481  
FXUS66 KMTR 232327  
AFDMTR  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE SAN FRANCISCO BAY AREA  
427 PM PDT SUN SEP 23 2018  
   
SYNOPSIS
 
HIGH PRESSURE WILL BEGIN TO BUILD OVER THE REGION  
THROUGH THE FIRST PART OF THE WEEK AND RESULT IN A WARMING, DRYING  
TREND. THIS WILL WILL BRING DRY OFFSHORE WINDS AT TIMES AND  
INCREASED FIRE WEATHER CONCERNS, ESPECIALLY FOR THE NORTH BAY  
HILLS TONIGHT INTO TUESDAY. TEMPERATURES ARE FORECAST TO COOL LATE  
IN THE WEEK WITH THE POTENTIAL FOR UNSETTLED WEATHER CONDITIONS  
DURING THE UPCOMING WEEKEND.  
 

 
   
DISCUSSION...AS OF 01:50 PM PDT SUNDAY
 
BREEZY ONSHORE FLOW  
EARLY THIS AFTERNOON HAS ALLOWED FOR COOL CONDITIONS TO PREVAIL AT  
THE COAST DESPITE THE LACK OF CLOUD COVER. ONLY LOW CLOUDS/PATCHY  
FOG BEING REPORTED IS ALONG HIGHWAY 1 FROM MONTEREY UP TO THE  
APTOS AREA AND ALONG THE SOUTHERN BIG SUR COASTLINE. WITH  
TEMPERATURES IN THE 50S AND 60S AT THE COAST, INLAND AREAS HAVE  
ALREADY WARMED INTO THE 70S AND 80S FOR THE MOST PART. LOOKING THE  
REST OF TODAY TO FEEL VERY AUTUMN LIKE WITH A FEW DEGREES OF  
ADDITIONAL WARMING UNDER A MAINLY CLEAR SKY.  
 
AS THE PASSING MID/UPPER LEVEL SHORT-WAVE TROUGH SHIFTS INLAND  
TONIGHT, UPPER LEVEL RIDGING WILL BEGIN TO BUILD OFF OF THE PACIFIC  
NORTHWEST COAST AND OVER NORTHERN CALIFORNIA. THIS WILL ALLOW FOR  
WEAK OFFSHORE FLOW TO DEVELOP OVER THE INTERIOR AND IN THE HIGHER  
ELEVATIONS ACROSS THE REGION USHERING IN A WARMER, DRIER AIR MASS.  
MEANWHILE, A WEAK MARINE LAYER IS LIKELY TO PERSIST ALONG THE COAST  
AND ALLOW FOR SOME STRATUS TO RETURN ALONG COASTAL AREAS. THE  
WARMING AND DRYING CONDITIONS WILL BE MOST NOTED IN THE NEXT 12 TO  
48 HOURS IN THE HILLS/HIGHER ELEVATIONS OF THE NORTH AND EAST BAY  
REGIONS WHICH WILL RESULT IN NEAR CRITICAL FIRE WEATHER CONDITIONS.  
ADDITIONAL INFORMATION CAN BE FOUND IN THE FIRE WEATHER DISCUSSION  
BELOW.  
 
THE RIDGE OF HIGH PRESSURE WILL CONTINUE TO BUILD THROUGH LATE WEEK  
WITH WEAK OFFSHORE FLOW AT TIMES. THIS WILL ALLOW FOR THE INTERIOR  
TO SEE AFTERNOON TEMPERATURES BACK INTO THE 80S AND 90S BY WEDNESDAY  
WITH COASTAL LOCATIONS IN THE 70S TO LOWER 80S (LOCATIONS SUCH AS  
SANTA CRUZ). BY MIDWEEK, THE MARINE LAYER MAY ALSO BECOME WELL MIXED  
ENOUGH TO PREVENT WIDESPREAD STRATUS DEVELOP DURING THE OVERNIGHT  
AND EARLY MORNING HOURS. WHILE DAYTIME TEMPERATURES WILL WARM TO  
ABOVE SEASONAL AVERAGES, OVERNIGHT LOWS WILL BE ALLOWED TO COOL  
DECENTLY GIVEN THE LONG AUTUMN NIGHTS. THUS, HEAT RISKS WILL REMAIN  
LOW TO MODERATE FOR JUST ABOUT ALL MAJOR URBAN AREAS.  
 
