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

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
000
FXUS66 KMTR 211702
AFDMTR

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SAN FRANCISCO BAY AREA
1002 AM PDT SUN SEP 21 2014

.SYNOPSIS...SLIGTHLY WARMER WEATHER IS FORECAST OVER THE NEXT
FEW DAYS. OTHERWISE LITTLE CHANGE IS EXPECTED WITH A RELATIVELY
DEEP MARINE LAYER RESULTING IN WIDESPREAD NIGHT AND MORNING LOW
CLOUDS. A PACIFIC STORM SYSTEM WILL LIKELY BRING RAIN TO THE
NORTHERN PART OF THE SAN FRANCISCO BAY AREA FROM LATE WEDNESDAY
INTO THURSDAY. RAIN MAY EXTEND AS FAR SOUTH AS THE MONTEREY BAY
AREA BY THURSDAY. SHOWERS ARE POSSIBLE FROM THURSDAY AFTERNOON
INTO FRIDAY EVENING.

&&

.DISCUSSION...AS OF 8:30 AM PDT SUNDAY...WIDESPREAD CLOUDS ACROSS
OUR CWA THIS MORNING WITH A 2.5 MB WESTERLY SURFACE GRADIENT PLUS
A A MARINE LAYER IN EXCESS OF 2,000 FEET. CLOUDS ARE FORECAST TO
BURN-OFF LATE THIS MORNING IN ALL SPOTS AWAY FROM THE COAST. AT
THE COAST, IT WILL LIKELY BE VERY SIMILAR TO YESTERDAY WITH THE
FLOW FORECAST TO REMAIN FROM THE WEST. HIGHS WILL BE IN THE 60S
AND 70S AT THE COAST WITH 70S AND 80S INLAND. ALSO WORTH NOTING
THAT POPS WERE GREATLY REDUCED OVER THE NORTH BAY, SO THE CHANCE
OF RAIN WAS REMOVED FOR THIS MORNING IN THAT SPOTS.

FORECAST WISE, LATEST MODELS CONTINUE TO BRING RAIN BACK TO OUR
AREA FOR WEDNESDAY INTO THURSDAY AS A COLD FRONT WILL MOVE ACROSS.
06Z RUN OF THE GFS IS MORE BULLISH WITH RAINFALL AMOUNTS COMPARED
TO PREVIOUS RUNS, SO WILL BE INTERESTING TO SEE HOW ALL OF THE 12Z
GUIDANCE LINES UP.

.PREVIOUS DISCUSSION...AN UPPER LOW HAS CONTINUED TO TRACK VERY
SLOWLY ACROSS CENTRAL CALIFORNIA OVERNIGHT AND IS CURRENTLY
CENTERED NEAR FRESNO. ISOLATED SHOWERS ARE WRAPPING AROUND THE
NORTH AND NORTHWEST SIDE OF THE LOW AND A FEW SHOWERS CONTINUE TO
MOVE INTO THE NORTH BAY AT THIS HOUR. THERE WAS EVEN AN ISOLATED
LIGHTNING STRIKE DETECTED NEAR THE NORTHERN TIP OF NAPA COUNTY
JUST AFTER MIDNIGHT. THE MODELS DID A VERY POOR JOB OF FORECASTING
THE CONVECTION THAT MADE ITS WAY INTO THE NORTHERN AND EASTERN
PORTIONS OF OUR AREA LATE YESTERDAY AND OVERNIGHT. ONLY THE MOST
RECENT RUN OF THE GFS (06Z) APPEARS TO HAVE CAUGHT ON. THE MODELS
AGREE THAT THE LOW WILL BEGIN TO MOVE QUICKLY TO THE ENE AND OUT
OF OUR AREA WITHIN THE NEXT FEW HOURS...ENDING ALL PRECIP CHANCES
BY SUNRISE. BUT WILL MONITOR CLOSELY FOR SIGNS OF ISOLATED SHOWERS
LINGERING BEYOND 12Z.

