099
FXUS66 KMTR 201559
AFDMTR
AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SAN FRANCISCO CA
759 AM PST WED NOV 20 2024
..NEW UPDATE
SYNOPSIS
ISSUED AT 157 AM PST WED NOV 20 2024
A STRONG ATMOSPHERIC RIVER HAS ARRIVED IN THE NORTH BAY AND WILL
BRING SIGNIFICANT IMPACTS TO THE REGION OVER THE NEXT SEVERAL DAYS.
KEY MESSAGES:
-FLOOD WATCH IS EFFECT FOR THE NORTH BAY THROUGH SATURDAY MORNING
-5-12" OF RAINFALL IN THE NORTH BAY WED-SUN
-1-6" OF RAINFALL SOUTH OF THE GOLDEN GATE, WITH THE HEAVIEST RAIN
EXPECTED ON FRIDAY
-WIND ADVISORY IN EFFECT FOR THE NORTH BAY, SAN FRANCISCO BAYSHORE,
AND PACIFIC COAST THROUGH EARLY THU FOR GUSTS UP TO 45-50 MPH
-A HIGH SURF ADVISORY IS IN EFFECT FROM WEDNESDAY MORNING THROUGH
FRIDAY MORNING FOR BREAKING WAVES OF 14-22 FT.
UPDATE
ISSUED AT 758 AM PST WED NOV 20 2024
THE LOW PRESSURE SYSTEM CONTINUES TO BRING SHOWERS MOSTLY OVER THE
NORTH BAY. RAINFALL TOTALS FOR THE LAST 3 HOURS RANGES FROM 0.20 TO
1.2 WITH HIGHER TERRAINS SEEING 1.6 INCHES! WITH THE EXPECTED
CONTINUOUS RAINFALL THROUGH THE AFTERNOON, A FLOOD ADVISORY HAS BEEN
ISSUED FOR AREAS IN CENTRAL SONOMA COUNTY, ESPECIALLY NEAR CAZADERO
AND GUERNEVILLE UNTIL 1:30 PM PST. SOUTHERLY WINDS CONTINUE TO
INCREASE WITH SOME OF THE MOUNTAIN TERRAINS SEEING UP TO 52 MPH THIS
MORNING. TEMPERATURES ALSO CONTINUE TO RISE WITH THE 24 HOUR
DIFFERENCE SHOWING A 9-14 DEGREES INCREASE. OTHERWISE, FORECAST
REMAINS ON TRACK AS OF NOW, AS WE CONTINUE TO MONITOR OUR RADAR FOR
ANY ADDITIONAL UPDATES TO PRODUCTS AND FORECAST.
SO
SHORT TERM
(TODAY AND TONIGHT)
ISSUED AT 157 AM PST WED NOV 20 2024
THE BOMB CYCLONE OFF THE COAST OF WASHINGTON REACHED IT'S MINIMUM
ANALYZED CENTRAL PRESSURE OF 942 MB ON THE WEATHER PREDICTION CENTER
ANALYSIS AT 20/03Z. THIS TIES THE LOWEST ANALYZED PRESSURE IN THE NE
PACIFIC OUTSIDE OF A HURRICANE (942 MB LAST ANALYZED ON OCTOBER 24,
2021). INTERESTINGLY THE PREVIOUS 942 CYCLONE WAS IN A NEARLY
IDENTICAL LOCATION AND THE SYNOPTIC SET UP WAS VERY SIMILAR. THE
BIGGEST DIFFERENCE WAS THAT THE LAST ONE HAPPENED IN OCTOBER. FOR
WHAT IT'S WORTH, SANTA ROSA RECORDED 7.83" OF RAIN THAT DAY, THE ALL
TIME RECORD (BY A 2+" MARGIN) GOING BACK TO 1902. THIS LED TO SWIFT
WATER RESCUES, EVACUATIONS, AND EXTENSIVE URBAN FLOODING. THIS
MATURE, STRONG LOW PRESSURE SYSTEM IS OCCLUDED, WITH A TRIPLE POINT
OFF THE COAST OF THE CA/OR BORDER, A WARM FRONT EXTENDING OFF THE
COAST OF N. CALIFORNIA, AND A COLD FRONT EXTENDING SE TOWARDS THE
CENTRAL SUBTROPICAL PACIFIC.
THE RADAR IS SHOWING LIGHT TO MODERATE STRATIFORM RAIN ACROSS THE NW
CORNER OF SONOMA COUNTY. THE RAIN LINE HASN'T PUSHED ANY FARTHER
SOUTH OVER THE LAST SEVERAL HOURS, AND SANTA ROSA HAS YET TO REPORT
RAIN AS OF 1 AM. THIS INITIAL BOUT OF RAIN IS ASSOCIATED WITH A WARM
FRONT CURRENTLY ALONG THE COAST WHICH WILL SLOWLY PUSH INLAND
THROUGH THE DAY. BEHIND THE WARM FRONT, PERSISTENT SE WINDS IN THE
WARM SECTOR WILL CONTINUE TO PUMP SUBTROPICAL MOISTURE TO THE BAY
AREA THROUGH THE NEXT 48 HOURS.
