FXUS66 KLOX 231808
AFDLOX

SOUTHWEST CALIFORNIA AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOS ANGELES/OXNARD CA
1007 AM PST WED DEC 23 2009

UPDATED AVIATION SECTION

.SYNOPSIS... 
DRY AND SEASONABLY COOL WEATHER WILL PERSIST THROUGH CHRISTMAS.
A SERIES OF MOSTLY WEAK STORM SYSTEMS MOVING THROUGH THE EASTERN 
PACIFIC WILL BRING A SLIGHT CHANCE OF RAIN TO THE REGION LATE 
SATURDAY OR SATURDAY NIGHT THROUGH MONDAY...THEN DRY WEATHER IS 
EXPECTED AGAIN THROUGH WEDNESDAY. 

&&

.SHORT TERM (TODAY-FRI)...COLD AND VIGOROUS UPPER LEVEL LOW PRESSURE
SYSTEM CURRENTLY CENTERED OVER ARIZONA...WITH CONTINUED STRONG NORTHERLY
FLOW ALOFT OVER SOUTHERN CALIFORNIA ON THE BACK SIDE OF TROUGH. CURRENT
OBSERVATIONS AND 12Z NAM MODEL GUIDANCE SUPPORT EXTENSION OF WIND
ADVISORY IN THE MOUNTAINS OF LOS ANGELES AND VENTURA COUNTIES 
THROUGH AT LEAST NOON TODAY. TAKING A LOOK AT RAWS OBSERVATIONS
THIS MORNING...STILL SEEING SOME WINDS GUSTING IN THE 50 TO 60 MPH
RANGE AT FAVORED LOCATIONS...SUCH AS WHITAKER PEAK...CAMP 9...AND 
CHILAO. WINDS ARE ALSO EXPECTED TO REMAIN GUSTY THIS MORNING ACROSS 
THE VALLEYS AND SANTA MONICA MOUNTAINS...BUT GENERALLY EXPECTED
TO REMAIN BELOW ADVISORY LEVELS. UPPER LEVEL LOW IS EXPECTED TO 
CONTINUE TRACK EASTWARD THROUGH THE DAY...WITH UPPER LEVEL SUPPORT 
EXPECTED TO GRADUALLY WEAKEN THIS AFTERNOON. THE STRONG UPPER LEVEL
WINDS HAVE ALSO GENERATED A LARGE AREA OF ASH AND DUST ACROSS MUCH
OF THE LA BASIN THIS MORNING.

OTHER BIG STORY TODAY CONTINUES TO BE COOL AIR MASS IN PLACE. SOME
WIND SHELTERED LOCATIONS EXPERIENCED SUB-FREEZING TEMPERATURES WITH
FROST THIS MORNING. WHILE THE AIR MASS IS EXPECTED TO SLOWLY MODIFY 
TODAY...THE COMBINATION OF LIGHTER WINDS AND A DRY AIR MASS WILL 
BRING ANOTHER ROUND OF CHILLY TEMPERATURES TONIGHT...WITH SOME 
ADDITIONAL FROST ADVISORIES POSSIBLY BEING ISSUED WITH AFTERNOON 
PACKAGE.

*** FROM PREVIOUS DISCUSSION ***

AN UPPER RIDGE IN THE ERN PACIFIC WILL SHIFT SLOWLY INTO THE W CST 
TONIGHT AND THU. RISING HEIGHTS/THICKNESSES...AND WARMING AT 850 AND 
950 MB SHOULD LEAD TO SEVERAL DEGREES OF WARMING IN MOST AREAS ON 
THU...WITH TEMPS NOT TOO FAR FROM SEASONAL LEVELS. THIS RIDGE WILL 
HOLD ACROSS THE W CST THROUGH CHRISTMAS...KEEPING SKIES MOSTLY 
CLEAR. LOW TEMPS LIKELY WILL NOT BE QUITE AS COLD THU NIGHT. SOME 
ADDITIONAL SLIGHT WARMING IS POSSIBLE ON CHRISTMAS...BUT NOTHING TOO 
SIGNIFICANT.

.LONG TERM (SAT-TUE)...THE RIDGE ACROSS THE REGION WILL BE KNOCKED 
DOWN AND PUSHED EWD SAT AS AN UPPER TROUGH MOVES TOWARD THE W CST. 
HOWEVER...THIS SYSTEM IS FCST TO WEAKEN QUICKLY AS IT PUSHES INTO 
THE RIDGE...SO ITS IMPACTS ON SRN CA LOOK MINOR AT BEST...CAUSING AN 
INCREASE IN CLOUDS AND SOME COOLING. FOR NOW...WILL LEAVE THE SLIGHT 
CHANCE OF RAIN ACROSS SLO COUNTY BY SAT AFTERNOON...BUT THAT COULD 
EVEN BE A STRETCH. THE ECMWF SHOWS THE SYSTEM HOLDING TOGETHER TO 
BRING A CHANCE OF RAIN TO THE ENTIRE AREA SAT NIGHT...WHILE THE GFS 
SHOWS NO RAIN AT ALL.

BEYOND THAT...THERE IS A FAIR AMOUNT OF UNCERTAINTY IN THE FORECAST 
AS THE MODELS ARE IN DISAGREEMENT WITH EACH OTHER AND FROM THEIR 
PREVIOUS RUNS. HOWEVER...BOTH THE GFS AND THE ECMWF SHOW THE SAME 
GENERAL IDEA...WITH ANOTHER TROUGH DROPPING SEWD MOVING THRU THE ERN 
PAC MOVING TO THE SW OF THE FCST AREA SUN THROUGH EARLY MONDAY. AT 
THIS POINT...IT APPEARS THAT THE BULK OF THE ENERGY AND MOISTURE 
WILL REMAIN TO THE S AND W OF THE REGION...BUT SLIGHT CHANCE POPS 
CERTAINLY LOOK WARRANTED SUN THRU MON. IT LOOKS LIKE NLY FLOW ALOFT 
WILL BRING DRIER CONDS MON NIGHT AND TUE. 

&&

.AVIATION...23/1800Z.

MID/UPPER LEVEL RIDGE OF HIGH PRESSURE UPSTREAM WILL MOVE OVER THE 
AREA. STRONG UPPER LEVEL NORTH WINDS WILL BECOME MODERATE WEST AFTER 
24/12Z WHILE LIGHT MID LEVEL NORTH WINDS PREVAIL OVER THE AREA. MODERATE 
NORTH GRADIENT WILL BECOME WEAK AFTER 23/21Z AND PERSIST THROUGH THE 
REMIANDER OF THE PERIOD. 

KLAX...SCATTERED 050 ASH WILL DISSIPATE THROUGH THE AFTERNOON. 
OTHERWISE VIRTUALLY CERTAIN CLEAR SKIES WILL PERSIST. IT IS VERY 
LIKELY EAST LOW LEVEL WINDS WILL REMAIN LESS THAN 10 KT.

KBUR...IT IS LIKELY LLWS WILL DIMINISH AFTER 23/21Z...OTHERWISE 
VIRTUALLY CERTAIN CLEAR SKIES WILL PERSIST.

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

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
WIND ADVISORY (SEE LAXNPWLOX).
SMALL CRAFT ADVISORY (SEE LAXMWWLOX).

&&

$$

PUBLIC...GOMBERG
AVIATION...30
SYNOPSIS...DB

WWW.WEATHER.GOV/LOSANGELES