749
FXUS66 KMTR 210805
AFDMTR
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SAN FRANCISCO CA
105 AM PDT TUE JUL 21 2026
..NEW SHORT TERM, LONG TERM
KEY MESSAGES
UPDATED AT 154 PM PDT SUN JUL 19 2026
- HIGH BASED SHOWERS THROUGH TUESDAY.
- BREEZY ONSHORE WINDS RETURN FOR THE SECOND HALF OF THE WEEK.
- TEMPERATURES WILL HOVER AROUND NORMAL THROUGH THE WEEK.
SHORT TERM
ISSUED AT 105 AM PDT TUE JUL 21 2026
(TODAY AND TONIGHT)
ISOLATED TO WIDELY SCATTERED HIGH BASED SHOWERS MAINLY CONTINUE
OVER THE COASTAL WATERS AND THE BAY AREA. A FEW REPORTS VIA SOCIAL
MEDIA AND WEBCAMS MONDAY SHOWED PATCHY LIGHT SPRINKLES RESULTING
IN A MIX OF TRACE REPORTS AND A COUPLE ISOLATED 0.01". ALMOST ALL
OF THE FORECAST AREA HAS REMAINED DRY. THE PRECIPITABLE WATER (PW)
ON LATE MONDAY AFTERNOON'S OAKLAND UPPER AIR SOUNDING WAS 1.42",
THE MAX MOVING AVERAGE FOR JULY 20. EARLIER OVER THE WEEKEND
SATURDAY EVENING'S PW WAS 0.50", 10TH PERCENTILE. DEEP SOUTHERLY
WINDS CURRENTLY TRANSPORTING REMNANT TROPICAL MOISTURE INTO THE
FORECAST AREA IS LARGELY DISTRIBUTING THE MOISTURE BETWEEN 15,000
FEET AND 26,000 FEET. BETWEEN 15,000 FEET AND THE GROUND, THE LAYER
IS RELATIVELY DRIER, LIKELY RESULTING IN SOME EVAPORATION OF RAIN
DROPS BEFORE REACHING THE GROUND.
NO LIGHTNING HAS BEEN OBSERVED SO FAR OVER OUR FORECAST AREA,
INCLUDING NO LIGHTNING ACTIVITY SINCE SUNDAY EVENING OVER THE
PACIFIC ALL THE WAY OUT ACROSS THE REMNANTS OF ELIDA. AN ASCAT
WIND PASS AT 0511Z (10:11 PM PDT MONDAY) SHOWED THE REMNANT
SURFACE LOW PRESSURE CIRCULATION ~ 500 MILES SOUTHWEST OF MONTEREY
WITH WINDS 15 TO 30 KNOTS WITH A FEW HIGHER GUSTS TO 35 KNOTS.
THE SURFACE LOW CENTER WILL SLOWLY FILL AND WEAKEN WHILE MOVING
NORTHWARD. THE AREA OUT AHEAD OF THE LOW HAS PW UPWARDS OF 1.50"
TO 1.60" WHICH IS NOT FAR FROM THE JULY DAILY MAXIMUM PW (1.74")
ON THE PERIOD RECORD SINCE 1948. THIS PW ARRIVES TODAY AND
FORECAST SOUNDINGS SHOW THIS WILL CONTINUE TO BE LARGELY
DISTRIBUTED IN THE MIDDLE TO UPPER TROPOSPHERE. THE HIGHEST MID TO
UPPER LEVEL PW WILL BE IN OUR AREA WHEN THERE IS WEAK POSITIVE
VORTICITY AT AND ABOVE THE 500 MB LEVEL THIS AFTERNOON/EVENING
OTHERWISE NEGATIVE VORTICITY WILL BE PRESENT IN THE 700-500 MB
LAYER. GIVEN MODEL TRENDS, A LACK OF LIGHTNING SEEN SO FAR,
TEMPORARY WEAK POSITIVE VORTICITY AT AND ABOVE 500 MB AND WEAK
CONVERGENCE AT 250 MB LATER TODAY, IT'S A PERSISTENCE FORECAST
TODAY. THE ATMOSPHERE IS VERY COMPLEX THOUGH, COULD IT STILL
CONVECT AND PRODUCE AN ISOLATED LIGHTNING STRIKE? YES. STAY TUNED
TO THE LATEST UPDATES.
