SOUTHWEST CALIFORNIA AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE LOS ANGELES/OXNARD CA
450 PM PDT SUN MAY 19 2013

UPDATED AVIATION DISCUSSION

.SYNOPSIS...
GUSTY NORTH WINDS ARE POSSIBLE THROUGH MONDAY MORNING FOR THE 
MOUNTAIN AREAS. WARMING IS EXPECTED THROUGH MONDAY AS A RIDGE OF 
HIGH PRESSURE BUILDS AND WEAK OFFSHORE FLOW DEVELOPS. AN INCREASE IN 
NIGHT THROUGH MORNING LOW CLOUDS AND FOG...ALONG WITH A COOLING 
TREND IS THEN EXPECTED TUESDAY AND BEYOND AS LOW PRESSURE TO THE 
NORTH INCREASES ONSHORE FLOW. COOL WEATHER AND PARTLY TO MOSTLY 
CLOUDY CONDITIONS WILL LIKELY CONTINUE INTO NEXT WEEKEND.

&&

.SHORT TERM (TODAY-WED)...
AN UPPER LEVEL RIDGE IS MOVING INTO THE PACIFIC NORTHWEST...AND WILL 
CONTINUE TO SHIFT ONSHORE OVERNIGHT. AT THE SURFACE...HIGH PRESSURE 
OVER THE WATERS AND LOW PRESSURE OVER FAR INLAND SE CALIFORNIA IS 
PRODUCING WEAK TO LOCALLY MODERATE ONSHORE FLOW ACROSS THE AREA THIS 
AFTERNOON. AS THE UPPER RIDGE SHIFTS ONSHORE OVER THE WEST 
COAST...SURFACE HIGH PRESSURE WILL BEGIN TO BUILD INTO THE GREAT 
BASIN. THIS WILL PRODUCE GUSTY NORTHWEST TO NORTH WINDS OVER THE 
AREA IN THE EVENING...WITH OFFSHORE FLOW DEVELOPING LATE TONIGHT 
INTO MONDAY MORNING. THE NORTHWEST TO NORTH WINDS WILL AFFECT 
PORTIONS OF SBA COUNTY AND THE INTERIOR MOUNTAINS OF LA AND VTA 
COUNTIES...WITH GUSTS GENERALLY 30 TO 35 MPH. 

AS THE SURFACE HIGH STRENGTHENS OVER NEVADA AND THERMAL LOW PRESSURE 
SHIFTS TO THE COAST...OFFSHORE WINDS WILL BEGIN TO DEVELOP OVER THE 
AREA AROUND DAYBREAK. WITH FAIRLY WEAK OFFSHORE GRADIENTS FORECAST 
BY THE WRF AND WEAK UPPER SUPPORT...THIS EVENT IS EXPECTED TO REMAIN 
JUST BELOW ADVISORY LEVEL. EXPECT WIND GUSTS UP TO 40 MPH IN THE 
MOUNTAINS WITH LOCAL GUSTS TO 35 MPH IN FAVORED VALLEYS SUCH AS 
SANTA CLARITA AND THE EASTERN VTA VALLEYS. EXPECT TO SEE WINDS 
SUBSIDE RAPIDLY BY LATE MORNING.  

THE BUILDING RIDGE ALOFT WILL RESULT IN MINIMAL MARINE LAYER 
DEVELOPMENT TONIGHT...WITH LOW CLOUDS AND FOG LIMITED TO THE PALOS 
VERDES PENINSULA. THE COMBINATION OF OFFSHORE FLOW AND HIGH PRESSURE 
ALOFT WILL MAKE MONDAY THE WARMEST DAY OF THE COMING WEEK WITH 70S 
WIDESPREAD ALONG THE COAST AND 80S TO LOWER 90S INLAND.

A DEEP UPPER LOW WILL BEGIN TO MOVE SOUTH ALONG THE EASTERN GULF OF 
ALASKA MONDAY INTO TUESDAY. FOR SOUTHWEST CALIFORNIA...WINDS WILL 
SHIFT BACK TO ONSHORE ALLOWING A MORE WIDESPREAD MARINE LAYER TO 
FORM MONDAY NIGHT. THE NAM12 SHOWS A COASTAL EDDY FORMING NEAR 
CATALINA ISLAND BUT HEIGHTS ALOFT WILL STILL BE HIGH ENOUGH TO KEEP 
THE MARINE LAYER SHALLOW...SO THAT LOW CLOUDS AND FOG WILL LIKELY BE 
LIMITED TO NEAR-COASTAL AREAS. 

