FXUS65 KREV 141905
AFDREV

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RENO NV
1105 AM PST MON DEC 14 2009

.UPDATE...
FOG HANGING IN TOUGHER THAN EXPECTED WITH MANY VALLEYS AROUND
RENO-CARSON STILL SITTING 1/4 MILE OR LESS. KSVE IS IMPROVING BUT
DOYLE AREA IS STILL SOCKED IN A BIT. WILL EXTEND FREEZING FOG ADVY
THROUGH 1 PM FOR RENO-CARSON...BUT EXPIRE LASSEN ALTHOUGH SOME
AREAS WILL REMAIN PATCHY THERE. ALSO SEEING LOCALLY DENSE FOG ON
PARTS OF I-80...BUT IT DOES NOT APPEAR TO BE AS WIDESPREAD THERE.
WILL HANDLE THAT ONE WITH SHORT TERM FORECASTS AS HWY 50 IS NOT
SEEING AS BIG AN IMPACT FROM THE PATCHY FOG AS I-80. WALLMANN

&&

.PREVIOUS DISCUSSION... /ISSUED 905 AM PST MON DEC 14 2009/ 

UPDATE...
QUICK UPDATE TO LOWER HIGHS FOR AREAS IMPACTED BY FOG. FOG IS
DENSE IN MANY AREAS WITH THE FREEZING FOG ADVY AND ALSO QUITE
DEEP. DEPTH OF FOG IS UP TO 500 FEET IN SOME AREAS AND THIS WILL
TAKE SOME TIME TO BURNOFF. TEMPS WILL BE SLOW TO RESPOND UNTIL
THIS OCCURS. ADVY ENDING TIME OF 19Z LOOKS GOOD FOR NOW...BUT IT
WILL BE A SLOW BURNOFF WITH SOME AREAS NOT COMPLETELY CLEARING
UNTIL 2 PM ALTHOUGH VIS WILL SLOWLY IMPROVE. NO OTHER CHANGES
PLANNED ATTM. WALLMANN

PREVIOUS DISCUSSION... /ISSUED 517 AM PST MON DEC 14 2009/ 

UPDATE...
FREEZING FOG HAS BECOME A BIT MORE WIDESPREAD IN THE RENO/SPARKS
AREA AND ALSO IN THE SUSANVILLE AREA AND TO THE SOUTHEAST ALONG
HIGHWAY 395 IN LASSEN COUNTY. FOG HAS STARTED TO LIFT A BIT IN THE
TRUCKEE VICINITY. WITH LOW LVL COLD AIR REMAINING IN THE
RENO/SPARKS AREA THROUGH MUCH OF THE MORNING AND ALSO IN AND NEAR
SUSANVILLE WOULD EXPECT THOSE AREAS TO SEE FREEZING FOG PERSIST
THE LONGEST THIS MORNING. RIME ICING ON AREA ROADS SHOULD CREATE
DANGEROUS DRIVING CONDITIONS FOR THE MORNING COMMUTE. THUS WILL
ISSUE A FREEZING FOG ADVISORY FOR ZONE 071 AND ZONE 003. MLF 

PREVIOUS DISCUSSION... /ISSUED 409 AM PST MON DEC 14 2009/ 

SHORT TERM...
AN AREA OF LIGHT SNOW SHOWERS HAS MOVED INTO W-CNTRL NV SO ISOLD
POPS WERE ADDED FOR THIS AREA THRU 15Z THIS MORNING. AREAS OF FZFG
HAVE BEEN REPORTED IN PARTS OF THE TAHOE BASIN AND THE TRUCKEE
AREA...WITH SOME DENSE FOG INDICATED BY THE LOWER ELEVATION WEB
CAMERAS AROUND SUSANVILLE. TEMPS ARE BELOW FREEZING ACROSS THE
REGION SO WX TYPE WAS CHANGED FROM FG TO FZFG FOR THE UPCOMING 
FORECAST THIS MORNING...AND THE MORNING COMMUTERS WILL NEED TO USE
EXTRA CAUTION AS SLUSHY ROADS REFROZE OVERNIGHT...AND SOME BLACK
ICE POSSIBLE ON TREATED ROAD SURFACES THAT DID NOT HAVE A CHANCE
TO DRY OUT YESTERDAY.

