516
FXUS66 KMTR 121858
AFDMTR
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SAN FRANCISCO CA
1158 AM PDT WED MAR 12 2025
..NEW AVIATION
SYNOPSIS
ISSUED AT 1212 AM PDT WED MAR 12 2025
AN ACTIVE WEATHER PATTERN ARRIVES TODAY WITH PERIODS OF RAIN,
STRONG WIND, THUNDERSTORM CHANCES, AND SOME SNOW TO THE HIGHER
REGIONAL MOUNTAINS. WHILE THE BULK OF THE IMPACTS ARRIVE TODAY
THROUGH THURSDAY, DISTURBED WEATHER WILL CONTINUE INTO EARLY NEXT
WEEK.
UPDATE
ISSUED AT 900 AM PDT WED MAR 12 2025
PREFRONTAL RAIN CONTINUES ACROSS THE SANTA LUCIA RANGE, SANTA CRUZ
MOUNTAINS, AND MUCH OF SAN FRANCISCO BAY AREA. DURING THE PAST 12
HOURS MT. TAMALPAIS HAS PICKED UP 0.41", 0.48" AT MINING RIDGE (IN
THE SANTA LUCIA RANGE), WITH GENERALLY LESS THAN 0.10" ELSEWHERE
(FOR THOSE WHO HAVE SEEN RAIN THIS MORNING). THE MAIN FRONT WILL
MOVE THROUGH LATE MORNING AND EARLY AFTERNOON ACROSS THE NORTH BAY,
EARLY AFTERNOON ACROSS THE BAY AREA, AND ACROSS THE CENTRAL COAST
LATER THIS AFTERNOON. STRONG AND GUSTY WINDS WILL ACCOMPANY THE
FRONTAL PASSAGE WITH GUSTS AS HIGH AS 55 MPH (LOCALLY GREATER IN THE
HIGHER ELEVATIONS). MODERATE TO LOCALLY HEAVY RAIN WILL ALSO
ACCOMPANY THE FRONT WITH RAIN RATES UP TO 0.50" PER HOUR, HOWEVER
FLOODING CONCERNS REMAIN LIMITED TO PONDING OF WATER ON ROADWAYS,
RISES OF CREEKS/STREAMS, AND MINOR URBAN FLOODING IN POORLY DRAINED
AREAS. OTHERWISE, THE FORECAST REMAINS ON TRACK FOR THE DAY WITH NO
UPDATES ANTICIPATED AT THIS TIME.
RGASS
SHORT TERM
(TODAY AND TONIGHT)
ISSUED AT 1212 AM PDT WED MAR 12 2025
PREFRONTAL RAIN OVER THE COASTAL WATERS IS JUST NOW GETTING INTO
RADAR RANGE. THIS FIRST ROUND WILL ARRIVE IN THE NORTH BAY BAY
AROUND MID TO LATE MORNING AND CONTINUE SLIDING SOUTH THROUGH THE
AFTERNOON. STRONG SOUTHERLY WINDS IN THE WARM SECTOR WILL
COINCIDE WITH THE MAIN RAIN BAND TODAY, MAKING FOR NASTY
CONDITIONS OUTSIDE. THE WIND HAS TRENDED STRONGER ON THE LATEST
HIGH RESOLUTION GUIDANCE. THE OFFICIAL FORECAST LEANS HEAVILY ON
THE 12/00Z LOCAL WRF, AND BRINGS A SEVERAL HOURS OF 30-55 MPH
GUSTS TO THE ENTIRE BAY AREA AND CENTRAL COAST. AS SUCH, WE'VE
ISSUED A WIND ADVISORY FROM 8 AM THROUGH 8 PM TODAY. THE RAIN
FORECAST HAS CHANGED SLIGHTLY WITH A LATER ARRIVAL, BUT OVERALL
SIMILAR ACCUMULATION AS THE PREVIOUS FEW FORECASTS. COASTAL AREAS
WILL SEE 1-1.5", WITH 2-4" IN THE COASTAL MOUNTAINS. INLAND
VALLEYS WILL SEE ANYWHERE FROM 0.5-1", WITH 1-2" IN THE INLAND
MOUNTAINS. AFTER THE COLD FRONT PUSHES THROUGH THIS
AFTERNOON/EVENING, COLD AIR ALOFT WILL SETTLE IN, CREATING AN
UNSTABLE ATMOSPHERE. THIS WILL CHANGE THE PRECIPITATION MODE FROM
STEADY STRATIFORM RAIN TO CONVECTIVE SHOWERS. MOST OF THE
PRECIPITATION WILL FALL AS RAIN TODAY, WITH HIGHER ELEVATIONS
SWITCHING TO SNOW OVERNIGHT AFTER THE COLD AIR STARTS TO SETTLE IN
BEHIND THE FRONT. THERE IS GOOD AGREEMENT THAT THE 500 MB HEIGHT
WILL DROP TO AROUND 536-538 DM BY THURSDAY MORNING. WHILE THAT'S
ENOUGH TO BRING SOME HIGH ELEVATION SNOW, IT'S NOT QUITE GETTING
COLD ENOUGH FOR ANYTHING EXTREME. FOR REFERENCE, THE LAST TIME
HIGHWAY 17 WAS SHUT DOWN DUE TO SNOW (2/24/23), THE 500 MB HEIGHT
WAS 533 DM. THE LAST TIME IT SNOWED IN SAN FRANCISCO (2/5/76), THE
500 MB HEIGHT WAS 528 DM. THE BIGGEST CONCERN FOR IMPACTS TODAY
IS THE COMBINATION OF WIND AND RAIN, WHICH COULD LEAD TO SOME
DOWNED TREES AND RELATED IMPACTS TO ROADWAYS AND POWER GRIDS. WE
ARE NOT EXPECTING THE LARGE RIVERS TO FLOOD, BUT URBAN AND SMALL
STREAM FLOODING, AS WELL AS SHALLOW LANDSLIDES, ARE POSSIBLE AS
THE NARROW BAND OF HIGH INTENSITY RAIN PUSHES THROUGH TODAY.
