304
FXUS66 KMTR 211158
AFDMTR
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SAN FRANCISCO CA
358 AM PST SAT DEC 21 2024
..NEW AVIATION, BEACHES
SYNOPSIS
ISSUED AT 250 AM PST SAT DEC 21 2024
A PARADE OF SYSTEMS WILL BRING COMPOUNDING IMPACTS TO THE BAY AREA
AND CENTRAL COAST. LIFE-THREATENING BAY, OCEAN, AND BEACH
CONDITIONS LINGER THROUGH NEXT WEEK.
SHORT TERM
(TODAY AND SUNDAY)
ISSUED AT 250 AM PST SAT DEC 21 2024
THE FIRST SHORTWAVE AND ITS ASSOCIATED SURFACE COLD FRONT WILL MOVE
THROUGH THE REGION THIS MORNING. RAINFALL TOTALS WILL BE AS HIGH AS
1.25" IN THE HIGHER TERRAIN OF THE NORTH BAY WITH AMOUNTS TAPERING
OFF SIGNIFICANTLY FROM NORTH TO SOUTH. SOUTHERLY WINDS WILL INCREASE
THROUGH THE MORNING AS THE COLD FRONT ENCROACHES THE AREA WITH
OCCASIONAL GUSTS UP TO 40 MPH LIKELY IN THE HIGHER TERRAIN AND
ALONG THE COAST, PARTICULARLY IN THE NORTH BAY. WHILE THIS FIRST
SYSTEM IS LARGELY FORECAST TO BE UNIMPACTFUL IN TERMS OF RAIN AND
WIND, IT WILL HAVE AN IMPACT ON WHAT TRANSPIRES WITH THE MORE
SIGNIFICANT SYSTEMS NEXT WEEK BY PROVIDING THE ANTECEDENT
CONDITIONS. THE REGION IS ALSO COMING OFF THE HEELS OF AN IMPACTFUL
STORM LAST WEEKEND, SO WHILE WINDS ARE NOT EXPECTED TO BE ANYWHERE
NEAR WHAT WAS EXPERIENCED LAST WEEKEND, SOILS APPROACHING SATURATION
WITH WINDS FROM AN ABNORMAL DIRECTION MAY ALLOW FOR TREES THAT WERE
SUFFICIENTLY DAMAGED LAST WEEKEND TO LOSE DAMAGED BRANCHES OR COME
DOWN ALTOGETHER.
SPC HAS HIGHLIGHTED THE NORTH BAY FOR GENERAL OR NON-SEVERE
THUNDERSTORMS THROUGH 4 AM SUNDAY. LET'S TAKE A DEEPER DIVE INTO THE
ECMWF POINT SOUNDING FORECAST FOR STS (CHARLES M. SCHULZ COUNTY
AIRPORT) AT 15Z (7 AM) THIS MORNING. THE THREE ESSENTIAL INGREDIENTS
FOR CONVECTION ARE LIFT, INSTABILITY, AND MOISTURE. THE PRIMARY
LIFTING MECHANISM WILL BE THE SURFACE COLD FRONT WITH THE SECONDARY
LIFTING MECHANISM BEING OROGRAPHIC LIFT. SURFACE BASED CAPE
(CONVECTIVE AVAILABLE POTENTIAL ENERGY) HARDLY RISES ABOVE 0 J/KG.
WHEN SURFACE BASED CAPE DOES NOT SHOW INSTABILITY WE LOOK AT MOST
UNSTABLE CAPE WHICH IN THIS CASE SHOWS 40 J/KG. MOISTURE WILL BE
PROVIDED FROM PRECEDING RAINFALL. ALL THAT BEING SAID, THE
ENVIRONMENT WILL BE THAT OF LOW CAPE AND HIGH SHEAR WHICH IS TYPICAL
FOR THE AREA.
A HIGH SURF ADVISORY WILL GO INTO EFFECT AT 6 AM SATURDAY AND RUN
THROUGH 4 PM SUNDAY WITH LIFE-THREATENING BAY, OCEAN, AND BEACH
CONDITIONS LINGERING INTO NEXT WEEK.
SUNDAY'S RAINFALL TOTALS WILL LARGELY BE THE PRODUCT OF WARM FRONTAL
PROCESSES. QPF (QUANTITATIVE PRECIPITATION FORECAST) HAS TRENDED
DOWN WITH THIS SECOND SHOT OF RAINFALL WITH TOTALS UP TO 0.75" IN
THE HIGHER TERRAIN OF THE NORTH BAY WITH NO WETTING RAINS (0.10"
OR GREATER) EXPECTED SOUTH OF THE GOLDEN GATE BRIDGE.
