588  
FXUS66 KMTR 231135  
AFDMTR  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE SAN FRANCISCO BAY AREA  
435 AM PDT SAT SEP 23 2017  
   
SYNOPSIS
 
HIGH PRESSURE BUILDING NEAR THE WEST COAST WILL RESULT  
IN DRY WEATHER, ALONG WITH A WARMING TREND, THROUGH AT LEAST THE  
MIDDLE OF NEXT WEEK.  
 

 
   
DISCUSSION...AS OF 3:30 AM PDT SATURDAY
 
CLEAR SKIES ACROSS OUR  
ENTIRE CWA THIS MORNING ALONG WITH A SQUASHED MARINE LAYER AND A  
MODERATE OFFSHORE FLOW. TEMPERATURES ARE GENERALLY RUNNING A FEW  
DEGREES COOLER THAN YESTERDAY THANKS IN LARGE PART TO DRIER AIR  
AND LIGHT WINDS. ONE EXCEPTION IS NAPA WHERE SPEEDS ARE STILL  
JUST ENOUGH TO KEEP THEM CURRENTLY AT 50. LOOK FOR PARTS OF THE  
NORTH BAY TO DROP INTO THE UPPER 30S (40S WILL BE COMMON) WITH THE  
REMAINDER OF OUR CWA GENERALLY IN THE 40S. LOW 50S WILL BE MOSTLY  
CONFINED TO THE COAST.  
 
TEMPERATURES WILL QUICKLY REBOUND AS ABUNDANT SUNSHINE WILL  
COMBINE WITH A SYNOPTIC PATTERN THAT WILL FEATURE THE LONGWAVE  
TROUGH OVERHEAD STARTING TO PROGRESS TO THE EAST. IN ADDITION, 850  
MB VALUES WILL INCREASE 1-2C BY THE AFTERNOON. HIGHS WILL  
GENERALLY BE IN THE 60S AT THE COAST WITH 70S TO LOWER 80S  
INLAND. WINDS WILL MOSTLY BE FROM THE NORTH OR NORTHEAST WITH  
GUSTY CONDITIONS POSSIBLE OVER PARTS OF THE NORTH BAY. THAT HAS  
LEAD TO AN ISSUANCE OF A RED FLAG WARNING STARTING THIS AFTERNOON  
FOR THE NORTH BAY HILLS. PLEASE SEE THE FIRE WEATHER SECTION  
BELOW FOR MORE INFORMATION.  
 
TEMPERATURES WILL CONTINUE TO WARM EACH DAY FOR SUNDAY INTO  
WEDNESDAY AS A RIDGE NEAR THE COAST BUILDS INTO OUR REGION AND  
850 MB VALUES GO UP EACH DAY. THE FLOW SHOULD REMAIN OFFSHORE  
WHILE THE MARINE LAYER WILL BE NON-EXISTENT. THIS WILL TRANSLATE  
INTO THE WARM WEATHER MAKING IT DOWN TO THE BEACHES. BY TUESDAY  
HIGHS WILL BE IN THE 80S AT MOST COASTAL SPOTS WITH 80S AND 90S  
INLAND. BEYOND THAT THE RANGE IN SOLUTIONS GREATLY INCREASES  
ALTHOUGH THE MAJORITY OF THE MODELS KEEP VERY WARM READINGS GOING  
AT LEAST INTO FRIDAY.  
 
