FXUS66 KMTR 051734
AFDMTR

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

.DISCUSSION...AS OF 9:15 AM PST FRIDAY...PLENTY OF LOW LEVEL
MOISTURE HAS ALLOWED THE MARINE LAYER TO PENETRATE INTO MOST VALLEY
LOCATIONS THIS MORNING. SINCE A WEAK 2000 FOOT INVERSION REMAINS
OVER THE AREA EXPECT THE LOW CLOUDS TO EVAPORATE TO THE COAST BEFORE
NOON. A FEW HIGH CLOUDS PASSED THROUGH THE SKY LAST NIGHT HELPING TO
KEEP OVERNIGHT TEMPERATURES WARMER THAN THE NIGHT BEFORE.

BEGINNING TONIGHT HOWEVER...A MAINLY CLEAR SKY AND A COLDER AIR
MASS MOVING TOWARD THE SAN FRANCISCO AND MONTEREY BAY AREAS
WILL BRING LOWER TEMPERATURES BOTH DURING THE DAY AND OVERNIGHT. A
BLOCKING UPPER LEVEL HIGH OVER IN THE GULF OF ALASKA IS TO BLAME. IT
HAS OPENED US UP TO NORTHERLY FLOW AND WILL ALLOW A SHORT WAVE TROUGH
TO DROP OUT OF CANADA. THIS TROUGH WILL BRING MODIFIED ARCTIC AIR TO
NORTHERN AND CENTRAL CALIFORNIA SUNDAY NIGHT INTO TUESDAY. IT WILL
INITIAL MIX WITH LIMITED MOISTURE FROM THE WEST CAUSING A FEW
SHOWERS OVER THE REGION BEGINNING AS EARLY AS SUNDAY EVENING. THE
SHORT WAVE WILL CONTINUE THE POSSIBILITY OF SHOWERS THROUGH MONDAY
AS SNOW LEVELS LOWER SIGNIFICANTLY. BY MONDAY MORNING THE SNOW LEVELS
WILL LOWER TO BELOW 1000 FEET IN THE NORTH BAY...AROUND 1500 FEET IN
SAN FRANCISCO AND TO AROUND 2500 FEET IN THE MONTEREY BAY AREA.
HOWEVER...IF CONVECTIVE SHOWERS DEVELOP LOCALLY LOWER SNOW LEVELS
WILL OCCUR.

AS CLEARING TAKES PLACE LATE ON MONDAY THE COLD AIR WILL CAUSE
TEMPERATURES TO PLUMMET TUESDAY MORNING. HOW QUICKLY THE CLOUDS
CLEAR IS STILL IN QUESTION AND WILL LOOK AT THIS MORE CLOSER TO
DETERMINE IF A HARD FREEZE WATCH IS NEEDED.

BEYOND TUESDAY...THE BLOCKING HIGH PROVIDES A STEADY STREAM OF
DISTURBANCES TO MARCH ACROSS CALIFORNIA IN MOIST WESTERLY FLOW.
PERIODS OF RAIN AND HIGH ELEVATION SNOW WILL BE POSSIBLE INTO NEXT
WEEKEND.

&&

.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 PT ARENA TO PT PIEDRAS BLANCAS 10-60 NM
           SCA PT ARENA TO PT REYES 0-10 NM

$$

PUBLIC FORECAST: EVANS
AVIATION/MARINE: TENTINGER

NWS ON THE WEB AT WEATHER.GOV/SANFRANCISCO