FXUS66 KLOX 022131
AFDLOX

SOUTHWEST CALIFORNIA AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOS ANGELES/OXNARD CA
130 PM PST WED DEC 2 2009

.SYNOPSIS...

MOSTLY CLEAR AND COOL...EXCEPT FOR OVERNIGHT COASTAL LOW CLOUDS AND 
FOG INTO THURSDAY. HIGH PRESSURE WILL MOVE IN LATE THURSDAY AND WARM 
THE TEMPERATURES FOR THE END OF THE WEEK. LOW PRESSURE BRINGS A GOOD 
CHANCE OF RAIN FOR SUNDAY NIGHT AND INTO EARLY NEXT WEEK. THE 
DAYTIME TEMPERATURES NEXT WEEK WILL BE BELOW THE SEASONAL VALUES.

&&

.SHORT TERM (TODAY-SAT)...GRADIENTS THIS AFTERNOON ALREADY STARTING 
TO SHOW SIGNS OF TURNING OFFSHORE AND THIS HAS HELPED TO CLEAR OUT 
THE DEEP MARINE LYR IN MOST AREAS. HOWEVER, LINGERING MOISTURE AT 
LOWER LEVELS ALONG WITH A STRENGTHENING/LOWERING INVERSION SHOULD 
HELP GENERATE AT LEAST SOME LOW CLOUDS AGAIN OVERNIGHT ACROSS THE 
COAST AND VALLEYS. SHOULD SEE SLIGHTLY WARMER TEMPS MOST AREAS THU 
WITH EARLIER SUNSHINE AND WEAK ONSHORE GRADIENTS. 

A RIDGE OF HIGH PRESSURE ALOFT ACROSS THE ERN PACIFIC WILL INCREASE 
HEIGHTS ACROSS THE AREA THURSDAY NIGHT INTO FRIDAY. AT THE SAME 
TIME, BUILDING SURFACE HIGH PRESSURE OVER THE GREAT BASIN WILL 
GENERATE SOME LOCALLY GUSTY NORTHEAST WINDS, ESPECIALLY ACROSS 
VENTURA AND LA COUNTIES. AT THIS POINT WE'RE THINKING WINDS SHOULD 
STAY BELOW ADVISORY LEVELS IN MOST AREAS, THOUGH THE USUAL FAVORED 
SPOTS WILL LIKELY SEE GUSTS INTO THE 40S BY FRIDAY MORNING. HIGHS AT 
LOWER ELEVATIONS SHOULD JUMP BACK INTO THE LOW OR EVEN MID 70S.

OFFSHORE FLOW WILL BE SHORT-LIVED HOWEVER. ON SATURDAY ALL THE 
MODELS ARE SHOWING A HIGH AMPLITUDE OMEGA BLOCKING PATTERN SETTING 
UP ACROSS THE ERN PACIFIC WITH A PRETTY COLD UPPER LOW DROPPING INTO 
IDAHO. AT THE SAME TIME, A WEAK UPPER LEVEL SYSTEM WILL MOVE THROUGH 
THE SOUTHERN BRANCH OF THE JET AND BRING AN INCREASE IN MID AND HIGH 
CLOUDS AND COOLER TEMPS TO THE AREA. THE LOWER LEVELS ARE STILL 
QUITE DRY SO NO PRECIPITATION IS EXPECTED, BUT HIGHS SHOULD FALL AT 
LEAST 5-10 DEGREES MOST AREAS. 

.LONG TERM (SUN-WED)...PRETTY ACTIVE WEATHER PATTERN NEXT WEEK. ON 
SUNDAY, THE MODELS ARE NOW IN DECENT AGREEMENT SHOWING THE COLD 
UPPER LOW ON THE EAST SIDE OF THE OMEGA BLOCK ACTUALLY RETROGRADING 
OFFSHORE. THE GFS HAD EARLIER INDICATED THIS LOW DROPPING SOUTH INTO 
THE GREAT BASIN SO MODEL AGREEMENT HAS IMPROVED DRAMATICALLY. BUT 
THIS ACTUALLY SLOWS THE ARRIVAL OF THE MOISTURE INTO OUR AREA, SO IT 
NOW APPEARS THAT THE MAJORITY OF SUNDAY WILL REMAIN DRY. HOWEVER, 
HEIGHTS WILL BE FALLING PRETTY RAPIDLY, ALLOWING THE MARINE 
INVERSION TO DEEPEN SIGNIFICANTLY, PERHAPS SIMILAR TO TODAY'S LEVEL. 
THUS, A COOL AND CLOUDY DAY EXPECTED SUNDAY WITH JUST PARTIAL 
AFTERNOON CLEARING AT BEST.