STRONG SOUTHERLY WINDS ARE BEING REPORTED ACROSS HIGHER ELEVATIONS
NORTH OF THE GOLDEN GATE. NEAR MT. ST. HELENA GALE FORCE WINDS
(37G48KT) ARE BEING REPORTED AT 3,200 FT. AS THE SUN COMES UP, THE
RAIN COOLED AIR WILL HELP STRONGER WINDS MIX DOWN TO THE SURFACE. A
WIND ADVISORY REMAINS IN EFFECT THROUGH 6 AM THURSDAY. THE
COMBINATION OF HEAVY RAIN AND STRONG WINDS WILL LIKELY RESULT IN
TREE DAMAGE AND RELATED IMPACTS.
LONG TERM
(THURSDAY THROUGH TUESDAY)
ISSUED AT 157 AM PST WED NOV 20 2024
ON THURSDAY, THE PARENT LOW PRESSURE SYSTEM WILL GRADUALLY FILL AND
MEANDER NORTH AS THE JET STREAM UNDERCUTS THE OLD CYCLONE. THIS WILL
ALLOW A NEW LOW PRESSURE TO FORM AT THE TRIPLE POINT BETWEEN THE
OCCLUDED, COLD, AND WARM FRONTS. THERE MAY BE A SLIGHT LULL IN
THE INTENSITY ON THURSDAY AS THIS PROCESS UNFOLDS. BY FRIDAY, THE
NEW, RAPIDLY DEEPENING CYCLONE WILL TAKE OVER AND GIVE THE COLD
FRONT THE ENCOURAGEMENT IT NEEDS TO FINALLY MAKE LANDFALL BY
FRIDAY. FRONTAL PASSAGE WILL BRING ANOTHER ROUND OF HEAVIER RAIN,
BUT AS IT PASSES, WILL END THE ATMOSPHERIC RIVER. THAT DOESN'T
MEAN THE RAIN WILL END. INSTABILITY WILL INCREASE IN THE NEW COLD
AIR MASS, AND THE RAIN WILL SHIFT FROM STEADY, STRATIFORM RAIN TO
SHOWERS AND POTENTIAL THUNDERSTORMS LATE FRIDAY THROUGH SATURDAY.
RAIN CHANCES, WHILE MUCH LIGHTER, WILL CONTINUE TO LINGER THROUGH
EARLY NEXT WEEK.
HOW MUCH RAIN ARE WE EXPECTING? THE FORECAST FOR THE INITIAL PUSH
TODAY HASN'T CHANGED MUCH, BUT THERE HAS BEEN AN INCREASE IN THE
EXPECTED RAIN LATER IN THE WEEK. THIS CHANGE IN THE NBM SEEMS TO BE
DOMINATED BY THE CANADIAN MODEL, WHICH IS BY FAR THE MOST BULLISH.
EVERY MEMBER OF THE 00Z CMC ENSEMBLE HAS AT LEAST 10" AT SANTA ROSA
BY SATURDAY. ALL 20 MEMBERS. THAT'S LIKELY A SEVERE UNDER
DISPERSION, AND DESPITE THE TIGHT SPREAD, DOESN'T ACTUALLY
INCREASE THIS FORECASTER'S CONFIDENCE. HERE'S WHAT WE KNOW, THIS
WILL BE A MAJOR EVENT FOR THE NORTH BAY. THERE IS NOW A 90% CHANCE
THAT SANTA ROSA WILL RECORD MORE THAN 5" AND A 10% CHANCE OF
EXCEEDING 12" BY THE END OF THE WEEKEND. THE NORMAL FOR THE ENTIRE
MONTH OF NOVEMBER IS 3.25". FOR SAN FRANCISCO, THERE IS A 90%
CHANCE OF EXCEEDING 0.75" AND A 10% CHANCE OF EXCEEDING 6". I KNOW
THAT'S A BIG SPREAD, BUT WITH THE SHARP, NEARLY STATIONARY
GRADIENT BETWEEN HEAVY RAIN AND MUCH LIGHTER RAIN, THE UNCERTAINTY
IS PARTICULARLY HIGH FOR SAN FRANCISCO. FOR SAN JOSE, THERE IS A
90% CHANCE OF EXCEEDING 0.5" AND A 10% CHANCE OF EXCEEDING 4".
WHILE THIS PARAGRAPH COVERS THE HEAVIEST RAIN, WHICH WILL BE DONE
BY THIS WEEKEND, THE CHANCE FOR LIGHTER RAIN DOESN'T GO AWAY UNTIL
WEDNESDAY. SINCE THIS IS STILL EARLY IN THE WATER YEAR, THE LARGE
RIVERS FORTUNATELY HAVE A LOT OF CAPACITY AND ARE NOT EXPECTED TO
FLOOD. FLOODING OF SMALLER CREEKS AND STREAMS, AS WELL AS URBAN
AND ROADWAY FLOODING, ARE ALL EXPECTED AND CAN BRING SIGNIFICANT
IMPACTS.
AVIATION
(12Z TAFS)
ISSUED AT 411 AM PST WED NOV 20 2024
STRONG ATMOSPHERIC RIVER UNDERWAY ACROSS THE NORTH BAY. S WINDS
EXPECTED TO INCREASE THROUGH TODAY, PEAKING THIS AFTERNOON.
