Area Forecast Discussion
Issued by NWS San Francisco Bay Area, CA

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FXUS66 KMTR 271737
AFDMTR

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SAN FRANCISCO BAY AREA
937 AM PST THU NOV 27 2014

...PERIODS OF RAIN FORECAST FROM LATE FRIDAY THROUGH THE MIDDLE OF
NEXT WEEK...

...POTENTIAL HEAVY RAIN...HYDRO CONCERNS...AND STRONG AND GUSTY
WINDS NEXT WEEK...

.SYNOPSIS...HIGH PRESSURE WILL MAINTAIN DRY AND MILD WEATHER
THROUGH TODAY. RAIN WILL DEVELOP IN THE NORTH BAY BY
LATE FRIDAY AND PRESS SOUTH THROUGH MOST OF THE REST OF THE REGION
FRIDAY NIGHT INTO SATURDAY. PERIODS OF RAIN ARE EXPECTED TO CONTINUE
INTO SUNDAY. MORE RAIN...POSSIBLY HEAVY...IS THEN LIKELY AROUND
THE MIDDLE OF NEXT WEEK.

&&

.DISCUSSION...AS OF 08:04 AM PST THURSDAY...LATEST OBSERVATIONS
SHOW A VERY MOIST BOUNDARY LAYER THIS MORNING. THE AMPLE LOW LEVEL
MOISTURE AND COOL TEMPERATURES LED TO PATCHY FOG AROUND THE BAY
AREA THIS MORNING. VISIBILITY RESTRICTIONS HAVE BEEN GREATEST
AROUND SF BAY...N BAY AND E BAY RANGING 3 TO 6 MILES. A FEW SPOTS
OF DENSE FOG HAVE ALSO BEEN REPORTED ACROSS THE N BAY. A QUICK
UPDATE TO THE FORECAST WAS MADE TO INCLUDE PATCHY FOG THROUGH THIS
MORNING.

OTHERWISE...HIGH PRESSURE WEST OF BAJA AND LOW PRESSURE
HEADED TOWARD THE PAC NW WILL KEEP A MOIST WESTERLY FLOW OVER THE
BAY AREA TODAY. AS A RESULT...FILTERED SUNSHINE CAN BE EXPECTED
THIS AFTERNOON AS AMPLE MID-HIGH LEVEL CLOUDS STREAM OVER THE
REGION. REGARDLESS OF THE HIGH CLOUDS...TEMPERATURES THIS
AFTERNOON WILL BE PLEASANT FOR THE HOLIDAY. IN FACT...MUCH OF THE
REGION WILL BE A FEW DEGREES ABOVE NORMAL AND IN THE 60S TO LOWER
70S. A FEW SPOTS IN INTERIOR MONTEREY AND SAN BENITO COUNTIES MAY
EVEN HIT 80.

THE REST OF THE FORECAST WILL FOCUS ON A VERY ACTIVE PATTERN
SHAPING UP FRIDAY THROUGH NEXT WEEK. FORE SPECIFIC DETAILS SEE
PREVIOUS DISCUSSION. WILL FINE TUNE FORECAST AFTER ALL THE 12Z
MODEL SUITE COMES IN.

&&

.PREV DISCUSSION...AS OF 3:00 AM PST THURSDAY...AFTER ANOTHER
CLEAR AND SEASONABLY COOL NIGHT OVERNIGHT, THE MUCH ANTICIPATED
RAIN EVENT WILL BEGIN ACROSS OUR AREA STARTING FRIDAY EVENING AS
MOISTURE ASSOCIATED WITH A COLD FRONT TO OUR WEST MAKES ITS WAY
DOWN OUR CWA ALONG WITH A SHORTWAVE TROF ALOFT. MODELS HAVE BEEN
IN EXCELLENT AGREEMENT OVER THE PAST COUPLE OF RUNS AND REMAIN SO
THIS MORNING WITH THE START TIME. RAIN WILL BEGIN OVER THE NORTH
BAY FRIDAY EVENING AND PROGRESS DOWN TO SF BAY AREA FRIDAY NIGHT
INTO SATURDAY MORNING WHILE THE MONTEREY BAY REGION WILL SEE
PRECIP START CLOSE TO NOON ON SATURDAY. IN MANY RESPECTS THE
TIMING IS A NEAR CARBON COPY OF LAST WEEKEND! RAINFALL AMOUNTS
WITH THE FIRST PUSH ARE EXPECTED TO BE 1-2" OVER THE NORTH BAY
WITH GENERALLY 1/4"-3/4" FOR MOST URBAN AREAS.

