799
FXUS66 KMTR 040851
AFDMTR
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SAN FRANCISCO CA
151 AM PDT MON MAY 4 2026
..NEW KEY MESSAGES, SHORT TERM, LONG TERM
KEY MESSAGES
UPDATED AT 1210 AM PDT MON MAY 4 2026
- COOLER TEMPERATURES THROUGH TUESDAY WITH A CHANCE FOR DRIZZLE
AND LIGHT RAIN, MAINLY ALONG THE COASTLINE AND THE BAY AREA
- WARMER AND DRIER CONDITIONS RETURN MID TO LATE WEEK
SHORT TERM
ISSUED AT 151 AM PDT MON MAY 4 2026
(TODAY AND TONIGHT)
THE 500 MB LOW CENTER WE'VE BEEN MONITORING IS CURRENTLY DRIFTING
AND WOBBLING AROUND APPROX 100 MILES SOUTHWEST OF MONTEREY. IT'S
TEMPORARILY QUASI-STATIONARY WITH THE POLAR JET STREAM WINDS ON THE
BACK SIDE OF THE LOW WHILE SUPPORTED BY SUBTROPICAL JET STREAM WINDS
AT THE BASE OF THE LOW. THE GEOPOTENTIAL HEIGHT OF THE LOW CENTER IS
~ 559 DECAMETERS, FOR REFERENCE THAT'S NEAR THE 10TH PERCENTILE
COMPARED TO OAKLAND UPPER AIR LONG TERM CLIMATOLOGY FOR EARLY MAY.
HIGHER SURFACE PRESSURES EXTEND UNDERNEATH THE UPPER LOW CORRESPONDING
TO CHILLY TO COLD LOWER LEVEL TEMPERATURES (925 MB AND 850 MB TEMPS).
THE COLD AIR INTRUSION LIFTED THE TEMPERATURE INVERSION AND ULTIMATELY
DID CAUSE DEEPER VERTICAL MIXING TO PARTIALLY CLEAR THE SKY FOR SUNNY
BREAKS SUNDAY THANKS TO THE MAY SUN ANGLE (HEAT ENERGY INTO THE SYSTEM
I.E. THE EARTH-ATMOSPHERE SYSTEM).
AND IT'S CHILLY ACROSS THE AREA WITH TEMPERATURES IN THE 50S, 40S AND
A FEW UPPER 30S AT HIGHER ELEVATIONS. PARTIAL CLOUDINESS CONTINUES
THROUGH THIS MORNING.
WITH THE LOW MOVING SLOWLY TODAY AND TONIGHT, WARM AIR ADVECTION
TO THE NORTH OF THE LOW CENTER WILL HAVE A CHANCE TO INTERACT WITH
THE COLDER AIR ALOFT RESULTING IN LIGHT TO MODERATE STRATIFORM RAIN
BEGINNING LATER TODAY OVER THE NORTH BAY THEN SPREADING ACROSS MUCH
OF THE BAY AREA TONIGHT. RAINFALL FORECASTS HAVE INCREASED. SEVERAL
HUNDREDTHS TO TWO TO THREE TENTHS OF AN INCH OF RAIN (POSSIBLY SLIGHTLY
HIGHER AMOUNTS MAY OCCUR) ARE FORECAST ACROSS MUCH OF THE BAY AREA.
ON FIRST GLANCE, ONE MAY THINK THE COUNTER-CLOCKWISE WIND FLOW TO
THE NORTH OF LOW CENTER (EAST TO NORTHEAST WINDS) FROM THE SIERRA
NEVADA WOULD PRODUCE DOWNSLOPING, A DRIER ATMOSPHERE AND NEAR ZERO
OR ZERO RAIN CHANCES. AND IT IS TRUE IT MOST CERTAINLY CAN DO THIS.
