FXUS66 KMTR 061106
AFDMTR

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SAN FRANCISCO BAY AREA CA
300 AM PST SUN DEC 5 2009

.DISCUSSION...AS OF 3:00 AM PST SUNDAY...SHORT TERM FOCUS IS ON
INCOMING MODIFIED ARCTIC AIRMASS PRESENTLY POISED OVER THE PACIFIC
NORTHWEST. OF COURSE IT'S ALREADY COLD ACROSS THE REGION THIS
MORNING BUT BELIEVE IT OR NOT A REINFORCEMENT IS ON THE WAY. AREA-
WIDE READINGS ARE IN THE 30S TO MID 40S AT THIS HOUR. A FEW INLAND
SITES REPORT MID 20S SUCH AS IN POPE VALLEY. MODIFIED ARCTIC AIR WILL
PLAY A ROLE IN THE WEATHER THROUGH MID-WEEK. THIS WILL BE THE COLDEST
AIR OF THE SEASON SO FAR. A FREEZE WATCH REMAINS IN EFFECT FOR INLAND
ZONES MONDAY NIGHT. A FEW RECORD LOWS COULD BE REACHED TUESDAY
MORNING. THE WRF MODEL SHOWS 850 MB LEVEL TEMPERATURES COOLING TO
-1C TO -3C TODAY AND BOTTOMING OUT BETWEEN -2C AND -6C LATE MONDAY
INTO TUESDAY MORNING. THIS IS FASTER AND COLDER THAN YESTERDAY'S
MODEL RUN. RAIN SHOWERS WITH LOWERING SNOW LEVELS ARE EXPECTED LATE
TODAY INTO MONDAY. SNOW LEVELS RANGING 500-1000 FEET NORTH BAY TO
1500 FEET OVER THE CENTRAL COAST ARE CERTAINLY POSSIBLE. REGARDING
SNOW AMOUNTS IN THE HILLS AND MOUNTAINS...BRIEF BURSTS OF HEAVY SNOW
COULD PRODUCE LIGHT ACCUMULATIONS...GENERALLY AN INCH OR SO...BUT
WOULD NOT BE SURPRISED TO SEE LOCALLY HEAVIER ACCUMULATIONS LIKE ON
HIGHWAY 17 SUMMIT. WILL HAVE TO MONITOR THIS FOR A POSSIBLE WINTER
WEATHER ADVISORY MONDAY. THE AIRMASS WILL SPEND SOME TIME OVER THE
EASTERN PACIFIC WITH THE DEVELOPMENT OF A LOW OVER THE COASTAL WATERS
MONDAY. ENHANCED JET DYNAMICS AND STEEP LOW-MID LEVEL INSTABILITY
MAY CAUSE A FEW LOW TOPPED THUNDERSTORMS MONDAY/EARLY MONDAY EVENING
AS WELL. THE WRF INDICATES LOW LEVEL CYCLONIC ROTATION OVER THE AREA
MONDAY. POTENTIAL IS THERE FOR ISOLATED SEVERE DUE TO SHEAR AND
INSTABILITY SIMULTANEOUSLY PRESENT MONDAY.

AFTER CLEARING MONDAY NIGHT EXPECT SEVERAL HOURS OF SUB-FREEZING
TEMPERATURES OVER INLAND LOCATIONS AND POSSIBLY AGAIN TUESDAY NIGHT.

MODEL DISAGREEMENTS CONTINUE. THE LATEST CHANGE...WET WEATHER DURING
MIDWEEK IS NOW SHIFTED OUT ONE MORE DAY TO THURSDAY AND IT SHOULD BE
A LOW QPF EVENT. ADJUSTMENTS TO POP/QPF/SKY GRIDS WERE MADE FOR MID-
WEEK. CLIMO OR NEAR CLIMO BEST APPROACH FOR NOW. LOW LEVEL WARM AIR
ADVECTION LIGHT RAINS ARE POSSIBLE THURSDAY BUT THE SYSTEM LACKS
ORGANIZED JET DYNAMICS. THE 06Z GFS IS DRIER BY THE WAY. BY LATE
FRIDAY OR SATURDAY THE ECMWF AND GFS AGREE A STRONG WESTERLY JET WILL
ENTER OUR REGION W/RAIN...POSSIBLY HEAVY. THIS IS MANY DAYS AWAY AND
RECENT MODEL PERFORMANCE EQUATES TO VERY LOW CONFIDENCE. AGREEMENT
WITH RUN TO RUN CONSISTENCY IS NOW VERY MUCH NEEDED IN ORDER TO
RESTORE CONFIDENCE IN EXTENDED RANGE.

&&

.AVIATION...AS OF 10:15 PM PST SATURDAY...DEW POINTS HAVE DROPPED
CONSIDERABLY UNDER A LIGHT OFFSHORE WIND. THEREFORE EXPECT CLEAR
SKIES TONIGHT IN THE SFO BAY AREA. THE MRY BAY AREA IS NOT AFFECTED
AS MUCH BY THE OFFSHORE WIND AND COULD STILL SEE PATCHY LOW CLOUDS
EARLY SUNDAY.

VICINITY OF KSFO...VFR CONDITIONS THROUGH SUNDAY WITH LIGHT EAST TO
NORTHEAST WINDS.

KSFO BRIDGE APPROACH...SAME AS KSFO.

MRY BAY AREA TERMINALS...PATCHY CLOUDS EXPECTED OVERNIGHT BUT ANY
CEILINGS WILL BE TEMPORARY.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
    .LATE MON NIGHT-TUE MORNING...FREEZE WARNING FOR VALLEY ZONES
                                  AND N BAY MOUNTAINS.

$$

PUBLIC FORECAST: CANEPA
AVIATION/MARINE: W PI/TENTINGER

NWS ON THE WEB AT WEATHER.GOV/SANFRANCISCO