00Z GFS AND ECMWF FORECAST THE 700-500 MB LAYER DRYING OUT
TONIGHT. HOWEVER, ELEVATED PW WILL REMAIN FROM THE SURFACE TO
LOWER LEVELS.
THE RECENT INCREASE IN PW (WATER VAPOR) WILL LARGELY SLOW RADIATIONAL
COOLING THROUGH MORNING, POSSIBLY SLOWING HOW WARM IT GETS TODAY TOO
AS WELL, THOUGH LOWER LEVEL TEMPERATURES AROUND THE 925 MB LEVEL ARE
FORECAST TO INCREASE A FEW DEGREES CELSIUS TODAY. DAYTIME HIGHS MAY
GET ABOVE NORMAL TODAY, VARYING FROM THE 70S/80 COASTSIDE TO THE 80S
BAYSIDE AND 90S/100 FAR INLAND.
LONG TERM
ISSUED AT 105 AM PDT TUE JUL 21 2026
(WEDNESDAY THROUGH MONDAY)
AS MENTIONED IN THE SHORT TERM DISCUSSION, ELEVATED PW WILL REMAIN
FROM THE SURFACE TO THE LOWER LEVELS TONIGHT. THIS WILL CARRY OVER
THROUGH WEDNESDAY. THIS IS TROPICAL MOISTURE THAT'LL KEEP SURFACE
DEWPOINT TEMPERATURES IN THE UPPER 50S/60S WEDNESDAY. DEWPOINTS IN
THIS RANGE WILL MAKE IT FEEL MUGGY WEDNESDAY WHILE THE MID TO UPPER
LEVELS WILL BE DRIER. IF LOW CLOUDS RETURN BY EARLY WEDNESDAY, PATCHY
LIGHT MEASURABLE DRIZZLE MAY DEVELOP IN THE TROPICAL AIR MASS. WEAK
COOL AIR ADVECTION, ONSHORE WINDS AND GENTLE UPSLOPING MAY HELP WITH
THE DEVELOPMENT OF PATCHY DRIZZLE.
DAYTIME ONSHORE BREEZES RETURN MID TO LATE WEEK. IT'LL OTHERWISE
BE DRY, WARM TO HOT INLAND AND COOLEST AT THE COAST AND BAYS LATER
IN THE WEEK. THE HIGH PRESSURE SYSTEM OVER THE INTERIOR WEST WILL
GRADUALLY SHIFT SOUTHWARD THEN SLOWLY MOVE TO THE FOUR CORNERS BY
THE WEEKEND.
AVIATION
(06Z TAFS)
ISSUED AT 923 PM PDT MON JUL 20 2026
MID TO HIGH LEVEL CLOUDS ARE INTERRUPTING NORMAL RADIATIVE PROCESS
FOR WIDES SPREAD STRATUS. LOW CONFIDENCE THAT STRATUS MAY DEVELOP
DURING THE OVERNIGHT THROUGH SUNRISE TUESDAY MORNING. EXPECTING
MAINLY VFR THROUGH THE TAF PERIOD FOR MAJORITY OF THE TERMINALS
WITH THE EXCEPTION OF KHAF WHICH IS ON THE COASTLINE. BE AWARE OF
THE POTENTIAL FOR ELEVATED SHOWERS TO PRODUCE LOCALLY GUSTY AND
ERRATIC WINDS IN THEIR VICINITY THROUGH THE FORECAST PERIOD.