COMPLEX FORECAST IS EXPECTED FOR NEXT FEW DAYS AS WIDESPREAD SNOW
COVER...A THICKENING AREA OF MID AND HIGH LEVEL CLOUD COVERAGE IS
MET BY A MOIST SUBTROPICAL FLOW AND INCREASED WINDS AT RIDGE
LEVEL. THIS PRESENTS A DILEMMA FOR SNOW LEVELS AS THE VALLEYS MAY
FAIL TO MIX OUT SUFFICIENTLY. WHILE THE WARMING TEMPS AT 700 MB
WOULD SUGGEST A HIGH SNOW LEVEL...MODEL SOUNDINGS HAVE CONSISTENTLY
KEPT A NEARLY ISOTHERMAL LAYER BELOW 700 MB TO THE SURFACE. THE
NAM IS EVEN MORE AGGRESSIVE IN KEEPING COLD AIR IN PLACE DOWN TO
THE SFC THRU WED MORNING...FAVORING SNOW AS THE PRECIP TYPE EVEN
FOR LOWER VALLEYS OF NERN CA SUCH AS SUSANVILLE. WHILE THE
FORECAST WILL GENERALLY INDICATE SNOW LEVELS BTWN 5500-6000 FT
THRU WED MORNING...THIS DOES NOT NECESSARILY RULE OUT FROZEN
PRECIP IN ALL AREAS BELOW THIS ELEVATION DUE TO THE UNCERTAINTY IN
THE SCOURING OUT OF THE LOW LEVEL COLD AIR MASS.

PRECIP AMOUNTS HAVE WAVERED FOR NERN CA AS GUIDANCE SOURCES HAVE
VARIED WITH THE ORIENTATION AND TIMING OF THE MAX PRECIP AXIS. THE
COASTAL AREAS OF NWRN CA APPEAR TO BE IN LINE FOR THE HEAVIEST
PRECIP WHERE QPF VALUES ARE WELL OVER 2 INCHES...WITH AMOUNTS
DROPPING OFF TO NEAR 0.50 AS THE MOISTURE AXIS REACHES CNTRL
PLUMAS AND SWRN LASSEN COUNTIES. QPF DROPS OFF QUICKLY ACROSS THE
REMAINDER OF NERN CA TUES NGT AND WED AS THE PRECIP DRIFTS SOUTH
ACROSS THE TAHOE BASIN. WITH THE POSSIBILITY THAT MORE PRECIP
COULD FALL AS SNOW AS THIS SYSTEM PEAKS ITS MOISTURE DURING THE
NIGHT OR MORNING...THE THREAT OF SIGNIFICANT RUNOFF CONCERNS
APPEARS TO BE LOW. 

TEMPS WILL STRUGGLE TO RISE TODAY AND TUESDAY AS THICK MID AND
HIGH CLOUD COVERAGE MOVES INTO THE REGION...BUT ENOUGH COOLING
SHOULD OCCUR AT NIGHT TO BRING TEMPS BELOW FREEZING. BY THE TIME
WARMER AIR IS MORE LIKELY TO REACH LOWER ELEVATIONS DURING THE
DAY WEDNESDAY...THE PRECIP BAND ALSO DIMINISHES. BY WED
NIGHT...LITTLE OR NO MEASURABLE PRECIP IS INDICATED ACROSS THE
REGION AS THE MOISTURE PLUME BREAKS DOWN. MJD

LONG TERM...THURSDAY THROUGH SUNDAY...
OVERALL HAVE MADE ONLY MINIMAL CHANGES TO THE EXTENDED PERIOD THIS 
FORECAST AS MEDIUM RANGE MODELS ARE IN DECENT AGREEMENT EARLY IN THE 
PERIOD BUT BEGIN TO DIVERGE QUITE A BIT BY THE END OF THE WEEKEND.

OPERATIONAL MODELS AND MOST ENSEMBLE MEMBERS START THE EXTENDED 
PERIOD WITH A FLAT RIDGE OVER THE RGN. THIS SHUD ALLOW SOME MSTR TO 
WORK INTO THE NRN PORTIONS OF THE CWA LATE THURSDAY INTO FRIDAY. GFS 
LINGERS THE FLAT RIDGE A LITTLE LONGER THAN THE ECWMF AND CANADIAN. 
ENSEMBLE SUPPORT IS SOMEWHAT MORE IN FAVOR OF THE ECMWF SOLUTION. WE 
HAVE LOW POPS IN FAR NW PORTIONS OF THE CWA ALREADY FOR 
THURSDAY/THURSDAY NIGHT AND WILL LEAVE THESE GIVEN PRESENCE OF THE 
FLAT RIDGE AND SHORT WAVES RIDING OVER THE TOP.

