Area Forecast Discussion
Issued by NWS Los Angeles, CA

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000
FXUS66 KLOX 300333
AFDLOX

SOUTHWEST CALIFORNIA AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOS ANGELES/OXNARD CA
833 PM PDT WED JUL 29 2015

.SYNOPSIS...

APPROACHING HIGH PRESSURE WILL ALLOW FOR SOUTHEASTERLY MONSOONAL
FLOW ALOFT THROUGH SATURDAY. THUNDERSTORM POTENTIAL WILL SPREAD
NORTHWESTWARD ACROSS THE AREA ON THURSDAY. THE POSSIBILITY OF
THUNDERSTORMS WILL LINGER UNTIL SATURDAY FOR THE INTERIOR. BY MONDAY
DRY SOUTHWEST WINDS WILL LIKELY REMOVE THE THUNDERSTORM THREAT.

&&

.SHORT TERM (TONIGHT-SAT)...
THE LEADING EDGE OF THE MONSOON FRONT ARRIVED INTO THE EASTERN PORTION OF
OUR FORECAST AREA THIS AFTERNOON...WITH AN ACTIVE DAY OF THUNDERSTORMS
IN THE ANTELOPE VALLEY AND SAN GABRIEL MOUNTAINS. A FLASH FLOOD WARNING
WAS ISSUED FOR THE EASTERN PORTION OF THE ANTELOPE VALLEY INCLUDING THE
PALMDALE METRO AREA...WITH REPORTS OF FLASH FLOODING FROM LAKE LOS ANGELES.
A SECOND WARNING WAS ISSUED FOR THE EASTERN SAN GABRIEL MOUNTAINS NEAR
WRIGHTWOOD.

THUNDERSTORM ACTIVITY ACROSS OUR FORECAST AREA HAS DIMINISHED THIS EVENING
BUT REMAINS ACTIVE ACROSS THE DESERTS OF SAN BERNARDINO COUNTY. A LARGE
HIGH PRESSURE SYSTEM OVER THE DESERTS SOUTHWEST WILL CONTINUE TO STEER
MONSOONAL MOISTURE INTO SOUTHERN CALIFORNIA THE NEXT FEW DAYS. IN THE
SHORT TERM...THE KEY FEATURE TO WATCH WILL BE AN EMBEDDED UPPER LEVEL WAVE
THAT IS EXPECTED TO ADVECT INTO LOS ANGELES COUNTY BY EARLY THURSDAY MORNING...
SHIFTING NORTHWESTWARD INTO VENTURA AND SANTA BARBARA COUNTIES DURING THE LATE
MORNING AND AFTERNOON HOURS. MODEL CROSS SECTIONS SHOWING A GOOD AREA OF
LIFT ASSOCIATED WITH THIS FEATURE...ESPECIALLY ACROSS LOS ANGELES COUNTY
EARLY TOMORROW MORNING. IN ADDITION...CROSS SECTIONS SHOWING A DEEPER SURGE OF
MONSOONAL MOISTURE COINCIDING WITH THIS WAVE. ESRL GPS SENSORS ALREADY SHOWING
PRECIPITABLE WATER VALUES UP TO 1.6 INCHES ACROSS PORTIONS OF LA
COUNTY AS OF 8 PM THIS EVENING. LATEST 00Z NAM SHOWING AN IMPRESSIVE PRECIPITABLE
WATER VALUE UP TO 2 INCHES BY EARLY THURSDAY MORNING ACROSS THE LA BASIN...INDICATIVE
OF THIS SURGE OF DEEPER MONSOONAL MOISTURE!

