FXUS66 KMTR 110454
AFDMTR

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SAN FRANCISCO BAY AREA CA
855 PM PST THU DEC 10 2009

.DISCUSSION...AS OF 8:55 PM PST THURSDAY...THE MOIST FLOW ACROSS THE
EASTERN PACIFIC HAS FINALLY BEGUN TO BREAK THROUGH THE RIDGE ALONG
THE WEST COAST OF NORTH AMERICA...AND MAKE ITS WAY INTO CALIFORNIA.
IT HAS TAKEN A WHILE TO MOISTEN UP THE PRE-EXISTING DRY AIRMASS...
ESPECIALLY THE LOW LEVELS...AND PRECIP HAS BEEN SLOW TO DEVELOP OVER
THE PAST 24 HOURS. INCREASED CLOUD COVER LAST NIGHT HELP HOLD TEMPS
UP AFTER TWO VERY COLD NIGHTS. THOSE SAME CLOUDS HELD TEMPERATURES
WELL BELOW NORMAL TODAY...ABOUT TEN DEGREES BELOW NORMAL. WARM
ADVECTION OVER THE NEXT 48 HOURS WILL LEAD TO A GRADUAL INCREASE IN
TEMPERATURES THROUGH SATURDAY. BY SATURDAY...HIGHS WILL BE IN THE
UPPER 50S TO NEAR 60 AND LOWS WILL MOSTLY RANGE FROM THE MID 40S TO
AROUND 50. HOWEVER...MOST OF THE FOCUS THROUGH THE WEEKEND WILL NOT
BE ON TEMPERATURES...BUT RATHER ON RAINFALL AND WIND.

WARM ADVECTION CONTINUES TO GENERATE LIGHT PRECIPITATION ACROSS
PORTIONS OF OUR FORECAST AREA THIS EVENING. HOWEVER...IN THE PAST
HOUR RAIN RATES HAVE MOVED INTO THE MODERATE CATEGORY ACROSS
PORTIONS OF THE SANTA CRUZ AND SANTA LUCIA MOUNTAINS AS A VORT MAX
APPROACHES THE CENTRAL CA COAST. THUS FAR THE BEST LIFT AND MOST
MOISTURE HAS BEEN FOCUSED ON THE SOUTHERN HALF OF OUR FORECAST AREA
AND RAINFALL HAS MOSTLY BEEN CONFINED TO AREAS FROM SANTA CRUZ COUNTY
SOUTHWARD. IN THE PAST 24 HOURS UP TO THREE-QUARTERS OF AN INCH OF
RAIN HAS FALLEN IN THE SANTA LUCIA MOUNTAINS OF MONTEREY COUNTY.
ISOLATED CONVECTION HAS BEEN OCCURRING OFF THE CENTRAL CALIFORNIA
COAST SINCE MID-AFTERNOON. THUS FAR THIS CONVECTION HAS SHOWN NO
SIGNS OF REACHING THE COAST...WHICH IS CONSISTENT WITH THE LATEST NAM
OUTPUT WHICH KEEPS ANY SIGNIFICANT INSTABILITY WELL OFFSHORE IN THE
SHORT-TERM.