BEYOND FRIDAY MOST MODELS SHOW RIDGE BUILDING WITH FAIRLY MILD TEMPS 
RETURNING. ALTHOUGH QUESTION REMAINS AS TO HOW WARM WE MIGHT GET. 
FOR NOW KEPT TEMPS AOB MODEL GUIDANCE AS WINDS IN THE MID LVLS 
BECOME LIGHT WITH LIMITED MIXING. THIS MAY HOLD VALLEY HIGH TEMPS 
DOWN JUST A BIT THROUGH SATURDAY.  SUNDAY IS A BIT MORE PROBLEMATIC 
AND AT THIS POINT CONFIDENCE IN THE FORECAST BECOMES VERY LOW. BY 
EARLY SUNDAY MRNG OPERATIONAL GFS IS SLIDING THE RIDGE AXIS EAST AND 
TRYING TO BRING A DEVELOPING LOW OFF THE WEST COAST TO NEAR THE 
OREGON COAST BY LATE IN THE DAY. ECMWF HOLDS THE RIDGE AXIS OVER THE 
RGN LONGER AND THIS IS SUPPORTED BY MORE OF THE ENSEMBLE MEMBERS. 
GFS WOULD RESULT IN HIGHER POPS OVER THE NW 1/4 OF THE CWA BY LATE 
SUNDAY WHILE ECMWF MEANS MILDER WEATHER AND DRY. THERE WERE ALREADY 
SLIGHT CHANCE POPS IN THE FORECAST FOR TAHOE/LASSEN AREAS AND WILL 
OPT TO KEEP THESE IN FOR NOW. AS STATED...FORECAST CONFIDENCE IS LOW 
BY THE END OF THE EXTENDED PERIOD GIVEN MODEL DIFFERENCES. BUT WOULD 
NOT BE SURPRISED IF GFS BEGINS TO TREND TOWARD MORE OF ITS ENSEMBLE 
MEMBERS AND THE ECMWF FORECAST WITH SUBSEQUENT RUNS. MLF

AVIATION...
LINGERING LOW/MID CLOUDS AND HAVE KEPT FOG FROM BECOMING VERY 
WIDESPREAD THIS MRNG. HAVE SEEN VSYBS AT TAF SITES BOUNCE UP AND 
DOWN THOUGH AND A FEW SNOW SHOWERS EVEN REMAIN IN THE SIERRA. THE 
SNOW SHOWERS WILL END THIS MRNG AND ALL AREAS SHUD BECOME VFR BY 
LATE MRNG. MVFR VSBYS STILL LIKELY ALL TAF SITES THROUGH 14/17 UTC 
AS PATCHY FOG DEVELOPS. BRIEF IFR CONDITIONS POSSIBLE AS WELL FOR 
KTVL/KTRK IN PASSING SNOW SHOWERS. KLOL/KRNO COULD ALSO BRIEFLY GO
BELOW 1 MILE VSBY IF MID CLOUDS BREAK.

CLOUDS BREAK BRIEFLY THIS EVENING ALLOWING FOG TO DEVELOP FOR KTRK 
AND KLOL BUT THEN CLOUDS ASCD WITH NEXT SYSTEM BEGIN TO MOVE INTO 
THE CWA. THIS SHUD LIMIT THE AMOUNT OF FOG AND KEEP IT RATHER 
LOCALIZED. NOT ANTICIPATING MUCH PCPN COVERAGE LATE TONIGHT. ONLY 
AREAS WITH ANY CHANCE WOULD BE TAHOE BASIN NORTHWARD. AND THIS WOULD 
BE MAINLY SCATTERED SHOWERS. STILL...KTRK ESPECIALLY CUD SEE MVFR TO 
IFR CIGS/VSBYS AFTER 15/09 UTC WITH SNOW SHOWERS. MLF

&&

.REV WATCHES/WARNINGS/ADVISORIES...
NV...FREEZING FOG ADVISORY UNTIL 1 PM PST THIS AFTERNOON FOR NVZ003.

CA...&&

$$

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