701
FXUS66 KMTR 181719
AFDMTR
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SAN FRANCISCO CA
919 AM PST THU DEC 18 2025
..NEW UPDATE, MARINE
KEY MESSAGES
UPDATED AT 143 AM PST THU DEC 18 2025
- DENSE FOG THROUGH THIS MORNING
- WIDESPREAD BENEFICIAL RAINFALL RETURNS FRIDAY
- IMPACTFUL RAIN AND WIND RETURN SATURDAY, PERSISTING AT LEAST
THROUGH WEDNESDAY
UPDATE
ISSUED AT 913 AM PST THU DEC 18 2025
NO UPDATE TO THE FORECAST THIS MORNING. FOG CONDITIONS ARE
IMPROVING BUT POCKETS OF DENSE FOG PERSIST IN MANY AREAS SO WILL
KEEP THE DENSE FOG ADVISORY IN PLACE UNTIL 11AM. LIGHT RAIN IS
EXPECTED TO ARRIVE IN THE NORTH BAY LATE TONIGHT WHICH WILL BEGIN
THE THE EXTENDED PERIOD OF RAINFALL WE'VE BEEN MESSAGING THROUGH
THIS WEEKEND AND ALL OF NEXT WEEK.
SHORT TERM
ISSUED AT 143 AM PST THU DEC 18 2025
(TODAY AND TONIGHT)
TODAY WILL START OUT FOGGY FOR SOME COASTAL AND VALLEY LOCATIONS. IF
DRIVING, SLOW DOWN, USE YOUR LOW-BEAM HEADLIGHTS, AND LEAVE PLENTY
OF DISTANCE AHEAD OF YOU. IF COMMUTING, ALLOW EXTRA DRIVE TIME TO
REACH YOUR DESTINATION SAFELY. COASTAL/HIGHER TERRAIN DRIZZLE IS
POSSIBLE FOR PLACES WITH FOG/STRATUS AND A FEW ISOLATED SHOWERS ARE
POSSIBLE FOR THE NORTH BAY THIS AFTERNOON, BUT GENERALLY SPEAKING
TODAY WILL BE DRY. THE SAME CANNOT BE SAID FOR THE REST OF THE
FORECAST PERIOD. TODAY IS YOUR LAST GUARANTEED DRY DAY FOR THE NEXT
SEVEN DAYS. ALL PREPARATIONS TO YOUR HOME SHOULD BE MADE BY TONIGHT
INCLUDING BUT NOT LIMITED TO CLEANING OUT YOUR GUTTERS AND STORM
DRAINS, SECURING LOOSE OUTDOOR ITEMS (LIKE CHRISTMAS INFLATABLES),
RAKING AND PROPERLY DISPOSING OF LEAVES, AND TRIMMING TREE BRANCHES
AWAY FROM YOUR HOME AND POWER LINES.
LONG TERM
ISSUED AT 143 AM PST THU DEC 18 2025
(FRIDAY THROUGH WEDNESDAY)
THE FIRST OF A SERIES OF SYSTEMS ARRIVES TOMORROW. SURFACE HIGH
PRESSURE OFF THE CALIFORNIA/MEXICO BORDER AND SURFACE LOW PRESSURE
IN THE GULF OF ALASKA/BRITISH COLUMBIA WILL ALLOW FOR A CONVEYOR
BELT OF SUBTROPICAL MOISTURE TO STREAM INTO THE STATE THROUGH
SATURDAY. A COLD FRONT WILL SWEEP ACROSS THE REGION FRIDAY BRINGING
WIDESPREAD BENEFICIAL RAINFALL EXCEPT FOR THE INTERIOR CENTRAL COAST
WHICH MAY STAY DRY OUTSIDE OF SOME HIGHER TERRAIN DRIZZLE. WINDS
WILL INCREASE WITH THE COLD FRONTAL PASSAGE WITH GUSTS UP TO 30 MPH
EXPECTED ALONG THE COAST, ACROSS RIDGELINES, AND THROUGH GAPS AND
PASSES. IT SEEMS AS IF THERE MAY BE A BRIEF BREAK IN THE RAINFALL
SATURDAY MORNING BEFORE IT INCREASES IN COVERAGE AND INTENSITY
SATURDAY AFTERNOON. WHILE THERE IS NO TRIGGER FOR THIS, THE REASON
SEEMS TO BE WARM AIR ADVECTION FROM THE AFOREMENTIONED SURFACE HIGH
PRESSURE, A PERSISTENT INFLUX OF IVT IN EXCESS OF 250 KG/MS, AND THE
AFOREMENTIONED COLD FRONT PROVIDING A CONVERGENCE ZONE FOR ALL OF
THESE TO MATERIALIZE. SATURDAY WILL LIKELY BE THE DAY WHERE THINGS
BEGIN TO TRANSITION FROM BENEFICIAL TO IMPACTFUL. THROUGH SATURDAY,
THE NORTH BAY IS EXPECTED TO RECEIVE THE BRUNT OF THE RAINFALL. AS
SUCH, IF YOU LIVE NEAR OR KNOW THAT YOU WILL BE TRAVELLING THROUGH
THE AREA, PLEASE BE AWARE OF PROBLEMATIC FLASHY AREAS OF MARK WEST
CREEK AND DO NOT DRIVE AROUND BARRICADES. WE ARE NOT QUITE WITHIN
THE RANGE OF HI-RES MODELS TO LOOK AT HOURLY PRECIPITATION RATES FOR
ALL OF SATURDAY YET, BUT LIKE ABOVE, RESIDENTS OR TRAVELERS NEAR THE
PICKETT FIRE BURN SCAR NEED TO BE AWARE OF AND PREPARE FOR THE
INCOMING WEATHER.
