705
FXUS66 KMTR 181752
AFDMTR
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SAN FRANCISCO CA
952 AM PST MON NOV 18 2024
..NEW UPDATE, AVIATION, MARINE
SYNOPSIS
ISSUED AT 231 AM PST MON NOV 18 2024
CHILLY AND QUIET MONDAY AND TUESDAY. STRONG ATMOSPHERIC RIVER TO
BEGIN IMPACTING THE NORTH BAY BY WEDNESDAY WITH RAINFALL LASTING
INTO THE WEEKEND. POTENTIAL FOR FLOOD IMPACTS IN THE NORTH BAY AND
NORTHERN PORTIONS OF THE BAY AREA BY LATE WEEK. MORE WIDESPREAD
RAINFALL EXPECTED BY LATE FRIDAY INTO THE WEEKEND.
UPDATE
ISSUED AT 945 AM PST MON NOV 18 2024
GENERALLY CLEAR SKIES THIS MORNING WITH CHILLY CONDITIONS. MORNING
LOWS ACROSS THE INLAND VALLEYS RANGED FROM THE MID 30S ACROSS THE
NORTH BAY TO THE LOW TO MID 40S ELSEWHERE, WITH THE COASTAL
REGIONS SEEING LOWS IN THE MID TO UPPER 40S. TODAY IS A GOOD DAY
FOR PEOPLE IN THE NORTH BAY TO CLEAR OUT GUTTERS AND STORM DRAINS
TO MITIGATE ANY FLOODING CONCERNS FROM THE UPCOMING STORMS, WHICH
ARE COVERED IN DETAIL IN THE PREVIOUS DISCUSSION. NO UPDATES TO
THE FORECAST.
DIALH
SHORT TERM
(TODAY AND TONIGHT)
ISSUED AT 231 AM PST MON NOV 18 2024
COOL AND QUIET DAY TODAY AS RAIN SHOWERS MOVE OUT OF THE REGION.
VERY LIGHT AMOUNTS OBSERVED OVER THE LAST 24 HOURS. FAIRLY UNIFORM
DISTRIBUTION WITH MOST LOCATIONS WITHIN THE 0.02-0.08" RANGE. THE
FREEZE WATCH HAS BEEN UPGRADED TO A FREEZE WARNING FOR INTERIOR
PORTIONS OF THE NORTH BAY AND CENTRAL COAST, AND A FROST ADVISORY
HAS BEEN ISSUED FOR THE SANTA CRUZ COAST AND WATSONVILLE AREAS WHERE
TEMPS AS LOW AS 34 ARE EXPECTED TONIGHT.
LONG TERM
(TUESDAY THROUGH SUNDAY)
ISSUED AT 231 AM PST MON NOV 18 2024
KEY POINTS:
-IS THIS AN ATMOSPHERIC RIVER? ABSOLUTELY, YES!! A STRONG ONE, TOO.
-IS IT GOING TO STALL? YES. (SEE METEOROLOGY SECTION BELOW)
-3-7" OF RAINFALL IN THE NORTH BAY WED-SUN
-LOCALLY UP TO 11" IN THE FAR NORTH BAY COASTAL RANGES
-MUCH LESS RAIN FURTHER SOUTH (SEE IMPACTS SECTION BELOW)
TUESDAY IS A TRANSITION DAY AND THE LAST FULLY DRY DAY TO FINISH UP
PRE-STORM PREPARATIONS FOR AREAS AROUND THE NORTH BAY AND BAY AREA.
BY THIS TIME, A ROBUST 150 KT JET ROUNDS THE BASE OF THE UPPER LEVEL
TROUGH, PROMOTING THE VERY RAPID STRENGTHENING OF THE ASSOCIATED
SURFACE LOW AND THE INTENSIFICATION OF DEEP MOISTURE ADVECTION FROM
THE SUBTROPICS. GIVEN THIS QUICK EVOLUTION OF NOTABLE UPPER LEVEL
FEATURES, WE ARE EXPECTING THE SURFACE LOW STRENGTHEN TO THE TUNE OF
ABOUT 44 MB IN 24 HOURS. AS A REMINDER, THE CRITERIA FOR "BOMB
CYCLOGENESIS" WHERE THIS STORM WILL BE LOCATED (~42N LATITUDE) IS
ABOUT 18 MB IN 24 HOURS. THEREFORE, GUIDANCE SUGGESTS THIS STORM
WILL MORE THAN DOUBLE THE INTENSIFICATION NEEDED TO REFER TO IT AS A
BOMB CYCLONE. VERY IMPORTANT TO NOTE THAT AS FAR AS WE ARE
CONCERNED, THIS IS JUST AN INTERESTING FACT. CERTAINLY AN IMPRESSIVE
ONE THAT WE GEEK OUT OVER, NO DOUBT. HOWEVER, WE ARE FOCUSED ON THE
IMPACTS, AND GETTING CAUGHT UP ON WHETHER A STORM IS A "BOMB
CYCLONE" OR NOT DOES NOT EFFECTIVELY COMMUNICATE THE ASSOCIATED
IMPACTS TO LIFE AND PROPERTY. WITH THAT OUT OF THE WAY, LET'S DIVE
INTO IT.
