062  
FXUS66 KMTR 240515  
AFDMTR  
 
AREA FORECAST DISCUSSION  
NATIONAL WEATHER SERVICE SAN FRANCISCO BAY AREA  
1015 PM PDT WED MAY 23 2018  
   
SYNOPSIS
 
COOL WEATHER CONDITIONS ALONG WITH WIDESPREAD NIGHT  
AND MORNING LOW CLOUDS WILL CONTINUE THROUGH THE END OF THE WEEK.  
AN UPPER LEVEL LOW PRESSURE SYSTEM WILL APPROACH THE COAST LATE  
THURSDAY AND BRING A CHANCE OF RAIN SHOWERS TO MUCH OF THE REGION,  
MAINLY ON THURSDAY NIGHT AND FRIDAY. SHOWERS MAY LINGER INTO  
SATURDAY MORNING BEFORE A WARMING AND DRYING TREND DEVELOPS FOR  
THE REMAINDER OF THE HOLIDAY WEEKEND.  
 

 
   
DISCUSSION...AS OF 9:15 PM PDT WEDNESDAY
 
'MAY GRAY' CONTINUED  
IN FULL FORCE TODAY, WITH LOW CLOUDS PERSISTING THROUGHOUT THE DAY  
ACROSS MUCH OF THE NORTH BAY, AND IN MOST COASTAL AREAS AS WELL.  
AFTERNOON HIGHS WERE ABOUT FIVE DEGREES COOLER THAN NORMAL IN  
COASTAL AREAS AND FROM 10 TO 15 DEGREES BELOW NORMAL INLAND. AREAS  
THAT DID MANAGE TO CLEAR THIS AFTERNOON WILL LIKELY SEE A RETURN  
TO CLOUDY CONDITIONS OVERNIGHT AS A DEEP MARINE LAYER REMAINS IN  
PLACE ACROSS THE REGION.  
 
AN UPPER LOW IS CURRENTLY CENTERED ABOUT 650 MILES WEST OF SAN  
FRANCISCO. THIS LOW IS FORECAST TO GRADUALLY APPROACH THE COAST  
FROM TONIGHT THROUGH THURSDAY NIGHT AND THEN MOVE INLAND OVER  
CENTRAL CALIFORNIA ON FRIDAY. AS THE LOW APPROACHES, THERE WILL  
LIKELY BE ADDITIONAL DEEPENING OF THE MARINE LAYER, ALONG WITH  
PATCHY NIGHT AND MORNING DRIZZLE. IT'S POSSIBLE THAT THE MARINE  
LAYER MAY DEEPEN TO THE POINT OF MIXING OUT BY TOMORROW AFTERNOON,  
IN WHICH CASE WE COULD SEE MORE WIDESPREAD SUNSHINE TOMORROW AND  
SLIGHTLY WARMER TEMPERATURES. BUT THE MORE LIKELY SCENARIO IS THAT  
TOMORROW WILL BE A DAY MUCH LIKE TODAY.  
 
AS THE LOW DRAWS NEARER ON THURSDAY NIGHT SHOWER CHANCES WILL  
DEVELOP ACROSS OUR REGION AS A DEEPER MOIST LAYER NEAR THE LOW  
CENTER ARRIVES ALONG THE COAST AND AS INSTABILITY INCREASES.  
BASED ON THE 00Z NAM AND MOST OTHER MODEL DATA, SHOWER CHANCES  
WILL BE HIGHEST FROM LATE THURSDAY NIGHT THROUGH FRIDAY. ALTHOUGH  
THERE APPEARS TO BE LITTLE OR NO CHANCE OF SURFACE-BASED  
THUNDERSTORM DEVELOPMENT IN OUR AREA FROM THIS SYSTEM (SURFACE-  
BASED CONVECTION IS EXPECTED BE LIMITED TO THE HIGHER MOUNTAINS TO  
OUR NORTH AND EAST), THERE IS AT LEAST A VERY SLIGHT CHANCE OF  
HIGH BASED THUNDERSTORM DEVELOPMENT LATE THURSDAY NIGHT INTO  
FRIDAY. THE 00Z NAM IS FORECASTING SUFFICIENT MID LEVEL (700-500  
MB LAYER) MOISTURE AND INSTABILITY FOR POSSIBLE THUNDERSTORM  
DEVELOPMENT IN THIS LAYER. THUNDERSTORM CHANCES ARE NOT CURRENTLY  
INCLUDED IN OUR FORECAST, BUT THIS IS SOMETHING THAT WILL NEED TO  
BE CONSIDERED DEPENDING ON WHAT THE REST OF THE 00Z MODEL DATA  
SHOWS.  
 
RAINFALL AMOUNTS FROM SHOWER ACTIVITY LATE THIS WEEK IS NOT  
EXPECTED TO EXCEED A TENTH OF AN INCH IN MOST PLACES, ALTHOUGH  
SOME MODELS ARE FORECASTING ISOLATED RAINFALL AMOUNTS OF MORE  
THAN A QUARTER OF AN INCH, ESPECIALLY IN THE NORTH AND EAST BAY.  
DUE TO THE SHOWERY NATURE OF THE PRECIPITATION, SOME LOCATIONS MAY  
SEE LITTLE OR NO RAIN.  
 
