000
FXUS66 KLOX 172134
AFDLOX

SOUTHWEST CALIFORNIA AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOS ANGELES/OXNARD CA
230 PM PDT WED SEP 17 2008

.SHORT TERM (TODAY-SAT)...MARINE LAYER HAS FALLEN TO NEAR 1000 FT
DEEP AT LAX THIS AFTERNOON. LOW CLOUDS HAVE DISSIPATED OFF THE
CENTRAL COAST...THE SBA S COAST AND VTU COAST. LOW CLOUDS WERE
BETWEEN THE PALOS VERDES PENINSULA AND CATALINA ISLAND BUT WERE
GENERALLY SHRINKING IN COVERAGE. CU BUILDUPS HAVE DEVELOPED OVER THE
VTU/L.A. COUNTY MOUNTAINS AND ANTELOPE VALLEY EARLY THIS AFTERNOON
BUT THUS FAR THUNDERSTORMS HAVE BEEN CONFINED TO AREAS E OF THE
REGION. THERE IS STILL A SLIGHT CHANCE OF SHOWERS AND THUNDERSTORMS
OVER THIS AREA THRU THE EVENING.

A BROAD SOUTHWESTERLY FLOW ALOFT WILL PREVAIL OVER THE FORECAST AREA
TONIGHT THROUGH FRI AS AN UPPER LEVEL TROF APPROACHES THE COAST. AT
LOWER TO MID LEVELS...THE FLOW WILL BE MORE SOUTHERLY THROUGH EARLY
THU WHICH WILL HELP TO BRING IN SIGNIFICANT LOWER TO MID LEVEL
MOISTURE FROM OFF THE BAJA COAST MAINLY TO VTU/L.A. COUNTIES. THE
12Z WRF WAS FORECASTING 850 MB DEWPOINTS TO INCREASE TO 12 TO 16 DEG
C OVER THE REGION...WITH PWAT VALUES INCREASING TO AS HIGH AS 1.6
INCHES ALONG THE COAST. THE INCREASING MOISTURE AT 850 MB SHOULD
HELP TO DESTABILIZE THE LOWER ATMOSPHERE ABOVE THE MARINE LAYER TO
THE MID LEVELS...AS SHOWN IN THE 12Z WRF SHOWALTER INDEX CHARTS (AS
LOW AT -3 ALONG THE VTU/L.A. COASTAL AREAS TONIGHT). THE 12Z WRF WAS
FORECASTING K-INDEX VALUES INCREASING TO 35 TO NEAR 40 TONIGHT FOR
MUCH OF THE AREA. IN ADDITION...THE GFS IS SUPPORTING THE IDEA OF
INCREASING LOWER LEVEL MOISTURE OVER THE REGION DURING THE PERIOD.
THE 18Z WRF WAS A BIT SLOWER IN BRINGING THE LOWER TO MID LEVEL
MOISTURE TO THE DISTRICT TONIGHT...AND SHOWALTER INDICES WERE NOT AS
LOW AS THE 12Z RUN FOR TONIGHT AS WELL. STILL...IT LOOKS LIKE THERE
WILL BE THE POSSIBILITY OF A FEW SHOWERS AND EVEN AN ISOLATED
THUNDERSTORM OVER VTU/L.A. COUNTIES TONIGHT AND EARLY THU. THIS WILL
BE REFLECTED IN THE AFTERNOON ZONE PACKAGE WITH A SLIGHT CHANCE OF
SHOWERS AND THUNDERSTORMS DEVELOPING THIS EVENING OVER MUCH OF
VTU/L.A. COUNTIES AND LINGERING INTO EARLY THU.

ALTHO SOME MID LEVEL MOISTURE SHOULD LINGER THU...IT LOOKS LIKE THE
THREAT OF AFTERNOON THUNDERSTORMS FOR THE MTNS OF VTU/L.A. COUNTIES
WILL BE LESS THAN 15 PERCENT WITH JUST SOME CU BUILDUPS POSSIBLE.
MARINE LAYER CLOUDS MAY BE HARD PRESSED TO BECOME ORGANIZED TONIGHT
S OF POINT CONCEPTION...OTHERWISE PLENTY OF LOW CLOUDS AND FOG NIGHT
AND MORNING HOURS CAN BE EXPECTED THRU FRI FOR THE COAST AND SOME
ADJACENT VALLEYS. HI TEMPS ARE FORECAST TO BE NEAR NORMAL FOR THE
MOST PART ON THU THEN COOL TO SLIGHTLY BELOW NORMAL FOR FRI.

