143  
FXUS66 KMTR 131209  
AFDMTR  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE SAN FRANCISCO BAY AREA  
409 AM PST FRI DEC 13 2019  
   
SYNOPSIS
 
AREAS OF DENSE FOG WILL PREVAIL ACROSS THE COAST AND  
THE COASTAL VALLEYS THIS MORNING, WITH A DISTURBANCE SPREADING  
LIGHT RAIN ACROSS THE REGION LATER TODAY AND TONIGHT. THE THREAT  
OF LIGHT RAIN WILL CONTINUE INTO THIS WEEKEND, AS A COLD FRONT  
TAKES AIM AT NORTHERN CALIFORNIA. DRIER AND COLDER CONDITIONS  
WILL PREVAIL ON SUNDAY, WITH FROST POSSIBLE SUNDAY NIGHT. AN  
UNSETTLED AND WET PATTERN WILL LIKELY TO RETURN BY THE MIDDLE OF  
NEXT WEEK.  
 

 
   
DISCUSSION...AS OF 03:55 AM PST FRIDAY
 
WATER VAPOR IMAGERY  
SHOWS A DIRTY RIDGE OVER NORTHERN CALIFORNIA THIS MORNING, WITH  
PLENTY OF HIGH CLOUD COVER SPREADING ACROSS THE NORTH BAY. THUS  
FAR, THIS HIGH CLOUD COVER IS NOT IMPEDING FOG DEVELOPMENT ACROSS  
THE NORTH BAY, AS ALL OF OUR NORTH BAY SITES ARE CURRENTLY  
REPORTING VISIBILITY AT OR BELOW A QUARTER MILE. FARTHER SOUTH,  
COASTAL AREAS AND THE COASTAL VALLEYS ARE ALSO DEALING WITH THIS  
DENSE FOG, INCLUDING SFO. THIS DENSE FOG WILL MAKE FOR A NASTY  
MORNING COMMUTE, WITH DELAYS POSSIBLE MOST OF OUR AIRPORTS.  
THEREFORE, A DENSE FOG ADVISORY REMAINS IN EFFECT THIS MORNING  
THROUGH 8 AM FOR ALL COASTAL REGIONS AND VALLEYS.  
 
LIGHT RAIN WILL ALSO GRADUALLY SPREAD ACROSS THE REGION TODAY, AS  
THE NEXT SYSTEM TAKES AIM AT NORTHERN CALIFORNIA. IN FACT, RADAR  
IS PICKING UP ON SOME LIGHT RETURNS ACROSS NORTHERN SONOMA ALREADY,  
WITH A FEW SITES REPORTING A HUNDREDTH HERE AND THERE. THE NORTH  
HALF OF THE BAY AREA WILL LIKELY SEE THE BRUNT OF THE RAIN OVER  
THE NEXT 24 HOURS, BUT AMOUNTS WILL REMAIN LIGHT FOR THE MOST. A  
COLD FRONT WILL THEN MOVE ACROSS THE REGION LATE IN THE DAY ON  
SATURDAY. ALTHOUGH THIS COLD FRONT WILL BRING SOME LIGHT RAIN TO  
THE REGION, THE BIGGEST IMPACT WILL BE WITH THE COLDER AND DRIER  
AIR MOVING IN BEHIND THE FRONT. ALTHOUGH SUNDAY WILL BE SLIGHTLY  
COOLER THAN SATURDAY, THE MAIN IMPACT WILL BE ON THE OVERNIGHT  
TEMPERATURES SUNDAY NIGHT. IN FACT, FROSTY CONDITIONS WILL BE  
POSSIBLE, AS TEMPERATURES FALL INTO THE LOWER TO MID 30S ACROSS  
THE INLAND VALLEYS.  
 
DRIER AND WARMER CONDITIONS WILL GRADUALLY DEVELOP FOR THE FIRST  
PART OF THE WORK WEEK, AS HIGH PRESSURE BUILDS OVER THE WEST  
COAST. HOWEVER, MODELS CONTINUE TO HINT THAT A WET AND UNSETTLED  
PATTERN WILL RETURN FOR THE LATTER PART OF THE WORK WEEK. AT THIS  
POINT IN TIME, THERE ARE STILL SOME SIGNIFICANT MODEL DIFFERENCES  
IN THE TIMING AND STRENGTH, BUT BELIEVE A MORE PROGRESSIVE  
PATTERN IS LIKELY LOOKING TO THE ENSEMBLES. PALMER  
 

 
   