THE MARINE LAYER REMAINS DEEP EARLY THIS MORNING AND WE WILL
PROBABLY SEE LOW CLOUDS CLEAR RATHER SLOWLY ONCE AGAIN TODAY.
TEMPERATURES TODAY ARE EXPECTED TO BE CLOSE TO WHAT THEY WERE
YESTERDAY...WITH INLAND AREAS REMAINING SLIGHTLY BELOW NORMAL AND
COASTAL AREAS HOLDING NEAR NORMAL. AS THE UPPER LOW MOVES OFF TO
THE EAST...A SHORTWAVE RIDGE WILL DEVELOP OVER CA BY LATE TODAY.
THAT RIDGE WILL THEN AMPLIFY OVER THE NEXT FEW DAYS IN RESPONSE TO
A DEEPENING UPPER TROUGH UPSTREAM OVER THE EASTERN PACIFIC. BUT
THE RIDGE WILL ALSO BE MOVING SLOWLY OFF TO THE EAST AND SO WE
PROBABLY WON`T SEE MORE THAN JUST SLIGHT WARMING THE NEXT FEW
DAYS. NIGHTS WILL CONTINUE TO BE BALMY...ESPECIALLY AS THE UPPER
TROUGH APPROACHES TUESDAY NIGHT (SIMILAR TO THE SITUATION THAT
OCCURRED THIS PAST WEDNESDAY NIGHT AHEAD OF AN APPROACHING COLD
FRONT WHEN OVERNIGHT LOWS WERE IN THE MID AND UPPER 60S). HAVE
WARMED OVERNIGHT LOWS ABOVE GUIDANCE...BUT MAY NOT HAVE GONE WARM
ENOUGH.

THE MODELS HAVE TRENDED SLOWER IN MOVING THE COLD FRONTAL RAIN
BAND THROUGH OUR AREA AROUND MIDWEEK AND THIS SEEMS REASONABLE GIVEN
THAT THE TROUGH WILL BE DEEPENING AS IT APPROACHES THE COAST. IT
NOW LOOKS LIKE RAIN WILL BEGIN IN THE NORTH BAY ON WEDNESDAY
BUT NOT MAKE IT SOUTH AND EAST THROUGH THE REST OF THE
SF BAY AREA UNTIL LATE WEDNESDAY NIGHT OR EVEN THURSDAY MORNING.
HAVE ADJUSTED THE POP GRIDS ACCORDINGLY. THE SOUTHERN EXTENT OF
THE FRONTAL RAIN BAND IS EXPECTED TO REACH TO ABOUT THE MONTEREY
PENINSULA ON THURSDAY MORNING. THEREAFTER...THE GFS AND ECMWF
AGREE QUITE WELL IN PRODUCING ONLY ISOLATED LIGHT SHOWERS AS THE UPPER
TROUGH MOVES THROUGH FROM LATE THURSDAY THROUGH FRIDAY EVENING.
THE GEM MODEL...ON THE OTHER HAND...INDICATES MUCH MORE MOISTURE
WITH THE TROUGH AND THIS MODEL SPREADS ANOTHER ROUND OF
WIDESPREAD RAIN ACROSS OUR AREA ON FRIDAY AND FRIDAY EVENING. HAVE
KEPT RAIN CHANCES GOING THROUGH FRIDAY NIGHT...BUT CONFIDENCE IN
THE FORECAST DECREASES CONSIDERABLY AFTER THURSDAY EVENING GIVEN
MODEL DIFFERENCES. THERE IS CURRENTLY NO EXPECTATION FOR
THUNDERSTORMS WITH THIS SYSTEM...BUT THIS WILL NEED TO BE LOOKED
AT MORE CLOSELY AS WE DRAW NEARER TO THE PRECIP EVENT.