THE BIG CHALLENGE FOR THE FORECAST IS JUST HOW WARM WILL IT GET  
BOTH AT THE COAST AND INLAND. 850 MB TEMPERATURES AND 500 MB  
HEIGHTS ARE NOT THAT IMPRESSIVE AND ANOMALY VALUES ARE GENERALLY 2  
STANDARD DEVIATIONS OR LESS, SO DID NOT FEEL COMFORTABLE GOING  
WITH EXTREMELY HIGH NUMBERS THAT SOME OF THE ENSEMBLE MEMBERS ARE  
SUGGESTION (SUCH AS 90S TO THE COAST). THE CURRENT FORECAST KEEPS  
THE HEAT THRESHOLD IN THE LOW CATEGORY AT THE WORST, HOWEVER IF  
THE FORECAST TRENDS UPWARDS THAT COULD EASILY MOVE TO THE MODERATE  
LEVEL. ANYONE WITH OUTDOOR PLANS OR AT A LOCATION WITHOUT AIR  
CONDITIONING SHOULD KEEP A CLOSE EYE ON THE FORECAST.  
 
LONGER RANGE GUIDANCE FAVORS WARMER THAN NORMAL CONDITIONS  
CONTINUING ALL OF THE WAY INTO THE MIDDLE OF OCTOBER. NO  
PRECIPITATION IS EXPECTED.  
 

 
   
AVIATION...AS OF 4:30 AM PDT SATURDAY
 
THE MARINE LAYER  
CONTINUES TO BE MIXED OUT AS A DEEP UPPER TROUGH OVER THE ROCKIES  
BRINGS COOL NORTHERLY FLOW TO THE AREA. VFR CONDITIONS EXPECTED  
THROUGH SUNDAY.  
 
VICINITY OF KSFO...VFR. WEST WINDS 14-16 KT AFTER 22Z.  
 
SFO BRIDGE APPROACH...SIMILAR TO KSFO.  
 
MONTEREY BAY TERMINALS...VFR.  
 

 
   
FIRE WEATHER...AS OF 3:30 AM PDT SATURDAY
 
A WARMING AND DRYING  
TREND WILL START THIS AFTERNOON AND CONTINUE AT LEAST INTO AT  
LEAST TUESDAY MORNING. IN ADDITION TO THE WARM TEMPERATURES THERE  
WILL BE LOWERING HUMIDITY VALUES AND PERIODS OF GUSTY NORTHEAST  
WINDS ACROSS THE NORTH BAY HILLS, ESPECIALLY ACROSS THE NAPA  
COUNTY HILLS AND THE HILLS OF NORTHERN SONOMA COUNTY. FINE FUELS  
WILL CONTINUE TO DRY GIVEN THE LACK OF MARINE AIR AND HUMIDITY.  
CLIMATOLOGY ALSO FAVORS A GREATLY CHANCE FOR CRITICAL FIRE  
WEATHER CONDITIONS FOR THE WEEKEND AND NEXT WEEK.  
 

 
   
MARINE...AS OF 3:30 AM PDT SATURDAY
 
HIGH PRESSURE WILL REMAIN  
OFF THE NORTHERN CALIFORNIA COAST WHILE LOW PRESSURE OVER THE  
ROCKIES MOVES EAST. WINDS WILL DECREASE SUNDAY EXCEPT OVER THE  
NORTHERN WATERS.  
 

 
   
MTR WATCHES/WARNINGS/ADVISORIES
 
 
.TDAY...SCA...MRY BAY FROM 2 PM  
SCA...PIGEON PT TO PT PINOS 0-10 NM FROM 2 PM  
SCA...PIGEON PT TO PT PIEDRAS BLANCAS 10-60 NM FROM 2 PM  
SCA...PT ARENA TO PIGEON PT 10-60 NM  
SCA...PT PINOS TO PT PIEDRAS BLANCAS 0-10 NM  
 

 
 

 
 
PUBLIC FORECAST: BELL  
AVIATION: W PI  
MARINE: W PI  
FIRE WEATHER: BELL/RWW  
 
VISIT US AT WWW.WEATHER.GOV/SANFRANCISCO  
 
FOLLOW US ON FACEBOOK, TWITTER, AND YOUTUBE AT:  
WWW.FACEBOOK.COM/NWSBAYAREA  
WWW.TWITTER.COM/NWSBAYAREA  
WWW.YOUTUBE.COM/NWSBAYAREA  
 
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