607  
FXUS66 KMTR 181734  
AFDMTR  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE SAN FRANCISCO BAY AREA  
934 AM PST WED JAN 18 2017  
   
SYNOPSIS
 
RAIN SPREADS FROM NORTH TO SOUTH TODAY WITH A PERIOD  
OF MODERATE TO HEAVY RAIN AS THE FRONT PASSES THROUGH THE REGION  
ALONG WITH GUSTY SOUTHERLY WINDS. SHOWERS LINGER ON THURSDAY BUT  
IN GENERAL THERE WILL BE A BREAK IN THE WEATHER. NEXT FRONT  
ARRIVES THURSDAY NIGHT INTO FRIDAY WITH ANOTHER ROUND OF WIND AND  
RAIN. LINGERING SHOWERS SATURDAY BEFORE THE FINAL FRONT BRINGS  
MORE WIND AND RAIN SUNDAY BEFORE TAPERING TO SHOWERS SUNDAY NIGHT  
AND MONDAY. DRY BUT COOL TUESDAY.  
 

 
   
DISCUSSION...AS OF 8:30 AM PST WEDNESDAY
 
BASED OFF OF RADAR DID  
A MINOR FORECAST UPDATE THIS MORNING TO MENTION AT LEAST A SLIGHT  
CHANCE OF RAIN FOR OUR ENTIRE CWA. HAVE BEEN SURPRISED AT THE  
LACKS OF TIPS FROM MANY OF OUR BUCKETS SINCE RADAR MIGHT SUGGEST  
OTHERWISE. COULD BE A CASE OF BRIGHT BANDING. LATEST RUN OF THE  
HRRR CONTINUES TO SHOW THE HEAVIEST RAINFALL AMOUNTS OCCURING THIS  
AFTERNOON AND EVENING.  
 
ANYWAY, CNRFC QPF VALUES ARE NOW IN SO WILL BE ISSUING AN UPDATE  
WITH THE LATEST NUMBERS IN THE NEXT FEW MINUTES. LOOKS LIKE VALUES  
ARE CLOSE TO 12 HOURS AGO, SO THE MAIN STORY REMAINS THREE SYSTEMS  
WILL GO THROUGH OUR REGION. THANKFULLY NEW CNRFC NUMBERS CONTINUE  
TO KEEP RIVER FORECAST POINTS BELOW MONITOR STAGE EVEN THROUGH  
SUNDAY MORNING. WILL BE INTERESTING TO SEE WHAT THE NUMBERS SHOW  
TOMORROW AS THE IMPACTS FROM THE THIRD SYSTEM ARE FELT.  
   
PREVIOUS DISCUSSION
 
LIGHT WARM ADVECTION RAINS ARE ALREADY BREAKING  
OUT ACROSS THE NORTH BAY WITH NUMEROUS STATIONS REPORTING LIGHT  
RAIN AND A FEW HUNDREDTHS OF RAINFALL OVERNIGHT. THE RADAR IS  
BRIGHT BANDING RIGHT NOW WITH FREEZING LEVELS AROUND 10,000 FEET  
SO RETURNS LOOK MORE IMPRESSIVE THAN THE RAINFALL THAT IS  
OCCURRING AT THIS TIME. HOWEVER THE TREND IS DEFINITELY FASTER AND  
WETTER WITH REALLY NONE OF THE MESOSCALE MODELS INDICATING THAT  
RAINFALL SHOULD ALREADY BE OCCURRING (NAM, HRRR, RAP). SO THE OLD  
RULE OF WHEN YOUR IN A WET PATTERN IT DOESNT TAKE MUCH TO KEEP  
THINGS GOING SEEMS TO BE HOLDING TRUE ONCE AGAIN. EXPECT MAINLY  
LIGHT RAIN THIS MORNING THAT WILL INCREASE IN INTENSITY BY MIDDAY  
AND THEN DRIFT SOUTHWARD THROUGH THE DISTRICT AS THE SURFACE COLD  
FRONT AND COLD AIR ALOFT SHIFTS SOUTHWARD. STILL EXPECT THE  
AFTERNOON AND EVENING COMMUTE TO BE MOST IMPACTED WITH  
MODERATE/HEAVY RAIN, PONDING ON ROADWAYS AND GUSTY SOUTH WINDS  
THAT ARE ACCOUNTED FOR WITH THE WIND ADVISORY. ONLY OTHER CHANGE  
FOR TODAYS FORECAST WAS TO INTRODUCE T-STORM CHANCES OVER MUCH OF  
THE REGION AS THE FRONT MOVES THROUGH. SPC DAY 1 OUTLOOK SHOWS  
THIS WITH SPC LIGHTNING PROBABILITY SHOWING HIGH PROBABILITY OF  
LIGHTNING STRIKES SO WONT BE SURPRISED TO GET REPORTS OF EMBEDDED  
T-STORMS TODAY AS THE FRONT ROLLS THROUGH. RAIN TURNS TO SHOWERS  
TONIGHT WITH LINGERING SHOWERS INTO THURSDAY MORNING. HAVE ALSO  
INTRODUCED SOME ISOLATED T-STORMS FOR THURSDAY AFTERNOON OVER THE  
HILLS AS THE COLD/UNSTABLE CORE OF THE UPPER LOW PASSES THROUGH  
THE REGION THURSDAY. RAIN TOTALS WITH THE FIRST STORM STILL SHOW  
0.50-1.50 INCHES FOR THE URBAN AREAS/VALLEY WITH 1-3 INCHES FOR  
THE NORTH BAY VALLEYS AND ALL COASTAL HILLS. A HEALTHY SHOT OF  
RAIN IN A FAIRLY SHORT WINDOW.  
 
