FXUS66 KMTR 051204
AFDMTR

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SAN FRANCISCO BAY AREA CA
404 AM PST SAT DEC 5 2009

.DISCUSSION...AS OF 2:45 AM PST FRIDAY...INITIALLY THE FOCUS IS ON
THE BEHAVIOR OF A MODIFIED ARCTIC AIRMASS PRESENTLY MIGRATING SOUTH
THROUGH OREGON AND SPILLING OVER THE BORDER INTO CALIFORNIA. IT'S
COLD ENOUGH TO SUPPORT A LITTLE OFF AND ON LIGHT SNOW AT MEDFORD FOR
INSTANCE. OVER THE BAY AREA AND CENTRAL COAST LOW LEVEL TEMPERATURES
WILL EDGE SLIGHTLY DOWNWARD TODAY BUT ENHANCED COOLING IS ON THE WAY
SUNDAY AFTERNOON INTO MONDAY. THE WRF MODEL SHOWS 850 MB LEVEL
TEMPERATURES COOLING TO 0C SUNDAY THEN BOTTOMING OUT BETWEEN -2C AND
-4C MONDAY. AN UPPER TROUGH AXIS WITH -29C AT 500 MB LEVEL ARRIVES
MONDAY AS WELL. COLDER AIR WILL LIKELY LINGER INTO TUESDAY BEFORE LOW
LEVEL WARM AIR ADVECTION ARRIVES. RAIN SHOWERS WITH LOW SNOW LEVELS
CAN BE EXPECTED LATE SUNDAY INTO MONDAY. SNOW LEVELS PRESSED DOWN TO
500-1000 FEET NORTH BAY AND 1500 FEET OVER THE CENTRAL COAST ARE
CERTAINLY POSSIBLE. WEATHER SYSTEMS THAT USHER IN COLDER AIR FROM THE
NORTH ARE TYPICALLY DRIER SYSTEMS BUT WILL HAVE TO MONITOR THE
AIRMASS TO SEE IF IT PICKS UP MOISTURE DURING ITS BRIEF TRAJECTORY
OVER THE EASTERN PACIFIC. THE ARRIVAL OF THE COLD AIRMASS HAPPENS TO
COINCIDE WITH THE SHORTEST DAYLIGHT HOURS OF THE YEAR.

ALSO IMPORTANT...THIS WILL BE THE COLDEST AIRMASS OF THE SEASON SO
FAR WITH A FEW HOURS OF LATE NIGHT/EARLY MORNING SUB-FREEZING
TEMPERATURES OVER INLAND LOCATIONS A PRETTY GOOD PROBABILITY BY
MONDAY NIGHT. A SPECIAL WEATHER STATEMENT ISSUED YESTERDAY AFTERNOON
WILL BE FRESHENED UP BY 5 AM. PLANS FOR EARLY WEEK SHOULD INCLUDE
PROVIDING PROPER SHELTER FOR ANYONE OR ANYTHING SENSITIVE TO COLD...
INCLUDING PETS. AS FOR SNOW AMOUNTS IN THE HILLS AND MOUNTAINS A
BRIEF BURST OF SNOW COULD AT LEAST PRODUCE A LIGHT COATING. WILL HAVE
TO MONITOR SUNDAY AND MONDAY FOR POSSIBLE WINTER WEATHER ADVISORY.

SIDE NOTE...DECIDED TO TRIM DOWN GRIDDED MAX/MIN TEMPERATURES A FEW
DEGREES INTO THE MIDDLE OF NEXT WEEK. SNOW LEVELS WERE ALSO LOWERED
SOMEWHAT.

BY MONDAY MORNING A TIGHTENING RIBBON OF COLDER TEMPERATURES WILL BE
POSITIONED OVER THE COASTAL WATERS. THE MAIN COLD FRONT WILL MOVE
INLAND BUT A SECONDARY TROUGH COULD LINGER OVER THE WATERS
TEMPORARILY. RELATIVELY MILDER OCEAN TEMPERATURES WILL CAUSE STEEP
1000-700 MB LAYER LAPSE RATES CLOSING IN ON 7.5C/KM MONDAY. WEATHER
SYSTEMS LIKE THIS CAN GENERATE LOW TOPPED CONVECTIVE CELLS TOO. THIS
COULD BECOME SIMILAR TO A SYNOPTIC/MESO SET UP IN DEC 2001. (SEE
BLIER 2003 - EXAMINATION OF A COOL-SEASON MONTEREY BAY AREA SEVERE
WEATHER EVENT USING WES). HIGHS MONDAY WILL PROBABLY HOVER IN THE
UPPER 30S TO MID 40S AREA-WIDE. BOTH ECMWF AND GFS SHOW RAPID
INTENSIFICATION OF THE UPPER LEVEL JET STREAM ONCE IT IS LOCATED OUT
OVER THE COASTAL WATERS MONDAY. IF LOW TOPPED CONVECTIVE CELLS
DEVELOP THEY COULD GAIN EXTRA SUPPORTING TILT NEEDED TO SUSTAIN CELL
GROWTH.