WARM ADVECTION WILL CONTINUE TO GENERATE PERIODS OF LIGHT TO
LOCALLY MODERATE RAINFALL THROUGH THE NIGHT. THE 00Z MODELS AGREE
THAT RAINFALL WILL GRADUALLY SPREAD NORTH ACROSS THE SF BAY AREA
OVERNIGHT. IT THEN APPEARS THERE WILL BE A SHORT LULL IN THE
PRECIPITATION ON FRIDAY MORNING...BEFORE THE NEXT IMPULSE APPROACHES
FROM THE WEST ON FRIDAY AFTERNOON AND TRIGGERS AN INCREASE IN
RAINFALL LATE FRIDAY AFTERNOON AND EVENING. FORCING WILL REMAIN
RATHER WEAK THROUGH FRIDAY NIGHT AND SO RAINFALL RATES WILL CONTINUE
TO MOSTLY BE LIGHT AS WILL WINDS. A MORE ROBUST FEATURE MOVING
THROUGH THE WESTERLIES WILL BRING THE POTENTIAL FOR HEAVY RAINFALL
AND STRONGER WINDS ON SATURDAY AND SATURDAY EVENING. IN FACT...BOTH
THE 00Z NAM AND 00Z GFS INDICATE THAT A BAND OF MODERATE TO HEAVY
RAIN WILL DEVELOP OVER OUR AREA JUST AHEAD OF A COLD FRONT ON
SATURDAY AFTERNOON. ALSO...BOTH MODELS DEVELOP A SURFACE LOW OFF CAPE
MENDOCINO LATE SATURDAY AFTERNOON (NAM 995 MB...GFS 998 MB). A LOW OF
THAT DEPTH AT THAT LATITUDE WILL TYPICALLY GENERATE LOCALLY STRONG
AND GUSTY SOUTH WINDS ACROSS OUR FORECAST AREA AND GUSTS TO 50 MPH
SEEM LIKE A GOOD POSSIBILITY JUST AHEAD OF THE FRONT ON SATURDAY...
PARTICULARLY NEAR THE COAST AND IN THE HILLS. WILL ISSUE A HAZARDOUS
WEATHER OUTLOOK TO INDICATE THE POTENTIAL FOR A WIND ADVISORY ON
SATURDAY. WILL ALSO MENTION THE POSSIBILITY OF MINOR URBAN FLOODING.
IN ADDITION...THE AIRMASS IS FORECAST TO BE UNSTABLE ENOUGH FOR
ISOLATED CONVECTION ON SATURDAY.

RAINFALL SHOULD TAPER OFF SATURDAY NIGHT THROUGH SUNDAY...AND
GENERALLY DRY WEATHER SHOULD RETURN TO OUR REGION BY MONDAY AS A
SHORTWAVE RIDGE DEVELOPS. RAIN MAY RETURN TO THE NORTH BAY AS EARLY
AS TUESDAY OF NEXT WEEK.

&&

.AVIATION...AS OF 3:50 PM PST THURSDAY...STEADY RAIN BAND IS OVER
KMRY AS OF 00Z. IT WILL MOVE NORTHEASTWARD TOWARDS THE 3 MAJOR BAY
AREA TERMINALS THRU THE EVENING HOURS. CURRENT EASTERLY WINDS AT
BAY AREA TERMINALS EXPECTED TO TREND SOUTHEAST IN DIRECTION THIS
EVENING. RAIN WILL BE FAIRLY LIGHT BUT STEADY WITH CIGS IN THE 3-4K
RANGE EXPECTED OVERNIGHT. MAIN IDEA IS TO EXPECT WET RUNWAYS WITH
LIGHT SE WIND FLOW.

VICINITY OF KSFO...EXPECT STEADY LIGHT RAIN THRU MOST OF THE NIGHT
WITH CIGS IN THE 3-4K RANGE. CURRENT EAST WINDS FORECAST TO SHIFT
MORE SOUTHEAST FOR MUCH OF THE EVENING AND OVERNIGHT HOURS. WET
RUNWAYS WITH CONTINUED LIGHT SE WINDS AND SHOWER CHANCES FOR FRIDAY
MORNING. MAY BE A BREAK IN THE WET WX FOR A TIME MIDDAY FRIDAY BUT
THEN FORECAST TO REDEVELOP AGAIN LATER FRIDAY AFTERNOON AND EVENING.

KSFO BRIDGE APPROACH...SAME AS KSFO VICINITY.

MRY BAY AREA TERMINALS...STRONG SE WINDS AT KSNS THIS EVENING WITH
LIGHTER SE WINDS FOR KMRY. STEADY LIGHT RAIN THRU 06Z THIS EVENING
WITH CIGS 3-5K. MORE SE WINDS WITH WET RUNWAYS AND CONTINUED -SHRA
FOR MUCH OF FRIDAY WITH BEST CHANCE AT A BREAK DURING THE MIDDLE OF
THE DAY WHEN WERE BETWEEN SYSTEMS.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .TNGT...SCA...PT ARENA TO PIGEON PT 10 TO 60 NM OFFSHORE
                ...PIGEON PT TO PT PIEDRAS BLANCAS 10 TO 60 NM
                ...PT PINOS TO PT PIEDRAS BLANCAS TO 10 NM

$$

PUBLIC FORECAST: DYKEMA
AVIATION/MARINE: RWW

NWS ON THE WEB AT WEATHER.GOV/SANFRANCISCO