A REX BLOCK MID/UPPER LEVEL PATTERN IS FORECAST TO DEVELOP WELL OFF  
OF THE WEST COAST LATE IN THE WEEK AS AN UPPER LEVEL LOW BEGINS TO  
UNDERCUT THE RIDGE OF HIGH PRESSURE. AS THIS OCCURS, THE AIR MASS  
ALOFT WILL BEGIN TO COOL AND ALLOW FOR A COOLING TREND ACROSS THE  
REGION THURSDAY INTO FRIDAY WITH AN INCREASE IN ONSHORE WINDS. THE  
MARINE LAYER MAY ALSO REBOUND AND ALLOW FOR LOW LEVEL CLOUDS TO  
RETURN TO THE COAST LATE IN THE WEEK. BOTH THE GFS AND ECMWF  
GENERATE SOME PRECIPITATION AT LEAST ACROSS NORTHERN CALIFORNIA AND  
AS FAR SOUTH AS THE NORTH BAY. CONFIDENCE REMAINS LOW AT THIS TIME  
FOR ANY MEASURABLE PRECIPITATION, AT LEAST FOR ANY WIDESPREAD  
RAINFALL. HOWEVER, THE MODELS DO HINT AT A CONTINUED COOL, UNSETTLED  
PATTERN OVER THE REGION HEADING INTO THE FIRST OF OCTOBER. SOMETHING  
WE WILL NEED TO KEEP AN EYE ON IN THE COMING DAYS.  
 

 
   
AVIATION...AS OF 04:27 PM PDT SUNDAY
 
FOR 00Z TAFS. SATELLITE  
IMAGERY SHOWING MAINLY CLEAR SKIES ACROSS THE REGION WITH SOME  
PASSING HIGH CLOUDS. VFR THROUGH THE EVENING WITH PATCHY IFR CIGS  
AROUND THE SF AND MONTEREY BAY AREA TAF SITES TONIGHT. PATCHY  
DENSE FOG WILL POSSIBLE IN THE EARLY MORNING IN THE NORTH BAY AND  
AROUND THE MONTEREY BAY. ONSHORE WINDS WILL DIMINISH IN THE  
OVERNIGHT HOURS BEFORE PICKING UP AGAIN TOMORROW AFTERNOON.  
 
LOW CONFIDENCE ON TIMING AND EXTENT OF STRATUS REDEVELOPMENT.  
 
VICINITY OF KSFO...VFR. GUSTY WINDS WILL CONTINUE INTO THE EARLY  
EVENING. MAY SEE SOME BRIEF LOW CIGS IN THE EARLY MORNING.  
 
SFO BRIDGE APPROACH...SIMILAR TO KSFO.  
 
MONTEREY BAY TERMINALS...VFR THROUGH THE EARLY EVENING. EARLY  
RETURN OF LOW CIGS TONIGHT, WITH PATCHY DENSE FOG DEVELOPING LATE  
TONIGHT INTO EARLY TOMORROW MORNING.  
 

 
   
FIRE WEATHER...AS OF 10:00 AM PDT SUNDAY
 
THE FIRE WEATHER  
WATCH HAS BEEN CANCELLED FOR THE NORTH BAY MOUNTAINS WHICH WAS  
PREVIOUSLY IN EFFECT FROM 11 PM TONIGHT THROUGH TUESDAY AFTERNOON.  
HOWEVER, DRY NORTHERLY WINDS REMAIN FORECAST TO DEVELOP OVER THIS  
REGION, ESPECIALLY OVER NAPA COUNTY, DURING THIS TIMEFRAME. THIS  
WILL DRIVE HUMIDITY LEVELS LOWER THROUGH TUESDAY AFTERNOON WITH  
OVERNIGHT RECOVERIES ONLY IN THE 20-25% RANGE. AFTERNOON HUMIDITY  
VALUES MAY FALL INTO THE SINGLE DIGITS TO LOWER TEENS MONDAY AND  
TUESDAY, YET WIND SPEEDS ARE FORECAST TO REMAIN BELOW RED FLAG  
CRITERIA.  
 
PRECAUTIONS SHOULD STILL BE TAKEN AS FUELS REMAIN CRITICALLY DRY  
AND ANY LOCALIZED GUSTY WINDS IN THE RIDGES/PEAKS COULD RESULT  
IN AN ELEVATED FIRE WEATHER RISK.  
 

 
   
MARINE...AS OF 01:46 PM PDT SUNDAY
 
MODERATE, LOCALLY GUSTY  
NORTHWEST WINDS ACROSS THE COASTAL WATERS THROUGH THE REMAINDER OF  
THE DAY, ESPECIALLY NORTH OF POINT REYES AND SOUTH OF POINT SUR  
AS WELL AS IN THE OUTER WATERS. LOCALLY STEEP FRESH SWELL WILL BE  
GENERATED BY THESE GUSTY WINDS. WINDS SUBSIDE BY TOMORROW MORNING  
THROUGH THE COMING DAYS.  
 

 
   
MTR WATCHES/WARNINGS/ADVISORIES
 
 
.TDAY...SCA...SF BAY  
SCA...MRY BAY  
SCA...PIGEON PT TO PT PINOS 0-10 NM  
SCA...PT ARENA TO PIGEON PT 10-60 NM  
SCA...PT ARENA TO PT REYES 0-10 NM  
SCA...PT PINOS TO PT PIEDRAS BLANCAS 0-10 NM  
SCA...PIGEON PT TO PT PIEDRAS BLANCAS 10-60 NM  
 

 
 

 
 
PUBLIC FORECAST: RGASS  
AVIATION: AS  
MARINE: DRP  
FIRE WEATHER: RGASS/DYKEMA  
 
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