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000 FXUS66 KLOX 180344 AFDLOX SOUTHWEST CALIFORNIA AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE LOS ANGELES/OXNARD CA 845 PM PDT WED SEP 17 2008 .SHORT TERM (TODAY-SAT)... WITH LESS MID LEVEL CLOUD COVERAGE ACROSS LA/VTU COUNTIES TODAY...TEMPS WERE ABLE TO CLIMB TO NEAR NORMAL FOR THIS TIME OF YEAR ACROSS THE VALLEYS WITH HIGHS MAINLY IN THE UPPER 80S TO LOWER 90S. THE ANTELOPE VALLEY WAS A BIT COOLER TODAY...LIKELY DUE TO ALTOCU PERSISTING ACROSS THE AREA MUCH OF THE AFTERNOON. AREAS TO THE NORTH WERE NOTICEABLY COOLER DUE TO LOWERING THICKNESS LVLS FROM UPPER TROF SLOWLY DIGGING DOWN THE NORTHEASTERN PACIFIC. THERE WERE SOME CU BUILDUPS ACROSS THE LA/VTU/SBA COUNTY MTNS TODAY...INCLUDING THE ANTELOPE VALLEY...BUT NO REPORTS OF ANY PRECIP. ONCE AGAIN...MOST OF THE CONVECTIVE ACTIVITY OCCURRED TO THE EAST OF THE AREA. FOR TONIGHT...BOTH NAM-WRF AND GFS CONTINUE TO ADVERTISE A RELATIVELY LOW LEVEL SWATH OF MOISTURE BETWEEN 850-700 MB ACROSS THE SOUTHERN COASTAL WATERS OVERNIGHT THROUGH TOMORROW. 850 MB DEWPOINTS WERE AS HIGH AS 12 DEGREES C ACROSS LA COUNTY. EARLIER SHIFT ADDED 20 PERCENT POPS FOR THE COASTAL AND VALLEY LOCATIONS OF LA/VTU COUNTIES FOR THIS EVENING THROUGH THE OVERNIGHT HOURS. I HAVE BEEN TORN TO KEEP SLIGHT CHANCE POPS IN THE FORECAST...BUT DECIDED TO DO SO...BUT MENTIONED 20 PERCENT POPS AFTER MIDNIGHT. THIS WAS BASED ON A FEW THINGS. ALTHOUGH INFRARED AND WATER VAPOR IMAGERY ARE NOT SHOWING MUCH IN THE WAY OF CLOUDS MOVING INTO THE FORECAST AREA FROM THE SOUTH...A COUPLE OF FEATURES DO STICK OUT. LATEST MIRAMAR SOUNDING INDICATED PWAT VALUES NEAR 1.5 INCHES WHICH WOULD INITIATE WELL WITH NAM-WRF. ALSO DECENT SOUTHERLY FLOW CONTINUES FROM THE SURFACE TO NEAR 700 MB WHERE WINDS BEGIN TO VEER FROM THE SOUTHWEST TO WEST TO 500 MB. IN THE UPPER LVLS...A DEFORMATION ZONE NEAR 300 MB DOES APPROACH THE SO CAL BIGHT BY MIDNIGHT ASSOCIATED FROM THE TROF TO THE NORTHWEST BRINGING SOME DIFFLUENT ALOFT...AS WELL AS MARGINAL DIVERGENCE NEAR 250 MB AS WELL. ALTHOUGH MODEL SOUNDINGS ARE NOT TO IMPRESSIVE...IF MID LEVEL CLOUDS DO DEVELOP AS MODELS SUGGEST...THERE COULD BE ENOUGH COOLING ALOFT AND INSTABILITY TO START A FEW SHOWERS AND ISOLATED THUNDERSTORMS OVER THE COASTAL WATERS AND MOVE ASHORE LATE TONIGHT. AGAIN...GUT FEELING IS THIS WONT OCCUR...BUT MODELS WERE CONSISTENT ENOUGH THAT SLIGHT CHANCE POPS SEEM REASONABLE. THERE WILL ALSO BE SOME STRATUS MOVING INTO LA/VTU COUNTY COASTAL AREAS OVERNIGHT AS MARINE LAYER WAS NEAR 1000 FT DEEP THIS EVENING...AND IF MID LVL MOISTURE DOES MOVE IN...LOW CLOUDS COULD BE DISRUPTED BY EARLY MORNING HOURS. THE CENTRAL COAST WILL STAY CLEAR THROUGH MIDNIGHT...THEN LOW CLOUDS SHOULD MOVE INTO COASTAL AREAS AFTER MIDNIGHT TO NEAR MORNING HOURS. FOR THU...WENT AHEAD AND ADDED SLIGHT CHANCE OF TSTMS IN THE AFTERNOON AND EVENING HOURS FOR LA/VTU COUNTY MTNS AND ANTELOPE VALLEY SINCE THEIR CONTINUES TO BE ENOUGH MID LEVEL MOISTURE ACROSS THE AREA TO JUSTIFY...BUT AGAIN...BEST CONVECTIVE PARAMETERS REMAIN TO THE EAST. OVERALL SKIES SHOULD BECOME MOSTLY SUNNY WITH A FEW DEGREES OF COOLING ACROSS THE INTERIOR REGIONS AS ONSHORE GRADS STRENGTHEN SOMEWHAT TOMORROW AFTERNOON. AS UPPER TROF BEGINS TO SWING THROUGH NRN CAL...EXPECT INCREASING LOW CLOUDS ACROSS THE COASTAL AREAS AND POSSIBLY ADJACENT VALLEYS OF LA/VTU COUNTIES LATE THU NIGHT INTO FRI. LOW CLOUDS SHOULD STAY SOUTH OF SANTA BARBARA THU NIGHT INTO FRI MORNING AS NORTHWEST TO NORTH WINDS SHOULD PERSIST ACROSS THE USUAL PASSES AND CANYONS. 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... 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...KAPLAN/SIRARD AVIATION...SWEET WWW.WEATHER.GOV/LOSANGELES