LONG TERM
(MONDAY THROUGH FRIDAY)
ISSUED AT 250 AM PST SAT DEC 21 2024
THE FIRST WIDESPREAD IMPACTFUL SYSTEM OF THE FORECAST PERIOD WILL
TAKE PLACE LATE-MORNING MONDAY THROUGH TUESDAY AFTERNOON. RAINFALL
TOTALS UP TO 2.50" CAN BE EXPECTED IN THE HIGHER TERRAIN OF THE
NORTH BAY WITH A GRADUAL TAPER FROM NORTH TO SOUTH. SOUTHERLY WINDS
WILL INCREASE WITH THE PASSAGE OF THE COLD FRONT WITH THE
PROBABILITY OF AT LEAST BRIEFLY REACHING WIND ADVISORY CRITERIA
(WIND GUSTS 45+ MPH) INCREASING TO 40%. THERE IS ALSO UP TO A 20%
CHANCE FOR THUNDERSTORMS WITH THIS SYSTEM. WPC HAS HIGHLIGHTED NAPA
AND SONOMA COUNTIES FOR A MARGINAL OR AT LEAST 5% RISK OF RAINFALL
EXCEEDING FLASH FLOOD GUIDANCE BETWEEN 4 AM MONDAY AND 4 AM TUESDAY.
THIS MEANS ISOLATED FLASH FLOODS ARE POSSIBLE AND THAT THEY WILL BE
LOCALIZED AND PRIMARILY AFFECTING PLACES THAT CAN EXPERIENCE RAPID
RUNOFF WITH HEAVY RAINFALL. AS THE RIVERS RISE SO WILL FLOODING
CONCERNS WITH FORECASTS SHOWING A RAPID RISE BEGINNING MONDAY NIGHT.
LIFE-THREATENING BAY, OCEAN, AND BEACH CONDITIONS WILL LINGER
THROUGH TUESDAY WITH A HIGH SURF WARNING LIKELY ISSUED ON THE HEELS
OF THE 6 AM SATURDAY THROUGH 4 PM SUNDAY HIGH SURF ADVISORY.
WEDNESDAY WILL SEE A BREAK IN THE RAIN, GIVING RIVERS AND THEIR
CREEKS AND STREAMS ABOUT 24 HOURS OR SO TO AT LEAST SOMEWHAT RECEDE.
THURSDAY AND FRIDAY WILL BRING THE SECOND WIDESPREAD IMPACTFUL
SYSTEM WITH RAINFALL TOTALS UP TO 3.50" IN THE HIGHER TERRAIN OF THE
NORTH BAY WITH A MORE ABRUPT TAPER FROM NORTH TO SOUTH WITH WPC
HIGHLIGHTING THE NORTH BAY FOR HEAVY PRECIPITATION THURSDAY AND
FRIDAY. IMPACTS FROM THIS SYSTEM WILL BE HEAVILY DEPENDENT ON WHAT
HAPPENS PRECEDING IT AND THE ANTECEDENT CONDITIONS. THE COMPOUNDING
EFFECTS OF SOILS APPROACHING SATURATION WILL MAKE IT MORE LIKELY
THAT TREES WILL FALL, MUD AND ROCK WILL SLIDE, AND CREEKS AND
STREAMS WILL RISE.
BELOW ARE MAINSTEM FORECAST POINTS AND THE ENSEMBLE FORECAST OF
EXCEEDING (ACTION/MONITOR STAGE), (MINOR FLOOD STAGE) BETWEEN MONDAY
AND NEXT SUNDAY.
SONOMA COUNTY:
RUSSIAN RIVER - GUERNEVILLE (GUEC1): 39%, 25%
RUSSIAN RIVER - HEALDSBURG (HEAC1): 9%, <2%
NAPA COUNTY:
NAPA RIVER - ST. HELENA (SHEC1): 25%, 16%
NAPA RIVER - NAPA (APCC1): 16%, 9%
AVIATION
(12Z TAFS)
ISSUED AT 357 AM PST SAT DEC 21 2024
MVFR/IFR CEILINGS AND/OR VISIBILITIES EXPECTED OVER THE BAY AREA
TERMINALS AND SHOULD PREVAIL THROUGH THE REMAINDER OF THE TAF
PERIOD, WITH THE EXCEPTION BEING KSJC WHICH IS FORECAST TO LOWER BY
EARLY THIS AFTERNOON. SOUTHERLY WINDS WILL INCREASE AHEAD OF AN
APPROACHING COLD FRONT THAT WILL BRING RAIN TO THE BAY AREA
TERMINALS BETWEEN NOW AND THIS AFTERNOON, THEN VCSH FOR KMRY AND
KSNS WHERE WE ARE NOT EXPECTING A STEADY RAIN. IN WAKE OF THE
FRONTAL PASSAGE, EXPECTING LOW CEILINGS TONIGHT INTO SUNDAY MORNING
ALONG WITH GENERALLY LIGHT WINDS.
VICINITY OF SFO...MVFR/IFR ARE EXPECTED TO PERSIST THROUGH THE TAF
PERIOD. PERIODS OF LIGHT RAIN CAN BE EXPECTED THIS MORNING AND
THROUGH THE EARLY AFTERNOON BEFORE DIMINISHING IN COVERAGE.
