THROUGH 22Z.

FOR KRNO...THE MAIN CONCERN FOR THE NEXT 12-24 HOURS WILL BE
INCREASING SURFACE WINDS TONIGHT AND SNOW ON THURSDAY. WINDS WILL
INCREASE AHEAD OF A COLD FRONT APPROACHING THE AREA. SOUTH TO
SOUTHWEST WINDS GUSTING UP 20-25 KTS THIS AFTERNOON WILL INCREASE
TONIGHT WITH GUSTS UP TO 35-45 KTS LIKELY...ESPECIALLY IN THE KRNO
AREA. WINDIEST PERIOD WILL BE FROM 04Z-15Z. MOUNTAIN WAVE ACTIVITY
AND ROTORS ARE LIKELY. AS THE COLD FRONT MOVES THROUGH THE AREA
AROUND 15Z WINDS SHOULD LIGHTEN SOMEWHAT AND CONVECTIVE SNOW SHOULD
BEGIN. AROUND 1 INCH OF QUICK SNOW ACCUMULATION IS POSSIBLE FROM
15Z-18Z WITH INSTABILITY SNOW SHOWERS EXPECTED THROUGH 00Z. MVFR TO
OCCASIONAL IFR POSSIBLE IN HEAVIER SNOW SHOWERS. MCGUIRE

&&

.REV WATCHES/WARNINGS/ADVISORIES...
NV...SNOW ADVISORY FROM 8 PM THIS EVENING TO 10 AM PST THURSDAY FOR
     NVZ005.

     LAKE WIND ADVISORY UNTIL 8 PM PST THIS EVENING FOR NVZ002.

     WINTER STORM WARNING FROM 8 PM THIS EVENING TO 4 PM PST THURSDAY
     FOR NVZ002.

     WIND ADVISORY FROM 9 PM THIS EVENING TO 10 AM PST THURSDAY FOR
     NVZ001.

     WIND ADVISORY FROM 9 PM THIS EVENING TO 4 AM PST THURSDAY FOR
     NVZ003.

     SNOW ADVISORY FROM 4 AM TO 10 AM PST THURSDAY FOR NVZ003.

CA...WINTER STORM WARNING UNTIL 4 PM PST THURSDAY FOR CAZ071.

     SNOW ADVISORY FROM 8 PM THIS EVENING TO 10 AM PST THURSDAY FOR
     CAZ070.

     WINTER STORM WARNING FROM 8 PM THIS EVENING TO 8 PM PST THURSDAY
     FOR CAZ073.

     LAKE WIND ADVISORY UNTIL 8 PM PST THIS EVENING FOR CAZ072.

     WINTER STORM WARNING FROM 8 PM THIS EVENING TO 4 PM PST THURSDAY
     FOR CAZ072.

&&

HTTP://WEATHER.GOV/RENO