499  
FXUS66 KMTR 280541  
AFDMTR  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE SAN FRANCISCO BAY AREA  
1041 PM PDT WED OCT 27 2021  
   
SYNOPSIS
 
WARMING AND DRYING TREND CONTINUES INTO FRIDAY. AN  
APPROACHING COLD FRONT LATE FRIDAY BRINGS COOLER CONDITIONS OVER  
THE WEEKEND, BUT THE AREA LOOKS TO REMAIN DRY. EARLY NEXT WEEK A  
FRONTAL SYSTEM BRINGS RAIN CHANCES TO THE BAY AREA, WITH HIGHER  
CHANCES IN NORTH BAY.  
 

 
   
DISCUSSION...AS OF 09:35 PM PDT WEDNESDAY
 
SATELLITE NIGHT FOG  
PRODUCT SHOWS COASTAL FOG MOVING SOUTHWARD FROM POINT REYES  
TOWARDS THE SF PENINSULA. IT HASN'T MADE MUCH INTRUSION OVER LAND  
AND NOT EXPECTING MUCH OVERNIGHT. HOWEVER, GIVEN THE MOIST AIRMASS  
IN PLACE (DEW PT'S IN THE UPPER 50S), CLEAR SKIES, AND LIGHT  
WINDS, EXPECTING SOME PATCHY FOG DEVELOPMENT OVERNIGHT.  
 
TOMORROW LOOKS TO BE THE WARMEST DAY OF THE WEEK WITH MOSTLY CLEAR  
SKIES IN THE AFTERNOON AND WARMING OF THE BOUNDARY LAYER AS A  
RIDGE MOVES OVER THE WEST COAST. A FEW DEGREES OF WARMING COMPARED  
TO TODAY WILL ALLOW MOST OF THE AREA TO REACH THE 70S, WITH SOME  
LOW 80S POSSIBLE IN OUR WARMEST INTERIOR SPOTS.  
 
FRIDAY WILL BRING SOME COOLING AS AN APPROACHING SHORTWAVE TROUGH  
REACHES THE COASTAL WATERS OF FAR NORTHERN CA. THIS SYSTEM WILL  
FORM A CUTOFF LOW THAT WILL TRANSIT THROUGH FAR NORCAL OR OREGON,  
MEANING OUR AREA WILL REMAIN DRY, WITH PERHAPS A FEW SPRINKLES IN  
THE NORTHERN REACHES OF NORTH BAY. THE MOST APPARENT CHANGES FOR  
US WILL BE CLOUDY AND COOL CONDITIONS THROUGH THE WEEKEND.  
 
OUR NEXT REAL RAIN CHANCE ARRIVES MONDAY, WITH AN APPROACHING  
FRONTAL BOUNDARY. THERE APPEARS TO BE A GOOD CHANCE THE NORTH BAY  
WILL SEE A WETTING RAIN, BUT FARTHER SOUTH, MODEL DISCREPANCIES  
GIVE US LESS CONFIDENCE. EURO IS MORE BULLISH THAN THE GFS IN  
BRINGING MOISTURE TO THE BAY AREA AND CENTRAL COAST. EITHER WAY,  
THIS WILL NOT BE AN AR TYPE EVENT, JUST A PROGRESSIVE FRONTAL  
SYSTEM. OFFICIAL FORECAST HAS CHANCE POPS COVERING MOST OF THE  
CWA.  
 
MORE RAIN CHANCES TOWARDS THE MIDDLE OF NEXT WEEK, BUT UNCLEAR IF  
THE MOISTURE WILL DIP SOUTH TOWARDS THE BAY AREA OR CUT OFF OVER  
NORCAL, UNDER MORE ZONAL FLOW. CPC PLACES THE BAY AREA IN A  
"LEANING ABOVE" PROBABILITY OF PRECIP CATEGORY FOR THEIR 6-10 AND  
8-14 DAY OUTLOOKS. SO EVEN THOUGH NO AR IS ON THE HORIZON, A  
WETTER PATTERN MAY BE COMING.  
 

 
   
AVIATION...AS OF 10:42 PM PDT WEDNESDAY
 
FOR THE 06Z TAFS.  
EARLY EVENING VISIBLE IMAGERY SHOWS FOG ALONG COASTAL SONOMA  
COUNTY TO POINT REYES IN MARIN COUNTY, AND ANOTHER PATCH ALONG SAN  
FRANCISCO AND COASTAL SAN MATEO COUNTY; FOG RESULTING IN VLIFR-  
IFR IN THESE LOCATIONS, VFR ELSEWHERE. WITH A NEUTRAL SFO-SAC  
PRESSURE GRADIENT AND WIND OVER LAND, THE ADVECTION OF FOG WILL  
LIKELY REMAIN CONFINED TO THE IMMEDIATE COAST WITH A VERY LOW PROB  
OF ONSHORE PUSH REACHING INLAND THIS EVENING, ESPECIALLY WITH  
LAND BREEZES DEVELOPING THIS EVENING AND OVERNIGHT.  
 
