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AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SAN FRANCISCO BAY AREA
1000 PM PST SUN DEC 28 2014

...A COLD STORM SYSTEM TO BRING STRONG AND GUSTY WINDS AND COLD
TEMPERATURES TO THE AREA FROM TUESDAY THROUGH WEDNESDAY...

.SYNOPSIS...DRY WEATHER IS FORECAST TO PERSIST THROUGH
MONDAY...ALONG WITH SEASONABLE TEMPERATURES...AS HIGH PRESSURE
REMAINS POSITIONED OVER THE AREA. A COLD AND MOSTLY DRY STORM
SYSTEM WILL THEN MOVE INTO THE REGION ON TUESDAY...BRINGING
WITH IT STRONG WINDS AND COLD TEMPERATURES. WINDS WILL DISSIPATE
BY WEDNESDAY EVENING. FREEZING TEMPERATURES ARE POSSIBLE BY LATE
TUESDAY NIGHT....BUT ARE MORE LIKELY TO OCCUR WEDNESDAY NIGHT AND
EARLY THURSDAY.

&&

.DISCUSSION...AS OF 8:55 PM PST SUNDAY...LITTLE WEATHER OF
SIGNIFICANCE IS FORECAST IN THE SHORT-TERM (THROUGH MONDAY).
LOW LEVEL MOISTURE CONTINUES TO INCREASE AND PATCHY LOW CLOUDS
HAVE DEVELOPED ALONG THE COAST FROM MARIN COUNTY SOUTH TO SANTA
CRUZ COUNTY THIS EVENING. A FEW PATCHES OF LOW CLOUDS HAVE ALSO
DEVELOPED AROUND SF BAY. EXPECT PATCHY FOG TO FORM IN THE NORTH
BAY VALLEYS BY LATE TONIGHT. OTHERWISE MOSTLY BENIGN WEATHER IS
FORECAST THROUGH MONDAY.

THE MODELS HAVE BEEN CONSISTENT IN DROPPING A COLD AND MOSTLY DRY
WEATHER SYSTEM SOUTH OUT OF BRITISH COLUMBIA BEGINNING TOMORROW.
THIS SYSTEM WILL PLUNGE SOUTH INTO THE PAC NW BY MONDAY MORNING.
THE LEADING EDGE OF A COLD AIRMASS WILL SWEEP ACROSS OUR REGION BY
LATE MONDAY NIGHT...WITH THE COLDEST AIR DUE TO ARRIVE ON TUESDAY
AS THE COLD UPPER TROUGH DIGS SOUTH INTO CENTRAL AND SOUTHERN CA.
WHAT LITTLE MOISTURE THIS SYSTEM HAS WILL MOSTLY FALL ACROSS THE
PAC NW...BUT SOME MODEL SOLUTIONS HAVE SHOWN LIGHT ISOLATED QPF
ACROSS OUR AREA...MAINLY ON TUESDAY. THE 00Z NAM SHOWS SOME
ISOLATED LIGHT SHOWER ACTIVITY ACROSS THE EASTERN PORTION OF OUR
FORECAST AREA AS EARLY AS MONDAY NIGHT...WITH ALMOST NO PRECIP
THEREAFTER. THIS SEEMS REASONABLE GIVEN THAT THE BEST CHANCE OF
PRECIP WILL PROBABLY BE WITH THE INITIAL COLD FRONT. HAVE
THEREFORE GONE AHEAD AND ADDED A SLIGHT CHANCE OF SHOWERS TO
EASTERN PORTIONS OF OUR AREA LATE MONDAY NIGHT...LOWERED POPS ON
TUESDAY...AND REMOVED MOST RAIN CHANCES COMPLETELY BY LATE TUESDAY
EVENING. SNOW LEVELS WITH THIS SYSTEM WILL BE LOW. SO...IF THERE
IS PRECIP...IT WILL FALL AS BRIEF SNOW SHOWERS IN THE HILLS ABOVE
2000 FEET.

