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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 WWW.WEATHER.GOV/LOSANGELES