Area Forecast Discussion
Issued by NWS Los Angeles, CA

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000
FXUS66 KLOX 280431
AFDLOX

SOUTHWEST CALIFORNIA AREA FORECAST DISCUSSION...UPDATED
NATIONAL WEATHER SERVICE LOS ANGELES/OXNARD CA
931 PM PDT MON JUL 27 2015

.SYNOPSIS...

A HIGH PRESSURE SYSTEM WILL BRING A COMBINATION OF VERY WARM AND
HUMID CONDITIONS TO THE REGION THROUGH EARLY NEXT WEEK...THEN A
COOLING TREND SHOULD BEGIN. THIS COMBINATION WILL GENERATE A SLIGHT
CHANCE OF THUNDERSTORMS AND SHOWERS FOR THE MOUNTAIN AND DESERT
AREAS FROM WEDNESDAY INTO EARLY NEXT WEEK. THE STORM ACTIVITY WILL
ALSO REACH THE COASTAL AND VALLEYS AREAS FROM LATE WEDNESDAY TO
FRIDAY.

&&

.UPDATE...

THE LATEST WATER VAPOR IMAGERY INDICATES A TROUGH OF LOW PRESSURE
OVER WESTERN MONTANA...WHILE A RIDGE OF HIGH PRESSURE REMAINS IN
PLACE NEAR THE TEXAS-OKLAHOMA BORDER. THE TROUGH WILL CONTINUE TO
MOVE EAST OF THE REGION TONIGHT AS THE RIDGE OVER GULF STATES BUILDS
WEST INTO THE DESERT SOUTHWEST. ONSHORE FLOW IN PLACE OVER THE LAST
COUPLE OF DAYS IS WEAKENING. A WARMING TREND WILL TAKE SHAPE ON
TUESDAY AS THE RIDGE OF HIGH PRESSURE BUILDS WEST AND THE ONSHORE
FLOW WEAKENS. SOUTHWEST FLOW ALOFT THIS EVENING IS STARTING WEAKEN
AND WILL BACK TO SOUTHEAST THROUGH BY THURSDAY MORNING. THE MARINE
LAYER IS STARTING TO THIN THIS EVENING...THINNING FROM AROUND 1700
FEET EARLIER THIS EVENING TO ABOUT 1000 FEET CURRENTLY. SOME
DEEPENING IS POSSIBLE TOWARDS TUESDAY MORNING OF THE MARINE LAYER
AND SOME COVERAGE INTO THE LOWER VALLEYS IS NOT OUT OF THE QUESTION
LATE TONIGHT OR EARLY TUESDAY MORNING. THE BEACHES AND IMMEDIATE
COASTAL LOCALES SHOULD HAVE CLOUDS HANGING AROUND A LITTLE LATER ON
TUESDAY...POSSIBLY LINGERING ALONG THE BEACHES INTO THE AFTERNOON
HOURS.

MODELS ARE HINTING AT A BIT MORE WEAKENING OF THE ONSHORE FLOW ON
TUESDAY. MOS GUIDANCE IS PICKING UP ON NORTHEAST WINDS AT PALMDALE
AND LANCASTER ON TUESDAY MORNING AND TUESDAY AFTERNOON. WITH ONSHORE
FLOW ALREADY INITIALIZED TOO STRONGLY ONSHORE THIS EVENING BY THE
MODEL SOLUTIONS...THE PACKAGE WILL BE WARMED SLIGHTLY. CLIMATOLOGY
STUDIES WOULD GIVE MUCH WARMER RESULT THAN THE CURRENT FORECAST. A
DEGREE OR TWO OF WARMING WILL INTRODUCED...ESPECIALLY FOR LOS
ANGELES COUNTY ON TUESDAY.

&&

.PREV DISCUSSION...

SHORT TERM...BY THURSDAY...MOST AREAS WILL BE IN THE 90S WITH THE
WARMEST VALLEY LOCATIONS SEEING LOW TRIPLE DIGITS. COASTAL AREAS
WILL BE LARGELY DEPENDENT ON HOW THE MARINE LAYER RESPONDS...WHICH
BY ALL INDICATION SHOULD DISSIPATE ON BY THURSDAY. WITH HUMIDITY ON
THE RISE THROUGH THE PERIOD AS WELL...IT WILL FEEL A TOUCH WARMER
THAN THE THERMOMETER READS ON THURSDAY AND FRIDAY...AND THE
ASSOCIATED HEAT INDEX VALUES MAY APPROACH EXCESSIVE HEAT THRESHOLDS
FOR THE VALLEYS AND MOUNTAINS. AT THIS POINT...IT DOES NOT LOOK LIKE
THOSE THRESHOLDS WILL BE MET OVER A LARGE ENOUGH AREA TO WARRANT AN
EXCESSIVE HEAT WATCH...BUT IF THE FORECAST TRENDS WARMER STILL...WE
MAY NEED ONE.