THE PRIMARY IMPACT OF THIS SYSTEM WILL NOT BE FROM ANY PRECIP IT
MAY OR MAY NOT PRODUCE...BUT RATHER FROM THE STRONG AND GUSTY
NORTHERLY WINDS IT'S EXPECTED TO GENERATE. WINDS WILL TURN TO THE
NORTH AND BEGIN TO INCREASE ON TUESDAY MORNING AFTER FRONTAL
PASSAGE. N-S PRESSURE GRADIENTS WILL THEN QUICKLY INCREASE ACROSS
CA ON TUESDAY AFTERNOON AND EVENING AS A STRONG SURFACE HIGH
BUILDS OVER NORTHERN NEVADA. THESE PRESSURE DIFFERENCES WILL DRIVE
STRONG NORTHERLY WINDS ACROSS OUR AREA BY TUESDAY AFTERNOON. IN
ADDITION...STRONG NORTH TO NORTHEAST WINDS WILL DEVELOP ALOFT BY
MIDDAY TUESDAY AS THE UPPER TROUGH AXIS SWINGS THROUGH. DOWNWARD
MOMENTUM TRANSFER FROM COLD AIR ADVECTION WILL HELP MIX THESE
STRONG WINDS ALOFT TO THE SURFACE. THE RESULT WILL BE STRONG AND
POSSIBLY DAMAGING WINDS ACROSS PORTIONS OF THE SF BAY AREA FROM
MIDDAY TUESDAY THROUGH MIDDAY WEDNESDAY. A HIGH WIND WATCH HAS
BEEN ISSUED FOR ELEVATIONS ABOVE 1000 FEET IN THE SF BAY
AREA...INCLUDING THE NORTH BAY MOUNTAINS...EAST BAY HILLS...AND
SANTA CRUZ MOUNTAINS. THE STRONGEST WINDS WILL LIKELY BE CONFINED
TO THE HILLS. HOWEVER...DRY AND COLD SYSTEMS OF THIS TYPE CAN
SOMETIMES PRODUCE STRONG WINDS DOWN TO NEAR SEA LEVEL AND SO WIND
ADVISORIES WILL NEED TO BE CONSIDERED FOR THE LOWER ELEVATIONS OF
THE SF BAY AREA AS WELL. WINDS WILL MOST LIKELY PEAK SOMETIME
BETWEEN MID AFTERNOON TUESDAY AND LATE TUESDAY NIGHT. WINDS ARE
FORECAST TO GRADUALLY SUBSIDE ON WEDNESDAY AS THE UPPER LOW SINKS
INTO FAR SE CA AND PRESSURE GRADIENTS RELAX.

THIS SYSTEM WILL SPREAD A COLD AIRMASS ACROSS OUR REGION BETWEEN
LATE MONDAY NIGHT AND TUESDAY EVENING. 850 MB TEMPERATURES ARE
FORECAST TO FALL TO AS LOW AS MINUS 4 DEG C BY TUESDAY AFTERNOON.
THE RESULT WILL BE UNCOMFORTABLE COLD CONDITIONS ON TUESDAY AND
TUESDAY NIGHT AS A VERY CHILLY AIRMASS COMBINES WITH STRONG AND
GUSTY WINDS. THIS WILL BE THE COLDEST AIRMASS SO FAR THIS SEASON.
MOS GUIDANCE FORECASTS LOW TEMPERATURES DOWN INTO THE MID AND
UPPER 20S IN THE INLAND VALLEYS FOR LATE TUESDAY NIGHT/WEDNESDAY
MORNING. THESE TUESDAY NIGHT MIN TEMPS SEEM TOO COLD GIVEN THE BRISK
WINDS AND MIXING THAT WILL LIKELY PERSIST THROUGH TUESDAY
NIGHT...EVEN IN THE VALLEYS. THE COLDER NIGHT WILL PROBABLY BE
WEDNESDAY NIGHT WHEN WINDS DROP OFF. WILL NEED TO CONSIDER FREEZE
WARNINGS FOR INLAND PORTIONS OF THE AREA BY MIDWEEK...AND PERHAPS
FROST ADVISORIES FOR AREAS CLOSE TO SF BAY. THE HIGHEST POTENTIAL
FOR FREEZING TEMPS WILL BE LATE WEDNESDAY NIGHT/EARLY THURSDAY
MORNING.

TEMPERATURES WILL MODERATE BY THE END OF THE WEEK AND INTO NEXT
WEEKEND AS THE COLD UPPER LOW MOVES OFF TO THE EAST AND A RIDGE
BUILDS OVER CA. THEREAFTER...THE LIKELIHOOD OF A RETURN TO A WET
WEATHER PATTERN IS BEGINNING TO LOOK LESS LIKELY...AT LEAST
ACCORDING TO THE 00Z GFS. INSTEAD OF DEVELOPING A WET ZONAL FLOW
INTO CA BY JANUARY 6 AS PREVIOUS GFS MODEL RUNS HAD SHOWN...THE
00Z GFS SHOWS A RIDGE HOLDING FIRM ALONG THE WEST COAST.

&&

.AVIATION...AS OF 10:00 PM PST SUNDAY...WEAK ONSHORE FLOW HAS
DEVELOPED ALLOWING SOME LOW CLOUDS TO FORM IN THE SFO BAY AREA. 
MOST OF THE CLOUDS ARE IN THE EAST AND SOUTH BAY BUT THERE IS A
CHANCE THE CLOUDS WILL IMPACT SFO LATER TONIGHT.

VICINITY OF KSFO...SCT CLOUDS. CHANCE FOR MVFR CIGS 12Z-16Z.

KSFO BRIDGE APPROACH...MVFR CIGS IN THE EAST BAY AND SOUTH OF THE
SAN MATEO BRIDGE. CIGS MAY OCCASIONALLY DRIFT NORTH OF THE BRIDGE
BETWEEN 12Z AND 16Z.

MONTEREY BAY AREA TERMINALS...VFR.

&&

.MARINE...AS OF 10:00 PM PST SUNDAY...STRONG SURFACE HIGH
PRESSURE WILL MOVE INTO THE GREAT BASIN RESULTING IN INCREASING
NORTHERLY WINDS OVER THE COASTAL WATERS MONDAY NIGHT THROUGH
WEDNESDAY. WINDS WILL DECREASE WEDNESDAY NIGHT AND THURSDAY AS THE
HIGH WEAKENS.

&&

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

&&

$$

PUBLIC FORECAST: DYKEMA
AVIATION: W PI
MARINE: W PI

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