THE SECOND SYSTEM ARRIVES SUNDAY BY WAY OF A SURFACE LOW MOVING INTO
NORTHERN CALIFORNIA. THERE IS STILL UNCERTAINTY IN EXACT LOCATION,
STRENGTH, AND TIMING OF THE LOW, BUT CONFIDENCE IS INCREASING THAT
THERE WILL BE ONE. NONETHELESS, THE PROXIMITY TO IT WILL BRING MORE
IMPACTS THAN THE ONES THAT HAVE REMAINED IN THE PACIFIC NORTHWEST.
THE SURFACE LOW WILL GET ITS MOISTURE FROM THE SAME SOURCE REGION IN
THE SUBTROPICS. THE DIFFERENCE THIS TIME AROUND IS THAT INSTEAD OF A
QUICK PEAK IN IVT, VALUES WILL REMAIN NEAR 500 KG/MS FOR NEAR 24
HOURS BASED ON THE GEFS AND ECMWF 80 MEMBER ENSEMBLE MEAN. MODERATE
TO HEAVY RAINFALL IS EXPECTED ACROSS MOST OF THE REGION ON SUNDAY.
RAINFALL TOTALS WILL BE LOCALLY HIGHER IN SOUTHWEST FACING TERRAIN
SUCH AS COASTAL NORTH BAY, SANTA CRUZ MOUNTAINS, AND BIG SUR COAST.
IN ADDITION TO THE RAIN, WIND WILL BE ANOTHER HAZARD AS IT INCREASES
SATURDAY NIGHT. PROBABILITIES ARE INCREASING FOR IMPACTFUL WIND
(GUSTS OF 45+ MPH) DURING THIS TIME FRAME. ANY WIND IMPACTS WILL BE
EXACERBATED BY THE RAIN AND VICE VERSA. THE COMBINATION OF WIND AND
MOIST SOILS WILL INCREASE THE RISK FOR DOWNED TREES. ADDITIONALLY,
ANY LEAVES THAT ARE LEFT ON TREES WILL POSE THE RISK OF
BLOCKING/CLOGGING STORM DRAINS IF/WHEN THEY FALL, POTENTIALLY
LEADING TO MORE FLOODING. THE ACCOMPANYING COLD FRONT WILL PASS
THROUGH ON MONDAY, BRINGING YET ANOTHER DAY OF RAIN AND WIND. AS OF
NOW MONDAY DOES NOT LOOK AS IMPACTFUL AS SUNDAY, BUT THERE IS STILL
UNCERTAINTY IN RAINFALL TOTALS AND THE CUMULATIVE IMPACTS THAT FOUR
DAYS OF RAIN WILL HAVE ON THE REGION AT THIS POINT.
THE THIRD SYSTEM ARRIVES TUESDAY AND WILL LINGER INTO WEDNESDAY
(CHRISTMAS EVE) AS A SURFACE LOW DEVELOPS OFF THE COAST. THERE IS A
GREAT DEAL OF UNCERTAINTY IN LOCATION, STRENGTH, AND TIMING. EVEN
SO, IT DOES LOOK LIKE IT WILL BE IMPACTFUL FOR OUR AREA IN TERMS OF
RAIN AND WIND WITH IVT REBOUNDING AND A LOW-LEVEL JET NOSING INTO
THE REGION. A POTENTIAL SILVER LINING IN TERMS OF RAINFALL IS THAT
THIS SYSTEM WILL SPREAD FROM SOUTH TO NORTH, BRINGING RAINFALL TO
PLACES THAT WILL NOT HAVE SEEN MUCH UP UNTIL THIS POINT SUCH AS THE
INTERIOR CENTRAL COAST.