HOWEVER, THIS LOW IS IN A DIFFERENT SYNOPTIC PATTERN. THE LOW IS IN
A TEMPORARY QUASI-BALANCE LONG ENOUGH FOR THE WARM AIR ADVECTION TO
ARRIVE WHILE MOST LIKELY IN NEAR BALANCE BECAUSE OF NEAR SURFACE
HIGHER PRESSURES (SURFACE PRESSURES ARE LOWER OVER THE GREAT BASIN,
MEANING NO OFFSHORE WINDS) AND THE NET FORCES DUE TO THE UPPER LEVEL
WINDS AROUND THE LOW. THIS IS A GREAT EXAMPLE AS TO HOW COMPLEX THE
ATMOSPHERE IS, WITH NUMEROUS MODES OF CIRCULATION, ETC.
LONG TERM
ISSUED AT 151 AM PDT MON MAY 4 2026
(TUESDAY THROUGH SUNDAY)
THE SUBTROPICAL JET STREAM AT THE BASE OF THE 500 MB LOW WILL
MOVE THE LOW EASTWARD AWAY FROM CALIFORNIA BY MID-WEEK. AFTER THIS,
HIGH PRESSURE WILL MOVE IN FROM THE WEST AND NORTHWEST BRINGING
WITH IT DRY CONDITIONS AND A WARMING TREND. IN GENERAL BECAUSE OF
THE STRENGTHENING LONG WAVE TROUGHING ON EACH SIDE OF THE HIGH
PRESSURE RIDGE, MERIDIONAL (NORTH-SOUTH) DISPLACEMENTS WILL ALSO
AMPLIFY/STRENGTHEN THE RIDGE, WITH GREATER COMPRESSIONAL WARMING
AND SURFACE WARMING RESULTING IN ABOVE TO WELL ABOVE NORMAL HIGH
TEMPERATURES IN OUR FORECAST AREA BY LATE WEEK AND WEEKEND. FAR
INLAND LOCATIONS ESPECIALLY BY NEXT WEEKEND WILL LIKELY REACH THE
80S AND 90S.
AVIATION
(06Z TAFS)
ISSUED AT 933 PM PDT SUN MAY 3 2026
NOT MUCH OF A DISCERNIBLE MARINE LAYER PER SE, BUT DEFINITELY A
SCT TO BKN CLOUD DECK FORMING OVER THE NEARBY TERRAIN WITH SOME
IMPACTS TO TERMINALS. TONIGHT THROUGH EARLY MONDAY WILL KEEP
MAINLY VFR CIGS WITH HEIGHTS IN THE 3000-5000 FT RANGE. TOMORROW
IS A TRICKY DAY AS MOISTURE TO THE EAST STREAMS WEST INCREASING
CHC FOR PRECIP AND LOWER CIGS. PRECIP INITIALLY LIKELY OVER THE N
BAY AND THEN SPREADING S TO SF BAY. FARTHER SOUTH THAN THAT IS
OUTSIDE OF THE TAF PERIOD. OVERALL CONF IS MEDIUM.
VICINITY OF SFO...VFR CIGS LOOKING MORE LIKELY THROUGH MUCH OF
MONDAY. LATE MONDAY INTO TUESDAY PRECIP CHC INCREASE WITH MVFR
CIGS.
SFO BRIDGE APPROACH...SIMILAR TO SFO.
MONTEREY BAY TERMINALS...MVFR CIGS SLOWLY DEVELOPING AROUND THE
PENINSULA. WILL HAVE A BETTER SHOT FILLING LATER IN THE NIGHT.
MARINE
(TONIGHT THROUGH NEXT SATURDAY)
ISSUED AT 933 PM PDT SUN MAY 3 2026
LIGHT TO GENTLE BREEZES OVER THE WATERS AS A WEAK DISTURBANCE
MOVES THROUGH THE REGION. THE LONGER PERIOD SWELL THAT ARRIVE
SUNDAY WILL CONTINUE TO DIMINISH THROUGH MONDAY. NORTHERLY WINDS
AND SEAS WILL BEGIN TO INCREASE THURSDAY AND INTO NEXT WEEKEND.
MTR WATCHES/WARNINGS/ADVISORIES
CA...BEACH HAZARDS STATEMENT UNTIL 11 PM PDT THIS EVENING FOR CAZ006-
505-509-529-530.
PZ...NONE.
SHORT TERM...CANEPA
LONG TERM....CANEPA
AVIATION...MM
MARINE...MM
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