AVIATION...AS OF 04:08 AM PST FRIDAY
 
LIFR CONDITIONS ARE  
CURRENTLY IMPACTING MOST TERMINALS ACROSS THE SAN FRANCISCO BAY  
AREA AND MONTEREY BAY REGION WITH IFR CEILINGS OVER INTERIOR EAST  
BAY TERMINALS. LOW CLOUDS AND AREAS OF DENSE FOG WILL PERSIST  
THROUGH ABOUT 15Z WITH IMPROVING CONDITIONS LIKELY AFTER 18Z.  
HOWEVER, LOW CEILINGS WILL LIKELY KEEP MVFR TO IFR CONDITIONS  
THROUGH MUCH OF THE DAY. A WEAK FRONTAL SYSTEM WILL THEN DROP  
SOUTHWARD ACROSS THE REGION RESULTING IN LIGHT RAIN DEVELOPING  
AROUND 21Z OVER THE NORTH BAY, 00Z ACROSS THE BAY AREA AND THEN  
03Z FOR THE MONTEREY BAY REGION. PERIODS OF LOWERING VISIBILITY  
AND CEILINGS WILL ALSO BE POSSIBLE DURING TIMES OF HEAVIER  
PRECIPITATION. PRECIPITATION SHOULD DIMINISH REGION-WIDE BY 06Z,  
YET WORSENING FLIGHT CONDITIONS ARE AGAIN LIKELY TO DEVELOP  
OVERNIGHT THROUGH SATURDAY MORNING DUE TO RESIDUAL MOISTURE AND  
LIGHT WINDS IN PLACE.  
 
VICINITY OF KSFO...PATCHY DENSE FOG OVER THE TERMINAL WILL RESULT  
IN LIFR CONDITIONS THROUGH EARLY MORNING WITH THE POTENTIAL  
IMPROVING VISIBILITIES AND CEILINGS AROUND 15Z AND MORE SO BY 18Z.  
MVFR CONDITIONS ARE THEN LIKELY THROUGH MUCH OF THE REMAINDER OF  
THE DAY WITH LIGHT RAIN LIKELY TO DEVELOP AFTER 21Z. GENERALLY  
LIGHT AND VARIABLE WINDS ARE LIKELY THROUGH THIS EVENING WITH AN  
INCREASE IN WEST-NORTHWEST WINDS LATE IN THE PERIOD.  
 
SFO BRIDGE APPROACH...SIMILAR TO KSFO.  
 
MONTEREY BAY TERMINALS...LIFR CONDITIONS IMPACTING REGIONAL  
TERMINALS WITH PATCHY DENSE FOG AND LOW CEILINGS. LIGHT WINDS ALSO  
PERSIST AND WILL LIKELY CONTINUE THROUGH MUCH OF THE DAY.  
CONDITIONS SHOULD IMPROVE AFTER 18Z TO IFR/MVFR WITH THE POTENTIAL  
FOR LIGHT RAIN TO DEVELOP AROUND 03Z.  
 

 
   
MARINE...AS OF 04:08 AM PST FRIDAY
 
SOUTHERLY WINDS WILL  
PERSIST TODAY AHEAD OF A WEAK FRONTAL BOUNDARY APPROACHING THE  
REGION. WINDS WILL SWITCH TO THE NORTHWEST TONIGHT AND INCREASE ON  
SATURDAY AS A COOL UPPER LEVEL TROUGH PASSES TO THE NORTH OF THE  
AREA. LARGE LONG-PERIOD NORTHWEST SWELL WILL IMPACT THE WATERS  
THROUGH SATURDAY.  
 

 
   
MTR WATCHES/WARNINGS/ADVISORIES
 
 
.TDAY...DENSE FOG ADVISORY...CAZ006-505-506-509-528>530  
HIGH SURF ADVISORY...CAZ006-505-509-529-530  
SCA...PT ARENA TO PT REYES 0-10 NM  
SCA...PT REYES TO PIGEON PT 0-10 NM  
SCA...PIGEON PT TO PT PINOS 0-10 NM  
SCA...PT PINOS TO PT PIEDRAS BLANCAS 0-10 NM  
SCA...PT ARENA TO PIGEON PT 10-60 NM  
SCA...PIGEON PT TO PT PIEDRAS BLANCAS 10-60 NM  
 

 
 

 
 
PUBLIC FORECAST: PALMER  
AVIATION: RGASS  
MARINE: RGASS  
 
VISIT US AT WWW.WEATHER.GOV/SANFRANCISCO  
 
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