Flash Flooding and Debris Flow in California, August 1 to 3, 2014

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SAN DIEGO CA
1000 AM PDT FRI AUG 1 2014

.SYNOPSIS...
WARM CONDITIONS WILL PREVAIL ACROSS SOUTHERN CALIFORNIA TODAY...AS
SOME SUBTROPICAL MOISTURE GRADUALLY INCREASES ACROSS THE REGION.  AS
A RESULT...ISOLATED AFTERNOON THUNDERSTORMS WILL BE POSSIBLE OVER
THE MOUNTAINS TODAY.  A BETTER SURGE OF MOISTURE AND A DISTURBANCE
WILL THEN LIFT ACROSS THE AREA THIS WEEKEND...BRINGING COOLER
CONDITIONS TO THE REGION AS WELL AS A CHANCE OF SHOWERS AND
THUNDERSTORMS DEVELOPING ACROSS THE ENTIRE AREA.  AS THIS UPPER
LEVEL LOW SHIFTS EAST LATE SUNDAY AND MONDAY...THE THREAT OF SHOWERS
AND THUNDERSTORMS WILL DECREASE.  AN UPPER LEVEL TROUGH WILL DEVELOP
ALONG THE WEST COAST FOR THE WORK WEEK...WITH DRIER CONDITIONS
PREVAILING.

&&

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

WATER VAPOR IMAGERY SHOWS HIGH PRESSURE LIFTING TO THE NORTH THIS
MORNING...WITH A CLEAR MOISTURE DEFORMATION ZONE OVER SOUTHERN
CALIFORNIA.  WATER VAPOR IMAGERY ALSO SHOWS A WEAK DISTURBANCE TO
OUR SOUTH OVER BAJA AS WELL AS A CLEARLY DEFINED UPPER LEVEL
CIRCULATION EVEN FARTHER SOUTH OVER CENTRAL BAJA.  THESE TWO
DISTURBANCES WILL LIKELY COMBINE BY THIS WEEKEND...BRINGING THE
RETURN OF UNSETTLED WEATHER TO THE REGION.

IN THE MEANTIME...THE 12Z KNKX SOUNDING SHOWS THAT THE PW HAVE
DECREASED A BIT THIS MORNING AS COMPARED TO YESTERDAY...BUT WILL
INCREASE AGAIN LATER THIS AFTERNOON.  THE SOUNDING ALSO SHOWS THE
MARINE LAYER DEEPENED OVERNIGHT TO AROUND 1600 FEET...WHICH ALLOWED
THE MARINE LAYER TO PUSH WELL INTO THE SAN DIEGO COUNTY VALLEYS THIS
MORNING.  VISIBLE IMAGERY NOW SHOWS THAT THE LOW CLOUDS HAVE MOSTLY
CLEARED BACK TO THE COAST.  WITH A 10 DEGREE INVERSION...THERE IS
THE POTENTIAL FOR SOME LOW CLOUDS TO CLING TO THE COAST TODAY.
OTHERWISE...JUST LOOKING FOR SOME ISOLATED ELEVATED THUNDERSTORMS TO
DEVELOP OVER THE MOUNTAINS THIS AFTERNOON.  TEMPERATURES WILL LIKELY
BE SIMILAR TO THOSE OF YESTERDAY...WITH THE MARINE LAYER HELPING TO
MODERATE TEMPERATURES THERE...WHILE INLAND AREAS SEE VERY WARM
CONDITIONS ONCE AGAIN.