THERE WONT BE MUCH BREAK BEFORE THE NEXT STORM BARRELS ONSHORE  
THURSDAY NIGHT INTO FRIDAY MORNING. GIVEN THE CURRENT SYSTEM  
TIMING AND THE PROPENSITY FOR THESE NW FLOW SYSTEMS TO ARRIVE  
FASTER WONT BE SURPRISED TO SEE THE SECOND FEATURE SPEED UP WITH  
THE PERIOD OF HEAVIEST RAIN LIKELY THURSDAY NIGHT INTO FRIDAY  
MORNING. PLENTY OF LEFT OVER MOISTURE AND INSTABILITY TO KEEP  
SHOWERS GOING FOR MUCH OF THE DAY FRIDAY INTO FRIDAY NIGHT AND  
EVEN EARLY SATURDAY MORNING. ITS THESE COLD POST-FRONTAL NW FLOW  
REGIMES WHERE SOME OF THE COASTAL SITES AND HILLS CAN REALLY OVER  
ACHIEVE ON FORECAST RAINFALL TOTALS. IN GENERAL, THE THU  
NIGHT/FRIDAY SYSTEM SHOULD PRODUCE SIMILAR RAINFALL TOTALS. IF  
ANYTHING THE MODELS AND ASSOCIATED MOISTURE PLUME SUGGEST IT MAY  
BE A TAD DRIER BUT POST-FRONTAL SHOWERS MAY OVERPRODUCE IN TERMS  
OF QPF.  
 
THERE SHOULD BE A BREAK ON SATURDAY BEFORE THE THIRD AND FINAL  
SYSTEM ARRIVES SATURDAY NIGHT INTO SUNDAY WITH ANOTHER ROUND OF  
STEADY RAINFALL. THE SUNDAY SYSTEM COULD BE THE WETTEST AS THE  
TRAJECTORY AND ASSOCIATED MOISTURE PLUME COULD BRING A LONGER  
PERIOD OF STEADY RAINFALL. ITS IMPORTANT TO REMEMBER THAT LAST  
WEEKS STORMS WERE ASSOCIATED WITH WARM AR STRETCHING BACK TOWARDS  
HAWAII. THE STORMS THIS WEEK ARE COMING FROM THE NW/W AND MORE  
PROGRESSIVE. NONETHELESS THE 5-6 DAY RAINFALL TOTALS WILL STILL BE  
IMPRESSIVE NO MATTER HOW YOU SLICE IT, ESPECIALLY ON TOP OF THE  
RAIN THAT FELL LAST WEEK. OUR WETTEST SPOTS COULD EASILY SEE 6-8  
INCHES OF RAIN BY MONDAY WITH WIDESPREAD 3-5 INCH AMOUNTS IN THE  
COASTAL HILLS AND 1-3 INCHES FOR THE VALLEYS/URBAN AREAS.  
HILLSIDES, TREES AND ROADS THAT ARE IN A PRECARIOUS SITUATION WILL  
BE STRESSED OVER THE NEXT 5-6 DAYS.  
 
