402
FXUS66 KMTR 180949
AFDMTR
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SAN FRANCISCO CA
149 AM PST THU DEC 18 2025
..NEW SHORT TERM, LONG TERM
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
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
(06Z TAFS)
ISSUED AT 921 PM PST WED DEC 17 2025
GENERALLY RADIATIVE STRATUS AND FOG IS DEVELOPING ACROSS THE
INTERIOR VALLEYS AND MONTEREY BAY REGION, WITH A PATCHWORK OF MVFR-
LIFR CONDITIONS ACROSS THE IMPACTED REGION. GENERAL EXPECTATION IS
FOR THE STRATUS DECK TO EXPAND THROUGH THE NIGHT WITH CONDITIONS
GRADUALLY IMPROVING THROUGH THURSDAY MORNING. AROUND THAT TIME, A
BAND OF MVFR CLOUDS WILL COME INTO THE NORTH BAY ASSOCIATED WITH AN
INCOMING FRONTAL SYSTEM, WITH STS NOT CLEARING OUT TO VFR AND APC'S
CLEARING A MODERATE CONFIDENCE FORECAST. THE REST OF THE REGION
SHOULD GET VFR CONDITIONS WITH LIGHT ONSHORE FLOW THURSDAY AFTERNOON
WITH STRATUS RETURNING LATE IN THE EVENING HOURS. RAIN CHANCES BEGIN
THURSDAY EVENING, FAVORING THE NORTH BAY AND WINDWARD SLOPES ACROSS
THE BAY AREA, EXPANDING IN COVERAGE AFTER THE END OF THE 24-HOUR TAF
PERIOD.
VICINITY OF SFO... VFR CONDITIONS CONTINUE THROUGH THE NEXT SEVERAL
HOURS WITH LIGHT WINDS. MODERATE CONFIDENCE THAT MVFR STRATUS
RETURNS TO THE TERMINAL AROUND MIDNIGHT WITH A GRADUAL DIMINISHING
TO IFR CONDITIONS THROUGH THE PRE-DAWN HOURS. THE LATEST HIGH
RESOLUTION MODEL OUTPUT DEPICTS THE STRATUS DEVELOPING MAINLY OVER
THE SF BAY AND THE EASTERN SHORE, BUT RETAINS ENOUGH OF A SIGNAL FOR
IMPACTS TO THE TERMINAL TO KEEP THE PREVIOUS FORECAST IN PLACE.
CLEARING IS EXPECTED LATE THURSDAY MORNING, WITH A RETURN TO BREEZY
WEST-NORTHWEST WINDS IN THE AFTERNOON. STRATUS RETURNS LATE THURSDAY
EVENING IN ADVANCE OF A RAIN BAND EXPECTED TO ARRIVE CLOSE TO THE
END OF THE TAF PERIOD.
SFO BRIDGE APPROACH... SOMEWHAT HIGHER CONFIDENCE IN DIRECT STRATUS
IMPACTS TO THE APPROACH PATH TONIGHT. OTHERWISE SIMILAR TO SFO.
MONTEREY BAY TERMINALS... LIFR STRATUS THROUGH THE NIGHT WITH
SHALLOW FOG POSSIBLE AT THE TERMINALS. VFR CONDITIONS RETURN LATE
THURSDAY MORNING WITH THE RETURN OF BREEZY NORTHEAST WINDS. MVFR
STRATUS RETURNS TO MRY NEAR THE END OF THE TAF PERIOD, AND TO SNS
SHORTLY AFTERWARDS, AS THE FRONTAL BAND APPROACHES THE REGION.
MARINE
(TONIGHT THROUGH NEXT TUESDAY)
ISSUED AT 921 PM PST WED DEC 17 2025
DENSE FOG CONTINUES ACROSS PARTS OF MONTEREY BAY AND IS EXPECTED
TO PERSIST THROUGH THE EARLY MORNING HOURS. LOW VISIBILITIES WILL
CREATE HAZARDOUS CONDITIONS FOR MARINERS, WHO SHOULD PROCEED WITH
CAUTION, USE PROPER FOG SIGNALS, AND MAKE SURE ALL RUNNING LIGHTS
ARE ON. LIGHT TO MODERATE ONSHORE BREEZES PREVAIL WITH MODERATE
SEAS THROUGH LATE WEEK. RAIN CHANCES RETURN TO THE NORTHERN WATERS
ON THURSDAY NIGHT AND PUSH SOUTHWARD THROUGH THE DAY ON FRIDAY.
ADDITIONAL RAIN ARRIVES SATURDAY NIGHT AND CONTINUES OFF AND ON
AT TIMES THROUGH MUCH OF NEXT WEEK.
MTR WATCHES/WARNINGS/ADVISORIES
CA...DENSE FOG ADVISORY UNTIL 7 AM PST THIS MORNING FOR CAZ510-529.
PZ...DENSE FOG ADVISORY UNTIL 7 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...DIALH
MARINE...DIALH
VISIT US AT WWW.WEATHER.GOV/SANFRANCISCO
FOLLOW US ON FACEBOOK, X, AND YOUTUBE AT:
WWW.FACEBOOK.COM/NWSBAYAREA
X.COM/NWSBAYAREA
WWW.YOUTUBE.COM/NWSBAYAREA
CLICK HERE TO GO TO PREVIOUS BULLETINS.
Main Text Page