LONG TERM
(THURSDAY THROUGH TUESDAY)
ISSUED AT 1212 AM PDT WED MAR 12 2025
THURSDAY LOOKS LIKE THE BEST DAY FOR THUNDERSTORM CHANCES, BUT
IT'S NOT A SLAM DUNK. AS THE COLD AIR CONTINUES TO RUSH IN
THROUGH THE MORNING, SUN BREAKS WILL ALLOW THE SURFACE TO WARM
MODERATELY. THIS COMBINATION OF COLD OVER WARM IS UNSTABLE. THE
WARM AIR NEAR THE SURFACE WILL RISE, EXPAND AND COOL TO THE DEW
POINT, AND CONDENSE OUT CLOUD DROPLETS. WITH RELATIVELY LOW
HUMIDITY AT THE SURFACE THURSDAY, THE CLOUD BASES WILL BE HIGHER
THAN NORMAL, AND THIS CLOUD FORMATION PROCESS WILL HAPPEN MOSTLY
ABOVE THE FREEZING LEVEL. THAT'S IMPORTANT BECAUSE IT MEANS THE
CLOUDS WILL CONTAIN A SIGNIFICANT AMOUNT OF ICE. ICE OBVIOUSLY CAN
MEAN HAIL, BUT IT'S ALSO A NECESSARY INGREDIENT FOR THUNDERSTORM
DEVELOPMENT. IT'S THE STATIC BUILDUP BETWEEN COLLIDING ICE
PARTICLES THAT BUILDS THE ELECTRICAL CHARGE NECESSARY FOR
LIGHTNING STRIKES. THE COLD AIR ALOFT AND STEEP LAPSE RATES (AS
HIGH AS 9 C/KM IN THE AFTERNOON) ARE FAVORABLE, BUT THE MOISTURE
CONTENT IS RELATIVELY LOW AND DECREASING IN THE POST-FRONTAL
ENVIRONMENT (PW AROUND 0.35"). THIS COULD BE THE LIMITING FACTOR
FOR THUNDERSTORM DEVELOPMENT AND MAY SKEW ANY ACTIVITY EARLIER IN
THE DAY WHEN THE HUMIDITY IS STILL ELEVATED FROM WEDNESDAY'S RAIN.
THERE LOOKS TO BE A BRIEF LULL IN SHOWERS AS THE DRY AIR FILTERS
IN LATE THURSDAY. IT WON'T LAST LONG, HOWEVER, AS THE NEXT SYSTEM
COMES ROLLING IN FROM THE EASTERN PACIFIC FRIDAY MORNING. THIS
WILL BRING ANOTHER ROUND OF STRATIFORM RAIN, BUT THE IMPACTS LOOK
LESS THAN THE FIRST SYSTEM. THE THIRD, AND WEAKEST, SYSTEM WILL
BRING ANOTHER ROUND OF RAIN MONDAY. THE REST OF THE WEEK IS MORE
COMPLICATED, WITH A LOT OF UNCERTAINTY BEYOND NEXT MID WEEK BUT
THE PATTERN WILL LIKELY REMAIN ACTIVE.
AVIATION
(18Z TAFS)
ISSUED AT 1156 AM PDT WED MAR 12 2025
A COLD FRONT IS BUILDING INTO THE REGION. PREFRONTAL SHOWERS AND
BREEZY SOUTH TO SOUTHEAST WINDS HAVE SPREAD ACROSS THE REGION WITH
SOME SPOTTY MVFR CIGS. THE FRONT WILL MOVE IN FROM THE NORTHWEST AND
SPREAD SOUTHWEST THROUGH THE AFTERNOON. THIS MEANS STS WILL BE THE
FIRST TO SEE THE MORE MODERATE TO AT TIMES HEAVIER RAINFALL TO
ARRIVING ALONG THE MAIN RAIN BAND, WHICH AFTERNOON FOR AREAS TO THE
SOUTH. IN ADDITION TO THE INCREASED RAIN RATES, STRONG AND GUSTY
SOUTHERLY WINDS WILL FLOW ALONG THE MAIN RAIN BAND. WINDS REDUCE BUT
REMAIN BREEZY TO GUSTY BEHIND THE FRONT WITH SCATTERED SHOWER
ACTIVITY THROUGH THE NIGHT. SPOTTY THUNDERSTORM CHANCES WILL BE
POSSIBLE OVERNIGHT AND INTO THURSDAY. THE MAIN AREA OF THUNDERSTORM
RISK WILL BE OVER THE MARINE ENVIRONMENT, WITH ONLY SLIGHT CHANCES
OVERLAND. WINDS SHIFT AND BECOME MORE WESTERLY AND NORTHWESTERLY
INTO EARLY THURSDAY, WITH SHOWER AND STORM CHANCES EXITING THAT
AFTERNOON.