ONGOING MOIST AIR PER DEW POINT TEMPS IN THE UPPER 50S/60. IT WAS  
JUST BELOW THE MAX MOVING AVG PER PRECIPITABLE WATER (PW) ON THE  
12Z WED OAKLAND UPPER AIR SOUNDING WITH A LITTLE DRYING ALOFT PER  
THE 00Z SOUNDING, THOUGH THE PW IS STILL WELL ABOVE LATE OCT  
OAKLAND NORMAL. RADIATIVE COOLING UNDER A CLEAR SKY (AND NAM  
FORECASTS VERY WEAK PRESSURE FALLS, ADDITIONAL COOLING) TONIGHT  
WILL NARROW TEMP-DEW POINT DEPRESSIONS LIKELY RESULTING IN AREAS  
VLIFR-IFR CONDITIONS IN FOG AND LOW CLOUDS TONIGHT AND THURSDAY  
MORNING; HREF MODEL LEANING THIS WAY. LATE NIGHT/MORNING COOL AIR  
DRAINAGE WINDS ALSO ADDING INTO THE MIX WHICH MAY HELP CLEAR OUT  
FOG AND/OR LOW CLOUDS AROUND THE MONTEREY PENINSULA EARLIER THAN  
USUAL. OVERALL, COOL AIR DRAINAGE WIND FLOW IS A BIT MORE OF A  
WILDCARD TONIGHT/THU MORNING SINCE THE ATMOSPHERE IS ALREADY  
FAIRLY MOIST, HAS A FIRM STEP IN THE DIRECTION OF FOG AND/OR LOW  
CLOUDS FORMATION OVERNIGHT AND THU MORNING. BY LATE THU MORNING  
AND AFTERNOON UNDER MAXIMUM DIURNAL MIXING, WHICH IS GETTING  
WEAKER AS WE MOVE INTO THE COOL SEASON, OFFERS BEST CHANCES FOR  
VFR RETURNING. FOG/LOW CLOUDS COULD BE STUBBORN TO MIX OUT THU  
MORNING.  
 
VICINITY OF KSFO...WATCHING CLOSELY A PATCH OF VLIFR-IFR IN FOG  
ALONG THE COAST THAT HAS SLOWLY APPROACHED FROM THE NORTHWEST  
THIS AFTERNOON. 5 MIN OBSERVATIONS INITIALLY SHOWING NW WIND, BUT  
NOW SHOWING WNW WIND DIRECTION WITH SLIGHT INCREASE IN SPEED.  
NORTHERLY WINDS ON THE COAST OVERALL FAVOR THE SOUTHWARD ADVECTION  
OF THIS FOG PATCH, BUT WILL MONITOR CLOSELY. MOIST AIR IN PLACE,  
CLEAR SKY AND WEAK PRESSURE FALLS TONIGHT SETTING UP FOR LIFR-IFR  
CONDITIONS TONIGHT AND THURSDAY MORNING. TEMPO BKN004 07Z-11Z THU,  
TO PREVAILING IFR THU MORNING WITH MIX OUT PROCESS NEAR 18Z THU. WNW  
WIND GRADUALLY DECREASING TO LIGHT AND VARIABLE TONIGHT AND THU  
MORNING, THEN NW TO WNW NEAR 10 KNOTS THU AFTERNOON AND EVENING.  
 
KSFO BRIDGE APPROACH...SIMILAR TO KSFO.  
 
MONTEREY BAY...VFR CURRENTLY, LATE EVENING AND OVERNIGHT FOG  
AND/OR LOW CLOUDS /LIFR-IFR/ LIKELY. COOL AIR DRAINAGE WINDS  
POSSIBLY STRONG ENOUGH TO MIX OUT FOG AND/OR LOW CLOUDS EARLIER  
THAN USUAL ON THU MORNING. OTHERWISE VFR THURSDAY.  
 

 
   
MARINE...AS OF 08:40 PM PDT WEDNESDAY
 
NORTHWEST WINDS WILL  
PERSIST INTO TONIGHT. LARGE WEST TO NORTHWEST SWELL REMAINS A  
CONCERN THROUGH THIS EVENING EVEN AS WAVE HEIGHTS AND PERIODS  
GRADUALLY SUBSIDE. HAZARDOUS SEAS ARE STILL EXPECTED AS A RESULT,  
PARTICULARLY FOR SMALLER VESSELS. LARGE BREAKING WAVES WILL ALSO  
STILL BE POSSIBLE ACROSS NEARSHORE SHOALS AND AROUND HARBOR  
ENTRANCES. SEAS WILL CONTINUE SUBSIDING AS WE GO INTO TOMORROW.  
 

 
   
MTR WATCHES/WARNINGS/ADVISORIES
 
 
.TNGT...SCA...PT PINOS TO PT PIEDRAS BLANCAS 0-10 NM UNTIL 3 AM  
SCA...PIGEON PT TO PT PIEDRAS BLANCAS 10-60 NM UNTIL 11 PM  
 

 
 

 
 
PUBLIC FORECAST: LORBER  
AVIATION: DIAZ  
MARINE: CANEPA  
 
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