FXUS66 KMTR 061319 AAA
AFDMTR

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SAN FRANCISCO BAY AREA CA
353 AM PST SUN DEC 6 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...UPDATE...AS OF 5:15 AM PST SUNDAY...LOW/MID-LEVEL
CLOUDS HAVE FORMED OFF THE COAST AND OVER NEAR-COAST TERMINALS
WITHIN THE LAST 1-2 HOURS. CURRENT THINKING IS THAT THE CIGS
WILL BE 1500-2000FT INITIALLY...THEN LIFT TO 2500-3500 FT DURING THE
LATE MORNING HOURS. EXACT TIMING OF THIS WILL BE VERY DIFFICULT...
HOWEVER WILL LEAN TOWARDS A MORE PESSIMISTIC OUTLOOK DUE TO WIDE
SPATIAL EXTENT OF THESE CLOUDS ON SATELLITE FOG PRODUCT. MID/HIGH-
LEVEL CLOUDS WILL CONTINUE TO FILTER IN LATER TODAY WITH GUSTY
AFTERNOON WESTERLY WINDS. AS EVEN MORE COOLER AIR SLIDES IN
TONIGHT...PRECIPITATION CHANCES WILL ALSO INCREASE. FOR MOST AREAS
THE RAIN WILL NOT BEGIN UNTIL AFTER MIDNIGHT. HAVE MENTIONED THE
POSSIBILITY FOR -RASN FOR THE KSTS TAF DUE TO THE COLD TEMPERATURES
THAT ARE EXPECTED TO BE IN PLACE. ELSEWHERE...THE RAIN SHOULD LAST
INTO MONDAY AFTERNOON WITH GENERALLY SSW WINDS.

VICINITY OF KSFO...MVFR CIGS AND VFR VSBYS DEVELOPED OFF THE COAST
AND PUSHED INLAND THIS MORNING. CLD BASES 1500-2000 FT...EXPECTED
TO LIFT DURING THE MID-LATE MORNING HOURS. ADDITIONAL MID/HIGH-LEVEL
CLOUDS AND ASSOCIATED RAIN MOVING IN LATE TONIGHT AFTER MIDNIGHT.
PERIODS OF MOD-HVY RAIN POSSIBLE INTO MONDAY MORNING.

KSFO BRIDGE APPROACH...SAME AS KSFO.

MRY BAY AREA TERMINALS...CLEAR SKIES CURRENTLY...HOWEVER THERE
IS A SLIGHT CHANCE OF STRATUS/FOG FORMATION AROUND KMRY OR KSNS THIS
MORNING. STRATUS HAS DEVELOPED ALONG THE COAST AND WILL POSSIBLY MOVE
OVER MRY BAY TERMINALS THIS MORNING. CLOUDS WILL MOST LIKELY STAY IN
TODAY...BUT CIGS SHOULD LIFT ABOVE 2500 FT IN THE LATE MORNING AND
EARLY AFTERNOON HOURS. OTHERWISE MID/HIGH-LEVEL CLOUDS MOVING IN
ASSOCIATED WITH APPROACHING LOW PRESSURE...AND EVENTUALLY RAIN
SHOWERS DEVELOPING LATE TONIGHT LASTING INTO MONDAY. PERIODS OF MOD-
HVY RAIN POSSIBLE INTO MONDAY MORNING.

&&

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

$$

PUBLIC FORECAST: CANEPA
AVIATION/MARINE: TENTINGER

NWS ON THE WEB AT WEATHER.GOV/SANFRANCISCO