METEOROLOGY AND TIMELINE:
BY EARLY WEDNESDAY MORNING, RAINFALL WILL HAVE BEGUN OVER PARTS OF
THE NORTH BAY, INTENSIFYING AND GRADUALLY NUDGING SOUTHWARD AS THE
DAY PROGRESSES. IMPRESSIVE SW FLOW IS PROGGED TO SET UP AND STALL
ALONG THE STAGNANT BAROCLINIC ZONE AIMED AT THE COAST BETWEEN EUREKA
AND BODEGA BAY. 50-70 KTS WITHIN THE 925-850 MB LAYER OVER THE SAME
LOCATION THROUGH MUCH OF THE DAY WEDNESDAY IS QUITE THE NUDGE
TOWARDS CALLING THIS A SIGNIFICANT STORM. THIS IS TRULY A STRONG
ATMOSPHERIC RIVER IN THE PUREST USE OF THE TERM. IN REGARD TO THE
WINDS, I DID BUMP WIND GUSTS UP TOWARDS THE 90TH PERCENTILE FOR
WEDNESDAY TO BE MORE IN LINE WITH 925 MB WINDS MIXING DOWN TOWARDS
THE SURFACE. GUSTS OF UP TO 40 MPH ARE POSSIBLE THROUGHOUT WEDNESDAY
ALONG THE SONOMA COUNTY COAST AND THE HIGH TERRAIN OF SONOMA AND
NAPA COUNTIES. GOING INTO THURSDAY, THE INITIAL SURFACE LOW WRAPS
NORTHWARD AROUND A NEWLY FORMED SURFACE LOW TO ITS SOUTHWEST THANKS
TO A NEW, EQUALLY IMPRESSIVE JET STREAK JUST DOWNSTREAM OF THE BASE
OF THE TROUGH. THIS NORTHWARD JOG OF THE INITIAL LOW WILL "PULL" THE
AXIS OF MOISTURE AND RESULTANT RAINFALL NORTHWARD A BIT. MEANWHILE,
THE NEWLY FORMED SURFACE LOW RAPIDLY DEEPENS AND WE UNDERGO ANOTHER
ROUND OF BOMB CYCLOGENESIS. BY THIS POINT, WE ARE IN THE THURSDAY
NIGHT/FRIDAY MORNING TIMEFRAME, WHEN THE UPPER LOW FINALLY BEGINS TO
TRANSLATE EASTWARD DUE TO THE OVERWHELMING INFLUENCE OF THE STRONG
DOWNSTREAM JET AND VERY LITTLE UPSTREAM SUPPORT. BASICALLY, THIS
MEANS THAT THE STORM SYSTEM WILL BEGIN TO BECOME ESSENTIALLY
"DISLODGED" FROM WHERE IT HAS STALLED, AND THE ASSOCIATED COLD FRONT
THEN MOVES DOWN THE COAST AND INLAND...MORE LIKE WHAT WE'RE USED TO
SEEING. THIS WILL RESULT IN A SECONDARY ROUND OF POTENTIALLY HEAVY
RAINFALL THAT WOULD AFFECT THE WHOLE REGION FROM NORCAL DOWN THROUGH
THE CENTRAL COAST FRIDAY INTO SATURDAY MORNING.
THE BIGGEST UNCERTAINTY WITH THIS SYSTEM IS HOW FAR SOUTH UPPER
LEVEL JET SETS UP. THE LATEST FORECAST HAS SHOWN A VERY SLIGHT SHIFT
TO THE SOUTH, BUT TRENDS ARE CONSISTENT ENOUGH TO EXHIBIT REASONABLE
CONFIDENCE IN THE FORECAST THUS FAR.