THE UPPER CLOSED LOW WILL CONTINUE TO SINK SOUTH ALONG THE WEST 
COAST TUESDAY INTO WEDNESDAY...RESULTING IN A COOLING TREND AND A 
DEEPENING MARINE LAYER FOR SW CALIFORNIA. BY TUESDAY NIGHT MARINE 
LAYER CLOUDS SHOULD ADVANCE INLAND THROUGH ALL BUT THE MOST INTERIOR 
VALLEYS...AND WEDNESDAY'S HIGHS WILL BE CAPPED NEAR 80 DEGREES.

.LONG TERM (THU-SUN)...
THE MEDIUM RANGE MODELS AGREE FAIRLY WELL IN PLACING THE CLOSED 
UPPER LOW OVER OREGON EARLY THURSDAY...WHERE IT IS FORECAST TO 
REMAIN NEARLY STATIONARY INTO FRIDAY. THE PROXIMITY OF THE UPPER 
CLOSED LOW WILL PRODUCE CYCLONIC CURVATURE ALOFT AND ONSHORE FLOW 
OVER THE CALIFORNIA COAST. THIS SCENARIO WILL CREATE A DEEP MARINE 
LAYER OVER SOUTHWEST CALIFORNIA...WITH LOW CLOUDS MOVING WELL INLAND 
TO THE INTERIOR VALLEYS THURSDAY AND FRIDAY NIGHTS. THE UPPER LOW IS 
FORECAST TO LIFT NORTHEAST OUT OF THE AREA LATE FRIDAY INTO SATURDAY 
AS AN UPPER TROUGH FORMS ALONG THE CALIFORNIA COAST. THIS SCENARIO 
WILL LIKELY PRODUCE LITTLE CHANGE THROUGH LATE IN THE 
EXTENDED...WITH A DEEP MARINE LAYER AND EXTENSIVE LOW CLOUDS FORMING 
EACH NIGHT TO MORNING PERIOD...ALONG WITH LOWER THAN NORMAL DAYTIME 
HIGH TEMPERATURES. 

&&

.AVIATION...19/2350Z.

VFR CONDITIONS ARE EXPECTED THROUGHOUT THE PERIOD EXCEPT FOR A 
CHANCE OF LIFR TO IFR CONDITIONS AT LOS ANGELES COUNTY COAST AND 
CENTRAL COAST TERMINALS BETWEEN 08Z AND 16Z. PERIOD OF MODERATE TO 
STRONG WIND SHEAR AND TURBULENCE ARE POSSIBLE AT KSBA THROUGH 08Z 
AND VENTURA COUNTY TERMINALS BETWEEN 10Z AND 18Z.
 
KLAX...VFR CONDITIONS ARE EXPECTED THROUGHOUT THE PERIOD EXCEPT FOR 
A 30 PERCENT CHANCE OF LIFR TO IFR CONDITIONS.

KBUR...VFR CONDITIONS ARE EXPECTED THROUGHOUT THE PERIOD WITH A 20 
PERCENT CHANCE OF MODERATE LOW-LEVEL WIND SHEAR AND TURBULENCE 
BETWEEN 10Z AND 16Z.

&&

.FIRE WEATHER...

GUSTY NORTHWEST TO NORTH WINDS WILL CONTINUE TO BRING LOWERING 
HUMIDITIES AND ELEVATED FIRE DANGER THROUGH THIS EVENING ACROSS THE 
MOUNTAINS...ANTELOPE VALLEY...SANTA CLARITA VALLEY...AND SBA SOUTH 
COAST. THESE AREAS WILL GENERALLY SEE GUSTS IN THE 30 TO 40 MPH 
RANGE. ALREADY SEEING HUMIDITIES LOWERING TO AROUND 10 PERCENT THIS 
AFTERNOON ACROSS THE SANTA CLARITA VALLEY...ALONG WITH TEMPERATURES 
CLIMBING INTO THE LOWER 90S.

