Area Forecast Discussion
Issued by NWS Los Angeles, CA

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000
FXUS66 KLOX 182028 AAB
AFDLOX

SOUTHWEST CALIFORNIA AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE LOS ANGELES/OXNARD CA
130 PM PDT SAT APR 18 2015

.SYNOPSIS...
EXPECT A STEADY COOLING TREND THROUGH THE MIDDLE OF NEXT WEEK WITH
LOW CLOUDS AND FOG IN THE COASTAL AND VALLEY AREAS.  THERE WILL BE A
SLIGHT CHANCE OF THUNDERSTORMS OVER SOME OF THE MOUNTAINS EARLY IN
THE WEEK AND A MORE WIDESPREAD CHANCE FOR SHOWERS AND ISOLATED
THUNDERSTORMS BY WEDNESDAY.

&&

.SHORT TERM (SUN-TUE)...
THE COOLING TREND THAT STARTED TODAY FOR MOST OF THE AREA WILL
CONTINUE THROUGH THE SHORT TERM AS AN UPPER LEVEL LOW DROPS DOWN OFF
THE COAST OF CALIFORNIA SUNDAY THEN SLIDES SOUTH AND INTO UPPER BAJA
BY TUES. A COUPLE OF PRIMARY IMPACTS AS A RESULT...ONE WILL BE THE
INCREASE IN THE MARINE LAYER AND COVERAGE OF OVERNIGHT CLOUDS AND
DENSE FOG WITH JUST ENOUGH OF AN INCREASE IN ONSHORE FLOW. EXPECT
COVERAGE TONIGHT ACROSS MOST COASTAL ZONES MINUS SOME OF THE SBA
SOUTH COAST...AND EVEN INTO THE LA COUNTY VALLEYS AND SANTA YNEZ
VALLEY. THEN ON MONDAY AND TUESDAY THE MARINE LAYER DEEPENS ENOUGH
FOR THE VENTURA COUNTY VALLEYS AND ALL OF THE SBA COUNTY SOUTH COAST
TO ALSO SEE SOME LOW CLOUDS AND FOG. BEST CHANCE OF DENSE FOG WILL
BE TONIGHT SINCE THE MARINE LAYER IS STILL SHALLOW ENOUGH. SKIES
SHOULD BE SUNNY TO MOSTLY SUNNY OVER INLAND AREAS DURING THE DAY
WITH GRADUAL CLEARING TO THE BEACHES.

SECOND IMPACT OF THIS TROUGH WILL BE CHANCES FOR SOME LATE AFTERNOON
AND EVENING CONVECTION MONDAY AND TUESDAY OVER THE VENTURA COUNTY
MOUNTAINS AND BLEEDING OVER INTO THE SBA COUNTY MOUNTAINS. MODELS
CONTINUE TO SHOW SOME INSTABILITY FOR THESE AREAS AND ENHANCED BY A
POOL OF MID LEVEL MOISTURE. LIFTED INDICIES FORECAST TUE BY THE NAM
ARE BETWEEN -4 TO -5 DEG C...SO SLIGHTLY MORE INSTABILITY FORECAST
FOR TUE COMPARED TO MONDAY. GFS AND ECMWF ALSO IN AGREEMENT AS FAR
AS TR CHANCES. NOT A LOT OF MOISTURE IN THE COLUMN TO WORK WITH SO
DO NOT EXPECT ANY SIGNIFICANT PRECIPITATION AMOUNTS WITH ANYTHING
THAT DOES DEVELOP. THE GFS PRODUCES QUITE A BIT OF PRECIP OVER THE
VENTURA COUNTY MOUNTAINS...AROUND AN INCH AND A HALF RUN TOTAL
THROUGH TUESDAY NIGHT WHICH LOOKS TO BE WELL OVERDONE. THE NAM ON
THE OTHER HAND SHOWS A MORE REASONABLE THIRD OF AN INCH. AT THIS
TIME. LOWER ELEVATIONS AND THE SAN GABRIELS WILL STAY DRY THROUGH
THIS PERIOD.

TEMPERATURES COOL AT LEAST SEVERAL DEGREES EACH DAY THROUGH THE
SHORT TERM. BY TUESDAY HIGHS WILL BE 10 DEGREES BELOW NORMAL IN SOME
AREAS...BUT WITH THE DESERTS STILL AROUND NORMAL. HIGHS WILL BE IN
THE 60S MOST COASTAL AREAS AND ONLY IN THE LOW 70S NEARBY VALLEYS.