THIS UPPER LEVEL WAVE WITH DEEPER MOISTURE WILL LIKELY TRIGGER NOCTURNAL CONVECTION
ACROSS THE EASTERN DESERTS TONIGHT...POTENTIALLY REACHING LOS ANGELES COUNTY LATE
TONIGHT OR EARLY THURSDAY MORNING. WHILE THERE IS STILL 15-20 KNOTS OF STEERING FLOW
WINDS BEING PROJECTED ACROSS MUCH OF LOS ANGELES AND VENTURA COUNTIES...THERE IS
CONCERN FOR TRAINING STORMS THAT COULD ENHANCE FLASH FLOODING THREAT ON THURSDAY.
IN COORDINATION WITH SURROUNDING OFFICES...WILL INCLUDE A FLASH FLOOD WATCH FOR
THURSDAY ACROSS THE MOUNTAINS AND ANTELOPE VALLEY DUE TO THE ARRIVAL OF DEEP MONSOONAL
MOISTURE AND POTENTIAL DYNAMICS ASSOCIATED WITH THE ABOVE MENTIONED UPPER LEVEL WAVE.
THERE WILL ALSO BE A THREAT OF SHOWERS AND THUNDERSTORMS ACROSS THE LOS ANGELES
BASIN BY LATE TONIGHT...THEN SPREADING NORTHWESTWARD ACROSS MOST COASTAL/VALLEY AREAS
ON THURSDAY AS THE WAVE AND DEEPER MOISTURE TRACK THROUGH THE FORECAST AREA.
IN FACT...00Z NAM SHOWING IMPRESSIVE K-INDEX VALUES OF 40 ACROSS THE LA BASIN
ON THURSDAY MORNING...A GOOD INDICATOR OF COASTAL/VALLEY THUNDERSTORM THREAT.

*** FROM PREVIOUS DISCUSSION ***

SHOWER/T-STORM ACTIVITY SHOULD DECREASE TOMORROW NIGHT INTO FRIDAY
MORNING...ESPECIALLY FOR THE COAST/VALLEY AREAS. IN FACT...ONLY REAL
CHANGE MADE TO THE PREVIOUS FORECAST WAS TO DECREASE POPS FOR THE
COASTAL/VALLEY AREAS TOMORROW EVENING AND NIGHT. ON FRIDAY AND
SATURDAY ANY THUNDERSTORM ACTIVITY WILL BE CONFINED TO THE
MOUNTAINS...INTERIOR VALLEYS...AND DESERT...AS SE FLOW ALOFT SHIFTS
TO THE SOUTH ON FRIDAY...THEN THE SW ON SATURDAY...RESULTING A
GRADUAL DRYING TREND.

ASIDE FROM THE T-STORM THREAT...ONLY OTHER SIGNIFICANT FORECAST
UNCERTAINTIES IN THE SHORT-TERM ARE THE COASTAL STRATUS TONIGHT AND
THE MAX TEMPS TOMORROW. VISIBLE SATELLITE IMAGERY SHOWS LOW CLOUDS
HANGING TOUGH AT MANY BEACHES THIS AFTERNOON...ESPECIALLY FROM
VENTURA NORTH. MOST BEACHES SHOULD CLEAR THIS AFTERNOON. THE LOW
CLOUD FORECAST TONIGHT WILL BE PROBLEMATIC WITH THE MID-LEVEL
MONSOON MOISTURE MOVING IN. BEST CHANCE FOR LOW CLOUDS AND FOG
TONIGHT AND TOMORROW MORNING WILL BE ON THE CENTRAL COAST. TOMORROW
COULD END UP BEING JUST AS HOT AS TODAY DEPENDING ON HOW MUCH MID
AND HIGH CLOUDS THERE AREA. OVERALL...WE ARE FORECASTING SLIGHTLY
COOLER CONDITIONS THURSDAY THROUGH SATURDAY AS HEIGHT/THICKNESS
VALUES FALL SOMEWHAT...BUT WITH THE ADDED HUMIDITY...IT WON`T FEEL
MUCH COOLER.

.LONG TERM (SUN-WED)...

THE THUNDERSTORM THREAT WILL CONTINUE TO DIMINISH ON SUNDAY AND
MONDAY AS DRY SW FLOW ALOFT BECOMES MORE ESTABLISHED...WITH A RIDGE
BUILDING OVER THE WESTERN U.S. AND A TROUGH OF LOW PRESSURE OFF THE
COAST. ON SUNDAY...A SLIGHT CHANCE OF T-STORMS WILL LINGER OVER THE
EASTERN SAN GABRIELS AND EASTERN ANTELOPE VALLEY. BY
MONDAY...THUNDERSTORMS ARE NO LONGER MENTIONED IN THE FORECAST. THE
BUILDING RIDGE OVER THE INTERIOR WESTERN U.S. WILL RESULT IN A
GRADUAL INCREASE IN HEIGHT/THICKNESS VALUES OVER SW CALIFORNIA
SUNDAY THROUGH WEDNESDAY...BRINGING SLIGHT WARMER CONDITIONS TO THE
REGION. AT THIS POINT...IT JUST LOOKS LIKE SLIGHTLY ABOVE NORMAL
TEMPERATURES.