A SECOND ROUND OF RAIN WILL THEN MOVE BACK TO OUR AREA AS ANOTHER
SHORTWAVE MOVES ACROSS. MODEL SOLUTIONS ARE GREATLY DIVERGING WITH
WHERE THE FOCUS OF THE RAIN WILL BE. GFS IS TRENDING TO THE NORTH
BAY, ECMWF TO THE MONTEREY BAY REGION, THE NAM IS VIRTUALLY DRY,
WHILE THE GEM HAS WIDESPREAD RAIN WITH THE HIGHEST AMOUNTS FROM
MONTEREY BAY UP TO SAN FRANCISCO BAY. DECIDED TO TREND TOWARD THE
ECMWF/GEM SOLUTIONS WITH THE NAM ENTIRELY DISCOUNTED. THAT LINE OF
THINKING BRINGS 1/2-1" FOR MANY URBAN SPOTS WITH 1.5-2.5" AROUND THE
SANTA CRUZ MOUNTAINS PLUS COASTAL RANGES IN MONTEREY COUNTY. RAIN
WILL END BY SUNDAY EVENING. LOCALLY GUSTY SOUTHERLY WINDS CAN BE
EXPECTED THROUGH THE WEEKEND ALTHOUGH THEY SHOULD REMAIN UNDER
ADVISORY LEVELS.

BEYOND SUNDAY MODELS START TO HAVE MORE DISAGREEMENT ON THE TIMING
OF THE MOISTURE AS THE PARENT LOW PROGRESSES TOWARD OUR AREA. THE
GFS REMAINS THE MOST BULLISH AND PROGRESSIVE WITH THE LOW AND HAS
IT ADVANCING TO OUR AREA BY TUESDAY NIGHT AND WELL TO THE EAST BY
WEDNESDAY. IN THAT SCENARIO, THE BREAK FROM SUNDAY IS SHORT AND IS
FOLLOWED BY AT LEAST TWO HEAVIER BANDS OF RAINFALL. THE SLOWER ECMWF
HAS SLOWED EVEN MORE FROM PREVIOUS RUNS AND NOW HAS THE LOW STAYING
NEARLY STATIONARY AND WELL TO OUR WEST ON MONDAY AND TUESDAY
BEFORE BRINGING BACK RAIN WEDNESDAY THROUGH POSSIBLY SATURDAY (AND
MAYBE BEYOND THAT). ECMWF ENSEMBLES WERE NOT AVAILABLE FOR THE
SHIFT, SO JUST GOING OFF THE GFS ENSEMBLES SHOWS THE MEAN VERY
CLOSE TO THE OPERATIONAL GFS SUGGESTING THAT THERE ARE TWO VERY
DISTINCT SOLUTIONS WITHOUT A CLEAR PATH TO CHOOSE. THEREFORE,
CONFIDENCE IN THE FORECAST REMAINS HIGH FOR SUBSTANTIAL RAIN NEXT
WEEK, HOWEVER THE TIMING OF THE RAIN CONTINUES TO HAVE A LOW
CONFIDENCE.

BY THE MIDDLE OF NEXT WEEK RAINFALL TOTALS ARE EXPECTED TO RANGE
FROM 4-7" OVER THE NORTH BAY PLUS COASTAL RANGES OF MONTEREY AND
SANTA CRUZ COUNTIES WITH GENERALLY 1.5"-3.5" FOR MANY URBAN AND
VALLEY LOCATIONS. LOCALLY HIGHER AMOUNTS ARE POSSIBLE IN BOTH
CASES. MODEL GUIDANCE IS ALSO ALL OVER THE PLACE FOR WIND SPEEDS
DUE TO VAST DIFFERENCES IN THE TIMING OF ANY SYSTEMS. WILL
CONTINUE TO ADVERTISE THE POTENTIAL FOR GUSTY WINDS AT SOME POINT
NEXT WEEK.

FINALLY, ALTHOUGH IT IS PAST THE SCOPE OF THE FORECAST PACKAGE,
THE 240 HOUR ECMWF PANEL BRINGS A SHARP LONGWAVE TROF TOWARD OUR
AREA WITH A LONG FETCH OF MOIST AIR FROM THE SW IN ADVANCE OF IT.
IF THE ECMWF VERIFIES, NEXT WEEKEND (ESPECIALLY SUNDAY) COULD BE
VERY WET AS WELL.
&&

.AVIATION...AS OF 9:37 AM PST THURSDAY...DRY AIRMASS CONTINUES
OVER THE AREA FOR ANOTHER DAY OF VFR CONDITIONS. HAZY CONDITIONS
AROUND THE SAN FRANCISCO BAY AREA MAY IMPACT SFO AND OAK.

VICINITY OF KSFO...VFR. HAZY CONDITIONS TODAY WITH LIGHT WINDS.

KSFO BRIDGE APPROACH...SIMILAR TO KSFO.

MONTEREY BAY AREA TERMINALS...VFR.

&&

.MARINE...AS OF 09:32 AM PST THURSDAY...WEAK HIGH PRESSURE OFF THE
SOUTHERN CALIFORNIA COAST WILL PERSIST TODAY WITH LIGHT AND
VARIABLE WINDS EXPECTED ACROSS THE COASTAL WATERS. SOUTHERLY WINDS
WILL DEVELOP FRIDAY AS A FRONTAL SYSTEM APPROACHES THE AREA. THESE
SOUTHERLY WINDS WILL INCREASE SUNDAY OR MONDAY AS ANOTHER STORM
SYSTEM APPROACHES CALIFORNIA.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .TDAY...NONE.

$$

PUBLIC FORECAST: MM/BELL
AVIATION/MARINE: SIMS

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