ALL MODELS NOW ON BOARD WITH THE FIRST SHOT OF RAIN ARRIVING INTO 
THE AREA EITHER SUNDAY NIGHT OR MONDAY MORNING. THE CANADIAN AND THE 
ECMWF SHOW THE SURFACE LOW ABOUT 300 MILES NORTHWEST OF SFO 12Z MON 
WITH A GOOD WARM FRONTAL PATTERN SETTING UP OVER SO CAL. THIS 
PATTERN TYPICALLY PRODUCES A STEADY, BUT MOSTLY LIGHT RAIN FOR OUR 
AREA, THOUGH IT REMAINS TO BE SEE HOW MUCH SOUTHERLY FLOW SETS UP TO 
POSSIBLY ENHANCE THE RAIN ALONG THE MOUNTAIN SLOPES. 

MODELS START TO DIVERGE TUE, THOUGH IT'S MAINLY THE GFS THAT IS THE 
OUTLIER HERE SHOWING THAT UPPER LOW ON THE EAST SIDE OF THE OMEGA 
BLOCK DROPPING SOUTH INTO THE GREAT BASIN AND PUSHING THE MOISTURE 
SOUTH OF THE AREA AND USHERING IN A LOT OF COLD AIR. FOR NOW HAVE 
DECIDED TO IGNORE THE GFS AND WENT WITH THE EC/CANADIAN SOLUTION. 
THIS SOLUTION MAINTAINS A RELATIVELY WARM, MOIST FLOW OF AIR ACROSS 
THE AREA TUE WITH PERIODS OF RAIN CONTINUING, AGAIN MOSTLY ON THE 
LIGHTER SIDE WITH THE CAVEAT THAT IT REMAINS TO BE SEEN HOW MUCH 
SOUTHERLY FLOW SETS UP. 

NOT TOO MUCH CHANGE ON WED, THOUGH IF THERE IS A BREAK IN THE WET 
WEATHER NEXT WEEK WED WILL LIKELY BE THE DAY WE SEE THE LEAST AMOUNT 
OF PRECIP. ALTHOUGH THURSDAY IS BEYOND THE CURRENT FORECAST CYCLE, 
THAT COULD BE THE WETTEST DAY NEXT WEEK AS THE ECMWF SHOWS THE BEST 
MOISTURE/SOUTHERLY FLOW COUPLING THAT COULD GENERATE HEAVIER RAIN.

&&

.AVIATION...02/1800Z. 

MID/UPPER LEVEL RIDGE OF HIGH PRESSURE UPSTREAM WILL APPROACH WHILE 
A BROAD MID/UPPER LEVEL TROUGH OF LOW PRESSURE CENTERED EAST OF THE 
AREA SHIFTS EAST. MODERATE UPPER LEVEL NORTHWEST WINDS AND MODERATE 
MID LEVEL WINDS WILL PREVAIL OVER THE AREA. A WEAK ONSHORE PRESSURE 
GRADIENT WILL CONTINUE THROUGH THE AFTERNOON. THERE IS A CHANCE A 
MODERATE NORTHERLY GRADIENT WILL DEVELOP TONIGHT. MARINE LAYER TOP 
WAS DISORGANIZED AND ASCENDING THROUGH THE MORNING AND OVER 4KFT AT 
THIS TIME...THERE IS A CHANCE TOPS WILL DESCEND SEVERAL HUNDRED FEET 
OVERNIGHT AS THE NORTHERLY GRADIENT DEVELOP. 

KLAX...IT IS LIKELY CIGS 040 WILL LINGER THROUGH 02/22Z...THERE IS A 
CHANCE CIGS 021 WILL REDEVELOP BY 03/05Z AND DESCEND TO AROUND 009 
AFTER 03/11Z AND PERSIST FOR SEVERAL HOURS THEN BECOME SCATTERED 
AFTER 03/18Z.  

KBUR...IT IS LIKELY CIGS 035 WILL LINGER THROUGH 02/21Z. THERE IS A 
CHANCE CIGS 012 WILL REDEVELOP BY 03/03Z AND PERSIST FOR SEVERAL 
HOURS. THERE IS A CHANCE CIGS WILL BECOME SCATTERED AFTER 03/08Z. 

VIRTUALLY CERTAIN - 95-100%
VERY LIKELY       - 80-95%
LIKELY            - 60-80%
CHANCE            - 30-60%
VERY UNLIKELY     - 20% OR LESS

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
NONE

&&

$$

PUBLIC...WOFFORD
AVIATION...30
SYNOPSIS...SETO

WWW.WEATHER.GOV/LOSANGELES