NOTED DRYING TREND STARTS TUESDAY AND LIKELY LASTS THROUGH MUCH OF  
NEXT WEEK IF NOT THE END OF THE MONTH.  
 

 
   
AVIATION...AS OF 9:28 AM PST WEDNESDAY
 
NOT THE BEST DAY  
WEATHERWISE FOR AVIATION AS A POTENT COLD FRONT SWEEPS THROUGH THE  
REGION. THE LEADING EDGE OF THE SYSTEM IS SPREADING LIGHT RAIN  
OVER THE REGION THIS MORNING WITH IFR TO VFR CONDITIONS. THE CAT  
REDUCTION IS MAINLY DUE TO VSBY CURRENTLY. FOR THE REST OF  
TODAY...EXPECT PRECIP TO INCREASE IN COVERAGE AND INTENSITY. THE  
HEAVIEST WILL LIKELY BE LATER THIS AFTERNOON. IN ADDITION TO THE  
RAINFALL...WINDS WILL INCREASE. LLWS WILL BE POSS WITH A 50-60 KT  
WIND ABOUT 2K FT. OAK AND SFO HAVE LLWS MENTIONED. DID NOT INCLUDE  
TSRA, BUT A THUNDERSTORM IS POSS BUT LOCATION/TIMING IS LOW CONF.  
RAIN WILL TRANSITION TO MORE SHOWERS OVERNIGHT INTO EARLY THURS.  
 
VICINITY OF KSFO....VFR THIS MORNING WITH MVFR TO POSS IFR LATER  
TODAY. LLWS FROM AROUND 21-22Z THROUGH 04Z. WIND SHIFT FROM S TO  
SW MORE LIKELY AFTER 03Z.  
 
SFO BRIDGE APPROACH...SIMILAR TO SFO.  
 
MONTEREY BAY TERMINALS...VFR THIS MORNING BECOMING MVFR TO IFR  
THIS AFTERNOON.  
 

 
   
MARINE...AS OF 08:30 AM PST WEDNESDAY
 
POTENT STORM SYSTEM  
MOVING THROUGH THE REGION WILL RESULT IN HAZARDOUS CONDITIONS AND  
ROUGH SEAS ALONG THE CENTRAL COAST TODAY AND TONIGHT. MARINERS ARE  
URGED TO USE CAUTION IF VENTURING TO OPEN WATERS. WINDS WILL  
DECREASE LATE WEDNESDAY NIGHT AS THE LOW LIFTS NORTHEAST BUT WILL  
INCREASE AGAIN BY FRIDAY AS ANOTHER STORM APPROACHES. VERY LARGE  
WEST SWELLS WILL ARRIVE LATER THIS WEEK WITH SWELLS BUILDING AS  
HIGH AS 26 FEET FRIDAY AND SATURDAY.  
 

 
   
MTR WATCHES/WARNINGS/ADVISORIES
 
 
.TDAY...WIND ADVISORY...CAZ006-508>513-516>518-528>530  
WIND ADVISORY...CAZ505>507  
SCA...PIGEON PT TO PT PINOS 0-10 NM  
SCA...PT PINOS TO PT PIEDRAS BLANCAS 0-10 NM FROM 1 PM  
GLW...PT ARENA TO PT REYES 0-10 NM  
GLW...PT ARENA TO PIGEON PT 10-60 NM  
SCA...PIGEON PT TO PT PIEDRAS BLANCAS 10-60 NM UNTIL 3 PM  
GLW...PIGEON PT TO PT PIEDRAS BLANCAS 10-60 NM FROM 3 PM  
SCA...PT REYES TO PIGEON PT 0-10 NM UNTIL 1 PM  
SCA...ROUGH BAR ADVISORY FOR SF BAR FROM 1 PM  
GLW...PT REYES TO PIGEON PT 0-10 NM FROM 1 PM  
SCA...MRY BAY  
SCA...SF BAY FROM 1 PM  
 

 
 

 
 
PUBLIC FORECAST: BELL/RWW  
AVIATION: MM  
MARINE: MM  
 
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