FXUS66 KMTR 271221
AFDMTR
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SAN FRANCISCO BAY AREA CA
400 AM PST FRI NOV 27 2009
.DISCUSSION...AS OF 4:00 AM PST FRIDAY...WEAK COLD FRONT IS MOVING
THROUGH THE NORTH BAY EARLY THIS MORNING. KMUX RADAR IS CURRENTLY
PICKING UP A FEW NARROW BANDS OF LIGHT PRECIP AND RAIN GAGES HAVE
RECORDED A COUPLE HUNDREDTHS ACROSS SONOMA AND MARIN COUNTIES THUS
FAR. INDICATIONS ARE THAT RAINFALL WITH THE FRONT WILL CONTINUE TO BE
VERY LIGHT AND THAT THE FRONT WILL ESSENTIALLY WASH OUT AS IT MOVES
SOUTH OF THE GOLDEN GATE AROUND SUNRISE. MEANWHILE...AN UPPER LOW IS
CURRENTLY DEVELOPING AT THE BASE OF A SHARP UPPER TROUGH ALONG THE
OREGON COAST. THAT LOW IS FORECAST TO DROP SOUTH ACROSS OUR
FORECAST AREA THIS AFTERNOON AND EVENING. THERE WILL BE ENOUGH
MOISTURE AND INSTABILITY WITH THE UPPER LOW TO PRODUCE SCATTERED
SHOWERS ALMOST ANYWHERE IN OUR CWA THROUGH THIS EVENING...ALTHOUGH
SHOWER CHANCES WILL TAPER OFF FROM NORTH TO SOUTH BEGINNING IN THE
NORTH BAY THIS AFTERNOON. THE 00Z ECWMF FORECASTS THE UPPER LOW TO
FOLLOW A MORE EASTERLY TRACK THAN EITHER THE NAM OR GFS AND THEREFORE
THE ECWMF FORECASTS LIGHTER AMOUNTS OF PRECIP ACROSS OUR AREA. BASED
ON CURRENT SATELLITE DATA...WOULD TEND TO FAVOR THE ECWMF SOLUTION...
ALTHOUGH IT'S DIFFICULT TO SAY PRECISELY WHERE THE LOW WILL TRACK. IN
ANY EVENT...RAINFALL TOTALS FOR MOST AREAS THROUGH THIS EVENING WILL
LIKELY BE LESS THAN A TENTH OF AN INCH...WITH PERHAPS UP TO A QUARTER
OF AN INCH WHERE HEAVIER SHOWERS DEVELOP. FORECAST STABILITY INDICES
ARE NOT VERY IMPRESSIVE AND BELIEVE THUNDERSTORM CHANCES THIS
AFTERNOON AND EVENING ARE VERY SMALL.
BRISK NORTHERLY FLOW WILL DEVELOP ACROSS OUR REGION TONIGHT ONCE THE
UPPER LOW DROPS INTO SOUTHERN CALIFORNIA AND A SURFACE HIGH RAPIDLY
BUILDS TO OUR NORTH ACROSS OREGON. THE TIGHTEST GRADIENTS AND
STRONGEST WINDS ALOFT ARE FORECAST TO BE ACROSS THE NORTHERN HALF
OF OUR AREA. BOTH THE 06Z NAM AND 06Z GFS FORECAST 925 MB WINDS
OF ABOUT 45 KNOTS ACROSS THE SF BAY AREA LATER TONIGHT INTO SATURDAY
MORNING. THIS LOOKS LIKE A MARGINAL WIND ADVISORY EVENT WITH ONLY
ISOLATED HIGHER PEAKS LIKELY TO REACH ADVISORY CRITERIA. HAVE KEPT
WIND GUSTS TO 40 MPH OR LESS IN THE FORECAST FOR TONIGHT AND SATURDAY
MORNING AND WILL HOLD OFF ON ANY ADVISORIES FOR NOW. IF THE 12Z MODEL
RUNS COME IN WITH STRONGER WINDS...THEN AN ADVISORY WOULD LIKELY NEED
TO BE ISSUED.
DAYTIME TEMPERATURES WILL WARM OVER THE WEEKEND...THANKS TO A DRY
OFFSHORE FLOW...PLENTY OF SUN...AND A WARMING AIRMASS ALOFT UNDER
AN UPPER RIDGE BUILDING OVER NORTHERN CA. TEMPERATURES WILL BE AT
LEAST A FEW DEGREES ABOVE NORMAL ACROSS THE ENTIRE REGION BY
SUNDAY. BRISK NORTH WINDS ON SATURDAY WILL MOSTLY BE CONFINED TO THE
HIGHER ELEVATIONS...AND COASTAL AND VALLEY LOCATIONS WILL SEE
LIGHT TO MODERATE WINDS. BY SUNDAY WINDS WILL BE LIGHT IN ALL AREAS.
A HIGH AMPLITUDE RIDGE IS FORECAST TO RESIDE ALONG THE WEST COAST OF
NORTH AMERICA THROUGH MUCH OF NEXT WEEK. THIS RIDGE MAY VERY WELL
MAINTAIN DRY CONDITIONS ACROSS OUR REGION THROUGH THE WEEK.
HOWEVER...THE GFS HAS BEEN HINTING AT THE DEVELOPMENT OF AT LEAST A
PARTIAL BREAKTHROUGH OF THE WESTERLIES UNDER THE RIDGE DURING THE
SECOND HALF OF NEXT WEEK. THE ECWMF HAD BEEN STEADFAST IN MAINTAINING
DRY WEATHER WITH NO SIGNS OF A BREAKTHROUGH DURING THE NEXT 7-9 DAYS.