BY TUESDAY NIGHT INTO WEDNESDAY THE DEVELOPMENT OF LOW LEVEL WARM AIR
ADVECTION WILL MEET ANY LINGERING COLD AIR TRAPPED ON THE IMMEDIATE
COAST AND INLAND VALLEYS. THE QUESTION OVER RECENT DAYS HAS BEEN
WILL THE WESTERLIES BRING IN A STRONG SHORTWAVE AND GENERATE A
SUBSTANTIAL QPF/HEAVY RAIN EVENT?? THE BASIS OF THIS LIES IN THE
DEVELOPMENT OF STRONGER WESTERLIES UNDERCUTTING THE RIDGE BUT
MORE IMPORTANTLY WHERE DOES THE JET SET UP NEXT WEEK? RECENTLY THE
ECMWF SHOWS THE MAIN JET SETTING UP MORE SO OVER SOCAL POSSIBLY
SETTING UP FOR HEAVY RAINS WELL TO OUR SOUTH NEXT WEEK. THE COLLISION
OF MILDER AIR AND MODIFIED ARCTIC OVER OUR FORECAST AREA COULD
GENERATE A PERIOD OF STEADY MODERATE RAINS/WINDS...BUT HOW LONG AND
HOW MUCH IS STILL NOT CERTAIN. A CLEARER PICTURE SHOULD PRESENT
ITSELF WITH SUBSEQUENT MODEL RUNS.

&&

.AVIATION...AS OF 4:04 AM PST SATURDAY...MAINLY MVFR CIGS AND VFR
VSBYS AROUND THE BAY AREA...WITH LOCALIZED IFR CIGS AND MVFR VSBYS
IN INLAND VALLEYS. A WEAK RIDGE WILL BUILD IN TODAY AHEAD OF A COLD
LOW PRESSURE DROPPING DOWN FROM THE NORTH. THIS SET-UP WILL PRODUCE
A DECENT 4-5MB COASTAL N-S GRADIENT. THEREFORE...WITH A SLIGHT
OFFSHORE FLOW AND A LACK OF A MARINE LAYER...CAN EXPECT CLEAR
CONDITIONS WITH VFR CIGS/VSBYS BY 16-17Z THIS MORNING AND LASTING
INTO THE NEXT 24-30 HOURS.

VICINITY OF KSFO...MVFR CIGS AND VFR VSBYS ARE EXPECTED TO LAST
UNTIL APPROX 15-16Z AS RESIDUAL MOISTURE LINGERS THIS MORNING. WITH
A WEAK RIDGE BUILDING IN...LOOK FOR SKIES TO CLEAR...AND REMAIN
CLEAR WITH GOOD VSBY THROUGH TONIGHT/TOMORROW MORNING.

KSFO BRIDGE APPROACH...CURRENTLY SAN MATEO BRIDGE IS CLEAR...BUT NOT
RULING OUT A COUPLE HOURS OF MVFR CIGS BEFORE 16Z.

MRY BAY AREA TERMINALS...MVFR CIGS AT KMRY AND KSNS...WITH IFR CIGS
AT KWVI. SKIES SHOULD CLEAR OUT BY 16Z AS A WEAK RIDGE OF HIGH
PRESSURE BUILDS IN AND MODERATE WINDS ALONG THE COAST HELP PUSH
STRATUS AWAY FROM THE COAST. A LIGHT OFFSHORE FLOW WILL ALSO HELP
KEEP CONDITIONS CLEAR WITH NO MARINE INVERSION AS WELL.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
    .TDA...SCA BEGINNING 7 AM PT ARENA TO PIGEON PT 10-60 NM
           SCA BEGINNING 7 AM PT ARENA TO PT REYES 0-10 NM
           SCA THIS AFTN AND TNGT PIGEON PT TO PT PIEDRAS BLANCAS
               10-60 NM
$$

PUBLIC FORECAST: CANEPA
AVIATION/MARINE: TENTINGER

NWS ON THE WEB AT WEATHER.GOV/SANFRANCISCO