AS FOR THIS WEEKEND...A GRADUAL COOLING TREND WILL DEVELOP...AS THE
UPPER LEVEL LOW FROM OVER CENTRAL BAJA LIFTS ACROSS THE REGION.
HOWEVER...THE MORE SIGNIFICANT CHANGE WILL BE IN THE INCREASE IN
SUBTROPICAL MOISTURE.  MODELS ARE SHOWING THAT PWS WILL INCREASE
FROM BETWEEN 1 AND 1.4 INCHES TODAY TO SOMEWHERE BETWEEN 1.5 AND 2
INCHES BY LATE SATURDAY.  THE INSTABILITY WILL ALSO INCREASE AS THIS
UPPER LEVEL SYSTEM DRIFTS NORTHWARD...WHICH WILL PROVIDE ENOUGH LIFT
FOR SHOWERS AND THUNDERSTORMS TO DEVELOP.  THE QUESTIONS REMAIN
THOUGH...WHAT WILL BE THE TRACK OF THIS UPPER LEVEL LOW AND HOW
QUICK WILL IT LIFT ACROSS THE AREA?  THE MODELS ARE STILL STRUGGLING
WITH THESE TWO QUESTIONS.  FOR EXAMPLE...THE 12Z NAM TODAY NOW SHOWS
THIS UPPER LEVEL LOW TRACKING RIGHT OVER SAN DIEGO COUNTY ON
SATURDAY NIGHT...BRINGING THE THREAT OF THESE SHOWERS AND
THUNDERSTORMS ALL THE WAY TO THE COAST.  THIS IS NEARLY A COMPLETE
180 FROM WHAT IT WAS SHOWING YESTERDAY.  THE 12Z GFS IS TRENDING
FARTHER WEST WITH THE LOW...WHICH WOULD ALSO INCREASE THE COASTAL
THREAT.  HOWEVER...THE 00Z ECMWF IS STILL MUCH FARTHER EAST WITH THE
LOW...WHICH WOULD NOT BRING SHOWERS ALL THE WAY TO THE COAST.  FOR
NOW...BELIEVE THAT THERE IS ENOUGH TO SUGGEST THAT SHOWERS AND
THUNDERSTORMS WILL BE POSSIBLE...EVEN IN THE COASTAL AREAS FOR LATE
SATURDAY THROUGH POTENTIALLY SUNDAY MORNING.  HOWEVER...IT APPEARS
THAT THIS LOW WILL QUICKLY LIFT TO OUR NORTH OR EAST BY
SUNDAY...REDUCING THE THREAT OF COASTAL OR EVEN VALLEY STORMS BY THE
AFTERNOON.  THE THREAT WILL LIKELY CONTINUE ACROSS OUR MOUNTAINS AND
DESERTS THOUGH THROUGH THE EVENING HOURS.

MUCH DRIER CONDITIONS SHOULD RETURN BY MONDAY...AS ANOTHER UPPER
LEVEL TROUGH DEVELOPS ALONG THE WEST COAST.  THIS MEANS THAT THE
UPPER LEVEL FLOW WILL BE SOUTHWESTERLY OR WESTERLY...WITH PWS
FALLING BELOW AN INCH.  THIS TROUGH WILL SHIFT INLAND BY
MIDWEEK...WITH GENERAL TROUGHINESS REMAINING OVER THE REGION THROUGH
FRIDAY.  THEREFORE...EXPECT DRY CONDITIONS AND ONSHORE FLOW TO
PREVAIL DURING THIS TIME...WITH TEMPERATURES NEAR OR BELOW NORMAL
FOR THE WEEK.  THE MARINE LAYER WILL DEEPEN AS WELL...WITH MORE
COASTAL LOW CLOUDS AND FOG POSSIBLE EACH NIGHT.  SOME LOCALLY GUSTY
WEST WIND WILL BE POSSIBLE THROUGH AND BELOW THE FAVORED CANYONS AND
PASSES EACH AFTERNOON.

&&

.AVIATION...
011500Z...A FEW THUNDERSTORMS OVER MOUNTAINS BETWEEN 20-01Z THIS
AFTERNOON AND LESS POSSIBLE OVER ADJACENT DESERTS AROUND 22-02Z.
THEY WILL BE ISOLATED...BUT WHERE THEY OCCUR BASES WILL BE AROUND
9000 FEET ASL WITH TOPS UP TO 35000 FEET. OTHERWISE...SCATTERED
CLOUDS AOA 15000 FEET ASL INCREASING SATURDAY.

LOW CLOUDS WILL RETURN TO COASTAL AIRPORTS THIS EVENING BETWEEN
04-08Z AT AROUND 1000 FEET ASL WITH TOPS TO 1500 FEET. THESE WILL
DISSIPATE SATURDAY MORNING BY 16Z.

&&

.MARINE...
800 AM...THERE WILL BE A SLIGHT CHANCE OF SHOWERS AND THUNDERSTORMS
FOR THE COASTAL WATERS SATURDAY NIGHT THROUGH SUNDAY MORNING. THESE
STORMS WILL BE CAPABLE OF PRODUCING BRIEF HEAVY RAIN...CLOUD TO
WATER LIGHTNING AND BRIEF GUSTY WINDS. OTHERWISE...NO MARINE HAZARDS
ARE EXPECTED THROUGH TUESDAY.

&&

.SKYWARN...
SKYWARN ACTIVATION IS NOT REQUESTED. HOWEVER WEATHER SPOTTERS ARE
ENCOURAGED TO REPORT SIGNIFICANT WEATHER CONDITIONS.

&&

.SGX WATCHES/WARNINGS/ADVISORIES...NONE.

&&

PUBLIC...PALMER
AVIATION/MARINE...MM