VICINITY OF SFO...VFR EXPECTED TO PREVAIL THROUGH THE TAF PERIOD
WITH A SLIGHT CHANCE OF IFR CEILINGS DEVELOPING LATE IN THE
OVERNIGHT THROUGH SUNRISE TUESDAY. BE AWARE OF THE POTENTIAL FOR
ELEVATED SHOWERS TO PRODUCE LOCALLY GUSTY AND ERRATIC WINDS IN
THEIR VICINITY THROUGH THE FORECAST PERIOD.
SFO BRIDGE APPROACH...SIMILAR TO SFO.
MONTEREY BAY TERMINALS...VFR IS EXPECTED TO PREVAIL WITH A SLIGHT
CHANCE OF IFR CEILINGS DEVELOPING OVERNIGHT BEFORE SUNRISE OR
SHORTLY AFTER. IF CEILINGS DON'T DEVELOP, EXPECT VFR THROUGH THE
REMAINDER OF THE FORECAST PERIOD. BE AWARE OF THE POTENTIAL FOR
ELEVATED SHOWERS TO PRODUCE LOCALLY GUSTY AND ERRATIC WINDS IN
THEIR VICINITY THROUGH THE FORECAST PERIOD.
MARINE
(TODAY THROUGH SATURDAY)
ISSUED AT 1049 AM PDT MON JUL 20 2026
LIGHT TO LOCALLY MODERATE NORTHWESTERLY WINDS WILL PREVAIL
THROUGH MIDWEEK, BECOMING MODERATE THURSDAY INTO FRIDAY. SLIGHT
TO MODERATE SEAS WILL PREVAIL. TROPICAL CYCLONE ACTIVITY IN THE
EASTERN PACIFIC OCEAN WILL SPUR SEVERAL ROUNDS OF MODERATE PERIOD
SOUTHERLY SWELLS; HAZARDOUS CONDITIONS FROM MIXED SEAS MAY RESULT.
INSTABILITY FROM THE REMNANTS OF THE TROPICAL CYCLONE WILL ALSO
BRING A SLIGHT CHANCE OF CONVECTION THAT MAY BRING SHOWERS AS WELL
AS GUSTY AND ERRATIC OUTFLOW WINDS THAT WOULD BE RESULT IN
HAZARDOUS CONDITIONS TO SMALL CRAFT.
FIRE WEATHER
ISSUED AT 154 PM PDT MON JUL 19 2026
NO SIGNIFICANT IMPACTS EXPECTED AS SUB-TROPICAL MOISTURE MOVES
OVER THE BAY AREA AND CENTRAL COAST. THERE HAS BEEN SOME
DISCUSSION OF DRY THUNDERSTORMS, BUT AS OF THIS AFTERNOON MODELS
ARE NOT INDICATING AN ENVIRONMENT THAT WOULD SUPPORT THEIR
DEVELOPMENT. AT THIS POINT I WOULD GIVE DRY THUNDERSTORMS A
PROBABILITY OF LESS THAN 1% THIS WEEK. WHAT SHOULD BE EXPECTED IS
AN ISOLATED SHOWER HERE OR THERE WITH LITTLE TO NO ACCUMULATION.
THE MARINE LAYER WILL REMAIN FAIRLY WELL MIXED OUT THROUGH TUESDAY
AFTERNOON, BUT SHOULD START TO BUILD BACK IN LATE TUESDAY AND
TUESDAY NIGHT. EXPECT IT TO REACH AROUND 1000 FEET BY MIDWEEK.
STRONG ONSHORE WINDS WILL DEVELOP EACH AFTERNOON FOR THE SECOND
HALF OF THE WEEK. THEY TYPICAL PLACES WILL GUSTS LIKE THEY DO
EVERY SUMMER AFTERNOON.
MTR WATCHES/WARNINGS/ADVISORIES
CA...NONE.
PZ...NONE.
SHORT TERM...CANEPA
LONG TERM....CANEPA
AVIATION...JM
MARINE...JM
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