FXUS66 KMTR 071307
AFDMTR

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SAN FRANCISCO BAY AREA CA
505 AM PST MON DEC 7 2009

.DISCUSSION...AS OF 5:00 AM PST MONDAY...A DEEP AND COLD UPPER LOW
IS NEARLY STATIONARY NEAR THE CA/OR BORDER EARLY THIS MORNING. A
POTENT SHORTWAVE HAS SWUNG OFFSHORE TO THE WEST OF THE LOW CENTER...
CARVING THE TROUGH WELL OFFSHORE...THEREBY DEVELOPING GOOD OVER-WATER
TRAJECTORY AND THUS A SOURCE FOR MOISTURE. IN ADDITION...A RICHER
SUPPLY OF MOISTURE STREAMING IN FROM THE WEST HAS MANAGED TO MAKE
IT'S WAY UNDER THE AMPLITUDE RIDGE OFFSHORE...AND SOME OF THIS
MOISTURE IS BEING DRAWN INTO CENTRAL AND SOUTHERN CALIFORNIA. THE
RESULT HAS BEEN STEADY LIGHT TO MODERATE PRECIPITATION ACROSS THE SAN
FRANCISCO BAY AREA OVERNIGHT. RAINFALL TOTALS THUS FAR HAVE
RANGED FROM A QUARTER TO THREE-QUARTERS OF AN INCH IN MOST PLACES...
WITH SOME LOCATIONS ON THE SAN FRANCISCO PENINSULA ACCUMULATING OVER
AN INCH. AS THE SHORTWAVE DROPS SLOWLY SOUTH ALONG THE COAST...THE
ASSOCIATED FRONTAL BAND HAS BEEN INGESTING INCREASING AMOUNTS OF
MOISTURE AND THUS PRECIPITATION AMOUNTS HAVE GENERALLY BEEN HIGHER
FARTHER TO THE SOUTH. OF COURSE...THE MOST IMPORTANT WEATHER STORY
AT THE MOMENT ARE THE LOW SNOW LEVELS. TEMPERATURES HAVE DROPPED TO
NEAR FREEZING AS LOW AS 1000 FEET IN THE SF BAY AREA AND SOME SNOW
HAS BEEN REPORTED LOCALLY DOWN TO NEAR SEA LEVEL IN THE EAST BAY.
EXPECT SNOW TO ACCUMULATE ABOVE 500 FEET ACROSS THE SF BAY THROUGH
THE MORNING COMMUTE...CAUSING DIFFICULT DRIVING CONDITIONS OVER
HIGHER ELEVATION ROADWAYS...INCLUDING HIGHWAY 17. A WINTER WEATHER
ADVISORY FOR SNOW REMAINS IN EFFECT UNTIL NOON TODAY FOR THE EAST BAY
HALLS AND SANTA CRUZ MOUNTAINS WHERE UP TO 6 INCHES OF SNOW MAY
ACCUMULATE BY LATE MORNING. MOST PRECIPITATION HAS ALREADY ENDED IN
THE NORTH BAY MOUNTAINS AND THEREFORE LOCATIONS NORTH OF SAN
FRANCISCO ARE NOT INCLUDED IN THE SNOW ADVISORY. FARTHER TO THE
SOUTH...A WINTER WEATHER ADVISORY FOR SNOW WILL REMAIN IN EFFECT
UNTIL 4 PM THIS AFTERNOON FOR ELEVATIONS ABOVE 1000 FEET IN MONTEREY
AND SAN BENITO COUNTIES.

