Flash Flood Warning 1, NWS San Francisco Bay Area, CA

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SAN FRANCISCO BAY AREA
900 AM PST SAT DEC 01 2001

RAINFALL REMAINS CONCENTRATED IN THE N BAY SO FAR THIS MRNG THOUGH
MEASURABLE PRECIP HAS FALLEN ALMOST EVERYWHERE N OF MRY BAY.  MOST
SPOTS IN THE N BAY HAVE RECEIVED ONE HALF TO ONE INCH BUT SEVERAL OF
THE USUALLY WETTER GAUGES...INCLUDING ROSS...KENTFIELD...AND VENADO
...ARE UP AROUND 2 INCHES.  SINCE THE HEAVY RAIN HAS NOT BEEN
WIDESPREAD...ISSUED A NOWCAST RATHER THAN AN URBAN FLOOD ADVISORY
FOR THE TIME BEING.

THE ETA AND AVN CONTINUE TO HAVE MAJOR PHILOSOPHICAL DIFFERENCES ON
HANDLING THE WEEKEND RAIN.  THE ETA CONTINUES TO CONCENTRATE THE
RAIN IN THE N BAY WITH BY FAR THE HEAVIEST QPF IN THAT AREA TDA AND
TNGT...THOUGH THE MODEL DOES BRING A LITTLE RAIN S TO MRY IN BOTH
PERIODS.  THE AVN...WHILE CORRECTLY CONCENTRATING THE RAIN IN THE N
BAY THIS MRNG...BRINGS THE HEAVIEST RAIN S THRU THE REST OF THE BAY
AREA THIS AFTN AND FOCUSES THE WETTEST CONDITIONS OVER MRY BAY THIS
EVE.  THE AVN THEN BRINGS THE HEAVIEST RAIN INLAND BY MIDDAY SUN
WHILE THE ETA CONTINUES TO SOAK THE COAST.

I AM STILL LEANING TOWARD THE AVNS HANDLING OF THE RAIN.  SAT PIX
AND RADAR SHOW THE ENHANCED CLOUDS AND GOOD RAIN SAGGING S INTO SAN
MATEO COUNTY AT THIS TIME...AND IT CERTAINLY APPEARS THAT THE
HEAVIEST RAIN WILL CONTINUE TO SHIFT S THIS AFTN AS THE FRONTAL BAND
APPROACHES.  THERE IS JET ACTION TO FACTOR INTO THE FORECAST.
ALTHOUGH THE JET CORE IN N OF OUR DISTRICT THIS MRNG...THE WINDS
ALOFT ARE PLENTY STRONG OVER US AND CLOUD ENHANCEMENT IS ALREADY
OCCURRING.  THIS COULD INCREASE RAINFALL INTENSITY TDA AND WILL MOST
LIKELY DO SO TNGT AS THE JET SHIFTS SWD OVER OUR DISTRICT AND
DYNAMICS REALLY KICK IN WITH GOOD THICKNESS DIFFLUENCE DEVELOPING.
THESE DYNAMICS WILL KEEP PERIODS OF RAIN GOING BETWEEN THE TWO
MAIN SYSTEMS...ESPECIALLY IN OROGRAPHICALLY FAVORED AREAS WHICH WILL
INCLUDE ALL OUR COASTAL MTNS.

AS FAR AS WIND IS CONCERNED IT APPEARS A BRISK 15-30 WILL COVER
THINGS TDA AS THE SURFACE LOW REMAINS FAR TO OUR N NEAR CAPE
FLATTERY IN NWRN WA.  STILL...WILL KEEP A CLOSE EYE ON THINGS.  AT
16Z SFO-ACV WAS 9 MB...DOWN FROM 14.4 FOUR HOURS EARLIER.  THE SUN
STORM STILL LOOKS LIKE A BETTER BET FOR WINDS AS THE SURFACE LOW...
THOUGH WEAKER THAN THE ONE TDA...WILL BE CENTERED MUCH FARTHER S...
JUST OFFSHORE IN THE VICINITY OF THE CA/OR BORDER AT 18Z SUN.

THE SUN STORM STILL LOOKS STRONGER OVERALL FOR OUR DISTRICT THAN TDAS
SYSTEM THOUGH THE N BAY WILL PROBABLY NOT GET MORE RAIN THAN TDA.
OTHER AREAS WILL AS THE JET WILL MOVE SWD THRU THE DISTRICT DURING
THE DAY.  WE WILL ALSO HAVE MUCH STRONGER COLD ADVECTION ON SUN TO
ADD TO THE DYNAMICS.  WITH A STRONG ONSHORE FLOW...COLD ADVECTION...
AND GOOD LOW LEVEL MOISTURE...SHOWERS WILL PROBABLY CONTINUE MOST OF
SUN NIGHT BEFORE ENDING MON.  SNOW LEVELS SUN NIGHT WILL DROP TO THE
2-3 THSD FOOT RANGE IF NOT LOWER AND WILL ADDRESS THIS IN AFTN
ZONES.

SHOWERS SHOULD END IN MOST AREAS MON MRNG AS THE UPPER TROF AXIS
FINALLY MOVES THRU THE DISTRICT.  AFTER THAT WE GO INTO A COOL NWLY
FLOW WHICH WILL RESULT IN MUCH DRIER CONDITIONS THOUGH A FEW
SHOWERS MAY SWEEP IN FROM THE NW THRU MIDWEEK.
ALL POPS +9+.  MARKKANEN

.SFO...TDA....GLW...PT ARENA TO PT REYES.
              SCA...PT REYES TO PIGEON PT.
       TNGT/SUN...HEAVY SURF ADVY...PT ARENA TO PIGEON PT.
       SUN........HEAVY SURF ADVY...PIGEON PT TO PT PIEDRAS BLANCAS.