&&

.AVIATION...30/0000Z...

AT 0000Z...THE MARINE INVERSION AT KLAX WAS BASED AT 1100 FEET. THE
TOP OF THE INVERSION WAS 3200 FEET WITH A TEMPERATURE OF 27 DEGREES
CELSIUS.

OVERALL...LOW CONFIDENCE IN THE 00Z TAFS AS MID-LEVEL MONSOONAL
MOISTURE MOVES INTO THE REGION TONIGHT AND LIKELY DISRUPTS THE
MARINE LAYER CLOUD FORMATION. TAF SITES ON THE CENTRAL COAST WILL
HAVE THE BEST CHANCE FOR LOW CLOUDS TONIGHT...BUT THERE IS A 40
PERCENT CHANCE OF NO IFR/LIFR CIGS FOR THESE LOCATIONS. IF LOW
CLOUDS FORM SOUTH OF POINT CONCEPTION...COVERAGE WILL LIKELY BE
PATCHY. THERE IS A 40 PERCENT CHANCE OF NO CIGS AT KSBA AND KOXR.
THUNDERSTORMS IN THE VICINITY OF KPMD AND WJF SHOULD DIMINISH WITHIN
THE NEXT SEVERAL HOURS. SHOWERS AND ISOLATED T-STORMS WILL
OVERSPREAD THE REGION FROM SOUTH TO NORTH LATE TONIGHT THROUGH
THURSDAY AFTERNOON.

KLAX...MODERATE CONFIDENCE IN THE FORECAST FOR NO LOW CLOUDS
TONIGHT...BUT THERE IS A 20 PERCENT CHANCE OF IFR CIGS FROM 06-12Z.
CAN NOT RULE OUT AN ISOLATED SHOWERS OR THUNDERSTORMS LATE TONIGHT
THROUGH EARLY THURSDAY AFTERNOON...SO VCTS WAS INCLUDED IN THE TAF.

KBUR...CAN NOT RULE OUT AN ISOLATED SHOWERS OR THUNDERSTORMS LATE
TONIGHT THROUGH EARLY THURSDAY AFTERNOON...SO VCTS WAS INCLUDED IN
THE TAF. CONDITIONS ARE EXPECTED TO REMAIN VFR FOR THE NEXT 24 HOURS
AS LONG AS NO THUNDERSTORMS DEVELOP.

&&

.MARINE...29/800 PM.

FOR THE OUTER WATERS...HIGH CONFIDENCE IN WINDS REMAINING BELOW SCA
LEVELS THROUGH FRIDAY MORNING. FROM FRIDAY AFTERNOON THROUGH THE
WEEKEND...NORTHWEST WINDS WILL INCREASE...WITH A CHANCE OF SCA LEVEL
WINDS DEVELOPING.

FOR THE INNER WATERS...HIGH CONFIDENCE IN WINDS REMAINING BELOW SCA
LEVELS THROUGH THE WEEKEND. THERE IS A CHANCE OF GUSTS REACHING SCA
LEVELS EACH AFTERNOON/EVENING FRIDAY THROUGH THE WEEKEND ACROSS
WESTERN SECTIONS OF SANTA BARBARA CHANNEL.

A MOIST AND UNSTABLE AIR MASS WILL BRING THE THREAT OF THUNDERSTORMS
LATE TONIGHT THROUGH THURSDAY EVENING...WITH THE BEST CHANCES OF
DEVELOPMENT THURSDAY MORNING.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
HIGH RIP CURRENT RISK (SEE LAXSRFLOX).
MARINE WEATHER STATEMENT (SEE LAXMWSLOX).
SPECIAL WEATHER STATEMENT (SEE LAXSPSLOX).

&&

$$

PUBLIC...GOMBERG/SUKUP
AVIATION...SUKUP
MARINE...RAT
SYNOPSIS...RM

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