TERMINALS NORTH OF THE SF BAY CAN EXPECT GUSTS 25-30 KTS, WHILE
COASTAL TERMINALS IN THE AREA, SUCH AS KHAF, CAN EXPECT GUSTS TO
40 KTS. RAINFALL THROUGH THE TAF PERIOD WILL REMAIN MOSTLY IN THE
NORTH BAY TERMINALS AS WELL, WITH AN EXTENDED PERIOD OF -RA/RA
PRETTY MUCH THROUGHOUT THE TAF PERIOD. LOWER CONFIDENCE TERMINALS
AROUND THE BAY AREA SINCE THEY'LL BE ON THE EDGE OF THE HEAVIER
PRECIP. VCSH IN BAY AREA TAFS CONVEYS THIS UNCERTAINTY.
VICINITY OF SFO...VERY WINDY TODAY AS S WINDS ARE EXPECTED TO GUST
TO 40 KTS THIS AFTERNOON, TAPERING OFF GRADUALLY THROUGH THIS
EVENING. KEPT -RA IN TAFS GIVEN THE POSSIBILITY OF SOME PERIODS OF
MORE PERSISTENT SHOWERS TODAY. HOWEVER, UNCERTAINTY REGARDING
PRECIP IS HIGH (CONFIDENCE LOW) AT KSFO THROUGH THIS TAF PERIOD.
ANY RAIN THAT DOES OCCUR WILL LIKELY BEGIN TO TAPER OFF AT THE END
OF THIS 30 HR TAF PERIOD (~18Z THU) BEFORE BECOMING HEAVY AGAIN
FRIDAY.
SFO BRIDGE APPROACH...SIMILAR TO SFO.
MONTEREY BAY TERMINALS...SHOULD REMAIN MOSTLY DRY TODAY AS MOST
PRECIP WILL BE WELL NORTH. HOWEVER, VERY SLIGHT CHANCE OF VCSH
THIS EVENING, BUT NOT ENOUGH CONFIDENCE TO INCLUDE IN TAF.
OTHERWISE, SOUTHERLY WINDS INCREASE THIS AFTERNOON WITH LOWERING
CIGS THROUGH THE PERIOD.
MARINE
(TODAY THROUGH MONDAY)
ISSUED AT 411 AM PST WED NOV 20 2024
A HURRICANE FORCE STORM OFF THE COAST OF THE PACIFIC NORTHWEST
HAS BEGUN TO SLOWLY WEAKEN, A PROCESS THAT WILL CONTINUE FOR THE
NEXT COUPLE DAYS. STRONG GALES CONTINUE IN THE NORTHERN WATERS, ON
A GRADUAL WEAKENING TREND. VERY ROUGH SEAS UP TO 20 FEET IN THE
NORTHERN ZONES ARE PROPAGATING SOUTH THROUGH THE COASTAL WATERS.
THESE WILL BEGIN TO ABATE THURSDAY AND CONTINUE ON THAT TREND
THROUGH THE WEEKEND. DESPITE THE GRADUAL IMPROVEMENT, CONDITIONS
WILL REMAIN INCREDIBLY DANGEROUS TO ALL BUT THE LARGEST VESSELS
OVER THE NEXT 48 HOURS.
MTR WATCHES/WARNINGS/ADVISORIES
CA...WIND ADVISORY UNTIL 6 AM PST THURSDAY FOR CAZ006-502>506-508-509.
HIGH SURF ADVISORY UNTIL 6 AM PST FRIDAY FOR CAZ006-505-509-530.
FLOOD WATCH THROUGH LATE FRIDAY NIGHT FOR CAZ502>506.
BEACH HAZARDS STATEMENT THROUGH LATE THURSDAY NIGHT FOR CAZ529.
PZ...GALE WARNING UNTIL 3 PM PST THIS AFTERNOON FOR SF BAY N OF BAY
BRIDGE.
SMALL CRAFT ADVISORY UNTIL 3 AM PST THURSDAY FOR MRY BAY.
GALE WARNING UNTIL 9 AM PST THURSDAY FOR PT ARENA TO PT REYES 0-
10 NM.
GALE WARNING UNTIL 3 AM PST THURSDAY FOR PT ARENA TO PT REYES 10-
60 NM-PT REYES TO PIGEON PT 0-10 NM.
SMALL CRAFT ADVISORY UNTIL 9 AM PST THIS MORNING FOR PIGEON PT
TO PT PINOS 0-10 NM-PIGEON PT TO PT PINOS 10-60 NM.
GALE WARNING UNTIL 3 AM PST THURSDAY FOR PIGEON PT TO PT PINOS 0-
10 NM-PIGEON PT TO PT PINOS 10-60 NM.
SMALL CRAFT ADVISORY UNTIL 9 PM PST THURSDAY FOR PT PINOS TO PT
PIEDRAS BLANCAS 0-10 NM.
SHORT TERM...FLYNN
LONG TERM....FLYNN
AVIATION...BEHRINGER
MARINE...FLYNN
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