SFO BRIDGE APPROACH...SIMILAR TO SFO.
MONTEREY BAY TERMINALS...MVFR/IFR CEILINGS ARE EXPECTED TO PERSIST
THROUGH THE TAF PERIOD. EXPECT SE WINDS AT KMRY AND KSNS BY SUNRISE
THIS MORNING, THEN EASING AFTER SUNSET. LIGHT RAIN IS EXPECTED TO BE
IN THE AREA FROM LATE MORNING THROUGH AT LEAST EARLY EVENING. MAY
HAVE A FEW LINGERING SHOWERS EARLY SUNDAY MORNING WITH LIGHT AND
VARIABLE WINDS.
MARINE
(TODAY THROUGH THURSDAY)
ISSUED AT 250 AM PST SAT DEC 21 2024
SOUTHERLY WINDS HAVE BEGUN TO INCREASE OVER COASTAL WATER THIS
THIS MORNING AS A STORM SYSTEM CONTINUES TO MOVE CLOSER TO THE
CALIFORNIA COAST. ONCE THE STORM MOVES ONSHORE, IT WILL BE
FOLLOWED BY A COLD FRONT THAT BEGINS TO SWEEP THROUGH EARLY
MORNING BRINGING RAIN, ISOLATED THUNDERSTORMS, GUSTY WINDS, AND
BUILDING SWELL. EXPECT PERIODS OF GALE FORCE GUSTS NORTH OF POINT
REYES THIS MORNING, THEN SPREADING SOUTH OVER THE WATERS OFF OF
THE SF PENINSULA. WAVE HEIGHTS WILL REMAIN ELEVATED THROUGH LATE
NEXT WEEK WITH 15 TO 20 FOOT WAVES MONDAY THROUGH WEDNESDAY.
EXPECT DANGEROUS CONDITIONS FOR SMALL CRAFTS THROUGH THE WEEKEND
INTO THE BEGINNING OF NEXT WEEK. THE ELEVATED SEAS WILL CONTINUE
TO PRODUCE ENHANCED SHOALING AT HARBOR ENTRANCES AND BARS.
BEACHES
ISSUED AT 357 AM PST SAT DEC 21 2024
ELEVATED SURF OF 20 TO 26 FEET, WITH OCCASIONAL BREAKERS UP TO 30
FEET AT FAVORED SPOTS SUCH AS MAVERICKS, WILL OCCUR THIS MORNING TO
SUNDAY AFTERNOON AS ANOTHER SYSTEM APPROACHES THE WEST COAST, WITH A
HIGH SURF ADVISORY ISSUED FOR THE PACIFIC COAST FROM 6 AM SATURDAY
THROUGH 4 PM SUNDAY.
THEN, A VERY LARGE AND POWERFUL, LONG PERIOD WEST SWELL TRAIN WILL
LIKELY ARRIVE ALONG OUR ENTIRE COASTLINE MONDAY INTO TUESDAY NEXT
WEEK. CURRENT GLOBAL WAVE MODELS ARE FORECASTING SWELL HEIGHTS TO
20+ FEET WITH VERY ENERGETIC SWELL PERIODS OF 20+ SECONDS, THIS WILL
LEAD TO MUCH HIGHER BREAKERS INCLUDING LOCALIZED COASTAL FLOODING
IMPACTS. AS MENTIONED ABOVE, THIS WILL RESULT IN LIFE-THREATENING
BAY, OCEAN, AND BEACH CONDITIONS LINGERING INTO NEXT WEEK. STAY
TUNED FOR FURTHER UPDATES.
MTR WATCHES/WARNINGS/ADVISORIES
CA...HIGH SURF ADVISORY UNTIL 4 PM PST SUNDAY FOR CAZ006-505-509-529-
530.
PZ...SMALL CRAFT ADVISORY FROM 9 AM THIS MORNING TO 3 PM PST THIS
AFTERNOON FOR SF BAY N OF BAY BRIDGE.
SMALL CRAFT ADVISORY FROM 3 PM THIS AFTERNOON TO 3 AM PST MONDAY
FOR MRY BAY.
GALE WARNING UNTIL 9 AM PST THIS MORNING FOR PT ARENA TO PT
REYES 0-10 NM-PT ARENA TO PT REYES 10-60 NM.
SMALL CRAFT ADVISORY UNTIL 3 AM PST MONDAY FOR PIGEON PT TO PT
PINOS 0-10 NM-PIGEON PT TO PT PINOS 10-60 NM-PT PINOS TO PT
PIEDRAS BLANCAS 0-10 NM-PT REYES TO PIGEON PT 0-10 NM.
SHORT TERM...SARMENT
LONG TERM....SARMENT
AVIATION...RGASS
MARINE...RGASS
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