Flash Flood Watch 5, NWS San Diego, CA

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SAN DIEGO CA
819 AM PDT TUE AUG 25 2015

.SYNOPSIS...
A WEAK DISTURBANCE AND MOISTURE IN A MONSOONAL FLOW ALOFT WILL
RESULT IN MORE CLOUDS...ALONG WITH A CHANCE OF SHOWERS AND ISOLATED
THUNDERSTORMS THROUGH TONIGHT. A DRIER SOUTHWEST FLOW WILL DEVELOP
ON WEDNESDAY...HELPING TO CLEAR THE HIGHER CLOUDS...BUT LINGERING
MOISTURE AND INSTABILITY WILL CONTINUE A CHANCE OF THUNDERSTORMS
OVER THE MOUNTAINS AND DESERTS. BUILDING HIGH PRESSURE OVER THE
SOUTHWEST WILL MEAN DRY AND VERY WARM WEATHER FOR THE REMAINDER OF
THE WEEK. A DEEPENING UPPER-LEVEL LOW PRESSURE TROUGH ALONG THE WEST
COAST WILL BRING COOLER WEATHER AND STRONGER ONSHORE FLOW EARLY NEXT
WEEK.

&&

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

FORECAST UPDATED TO TRIM MAX TEMPS IN THE LOWER DESERTS AND ADJUST
SHWR/TSTM WORDING...

MID AND HIGH LEVEL CLOUDS WERE WIDESPREAD THIS MORNING OVER THE
REGION. RADAR INDICATED SOME SCATTERED LIGHT RETURNS DRIFTING FROM SE
TO NW WHICH MAY BE REACHING THE GROUND AS A FEW SPRINKLES...BUT NO
MEASURABLE RAINFALL WAS REPORTED THROUGH 8 AM PDT. FARTHER TO THE
SE...IR SATELLITE IMAGERY SHOWED COOLING TOPS OVER NORTHERN BAJA
ASSOCIATED WITH AN MCS. THE COLDEST TOPS WERE DRIFTING WNW...BUT
EXPECT THE HIGHER CLOUDS TO BE DRAWN OVER THE CWA TODAY DUE TO THE
SE FLOW ALOFT.

THE 12Z MIRAMAR SOUNDING SHOWED A NICE PW JUMP TO 1.46 INCH...BUT
MUCH OF THIS WAS ABOVE 10K FT. WINDS WERE LIGHT AND VARIABLE BELOW
5K FT...BECOMING SE AROUND 10 KT ABOVE 8K FT. A 9 DEGREE C INVERSION
WAS BASED NEAR 1100 FT...WHICH WAS ANOTHER 200 FT LOWER THAN
YESTERDAY. THERE WAS SOME MID-LEVEL INSTABILITY BUT THE LFC WAS
ABOUT 14K FT AND POTENTIAL INSTABILITY PARAMETERS WERE MODEST...EVEN
AFTER MODIFYING FOR DAYTIME HEATING. SURFACE PRESSURE RADIENTS WERE
3-5 MBS ONSHORE TO THE LOWER DESERTS...AND WEAKER TO THE NE. WINDS
WERE LIGHT.

BASED ON THE 12Z SOUNDING...THE MAIN CONCERN FROM ANY CONVECTION
TODAY WILL AT LEAST BEGIN AS DRY LIGHTNING STRIKES AND MICROBURST
WINDS...UNTIL THE 10K FT DRY LAYER MOISTENS UP. ADDITIONALLY...THE
EXTENSIVE CLOUD COVER EXPECTED SHOULD BE AN INHIBITING FACTOR FOR
ANY DEEP CONVECTION IN MANY AREAS. IF WE DO GET SIGNIFICANT BREAKS
HOWEVER...A FEW THUNDERSTORMS SHOULD GET GOING. WE EXPECT THE
ATMOSPHERIC PROFILE TO TRANSITION TO ONE MORE SUPPORTIVE OF
THUNDERSTORMS WITH LOCALLY HEAVY RAIN. IF NOT TODAY...THEN ON
WEDNESDAY. THE HIRES MODELS CONFIRM THIS TREND IN ALL OF THE LATEST
RUNS. ANY THUNDERSTORMS ON WEDNESDAY WILL BE CONFINED TO THE HIGHER
MTN RIDGES AND DESERTS.

THE GLOBAL MODELS STILL SHOW THE STRONG SUBTROPICAL HIGH CENTER
DEAMPLIFYING MERIDIONALLY AND SHIFTING TO THE SW WITH AN EAS-
WEST AXIS ALIGNED ACROSS NORTHERN MEXICO AND TEXAS. 500 MB
HEIGHTS INCREASE MARGINALLY OVER SOCAL...PEAKING AROUND 593 DM
ON THU...THEN SLOWLY FALLING AS THE DEEP TROUGH OFF THE WEST COAST
SWINGS SLOWLY INLAND. THIS WILL MEAN VERY WARM AND DRY WEATHER
ACROSS THE SOUTHLAND INTO THE WEEKEND...FOLLOWED BY A BUILDING
MARINE LAYER AND COOLER CONDITIONS EARLY NEXT WEEK.

&&

.AVIATION...
251500Z...COAST/VALLEYS...AREAS OF LOW CLOUDS WITH BASES AROUND 1000
FEET MSL AND TOPS TO 2000 FEET THIS MORNING. EXPECT LOW CLOUDS TO
CLEAR BY 18Z. LOW CONFIDENCE IN RETURN OF COASTAL LOW CLOUDS
TONIGHT...WITH MOSTLY PATCHY COVERAGE.

ACROSS THE REGION...WIDESPREAD CLOUDS AOA 10000 FEET WILL CONTINUE
THROUGH THIS EVENING. LIGHT SHOWERS ARE POSSIBLE FROM THESE CLOUDS.
ISOLATED THUNDERSTORMS ARE EXPECTED MAINLY IN MOUNTAINS AND DESERTS
DURING THE AFTERNOON. TSTORM BASES WOULD BE AROUND 9000 FEET WITH
LAYERS TO 40000 FEET. GUSTY AND ERRATIC WINDS ARE POSSIBLE WITH
STORMS...ALONG WITH BRIEF PERIODS OF LOCALLY HEAVY RAINFALL. TSTMS
COULD ALSO PRODUCE AREAS OF REDUCED VIS IN THE DESERTS...WITH
BLOWING DUST AND SAND. CHANCES OF TSTMS WILL DECREASE BY 00-02Z.

&&

.MARINE...

800 AM...ISOLATED TSTMS ARE POSSIBLE OVER THE COASTAL WATERS TODAY.
OTHERWISE...NO HAZARDOUS MARINE WEATHER IS EXPECTED THROUGH
SATURDAY.

&&

.SKYWARN...SKYWARN ACTIVATION MAY BE REQUESTED LATER TODAY FOR THE
THREAT OF THUNDERSTORMS...PRIMARILY OVER THE MOUNTAINS. WEATHER
SPOTTERS ARE ENCOURAGED TO REMAIN ALERT TO THE LATEST WEATHER
FORECASTS AND POSSIBLE SKYWARN ACTIVATION.

&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...NONE.
PZ...NONE.
&&

PUBLIC...JAD
AVIATION/MARINE...PG