RAINFALL TOTALS WITH THIS SYSTEM LOOK TO BE MOSTLY AROUND A
QUARTER OF AN INCH OR LESS EXCEPT UP TO A HALF INCH OR MORE IN THE
COASTAL MOUNTAINS...ESPECIALLY IN THE NORTH BAY. THESE TOTALS ARE
BASED ON THE GFS AND ECMWF GUIDANCE AND HIGHER AMOUNTS WOULD
OCCUR IF THE 00Z GEM SCENARIO WERE TO VERIFY.

&&

.AVIATION...AS OF 9:57 AM PDT SUNDAY...THE STRATUS CLOUD COVERAGE
IS NEARLY INDENTICAL TO YESTERDAY MORNING BUT THE MARINE LAYER IS
SOMEWHAT DEEPER THIS MORNING. THE 12Z OAK SOUNDING SHOWS PRECIP
WATER VALUE UP TO 1.33" A POSITIVE CHANGE FROM LAST EVE AND
YESTERDAY MORNING WITH A HIGHER DISTRIBUTION OF HUMIDITY FROM THE
SURFACE TO LOW LEVELS AND THEN AGAIN IN THE MIDDLE LEVELS OF THE
ATMOSPHERE. ALL IN ALL THIS SUGGESTS THE STRATUS CLOUDS MAY TAKE A
LITTLE LONGER TO MIX OUT TODAY. FORECAST IS MOSTLY HINGING ON THE
ONSHORE (SFO-SAC) GRADIENT...PRESENTLY 2-3 MB...AND IS FORECAST
TO BE APPROX THE SAME THIS AFTERNOON AND AGAIN MONDAY AFTERNOON.
OVERALL THINK THE OCEAN BASED STRATUS CLOUD PATTERN WILL PERSIST
OVER THE NEXT 24-36 HOURS WITH THE USUAL LATE NIGHT/EARLY MORNING
INLAND STRATUS COVERAGE. THERE COULD BE A FEW PATCHES OF MOSTLY
NON-MEASURABLE VERY LIGHT DRIZZLE.

VICINITY OF KSFO...SLIGHTLY LONGER MVFR CIGS ARE FORECAST TODAY PER
COORD WITH THE NWS/CWSU OAKLAND OFFICE. AS OF THIS WRITING STRATUS
CLOUDS ARE BEGINNING TO SCATTER OUT SW OF MOFFETT FIELD OTHERWISE STILL
FAIRLY EXTENSIVE OVER THE BAY AREA. SCT015 VFR CONDITIONS ARE FORECAST
BY 1930Z.

KSFO BRIDGE APPROACH...STRATUS CLOUDS COULD LINGER UP TO 45 MIN
LONGER IN THE APPROACH.

MONTEREY BAY AREA TERMINALS...EXTENSIVE STRATUS CLOUD COVERAGE
NORTH CENTRAL COAST THIS MORNING. CLOUDS SHOULD MIX OUT BETWEEN
19Z-20Z.

&&

.MARINE...AS OF 9:57 AM PDT SUNDAY...LIGHT WINDS AND SEAS WILL
CONTINUE OVER THE COASTAL WATERS THROUGH EARLY TO MID WEEK. FOR
THE MID TO LATE WEEK TIME FRAME MODERATE TO LARGE NORTHWEST SWELL
WILL ENTER THE WATERS.

&&

.CLIMATE...AUTUMN OFFICIALLY BEGINS (AUTUMNAL EQUINOX) AT 7:29 PM
PDT ON MONDAY SEPTEMBER 22.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .TDAY...SCA...SF BAY NORTH OF THE BAY BRIDGE FROM 3 PM

&&

$$

PUBLIC FORECAST: BELL/DYKEMA
AVIATION/MARINE: CANEPA

VISIT US AT WEATHER.GOV/SANFRANCISCO

FOLLOW US ON FACEBOOK, TWITTER, AND YOUTUBE AT:
WWW.FACEBOOK.COM/NWSBAYAREA
WWW.TWITTER.COM/NWSBAYAREA
WWW.YOUTUBE.COM/NWSBAYAREA




USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.