Flash Flood Watch 12, NWS San Diego, CA

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SAN DIEGO CA
230 PM PDT MON AUG 25 2008

.SYNOPSIS...
MONSOONAL MOISTURE IN SOUTHEAST FLOW ALOFT WILL BRING A CHANCE OF
THUNDERSTORMS TO THE MOUNTAINS AND DESERTS FOR MUCH OF THE WEEK. A
FEW SHOWERS OR THUNDERSTORMS COULD DRIFT WEST OF THE MOUNTAINS
TOWARDS THE COAST. ONSHORE FLOW AND THE MARINE LAYER WILL CONTINUE
TO BRING PATCHY NIGHT AND MORNING LOW CLOUDS AND FOG TO THE COASTAL
AREAS AND WESTERN SAN DIEGO VALLEYS.

&&

.DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE...
SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO
COUNTIES...

.SHORT TERM (TONIGHT THROUGH THURSDAY)...
THUNDERSTORMS WITH HEAVY RAIN ARE ONGOING ACROSS THE MOUNTAINS AND
DESERTS THIS AFTERNOON. SEE LAXLSRSGX FOR STORM REPORTS. A FEW
SHOWERS AND THUNDERSTORMS ROLLED THROUGH THE VALLEYS AND COAST THIS
MORNING...BUT HAVE SINCE MOVED OFFSHORE. A SIGNIFICANT GULF SURGE
OCCURRED IN THE LOWER DESERTS WITH 70 DEGREE DEWPOINTS SPREADING ALL
THE WAY INTO BEAUMONT THROUGH THE BANNING PASS. MORE IMPRESSIVE IS
THE FACT THAT A 79 DEGREE DEWPOINT WAS MEASURED AT SEVERAL LOCATIONS
INCLUDING IMPERIAL AIRPORT...FISH CREEK...AND EVEN BEAUMONT. THE
SURGE APPEARS TO HAVE DEEPENED SINCE THIS MORNING AND DEWPOINTS IN
THE LOWER MOUNTAINS HAVE NOW CLIMBED INTO THE 60S.

THE 12Z NKX SOUNDING SHOWED CONSIDERABLE WARMING IN THE LOW LEVELS
WITH CONTINUED COOLING ALOFT. THIS HAS TRANSLATED INTO STEEP LOW/MID
LEVEL LAPSE RATES. THE INCREASE IN MOISTURE AND HEAT WILL FUEL
ADDITIONAL THUNDERSTORM DEVELOPMENT THIS AFTERNOON OVER THE
MOUNTAINS AND DESERTS. SOME OF THESE STORMS COULD DRIFT WEST INTO
THE VALLEYS. NO WIND DATA WAS AVAILABLE ON THE SOUNDING...BUT RADAR
OBS SUGGEST THAT STORM MOTION IS MAINLY WEST OR NORTHWEST AT 5-10
MPH. NAM12 MODEL SOUNDINGS INDICATE CAPE VALUES OF 2000-3500 J/KG
WITH LIS OF -6 TO -9 AND NO CIN THIS AFTERNOON OVER THE MOUNTAINS.
THESE NUMBERS ASSUME MAX HEATING THOUGH...AND WITH THE SUBSTANTIAL
INCREASE IN MOISTURE...TEMPS HAVE STRUGGLED TO REACH FORECAST HIGHS.
AS A RESULT...INSTABILITY MAY NOT BE AS GREAT AS THE NAM IS
ADVERTISING. WHILE A FEW PULSE-SEVERE TYPE STORMS ARE POSSIBLE...THE
GREATEST THREAT APPEARS TO BE HEAVY RAIN AND FLASH FLOODING.

MAX TEMPS MAY STILL EXCEED 100 DEGREES IN THE INLAND EMPIRE...AND
THUNDERSTORMS REMAIN POSSIBLE ALONG THE ELSINORE CONVERGENCE ZONE
FROM NEAR ELSINORE TO HEMET/SAN JACINTO THIS AFTERNOON. IF STORMS
CAN DEVELOP IN THE INLAND EMPIRE...THEY WILL HAVE THE POTENTIAL TO
PRODUCE STRONG GUSTY DOWNBURST WINDS AND FLASH FLOODING.