VICINITY OF SFO...MVFR CIGS AND SPOTTY SHOWERS ARE BUILDING INTO THE
SF BAY. EXPECT INCREASED RAIN RATES ALONG WITH STRONGER AND GUSTIER
WINDS AS THE MAIN RAIN BAND ARRIVES IN THE MID TO LATE AFTERNOON
AFTERNOON. WINDS TURN SLIGHTLY MORE SOUTHWEST IN THE WAKE OF THE
MAIN BAND, BUT REMAIN STRONG, HOWEVER RAIN RATES WILL REDUCE. EXPECT
SCATTERED SHOWERS THROUGH THE NIGHT WITH SLIGHT CHANCES FOR ISOLATED
THUNDERSTORMS. WINDS TURN MORE WESTERLY IN THE LATE NIGHT. SHOWER
AND STORM CHANCES TO EXIT BY THE END OF THE TAF PERIOD.
SFO BRIDGE APPROACH...SIMILAR TO SFO.
MONTEREY BAY TERMINALS...SPOTTY PRE-FRONTAL SHOWERS ARE MOVING
THROUGH THE AREA ALONG WITH BUILDING MID-LEVEL CLOUDS. BREEZY
SOUTHERLY WINDS BECOME STRONGER AND GUSTIER AS THE MAIN RAIN BAND
ARRIVE IN THE MID TO LATE AFTERNOON. RAIN RATES WILL BE MODERATE
TO, AT TIMES, HEAVY, LEADING TO REDUCED VISIBILITY. BEHIND THE
FRONT, WINDS TURN SOUTHWEST, WITH LINGERING SHOWER ACTIVITY EXPECTED
THROUGH THE NIGHT. CHANCES FOR ISOLATED STORMS INCREASE OVERNIGHT
OFF THE COAST AND AROUND THE MONTEREY BAY WITH AROUND A 20% CHANCE
AT THE TERMINALS EARLY THURSDAY. EXPECT WINDS TO TURN NORTHWEST MID-
THURSDAY MORNING WITH RAIN AND STORM CHANCES EXITING SLIGHTLY BEYOND
THE TAF PERIOD.
MARINE
(TODAY THROUGH MONDAY)
ISSUED AT 1156 AM PDT WED MAR 12 2025
WIDESPREAD GALE WARNINGS ARE IN EFFECT THROUGH THE DAY. ROUGH
SEAS WILL BUILD THROUGH THE DAY. OVERNIGHT, THERE IS A 30 PERCENT
CHANCE FOR THUNDERSTORMS AND ASSOCIATED STRONG GUSTS. SEAS WILL
REMAIN ROUGH INTO NEXT WEEK AS A SERIES OF STORMS MOVES THROUGH.
BEACHES
ISSUED AT 1156 AM PDT WED MAR 12 2025
HAZARDOUS BEACH CONDITIONS ARE EXPECTED WEDNESDAY AND THURSDAY
AS MODERATE PERIOD WESTERLY SWELL BUILDS TO SUPPORT BREAKING
WAVES UP TO 15 FEET. ADDITIONAL HAZARDOUS SURF WILL BE POSSIBLE
AGAIN THIS SUNDAY. AS ALWAYS, PLEASE REMEMBER TO STAY OFF OF
ROCKS AND JETTIES, KEEP PETS ON A LEASH, AND NEVER TURN YOUR BACK
ON THE OCEAN!
MTR WATCHES/WARNINGS/ADVISORIES
CA...WIND ADVISORY UNTIL 8 PM PDT THIS EVENING FOR CAZ006-502>506-
508>510-512>518-528>530.
PZ...GALE WARNING UNTIL 9 PM PDT THIS EVENING FOR MRY BAY-PIGEON PT
TO PT PINOS 0-10 NM-PIGEON PT TO PT PINOS 10-60 NM-PT ARENA
TO PT REYES 10-60 NM-PT PINOS TO PT PIEDRAS BLANCAS 0-10 NM-
SF BAY N OF BAY BRIDGE.
GALE WARNING UNTIL 3 AM PDT THURSDAY FOR PT ARENA TO PT REYES 0-
10 NM-PT REYES TO PIGEON PT 0-10 NM.
SHORT TERM...FLYNN
LONG TERM....FLYNN
AVIATION...MURDOCK
MARINE...MURDOCK
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