.LONG TERM (WED-SAT)...
A MORE COMPLICATED FORECAST FOR THE EXTENDED PERIOD. A SECOND UPPER
LOW DROPS SOUTHEAST THROUGH THE EASTERN PAC...AND BY MIDDAY WED THE
CENTER OF THIS LOW WILL BE AROUND 500 MILES TO THE SOUTHWEST OF
LAX...THE ECMWF FURTHER AWAY THAN THE GFS. FIRST THE EASIER PART OF
THE FORECAST...TEMPERATURES WILL CONTINUE TO COOL EACH DAY THROUGH
AT LEAST THURSDAY WITH PARTLY TO MOSTLY CLOUDY SKIES. ALSO EXPECT
ANOTHER HEALTHY MARINE LAYER WEDNESDAY MORNING. MODEL POPS ARE NOT
IN VERY GOOD AGREEMENT FOR WEDNESDAY. MOST DETERMINISTIC MODEL POPS
ARE RESTRICTED TO OVER THE VTU COUNTY MOUNTAINS WHILE STATISTICAL
AND ENSEMBLE GUIDANCE SPREADS POPS INTO THE VALLEYS. CURRENT
FORECAST HAS AT LEAST SLIGHT CHANCE POPS EVERYWHERE AND CHANCE POPS
OVER VTU COUNTY MOUNTAINS AND OVER L.A. COUNTY WHERE DYNAMICS MAY BE
BETTER. NOT ENOUGH CLARITY IN THE LATEST MODELS TO REALLY CHANGE
OFFICIAL FORECAST MUCH IF AT ALL. SOME LOWER ELEVATIONS MAY NOT SEE
ANY MEASURABLE.

BY WEDNESDAY NIGHT THE GFS FORECASTS THE LOW TO CLOSE OFF OVER SOCAL
WITH A COUPLE OF VORT LOBES ROTATING AROUND ITS PERIMETER TO THE
WEST AND SOUTHWEST. THE ECMWF MEANWHILE CLOSED IT OFF FURTHER TO THE
SOUTH OVER NORTHERN BAJA. THEN MIDDAY THURSDAY MODELS ARE IN BETTER
AGREEMENT AND GENERALLY AGREE ON CONTINUING TO SWING THE LOW INTO
THE DESERT SOUTHWEST. TECHNICALLY THE TROUGH AXIS IS THROUGH OUR
FORECAST AREA BY THIS TIME. NOTHING IS CLEAR CUT AS FAR AS THE
PRECIP FORECAST WEDNESDAY NIGHT INTO THURSDAY. AGAIN...WOULDN`T BE
SURPRISED IF SOME AREAS DON`T RECEIVE ANY MEASURABLE PRECIPITATION
THIS PERIOD...WHICH THE CHANCES OF THAT ARE GREATEST IF THE ECMWF
VERIFIES. FOR NOW WILL LEAVE FORECAST AS IT STANDS WITH LITTLE
CHANGE FROM WEDNESDAY`S FORECAST POPS...ESPECIALLY SINCE ENSEMBLES
SUPPORT AT LEAST A SLIGHT CHANCE OF MEASURABLE THIS PERIOD. WITH THE
COOLER TEMPS THERE IS SOME CHANCE OF SOME SNOW MIXED IN WITH THE
PRECIP DOWN TO THE GRAPEVINE BUT WITHOUT MUCH CHANCE AT ALL OF ANY
ACCUMULATION.

BIG DIFFERENCES BETWEEN THE ECMWF AND THE GFS FOR THURSDAY NIGHT
INTO FRIDAY. THE GFS KEEPS GENERAL TROUGHINESS OVER THE AREA AND THE
ECMWF ACTUALLY MOVES A SHORTWAVE RIDGE THROUGH THE AREA. CURRENT
FORECAST HAS SLIGHT TO CHANCE POPS RESTRICTED TO MAINLY SOUTHERN
AREAS AND MOUNTAINS ON FRIDAY AND FOR NOW WILL LEAVE THAT AS IS.
ECMWF IS DRIEST OF COURSE AND THIS IS SORT OF THE DIRECTION THE
ENSEMBLE POPS ARE TAKING.