THE SUBTROPICAL RIDGE AXIS WILL REMAIN JUST TO THE NORTH OF SOUTHERN
CALIFORNIA WITH EAST TO SOUTHEAST FLOW ALOFT DIRECTING MONSOONAL
MOISTURE INTO THE MOUNTAINS AND DESERTS THROUGH THURSDAY. THIS
SHOULD MAINTAIN AT LEAST A SLIGHT CHANCE OF AFTERNOON AND EVENING
THUNDERSTORMS IN THE MOUNTAINS AND DESERTS EACH DAY. THERE IS
HOWEVER THE POSSIBILITY THAT DIFFICULT TO TIME DISTURBANCES EMBEDDED
IN THE EAST SOUTHEAST FLOW ALOFT COULD ENHANCE THE THREAT OF
THUNDERSTORMS AT ANY TIME OVER THE NEXT SEVERAL DAYS. THE WILD CARD
IS TROPICAL STORM JULIO WHICH IS CURRENTLY ABOUT 450 MILES SOUTH OF
SAN DIEGO. THUNDERSTORMS ARE STILL ONGOING WITH JULIO...AND ALTHOUGH
IT SHOULD FALL APART OVER THE NEXT 24 HOURS...REMNANT MOISTURE MAY
WORK ITS WAY INTO THE FORECAST AREA OVER THE NEXT FEW DAYS. WOULD
NOT BE SURPRISED TO SEE A FEW SHOWERS OR THUNDERSTORMS DRIFT ALL THE
WAY TO THE COAST ON WEDNESDAY OR THURSDAY. MUCH DEPENDS ON THE
POSITION OF THE UPPER RIDGE...AND THIS WILL BE EXAMINED FURTHER ON
THE NEXT SHIFT.

TEMPS ARE EXPECTED TO REMAIN NEAR OR SLIGHTLY ABOVE SEASONAL
AVERAGES WITH PATCHY NIGHT/MORNING LOW CLOUDS AND FOG CONTINUING
ALONG THE COAST AND IN THE WESTERN VALLEYS...MAINLY IN SAN DIEGO
COUNTY.

&&

.LONG TERM (FRIDAY THROUGH MONDAY)...
FOR FRIDAY THROUGH SUNDAY...MID LEVEL MOISTURE WILL MAINTAIN THE
SLIGHT CHANCE OF AFTERNOON AND EARLY EVENING THUNDERSTORMS IN THE
MOUNTAINS AND DESERTS. THERES QUITE A SPREAD IN THE SHORTWAVE
DETAILS IN THE MODEL SOLUTIONS FOR THE WEEKEND. THE DETAILS OF
TROUGHING ALONG THE WEST COAST WILL INFLUENCE TO WHAT DEGREE ENOUGH
MOISTURE REMAINS TO SUPPORT CONTINUED AFTERNOON AND EARLY EVENING
MOUNTAIN AND DESERT THUNDERSTORMS OR WHETHER THERE IS ENOUGH DRYING
TO REDUCE THE THUNDERSTORM CHANCES. THE CURRENT FORECAST DOESNT
SHOW MUCH CHANGE FROM THE SLIGHT CHANCE POPS FOR MUCH OF THE WEEK.

&&

.AVIATION...
251900Z..MAIN CONCERN TODAY WILL BE WITH THUNDERSTORMS. SCATTERED
STORMS WITH ISOLATED TOPS TO FL500 EXPECTED UNTIL ABOUT 03Z. THE
STRONGER STORMS WILL CONTAIN STRONG UP AND DOWN DRAFTS...LARGE
HAIL...AND LLWS. THERE IS AN OUTSIDE CHANCE FOR A THUNDERSTORM NEAR
KONT/KRIV/KPSP THIS AFTERNOON/EVENING SO VCTS AND CB ARE CURRENTLY
IN THE KONT AND KPSP TAFS. OTHERWISE...SCT-BKN CIGS AT FL100-150
WITH MULTIPLE LAYERS TO FL300 AND VIS UNRESTRICTED.

FOR COASTAL AREAS...CONFIDENCE WILL BE LOW FOR TIMING AND HEIGHT OF
CIGS THIS EVENING AND OVERNIGHT DUE TO TROPICAL AIRMASS IN PLACE
ABOVE THE MARINE LAYER. EXPECT PATCHES TO REMAIN ALONG THE COAST ALL
DAY WITH LIMITED INLAND PENETRATION OVERNIGHT. MARINE LAYER DEPTH
EXPECTED TO REMAIN NEAR 1K FT MSL. AREAS OF 3-5SM VIS RESTRICTIONS
WILL CONTINUE NEAR THE COAST INTO EVENING. EXPECT LIMITED STRATUS TO
REDEVELOP OVERNIGHT WITH BASES AROUND 800 FT MSL AND AREAS OF 1-3SM
VIS RESTRICTIONS INTO THE MESAS AND COASTAL VALLEYS AFTER 06Z TUE.

&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...FLASH FLOOD WATCH FOR THE VALLEYS...MOUNTAINS...AND DESERTS
THROUGH 8 PM. SEE LAXFFASGX.
PZ...NONE.

&&

PUBLIC...SCV
AVIATION/MARINE...JAD

NWS ON THE WEB AT WEATHER.GOV/SANDIEGO

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