LATEST NAM AND GFS AGREE THAT RAIN/SNOW WILL TAPER OFF FROM N TO S
THIS MORNING AND MOSTLY END BY MID AFTERNOON. THEN...ATTENTION WILL
TURN TO VERY COLD TEMPERATURES TONIGHT/EARLY TUESDAY. JUST HOW FAR
TEMPS DROP WILL DEPEND ON HOW QUICKLY SKIES CLEAR AND HOW MUCH DRYING
DEVELOPS IN THE LOW LEVELS. BUT THE AIRMASS IS SO COLD THAT THERE
APPEARS LITTLE DOUBT THAT SEVERAL HOURS OF SUB-FREEZING TEMPERATURES
WILL OCCUR IN THE VALLEYS. THUS...THE FREEZE WATCH FOR THE VALLEYS
HAS BEEN CONVERTED TO A FREEZE WARNING. IN ADDITION...A FROST
ADVISORY HAS BEEN ISSUED FOR LATE TONIGHT/TUESDAY MORNING FOR THE
COASTAL AND BAYSIDE ZONES IN OUR CWA. THE AIRMASS SHOULD BEGIN TO
MODERATE SOMEWHAT BY TUESDAY NIGHT. HOWEVER...THE AIRMASS WILL
HAVE TIME TO DRY OUT FURTHER AND SO TEMPERATURES MAY VERY WELL BE
AS JUST COLD ON TUESDAY NIGHT AS MONDAY NIGHT. ANOTHER ROUND OF
FREEZE WARNINGS WILL LIKELY BE NEEDED.

THE MODELS CONTINUE TO STRUGGLE WITH THE PATTERN EVOLUTION DURING
THE SECOND HALF OF THE WEEK AND INTO THE UPCOMING WEEKEND. THE
CONSENSUS IS THAT A MOIST WESTERLY FLOW WILL FULLY BREAK UNDER THE
RIDGE DURING THE SECOND HALF OF THE WEEK...BUT EXACTLY WHERE THAT
FLOW WILL BE DIRECTED AND WHEN THE INDIVIDUAL RAIN EVENTS WILL OCCUR
IS STILL VERY MUCH IN QUESTION. IT APPEAR THAT THE INITIAL IMPULSE
WILL SPLIT AND BRING ONLY MODERATE AMOUNTS OF RAINFALL TO CENTRAL
AND SOUTHERN CA ON WEDNESDAY NIGHT AND THURSDAY. SUBSEQUENT SYSTEMS
LOOK TO BE WETTER AND WINDIER...BUT PRECISELY WHEN AND WHERE THEY
FOCUS THAT RAIN AND WIND IS STILL UNCERTAIN.

&&

.AVIATION...AS OF 4:00 AM PST MONDAY...COLD FRONT IS SWEEPING SOUTH
OVER THE SOUTH BAY AND MONTEREY BAY. LOW PRESSURE SPINS UP WEST OF
MONTEREY BAY AS MODIFIED ARCTIC AIR COMES DOWN THE COAST. WINDS ARE
TURNING SOUTHERLY AT THE TERMINALS IN RESPONSE WITH INCREASING RAIN
AND LOWERING CIGS. BOUNDARY WILL BE THROUGH MONTEREY BAY REGION BY
18Z.

VICINITY OF KSFO...WET RUNWAYS THIS MORNING. RAPIDLY IMPROVING
CONDITIONS THIS AFTERNOON. MVFR THIS MORNING IN SHOWERS. VFR THIS
AFTERNOON.

KSFO BRIDGE APPROACH...SAME AS KSFO.

MRY BAY AREA TERMINALS...INCREASING SHOWER ACTIVITY AT THE MOMENT AS
SURFACE LOW PRESSURE IS DEVELOPING JUST WEST OF MONTEREY BAY. EXPECT
WET RUNWAYS AND INCREASING SOUTHEAST WIND FLOW. MODERATE TO HEAVY
SHOWERS WILL LOWER CEILINGS TO 1500 FEET. WINDS SHIFT TO WESTERLY BY
LATE MORNING WITH RAPIDLY IMPROVING CONDITIONS.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
    .TDA...WINTER WEATHER ADVISORY FOR SNOW...ALL ZONES ABOVE 1000
           FEET EXCEPT NORTH BAY MOUNTAINS.
    .LATE MON NIGHT-TUE MORNING...FREEZE WARNING FOR VALLEY ZONES
                                  AND N BAY MOUNTAINS.
                               ...FROST ADVISORY FOR OCEANSIDE AND
                                  BAYSIDE ZONES

$$

PUBLIC FORECAST: DYKEMA
AVIATION/MARINE: RWW/CANEPA

NWS ON THE WEB AT WEATHER.GOV/SANFRANCISCO