BY LATE TONIGHT AND MONDAY...SURFACE HIGH PRESSURE BUILDS INTO THE 
GREAT BASIN...SETTING THE STAGE FOR A BRIEF OFFSHORE WIND EVENT FOR 
THE REGION. THE PROJECTED LAX-DAGGETT GRADIENT IS ONLY EXPECTED TO BE
AROUND -1.5 MB...WITH SOME UPPER LEVEL WIND SUPPORT. OFFSHORE 
WINDS ARE EXPECTED TO DEVELOP ACROSS THE MOUNTAINS LATE 
TONIGHT...THEN DESCEND INTO THE VALLEYS ON MONDAY MORNING. AT THIS 
TIME...WIND GUSTS IN THE 35 TO 40 MPH RANGE CAN BE EXPECTED IN THE 
MOUNTAINS ON MONDAY MORNING...WITH GUSTS IN THE 25 TO 35 MPH RANGE
ACROSS THE VALLEYS (MAINLY SANTA CLARITA AND EASTERN VENTURA 
VALLEYS). THERE WILL LIKELY BE WIDESPREAD HUMIDITIES BETWEEN 10 AND
15 PERCENT ON MONDAY ACROSS THE MOUNTAINS AND VALLEYS OF LOS ANGELES
AND VENTURA COUNTIES...WITH ISOLATED SINGLE DIGIT READINGS LIKELY.
WHILE THIS OFFSHORE WIND EVENT DOES NOT APPEAR TO BE STRONG ENOUGH OR
HAVE SUFFICIENT DURATION TO WARRANT A RED FLAG WARNING...THERE WILL 
BE ELEVATED FIRE DANGER ACROSS THE MOUNTAINS AND VALLEYS ON 
MONDAY...ESPECIALLY CONSIDERING THE VERY DRY FUELS FOR THIS TIME OF 
YEAR. THE OFFSHORE WINDS ARE EXPECTED TO DIMINISH FAIRLY RAPIDLY ON 
MONDAY AFTERNOON...WITH ONSHORE WINDS GRADUALLY SPREADING ACROSS 
COASTAL AND VALLEY AREAS THROUGH THE AFTERNOON AND EVENING HOURS.

&&

.MARINE...A THERMAL TROUGH OF LOW PRESSURE WILL INTENSIFY NEAR SHORE 
WHILE HIGH PRESSURE PERSISTS OVER THE EASTERN PACIFIC THROUGH EARLY 
MONDAY AND A GALE WARNING IS IN EFFECT OVER THE OUTER WATERS FROM 
SAN CLEMENTE ISLAND TO POINT PIEDRAS BLANCAS AND INCLUDING THE INNER 
WATERS FROM POINT PIEDRAS BLANCAS TO POINT SAL THROUGH LATE TONIGHT. 
THE RIDGE OF HIGH PRESSURE OVER THE EASTERN PACIFIC WILL WEAKEN 
MONDAY. A TROUGH OF LOW PRESSURE IS THEN EXPECTED TO MOVE SOUTHEAST 
OVER THE OFFSHORE AND COASTAL WATERS EARLY TUESDAY THROUGH EARLY 
WEDNESDAY WHILE A RIDGE OF HIGH PRESSURE STRENGTHENS OVER THE 
EASTERN PACIFIC. NORTHWEST WINDS WILL LIKELY INCREASE NEAR SHORE 
NORTH OF POINT CONCEPTION AND OVER THE OUTER WATERS SOUTH OF POINT 
CONCEPTION TUESDAY EVENING THROUGH WEDNESDAY. A GALE WATCH WILL BE 
ISSUED WHEN THE RISK OF GALE FORCE WINDS HAS SIGNIFICANTLY 
INCREASED.    

&&


.LOX WATCHES/WARNINGS/ADVISORIES...
GALE WARNING (SEE LAXMWWLOX).
SMALL CRAFT ADVISORY (SEE LAXMWWLOX).
HIGH RIP CURRENT RISK (SEE LAXSRFLOX).
MARINE WEATHER STATEMENT (SEE LAXMWSLOX).

&&

$$

PUBLIC...SMITH
AVIATION...HALL
SYNOPSIS...SWEET
MARINE...30
FIRE WEATHER...GOMBERG

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