THE BIGGEST QUESTION MARK OF THE EXTENDED PERIOD IS FRIDAY NIGHT
INTO SATURDAY. THE ECMWF BRINGS A VERY SHARP TROUGH ACROSS THE AREA
THIS PERIOD AND INCLUDING A WELL-DEFINED TROUGH PASSAGE. THE HIGHER
POPS ARE ALSO SUPPORTED BY THE ENSEMBLE SOLUTIONS. AND DESPITE THE
GFS DETERMINISTIC RUN THAT KEEPS ENERGY WELL TO THE SOUTH...ITS
ENSEMBLE MEAN HINTS AT SOMETHING CLOSER TO EC SOLUTION. THE EC MODEL
HAS ALSO SHOWN A LITTLE MORE RUN TO RUN CONSISTENCY. BUT DEFINITELY
NOT ENOUGH CLARITY TO MAKE SWEEPING CHANGES TO SATURDAY`S FORECAST
AT THIS POINT. FOR NOW AT LEAST A MENTION OF PRECIP WILL SUFFICE
UNTIL WE GET TO SAY MIDWEEK WHEN HOPEFULLY THINGS WILL BE CLEARER.
FOR NOW TEMPS ARE FORECAST TO MODERATE SOME ON FRIDAY AND SATURDAY
BUT THAT COULD CHANGE.

AS IT STANDS NOW WE DO NOT EXPECT MUCH IN THE WAY OF STORM TOTAL
PRECIPITATION AMOUNTS FOR WED THROUGH SATURDAY...LOWER ELEVATIONS
POSSIBLY IN THE TENTH TO QUARTER AND HIGHER AMOUNTS MAINLY OVER THE
VTU COUNTY MOUNTAINS. A LOT OF THIS DEPENDS ON THE DEVELOPMENT OF A
POSSIBLE SYSTEM ON SATURDAY WHICH COULD DOUBLE THESE AMOUNTS.

&&

.AVIATION...18/1800Z.

MID/UPPER LEVEL TROUGH OF LOW PRESSURE OVER THE AREA WILL PERSIST.
UPPER LEVEL MODERATE WEST WINDS WILL BECOME MODERATE WEST-SOUTHWEST
AFTER 19/13Z AND MID LEVEL LIGHT SOUTH WINDS WILL PREVAIL THROUGH
THE PERIOD OVER THE AREA. WEAK TO MODERATE ONSHORE PRESSURE GRADIENT
THROUGH 19/19Z THEN MODERATE ONSHORE GRADIENT. WEAK CAPPING LOW
LEVEL INVERSION BASE WAS APPROXIMATELY .7KFT WITH AN OVERCAST CLOUD
FIELD WEST AND NORTH OF THE CHANNEL ISLANDS AND CLEAR OVER THE BIGHT
THIS MORNING. THE INVERSION BASE IS EXPECTED TO DIFFER BY PLUS .7KFT
AND CLOUD FIELD WILL BECOME BROKEN TO OVERCAST OVER THE BIGHT SUNDAY
MORNING.

MARINE LAYER AT LAX AT 1550 IS 784 FEET DEEP AND THE INVERSION TOP
IS AT 2056 FEET WITH A TEMP OF 18 DEGREES C.

KLAX...CHANCE CIGS 007 VSBY 5SM BR BETWEEN 19/10-19/12Z AND CHANCE
CIGS 005 VSBY 1SM BR BETWEEN 19/12-19/17Z.

KBUR...CHANCE CIGS 005 VSBY 3SM BR BETWEEN 19/11-19/13Z AND CHANCE
CIGS 003 VSBY 1SM BR BETWEEN 19/13-19-17Z.

VIRTUALLY CERTAIN - 95-100%
VERY LIKELY       - 80-95%
LIKELY            - 60-80%
CHANCE            - 30-60%
VERY UNLIKELY     - 20% OR LESS

&&

.MARINE...18/130 PM.

NORTHWEST WINDS IN THE VICINITY OF THE NORTHERN CHANNEL ISLANDS ARE
EXPECTED TO INCREASE THIS AFTERNOON WITH OCCASIONAL GUSTS TO 25 KT.
THE GUSTS ARE EXPECTED TO DIMINISH AROUND MIDNIGHT. OTHERWISE SMALL
CRAFT ADVISORY CONDITIONS ARE NOT EXPECTED THROUGH TUESDAY. THERE IS
A CHANCE SCA CONDITIONS WILL EXIST NEXT WEEK WEDNESDAY MORNING
THROUGH THURSDAY FROM PIEDRAS BLANCAS TO POINT SAL 10-60NM
AND FROM POINT SAL TO SAN MIGUEL ISLAND. THERE IS A CHANCE AREAS OF
DENSE FOG WILL DEVELOP OVER THE SAN PEDRO AND SANTA MONICA BASINS AS
WELL AS ALONG THE CENTRAL COAST TONIGHT.


&&

.LOX WATCHES/WARNINGS/ADVISORIES...
SMALL CRAFT ADVISORY (SEE LAXMWWLOX).
HIGH SURF ADVISORY (SEE LAXCFWLOX).

&&

$$

PUBLIC...JACKSON
AVIATION/MARINE...30
SYNOPSIS...SWEET

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