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