HOWEVER...THE 00Z ECMWF NOW SHOWS A BIT OF ENERGY BREAKING THROUGH
AND SKIRTING THE COAST AROUND MIDWEEK...WITH A MORE COHESIVE SYSTEM
BREAKING THROUGH A FEW DAYS AFTER THAT. THE GFS FORECASTS A COUPLE OF
LIGHT PRECIP EVENTS BEGINNING AROUND MIDWEEK...BUT SPACED CLOSER
TOGETHER THAN THE ECMWF. OBVIOUSLY THERE IS A GREAT DEAL OF
UNCERTAINTY REGARDING PRECIP DURING THE SECOND HALF OF NEXT WEEK
GIVEN RECENT MODEL PERFORMANCE. BUT BELIEVE THERE SHOULD BE AT LEAST
A MENTION OF RAIN CHANCES DURING THE SECOND HALF OF THE WEEK GIVEN
THE LATEST MODEL OUTPUT. A MORE SIGNIFICANT BREAKTHROUGH OF THE
WESTERLIES AND DEVELOPMENT OF WET WEATHER IS MORE LIKELY TO DEVELOP
BEYOND OUR CURRENT FORECAST PERIOD...AND MOST LIKELY DURING THE
SECOND WEEK IN DECEMBER. RECENT STRENGTHENING OF THE MJO SIGNAL AS IT
MOVES INTO THE WESTERN PACIFIC SUPPORTS THIS IDEA.
&&
.AVIATION...AS OF 4:00 AM PST FRIDAY...LOW CLOUDS HAVE SPREAD INTO
THE SFO AND MRY BAY AREAS THIS MORNING. CEILINGS ARE BELOW 1000
FEET WITH A SECOND LAYER AROUND 2000 FEET. THERE ARE LOCAL AREAS
WITH VISIBILITIES DOWN TO 1-3 MILES. SCATTERED RAIN SHOWERS WILL MOVE
THROUGH THE SFO BAY AREA THIS MORNING AS A FRONTAL SYSTEM MOVES
THROUGH. THE FRONT WILL WEAKEN AS IT APPROACHES THE MRY BAY AREA
REDUCING SHOWER ACTIVITY. THE CEILINGS WILL GRADUALLY LIFT AND
CLEAR OUT DURING THE AFTERNOON AFTER THE FRONT PASSES. NORTHWEST
WINDS WILL INCREASE BEHIND THE FRONT AS WELL.
KSFO TERMINAL AND SFO APPROACH...A SHOWER HAS BEEN REPORTED IN THE
PAST HOUR. MULTIPLE LAYERS OF CLOUDS WITH LOWEST CEILINGS BELOW 1000
FEET AND ANOTHER LAYER AROUND 2000-2500 FEET. THE LOWEST
CEILINGS ARE EXPECTED TO CLEAR OUT BY 18Z LEAVING THE 2000-
2500 FOOT LAYER. VFR CONDITIONS EXPECTED AFTER 21Z AS CLOUDS
LIFT ABOVE 3000 FEET. SCATTERED SHOWERS THROUGH 20Z. WINDS GUSTS
INCREASING TO TO 30 KT THIS AFTERNOON.
MRY BAY AREA TERMINALS...MULTIPLE LAYERS OF CLOUDS WITH LOWEST
CEILINGS BELOW 1000 FEET AND ANOTHER LAYER AROUND 1500-2000 FEET. THE
LOWEST CEILINGS ARE EXPECTED TO CLEAR OUT BY 16Z LEAVING THE 1500-
2000 FOOT LAYER. MVFR CONDITIONS ARE EXPECTED THROUGH THE DAY DUE TO
INCREASING UNSTABLE NORTHWEST FLOW.
&&
.MARINE...AS OF 4:15 AM FRIDAY...BUOY REPORTS INDICATE SEAS DOWN TO
THE 11-13 FOOT RANGE AFTER BEING MUCH HIGHER YESTERDAY. MODELS
SHOW SEAS INCREASING TO 15-18 FEET LATER TODAY SO A HIGH
SURF ADVISORY REMAINS VALID FOR THE ENTIRE COAST FROM 10 AM
THROUGH 4 AM SATURDAY. SWELL PERIODS SHOULD BE IN THE 14-15 SECONDS
RANGE SO SOME IMPRESSIVE SWELL SETS COULD COME IN. CARE SHOULD BE
TAKEN IF VENTURING OUT TO THE BEACHES AS UNEXPECTED LARGE SWELL
SETS/RIP CURRENTS COULD PRESENT PROBLEMS ESPECIALLY FOR VISITORS NOT
FAMILIAR WITH WEST COAST SWELL BEHAVIOR.
&&
.MTR WATCHES/WARNINGS/ADVISORIES...
.TDA...HIGH SURF ADVISORY ON THE COAST 10 AM THROUGH 4 AM
SATURDAY.
...GALE WARNING...PT ARENA TO PIGEON PT 0-10 NM
PT ARENA TO PT PIEDRAS BLANCAS 10-60 NM
...SCA...PIGEON PT TO PT PIEDRAS BLANCAS 0-10 NM
SFO AND MRY BAYS
...SCA FOR ROUGH BAR...SF BAR
$$
PUBLIC FORECAST: DYKEMA
AVIATION/MARINE: W PI
NWS ON THE WEB AT WEATHER.GOV/SANFRANCISCO
