Heavy Rain in California, May 5, 2009

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SAN FRANCISCO BAY AREA CA
450 AM PDT TUE MAY 5 2009

.DISCUSSION...AS OF 3:55 AM PDT TUESDAY...WET ROADS FOR THE MORNING
COMMUTE AS RAIN SLOWLY TAPERS OFF ACROSS THE REGION. RAINFALL
GRADIENT CONTINUES TO BE DRAMATIC WITH THIS EVENT WITH GAGES IN THE
NORTH BAY IN EXCESS OF 1 INCH OVER THE LAST 24 HOURS. RADAR RETURNS
ARE NOT TOO IMPRESSIVE BUT PRECIP IS LOW CLOUD TOP BASED WITH 1 AND
3 HOUR RETURNS STILL FAIRLY IMPRESSIVE IN THE HILLS OF THE NORTH AND
EAST BAY AS WELL AS THE SANTA CRUZ MOUNTAINS. OLEMA VALLEY NEAR
POINT REYES REPORTS 1.23 INCH 24 HOUR TOTAL WITH 1.26 AT WOODACRE
AND 0.98 AT THE HAWKEYE RAWS. EAST BAY HILL GAGES SHOWING 0.25-0.50
TOTALS SO FAR WITH 0.65 AT BEN LOMOND IN THE SANTA CRUZ MOUNTAINS.
MEANWHILE GAGES FROM MONTEREY BAY SOUTHWARD ARE HAVING A HARD TIME
TIPPING SO FAR. HOWEVER LATEST TRENDS INDICATES SOME SHOWERS WILL
PASS OVER THE MONTEREY BAY REGION FOR THE MORNING COMMUTE. HAVE PUT
THE GRIDS TOGETHER WITH LIGHT RAIN THIS MORNING AND THEN KEEP
ISOLATED SHOWERS IN THE FORECAST THIS AFTERNOON. ALTHOUGH AFTER
MIDDAY LITTLE PRECIP SHOULD REMAIN AS THE BOUNDARY WASHES OUT.

STAGNANT AIRMASS WILL BRING SOME PATCHY DENSE FOG POTENTIAL LATER
TONIGHT ESPECIALLY IN THE NORTH BAY VALLEYS WITH RECENT RAIN AND ANY
SURFACE BASED INVERSIONS THAT DEVELOP. LATE NIGHT FOG WAS PUT INTO
THE GRIDS FOR LATE TONIGHT AND INTO WEDS MORNING.

NEXT AND LAST FRONTAL BOUNDARY IS FORECAST TO ARRIVE ABOUT 24 HOURS
FROM NOW...VERY LATE TONIGHT AND THROUGH THE MORNING HOURS OF WEDS.
SIMILAR TO PREVIOUS SYSTEMS...THE BEST RAIN CHANCES WILL STAY NORTH
OF THE GOLDEN GATE WITH ONLY LIGHT QPF UNDER 0.25 EXPECTED. MODELS
ARE PRETTY DECISIVE IN KEEPING THE RAIN/NO RAIN LINE SOMEWHERE AROUND
THE SF PENINSULA WITH LITTLE OR NO CHANCE FROM THE SANTA CLARA VALLEY
SOUTHWARD ON WEDS.

DRY WEATHER IS FORECAST THURSDAY THROUGH THE WEEKEND AND INTO EARLY
NEXT WEEK. MODELS INDICATE LARGE SURFACE PRESSURE GRADIENTS WITH
500 MB PATTERN FEATURING A 560 DM TROUGH OVER THE PACIFIC NW AND A
590 DM HIGH SOUTHWEST OF POINT CONCEPTION. THE ONLY SIGNIFICANT
RESULT WILL BE SOME PERIODS OF STRONG AND GUSTY WINDS...MOST LIKELY
OVER THE OCEAN...NEAR THE COAST AND IN THE HILLS SOMETIME FRIDAY
INTO SATURDAY. TEMPS WILL BE NEAR NORMAL AT THE COAST AND PERHAPS
SLIGHTLY ABOVE NORMAL INLAND. LONG RANGE ECMWF RUNS INDICATE
THERMAL TROUGH NEAR THE COAST PORTENDING PLEASANT SPRINGTIME WEATHER.

&&

.AVIATION...AS OF 4:50 AM PDT TUESDAY...KMUX IS INDICATING LIGHT RAIN
ACROSS THE SAN FRANCISCO BAY AREA THIS MORNING. RAIN AND MVFR
CIGS WILL REMAIN THROUGH LATE MORNING. MVFR CIGS ARE EXPECTED TO
PREVAIL THROUGH THIS AFTERNOON AT WHICH POINT A PERIOD OF VFR WILL
LAST THROUGH MIDNIGHT. A MOIST BOUNDARY LAYER WILL USHER IN IFR
CIGS BY MIDNIGHT.

.MONTEREY PENINSULA...LIGHT RAIN IS BEING REPORTED AROUND THE
PENINSULA WITH KMUX SHOWING SHOWERS THIS MORNING. SHOWERS WILL
LINGER THROUGH LATE MORNING WITH CIGS HOVERING AROUND 4000 FEET.
MOIST BOUNDARY CONDITIONS WILL AID IN THE FORMATION OF MVFR CIGS
TOWARDS MIDNIGHT.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...NONE.

&&

PUBLIC FORECAST: RWW
AVIATION/MARINE FORECAST: LARRY

NWS ON THE WEB AT WEATHER.GOV/SANFRANCISCO