COOL CONDITIONS WILL CONTINUE THROUGH FRIDAY AS THE LOW MOVES  
ACROSS CALIFORNIA, WITH MUCH COOLER THAN NORMAL CONDITIONS INLAND.  
A FEW SHOWERS MAY LINGER INTO SATURDAY MORNING. BUT BY SATURDAY  
AFTERNOON THE LOW IS PROJECTED TO MOVE WELL OFF TO OUR EAST AND  
WE SHOULD CLEAR OUT AND BEGIN TO WARM UP AS SUBSIDENCE DEVELOPS.  
THE WARMING AND DRYING TREND WILL THEN CONTINUE ON THROUGH THE  
REMAINDER OF LONG HOLIDAY WEEKEND AS A SHORTWAVE RIDGE DEVELOPS  
JUST OFFSHORE. TEMPERATURES ARE EXPECTED TO WARM TO NEAR NORMAL BY  
MEMORIAL DAY. COOLING IS THEN EXPECTED BY THE MIDDLE OF NEXT WEEK  
AS A TROUGH DEVELOPS NEAR THE WEST COAST.  
 

 
   
AVIATION...AS OF 10:15 PM PDT WEDNESDAY
 
MVFR CEILINGS, GOOD  
VISIBILITIES TONIGHT INTO THURSDAY MORNING. LOWER TO MID LEVEL  
THERMAL RIDGING EMBEDDED IN STEADILY WEAKENING 500 MB HEIGHTS  
TONIGHT WILL TRANSITION OVER TO DEEPER LAYERED THERMAL AND 500 MB  
HEIGHT TROUGHING THURSDAY AND THURSDAY NIGHT. HIGH BOUNDARY LAYER  
HUMIDITY AND COOLING TONIGHT AND THURSDAY MORNING GENERATES LOW  
CLOUDS TONIGHT AND THURSDAY MORNING.  
 
GUSTY WESTERLY WINDS IN THE EAST BAY HILLS/MOUNTAINS AND IN THE  
DELTA WILL DIMINISH TONIGHT INTO EARLY THURSDAY MORNING. WESTERLY  
WINDS BECOMING GUSTY AGAIN IN THE DELTA, OVER THE BAYS, AND TO THE  
GOLDEN GATE THURSDAY AFTERNOON AND EVENING.  
 
ISOLATED TO WIDELY SCATTERED CONVECTION LEADING TO ISOLATED  
THUNDERSTORM DEVELOPMENT IS SOMETHING TO KEEP IN MIND, A  
POSSIBILITY EITHER THURSDAY AFTERNOON AND/OR THURSDAY NIGHT.  
700-500 MB LAYER INSTABILITY STEEPENS A BIT, WITH A SWATH OF DRIER  
MID LEVEL AIR ARRIVING FROM THE SOUTHWEST THURSDAY, BUT NOT MUCH  
SURFACE BASED MUCAPE ON THE 00Z NAM RUN.  
 
VICINITY OF KSFO...MVFR TONIGHT INTO THURSDAY MORNING. THURSDAY  
IS A TRANSITION DAY TO UPPER LEVEL TROUGHING, LOW CLOUDS SHOULD  
LIFT TO SCATTERED COVERAGE WITH LATE DAY AND/OR EVENING SHOWER  
POSSIBLE.  
 
SFO BRIDGE APPROACH...SIMILAR TO KSFO.  
 
MONTEREY BAY TERMINALS...MVFR TONIGHT INTO THURSDAY MORNING.  
PARTIAL CLEARING THURSDAY AFTERNOON AND EVENING, UPPER LEVEL  
TROUGHING WILL SWING IN FROM THE SOUTHWEST POSSIBLY BRINGING LATE  
DAY AND/OR EVENING SHOWER.  
 

 
   
MARINE...AS OF 5:39 PM PDT WEDNESDAY
 
NORTHWEST WINDS WILL  
REMAIN LIGHT ACROSS THE WATERS TODAY AND TONIGHT. WINDS WILL TURN  
SOUTHERLY TOMORROW MORNING AS AN UPPER LOW AND WEAK SURFACE COLD  
FRONT APPROACH FROM THE WEST. THIS WILL BRING SHOWERS TO THE AREA  
TOMORROW AND FRIDAY. WINDS WILL TURN BACK OUT OF THE NORTHWEST  
ONCE THE UPPER LOW MOVES THROUGH LATE FRIDAY. WINDS ACROSS THE  
WATERS WILL ALSO INCREASE AND REMAIN GUSTY THROUGH THE HOLIDAY  
WEEKEND. LONG PERIOD SOUTHERLY SWELL WILL PERSIST THROUGH THE END  
OF THE WEEK. PERIODS AND SWELL HEIGHTS WILL GRADUALLY LOWER OVER  
THE NEXT FEW DAYS BEFORE A SECOND LONG PERIOD, ALTHOUGH WEAKER,  
SOUTHERLY SWELL MOVES IN TOWARDS THE END OF THE WEEKEND.  
 

 
   
MTR WATCHES/WARNINGS/ADVISORIES
 
 
.TNGT...SCA...SF BAY UNTIL 3 AM  
 

 
 

 
 
PUBLIC FORECAST: DYKEMA  
AVIATION: CANEPA  
MARINE: AS  
 
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  
 
CLICK HERE TO GO TO PREVIOUS BULLETINS.

The Nexlab CA Page Main Text Page