FXUS66 KMTR 250428
AFDMTR

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SAN FRANCISCO BAY AREA CA
830 PM PST THU DEC 24 2009

.DISCUSSION...AS OF 8:30 PM PST THURSDAY...SKIES ARE MOSTLY CLEAR
OVER LAND THIS EVENING. SATELLITE SHOWS A PATCH OF STRATUS OVER THE
COASTAL WATERS NEAR POINT ARENA. LOCALIZED WEAK OFFSHORE PRESSURE
GRADIENTS/WINDS SHOULD HELP KEEP THE STRATUS OUT OF THE BAY AREA.

HIGHS TODAY WERE GENERALLY THE SAME OR SLIGHTLY MILDER THAN THEY WERE
YESTERDAY RANGING FROM THE MID 50S TO LOW 60S. MORNING LOWS WERE IN
THE UPPER 20S INLAND VALLEYS TO MID 40S AT THE IMMEDIATE COASTLINE.

MAINLY CLEAR SKIES ARE EXPECTED TO CONTINUE AREA-WIDE TONIGHT FOR
LAST MINUTE HOLIDAY PREPARATIONS. DRY WEATHER WILL CONTINUE THROUGH
CHRISTMAS DAY AS WELL.

CLOUDS WILL BE ON THE INCREASE SATURDAY AHEAD OF AN EASTERN PACIFIC
LOW PRESSURE SYSTEM. THE MODELS CONTINUE TO HAVE A HARD TIME PINNING
DOWN THE LARGE TO SMALL SCALE DETAILS OF THIS SYSTEM. FORECAST
CONFIDENCE LARGELY HINGES ON MODEL AGREEMENT AND CONSISTENCY WHICH
APPEARS TO BE LACKING AGAIN AFTER SOME AGREEMENT SURFACED YESTERDAY.
FORECAST DIFFERENCES IN TIMING WERE ALSO SHOWING UP LAST NIGHT.
RECENT MODEL RUNS NOW INDICATE THE JET STREAM WILL SET UP FARTHER
SOUTH WHICH IMPLIES THE MAJORITY OF RAIN WILL SLIDE BY TO THE SOUTH
TO SOUTHERN CALIFORNIA INSTEAD OF THE BAY AREA. THIS HIGH LEVEL OF
INCONSISTENCY LEAVES ONE LAST METHOD TO USE IN ORDER TO HELP PIECE
TOGETHER SOME KIND OF FORECAST WITH SOME KIND OF CONFIDENCE BEHIND IT
ALBEIT LOW AND IT'S MODEL TRENDS. SINCE THE MODELS ARE TRENDING THE
STORM TRACK FARTHER SOUTH QPF TOTALS ARE NOW LIKELY TO BE LESS...
POSSIBLY MUCH LESS THAN ORIGINALLY THOUGHT FOR THE BAY AREA AND
CENTRAL COAST. THE NEW 00Z GFS SHOWS ABOUT 1/10 OF INCH QPF OVER THE
AREA FALLING MAINLY ON SUNDAY. PRESENT FORECAST INDICATES A CHANCE OF
RAIN BEGINNING LATE SATURDAY AND CONTINUING INTO SUNDAY WHICH IS FINE
FOR NOW. SINCE CONFIDENCE LEVELS ARE LOWERING EVEN WITH REGARDS TO
THE FIRST STORM IT'S VERY DIFFICULT TO FIGURE WHAT KIND OF PATTERN
THERE WILL BE THROUGH NEXT WEEK. IF THE MODELS STICK TO A MORE
SOUTHERLY STORM TRACK THEN THE PATTERN WILL BE SLIGHTLY UNSETTLED BUT
WITH MUCH LESS QPF THAN ORIGINALLY THOUGHT. THE MODELS WERE HINTING
AT A FOLLOW UP STORM TO ARRIVE BY THE MIDDLE OF THE WEEK WITH LESS
QPF THAN THE WEEKEND STORM...BUT NOW THERE IS DOUBT SURROUNDING
WETHER OR NOT THIS STORM WILL BE A FACTOR IN THE WEATHER AT ALL. IN
FACT THE GFS SHOWS RIDGING. SO LATE IN THE WEEK UPPER LEVEL RIDGING
DOES APPEAR TO RETURN BUT AGAIN IT'S DIFFICULT TO SAY AT THIS POINT
IN TIME. CLIMATOLOGY SAYS THE CLOSER WE GET TO JANUARY THE MORE
LIKELY WE'LL SEE THE NORMAL MID-WINTER BREAK FROM RAIN DEVELOP FOR A
2-3 WEEK PERIOD ON AVERAGE.

NO UPDATES ARE PLANNED THIS EVENING.

&&

.AVIATION...AS OF 4:00 PM THURSDAY...

HIGH PRESSURE OVER THE GREAT BASIN WILL MAINTAIN LIGHT OFFSHORE
FLOW AND A DRY LOW LEVEL AIR MASS OVER THE DISTRICT TONIGHT AND
FRIDAY. WITH THE EXCEPTION OF KSTS...VFR CONDITIONS ARE EXPECTED TO
PREVAIL AT ALL TERMINALS THROUGH CHRISTMAS DAY. AT KSTS...FOG
OR FREEZING FOG IS EXPECTED TO REDEVELOP LATE THIS EVENING AND
PERSIST INTO THE MORNING.

VICINITY OF KSFO...VFR CONDITIONS THROUGH CHRISTMAS DAY. WINDS WILL
BE GENERALLY EAST TO NORTHEAST AROUND 5 KT THROUGH FRIDAY MORNING...
AND THEN TURN WEAKLY ONSHORE AT TIMES IN THE AFTERNOON.

KSFO BRIDGE APPROACH...SAME AS KSFO.

MRY BAY AREA TERMINALS...VFR CONDITIONS THROUGH CHRISTMAS DAY.
LIGHT EASTERLY WINDS WILL SWITCH TO LIGHT NORTHWESTERLY DURING THE
AFTERNOON ON FRIDAY.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
   .TNGT...NONE

$$

PUBLIC FORECAST: CANEPA
AVIATION/MARINE: LARRY/BLIER

NWS ON THE WEB AT WEATHER.GOV/SANFRANCISCO