IT LOOKS LIKE A FLAT UPPER RIDGE WILL BUILD INTO SRN CA FRI NIGHT
AND SAT. DRY WEATHER CAN BE EXPECTED OVER THE DISTRICT DURING THE
PERIOD. THE MARINE LAYER CLOUDS WILL CONTINUE TO AFFECT THE COAST
EXCEPT THE SBA S COAST AS NORTHERLY GUSTY CANYON WINDS CAN BE
EXPECTED THERE FRI NIGHT. TEMPS OVER THE REGION WILL BE NEAR NORMAL
FOR THE MOST PART.

.LONG TERM (SUN-WED)...THERE ARE SOME DIFFERENCES IN THE UPPER LEVEL
FEATURES BETWEEN THE MEDIUM RANGE MODELS. THE GFS IS FORECASTING A
BROAD UPPER LEVEL TROF OVER CA SUN THRU MON...THEN THE UPPER TROF
MOVES E WITH AN UPPER RIDGE BUILDING IN BY TUE NIGHT INTO WED. THE
ECMWF HAS A FLAT UPPER RIDGE PERSISTING OVER SOCAL SUN AND MON THEN
A BROADER UPPER HIGH WILL BUILD INTO THE REGION BY WED. THE WRF 500
MB HEIGHT FORECAST FOR LATE SAT WOULD SUGGEST LEANING MORE TOWARD
THE ECMWF SOLUTION. DRY WEATHER CAN BE EXPECTED OVER THE FORECAST
AREA DURING THE PERIOD. THE MARINE INVERSION WILL FLUCTUATE IN DEPTH
DURING THE PERIOD AND WILL LIKELY BE SHALLOWER TUE AND WED...BUT
OVERALL VARYING AMOUNTS OF NIGHT AND MORNING LOW CLOUDS AND FOG WILL
PERSIST MAINLY ALONG THE COASTAL PLAIN. TEMPS OVER THE REGION WILL
BE NEAR NORMAL TO PERHAPS SLIGHTLY ABOVE NORMAL THRU TUE THEN WARM
TO A FEW DEGREES ABOVE NORMAL IN MANY AREAS FOR WED.

&&

.AVIATION...17/2040Z.
A RELATIVELY LOW CONFIDENCE FORECAST FOR TONIGHT AND THURSDAY. SOUTH
OF POINT CONCEPTION...MID LEVEL MOISTURE HAS THE POTENTIAL TO
DISRUPT THE MARINE LAYER CAUSING A DELAY IN THE ARRIVAL TIME OF THE
LOW CLOUDS AND FOG FOR THE COASTAL TAFS. THE MARINE LAYER IS
EXPECTED TO SHRINK IN DEPTH TONIGHT WHICH SHOULD PREVENT ANOTHER
UNEXPECTED LOW CLOUD INTRUSION INTO THE VALLEY TAF SITES. NORTH OF
POINT CONCEPTION...THE LOW CLOUDS AND FOG OFFSHORE WERE STARTING TO
ERODE AWAY A BIT IN THE LATEST SATELLITE TRENDS...SO THE NEXT
ISSUANCE OF CENTRAL COAST TAFS MAY BE ADJUSTED TO ACCOUNT FOR A
LATER ONSET OF LOW CLOUDS AND FOG...NOT TO MENTION THE POSSIBILITY
OF LITTLE OR NO FOG.

KLAX...CLEAR SKIES THROUGH THE EVENING...FOLLOWED BY THE ONSET OF
LOW CLOUDS AND FOG WITH MVFR TO IFR CONDITIONS AROUND MIDNIGHT OR
LATER. THERE IS A 40 PERCENT CHANCE OF CLEAR TO PARTLY CLOUDY SKIES
THROUGH THE NIGHT WITH VFR CONDITIONS. MID LEVEL MOISTURE HAS A 20
PERCENT CHANCE OF PRODUCING A BRIEF SHOWER OR TWO OVERNIGHT.

KBUR...THERE IS A 70 PERCENT CHANCE OF CLEAR SKIES THROUGH THE
NIGHT. A 30 PERCENT CHANCE EXISTS FOR A BRIEF ENCOUNTER OF LOW
CLOUDS AND FOG WITH IFR CONDITIONS AFTER 13Z THURSDAY.

&&

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

&&

$$

PUBLIC...SIRARD
AVIATION...SWEET

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