IMPACTS:
WEDNESDAY AND THURSDAY RAINFALL WILL LIKELY BE THE HEAVIEST FOR THE
NORTH BAY AND WILL SET THE STAGE FOR ANY FLOODING THAT MAY OCCUR
LATER IN THE EVENT. RAINFALL TOTALS OF 4-6" CAN BE EXPECTED BY
THURSDAY NIGHT IN THE NORTHERN HALF OF SONOMA AND NAPA COUNTIES,
WITH LOCALLY HIGHER AMOUNTS UP TO 7" IN THE COASTAL RANGES. A FAIRLY
QUICK TAPER TO THE SOUTH IS STILL EXPECTED SINCE THIS WILL BE A VERY
STAGNANT PERIOD FOR THE AR. AMOUNTS FALL OFF TO ABOUT 2-4" FOR AREAS
BETWEEN THE SAN PABLO BAY AND SANTA ROSA, 1-2" FROM THE SF BAY TO
BAYSIDE MARIN/SONOMA, 0.5-1.0" FROM SANTA CRUZ TO THE SF BAY, AND
GENERALLY 0.25" OR LESS SOUTH OF SANTA CRUZ WITH LITTLE TO NO
RAINFALL EXPECTED SOUTH OF MONTEREY/SALINAS. BEGINNING FRIDAY
MORNING THROUGH SUNDAY AFTERNOON, WE COULD SEE AN ADDITIONAL 2-3"
FOR THE NORTH BAY, 1-2.5" FOR THE SANTA CRUZ MTNS, BIG SUR COAST,
AND NORTHERN PORTIONS OF THE SF BAY AREA, INCLUDING THE CITY OF SF,
AND 0.5-1.0" ELSEWHERE. THE SILVER LINING FOR MOST OF OUR AREA IS
THAT THIS IS THE FIRST MAJOR STORM OF THE SEASON, AND SOILS SHOULD
BE ABLE TO HANDLE AT LEAST THE INITIAL RAINFALL. THE MORE WORRISOME
LOCATIONS WILL BE THOSE IN THE NORTH BAY THAT WILL LIKELY BECOME
SATURATED VERY QUICKLY. EVEN IF WE DON'T SEE TOO MANY FLOODING
IMPACTS WEDNESDAY, I WOULDN'T BE SURPRISED IF FLOODING GETS WORSE
FRIDAY WITH THE SECOND WAVE, EVEN THOUGH LESS RAIN IS EXPECTED
DURING THAT TIME. THIS IS VERY MUCH THE TYPE OF STORM WHERE MOST
FOLKS IN THE BAY AREA MAY BE UNAWARE OF ANY SEVERE WEATHER ONGOING,
BUT ASK THOSE IN THE NORTH BAY AND YOU'LL GET A DIFFERENT ANSWER.
PLEASE STAY TUNED TO THE FORECAST FOR THE LATEST INFORMATION AS THIS
POTENTIALLY SERIOUS SITUATION DEVELOPS.
AVIATION
(18Z TAFS)
ISSUED AT 945 AM PST MON NOV 18 2024
VFR CONDITIONS CONTINUE AT ALL TERMINALS THROUGH THE TAF PERIOD. THE
LOWER CONFIDENCE PORTION OF THE FORECAST WILL BE THE WIND. WHILE
THE WINDS WILL BREEZY AT TIMES THE DIRECTION WILL BE VARIABLE AND
SHIFT FROM NORTHERLY TO LIGHT DRAINAGE FLOW. ADDITIONALLY, BREEZY,
BUT NOT STRONG ENOUGH FOR LLWS OR AIRPORT WEATHER WARNING. WILL SEE
SOME MID-HIGH LEVEL CLOUDS STREAMING OVER THE REGION.
VICINITY OF SFO...VFR. NW WITH GUSTS UP TO 25 KT IN THE TAF. THERE
IS A 30-40% CHC OF WIND EXCEEDING 30 KT THIS AFTERNOON.
SFO BRIDGE APPROACH...SIMILAR TO SFO.
MONTEREY BAY TERMINALS...VFR.
MARINE
(TODAY THROUGH SATURDAY)
ISSUED AT 945 AM PST MON NOV 18 2024
CHAOTIC SEA STATE OF 10 TO 15 FEET WILL PEAK MONDAY BEFORE
GRADUALLY SUBSIDING TUESDAY. BREEZY NORTHERLY FLOW WILL PERSIST ON
MONDAY WITH LOCALLY STRONGER WINDS SOUTH OF POINT SUR. A POTENT
STORM SYSTEM DEEPENING IN THE NORTHERN PACIFIC WILL BRING STRONGER
SOUTHERLY WINDS, ELEVATED SEAS, RAIN, AND THUNDERSTORMS LATE THIS
WEEK.
MTR WATCHES/WARNINGS/ADVISORIES
CA...COASTAL FLOOD ADVISORY UNTIL 4 PM PST THIS AFTERNOON FOR CAZ006-
506-508.
FREEZE WARNING FROM MIDNIGHT TONIGHT TO 8 AM PST TUESDAY FOR
CAZ504-506-516>518-528.
FROST ADVISORY FROM MIDNIGHT TONIGHT TO 8 AM PST TUESDAY FOR
CAZ529.
PZ...SMALL CRAFT ADVISORY UNTIL 3 PM PST THIS AFTERNOON FOR SF BAY N
OF BAY BRIDGE.
SMALL CRAFT ADVISORY UNTIL 9 PM PST THIS EVENING FOR MRY BAY-
PIGEON PT TO PT PINOS 0-10 NM-PT PINOS TO PT PIEDRAS
BLANCAS 0-10 NM-PT REYES TO PIGEON PT 0-10 NM.
SMALL CRAFT ADVISORY UNTIL 3 AM PST WEDNESDAY FOR PT ARENA TO PT
REYES 0-10 NM.
SMALL CRAFT ADVISORY UNTIL 3 PM PST TUESDAY FOR PIGEON PT TO PT
PINOS 10-60 NM-PT ARENA TO PT REYES 10-60 NM.
GALE WARNING FROM 3 PM TUESDAY TO 3 AM PST WEDNESDAY FOR PT
ARENA TO PT REYES 10-60 NM.
SHORT TERM...BEHRINGER
LONG TERM....BEHRINGER
AVIATION...MM
MARINE...MM
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