THE ADDED HUMIDITY WILL BE A RESULT OF THE LARGE RIDGE OVER OKLAHOMA
SLIDING WESTWARD...RESULTING IN MOIST SOUTHEAST FLOW ALOFT. THIS
MONSOONAL FLOW SHOULD ARRIVE QUICKLY ON WEDNESDAY...AND PERSIST
THROUGH THE WEEK...WITH THE JUICIEST AIR ON THURSDAY AND FRIDAY.
WITH THIS MOISTURE...AND THE VERY HOT TEMPERATURES OVER THE
INTERIOR...SHOWERS AND THUNDERSTORMS WILL BECOME MORE LIKELY. ON
WEDNESDAY WITH THE INITIAL ARRIVAL...THE 1.2 INCH PWATS ARE NOT
CONDUCIVE TO FLASH FLOODING AND DRY LIGHTNING WILL BE POSSIBLE. BY
WEDNESDAY NIGHT AND THURSDAY HOWEVER...PWATS JUMP UP TO 1.6 INCHES
WITH STEERING FLOW BELOW 10 KTS. THESE TWO INGREDIENTS ALONE
HIGHLIGHT THE SIGNIFICANT POTENTIAL FOR FLASH FLOODING...AND IF THIS
WERE EXPECTED TOMORROW...WE WOULD PROBABLY HAVE A FLASH FLOOD WATCH
OUT NOW. IN ADDITION TO THE INTERIOR SECTIONS HAVING A 20-30 PERCENT
CHANCE FOR THUNDERSTORMS...SOME COASTAL AND VALLEY AREAS ALSO HAVE A
SLIGHT CHANCE FOR THURSDAY AND FRIDAY...ALTHOUGH WEDNESDAY CANNOT BE
COUNTED OUT EITHER.

.LONG TERM... THE RIDGE OF HIGH PRESSURE PARKED OVER NEW MEXICO ON
FRIDAY WILL SLIDE TO THE NORTH AND EAST OVER THE WEEKEND...WITH THE
FLOW SHIFTING TO SOUTHWESTERLY ALOFT BY SUNDAY OR MONDAY. WHEN THIS
HAPPENS...THE THUNDERSTORM POTENTIAL WILL QUICKLY DECLINE...AND
TEMPERATURES WILL LOWER AS ONSHORE FLOW RETURNS. STILL...THERE IS A
SLIGHT CHANCE FOR THUNDERSTORMS OVER THE INTERIOR AREAS ON SATURDAY
AND POSSIBLY SUNDAY AS THE FLOW SLOWLY SHIFTS.

&&

.AVIATION...28/0000Z.

AT 0000Z...THE MARINE INVERSION AT KLAX WAS BASED AROUND 1700 FEET.
THE TOP OF THE INVERSION WAS AROUND 2600 FEET WITH A TEMPERATURE OF
21 DEGREES CELSIUS.

MODERATE CONFIDENCE IN THE CURRENT FORECAST. IFR TO MVFR CONDITIONS
WILL LIKELY SPREAD INTO COASTAL TERMINALS THROUGH 10Z. THERE IS A
CHANCE OF IFR TO MVFR CONDITIONS AT VALLEY TERMINALS BETWEEN 10Z AND
15Z. NORTH OF POINT CONCEPTION...THERE IS A CHANCE OF VLIFR TO LIFR
CONDITIONS AT CENTRAL COAST TERMINALS BETWEEN 04Z AND 15Z.

KLAX...MVFR CONDITIONS COULD DEVELOP AS SOON AS 03Z...OR AS LATE AS
10Z. HIGHER CONFIDENCE EXISTS IN A LATER ARRIVAL TIME BETWEEN 08Z
AND 10Z. THERE IS A 40 PERCENT CHANCE OF IFR CONDITIONS. VFR
CONDITIONS COULD DEVELOP AS SOON AS 16Z...OR AS LATE AS 18Z. THERE
IS

KBUR...VFR CONDITIONS ARE EXPECTED THROUGH THE PERIOD...EXCEPT FOR
A 30 PERCENT CHANCE OF IFR TO MVFR CONDITIONS BETWEEN 10Z AND 16Z.

&&

.MARINE...27/900 PM.

FOR THE OUTER WATERS...MODERATE TO HIGH CONFIDENCE THAT SCA CONDS
WILL CONTINUE THROUGH LATE TONIGHT INTO EARLY TUESDAY MORNING. THE
WINDS ARE THEN FORECAST TO DECREASE SLOWLY THROUGH THE DAY TUESDAY
AND REMAIN BELOW SCA LEVELS THROUGH THE WEEK.

FOR THE INNER WATERS..VERY GOOD CONFIDENCE IN CURRENT FORECAST.
THERE IS JUST A 20 PERCENT CHANCE OF SCA LEVEL WINDS THIS EVENING
ACROSS THE NORTHERN INNER WATERS AND THE FAR WESTERN PORTION OF THE
SANTA BARBARA CHANNEL.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
SMALL CRAFT ADVISORY (SEE LAXMWWLOX).
SPECIAL WEATHER STATEMENT (SEE LAXSPSLOX).

&&

$$

PUBLIC...HALL/KITTELL
AVIATION...HALL
MARINE...SMITH
SYNOPSIS...SETO/HALL

WWW.WEATHER.GOV/LOSANGELES



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