ALL OF THIS TO SAY THAT THERE IS STILL A GREAT DEAL OF UNCERTAINTY
IN LOCATION, STRENGTH, AND TIMING OF THINGS AND SUBSEQUENTLY THEIR
IMPACTS. THERE IS CERTAINTY THAT IT IS GOING TO RAIN FOR THE NEXT
SEVEN DAYS AND LIKELY INTO THE CHRISTMAS HOLIDAY AND BEYOND. IMPACTS
WILL NOT ONLY BE FELT WITH EACH INDIVIDUAL SYSTEM, BUT ALSO THE
CUMULATIVE EFFECT OF THE ONES BEFORE IT. ALL PREPARATIONS FOR RAIN
AND WIND SHOULD BE DONE TODAY. IF YOU ARE TRAVELLING, PLEASE ACCOUNT
FOR THE WEATHER AND PLAN ACCORDINGLY. MINOR/NUISANCE FLOODING OF
URBAN, LOW-LYING, AND POOR DRAINAGE AREAS AS WELL AS FLASHY
CREEKS/STREAMS WILL LIKELY HAPPEN AT SOME POINT, POTENTIALLY AS SOON
AS SATURDAY.
AVIATION
(12Z TAFS)
ISSUED AT 325 AM PST THU DEC 18 2025
CONDITIONS ARE LARGELY VARIABLE ACROSS THE FORECAST AREA. FOG AND
LOW STRATUS CLOUDS /LIFR-IFR/ CONTINUE TO DEVELOP INCLUDING AREAS
OF DENSE FOG /VLIFR/ OTHERWISE IT'S VFR. GRADUAL CLEARING OCCURING
BY LATE MORNING AND AFTERNOON. FOG AND LOW STRATUS DEVELOP TONIGHT
AND FRIDAY MORNING.
VICINITY OF SFO...IFR-MVFR THROUGH 18Z THEN MVFR TO 18Z-20Z TODAY.
LIGHT AND VARIABLE WIND BECOMING WEST 5 TO 10 KNOTS LATE TODAY AND
TONIGHT.
SFO BRIDGE APPROACH...SIMILAR TO SFO.
MONTEREY BAY TERMINALS...CONDITIONS VARY FROM FOG AND STRATUS /VLIFR-
IFR/ INCLUDING SOMEWHAT HIGHER BASED STRATUS /MVFR/. GRADUAL CLEARING
OCCURING BY LATE MORNING AND AFTERNOON. IFR IN STRATUS REDEVELOPS
TONIGHT AND FRIDAY MORNING. LIGHT AND VARIABLE WINDS BECOMING ONSHORE
5 TO 10 KNOTS IN THE AFTERNOON AND EARLY EVENING.
MARINE
(TODAY THROUGH TUESDAY)
ISSUED AT 913 AM PST THU DEC 18 2025
A DENSE FOG ADVISORY ACROSS THE MONTEREY BAY IS IN EFFECT UNTIL
11 AM TODAY. LIGHT TO MODERATE ONSHORE BREEZES WILL PREVAIL WITH
MODERATE SEAS THROUGH LATE WEEK. PERIODIC RAIN CHANCES CONTINUE
THROUGH THE WEEK AS WELL WITH ADDITIONAL RAIN DEVELOPING OVER THE
WEEKEND. MORE SUBSTANTIAL RAINFALL AND STRONG STORM SYSTEM
DEVELOPING AROUND CHRISTMAS DAY.
MTR WATCHES/WARNINGS/ADVISORIES
CA...DENSE FOG ADVISORY UNTIL 11 AM PST THIS MORNING FOR CAZ506-510-
516-528>530.
PZ...DENSE FOG ADVISORY UNTIL 11 AM PST THIS MORNING FOR MRY BAY.
SMALL CRAFT ADVISORY FROM 9 AM TO 3 PM PST FRIDAY FOR PT ARENA
TO PT REYES 10-60 NM.
SHORT TERM...SARMENT
LONG TERM....SARMENT
AVIATION...CANEPA
MARINE...BEHRINGER
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