000
FXUS66 KLOX 172040
AFDLOX

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

NEW AVIATION DISCUSSION

.SHORT TERM (TODAY-FRI)...MARINE LAYER RANGED THIS MORNING FROM 1400
FT DEEP AT LAX TO NEAR 2200 FT AT VBG. LOW CLOUDS OVER MANY VALLEY
AND COASTAL AREAS WERE CLEARING RATHER QUICKLY BACK TO THE COAST
THROUGH MID MORNING. SOME LOW CLOUDS MAY LINGER AT THE BEACHES S OF
POINT CONCEPTION THIS AFTERNOON...OTHERWISE MOST OF THE LOW CLOUDS
SHOULD CLEAR OFF THE COAST. MOISTURE COMING IN FROM THE S AT LOWER
TO MID LEVELS WILL BRING PARTLY CLOUDY SKIES TO PORTIONS OF VTU/L.A.
COUNTIES THIS AFTERNOON. THERE IS ALSO A SLIGHT CHANCE OF
THUNDERSTORMS THIS AFTERNOON AND EARLY EVENING IN THE MOUNTAINS OF
VTU/L.A. COUNTIES AND THE ANTELOPE VALLEY...ALTHO AN INVERSION AT
AROUND 15000 FT AS SHOWN ON MORNING SOUNDINGS MAY INHIBIT CONVECTION
TO SOME DEGREE. TEMPS TODAY ACROSS THE REGION WILL BE GENERALLY NEAR
NORMAL FOR MOST AREAS.

A BROAD SOUTHWESTERLY FLOW ALOFT WILL PREVAIL OVER THE FORECAST AREA
THROUGH FRI AS AN UPPER LEVEL TROF APPROACHES THE COAST. AT LOWER TO
MID LEVELS...THE FLOW WILL BE MORE SOUTHERLY WHICH WILL HELP TO
BRING IN SIGNIFICANT LOWER TO MID LEVEL MOISTURE FROM OFF THE BAJA
COAST MAINLY TO VTU/L.A. COUNTIES TONIGHT INTO EARLY THU. THE WRF IS
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 WRF SHOWALTER INDEX CHARTS (AS LOW AT -3 ALONG
THE VTU/L.A. COASTAL AREAS TONIGHT). THE WRF IS 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. IT LOOKS MORE AND MORE
LIKE THERE WILL BE THE POSSIBILITY OF A FEW SHOWERS AND EVEN AN
ISOLATED THUNDERSTORM OVER VTU/L.A. COUNTIES TONIGHT. THIS WILL
LIKELY BE REFLECTED IN THE AFTERNOON ZONE PACKAGE. FOR NOW HAVE
OPTED TO INTRODUCE PARTLY CLOUDY SKIES TO THE REGION FOR THIS
AFTERNOON AND TONIGHT. 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 MINIMAL AT BEST 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 VALLEYS. HI TEMPS ARE FORECAST TO BE NEAR NORMAL FOR
THE MOST PART ON THU THEN COOL TO SLIGHTLY BELOW NORMAL FOR FRI.

...FROM PREVIOUS DISCUSSION...

.LONG TERM (FRI NIGHT-WED)...THE IMPULSE ON FRIDAY CONTINUES TO
FLUMMOX THE MEDIUM RANGE MODELS. SATURDAYS FORECAST LOOKS ON TRACK
WITH THE LOW MOVING OUT AND HGTS RISING THE MARINE LAYER SHOULD
SHRINK A LITTLE AND TEMPS SHOULD WARM A FEW DEGREES.

AFTER SATURDAY THE MDLS GO THEIR OWN WAYS AND THE GFS ENSEMBLES
BOMB-BURST INTO A SQUIGGLY MESS. WITH LITTLE OR NO CONFIDENCE IN THE
LONG RANGE FORECAST MADE A FEW ADJUSTMENT TOWARD CLIMATOLOGY BUT IN
GENERAL LEFT THINGS UNCHANGED FROM SUNDAY ON. HOPEFULLY THE MODELS
WILL GET A HANDLE ON FRIDAYS IMPULSE AND THE MEDIUM RANGE MDLS WILL
GET A HANDLE ON THINGS.

&&

.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/RORKE
AVIATION...SWEET

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