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Area Forecast Discussion

FXUS66 KMTR [Monterey CA] 072233 AFDMTR ∨

FXUS66 KMTR 072233
AFDMTR [HEAD]

National Weather Service San Francisco Bay Area 2:33pm PST Wed Dec 7 2022 ∨

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SAN FRANCISCO BAY AREA
233 PM PST WED DEC 7 2022 [HEAD]

Synopsis. Increasing high clouds ahead of the next front, but chilly morning temperatures expected Thursday morning. Chances for rain Thursday night into Friday morning, with highest amounts along the North Bay coastal ranges. Another system will bring even more rain again Saturday into Sunday. Cold morning temperatures are expected at the start of next week. ∨

.SYNOPSIS...INCREASING HIGH CLOUDS AHEAD OF THE NEXT FRONT, BUT
CHILLY MORNING TEMPERATURES EXPECTED THURSDAY MORNING. CHANCES FOR
RAIN THURSDAY NIGHT INTO FRIDAY MORNING, WITH HIGHEST AMOUNTS ALONG
THE NORTH BAY COASTAL RANGES. ANOTHER SYSTEM WILL BRING EVEN MORE
RAIN AGAIN SATURDAY INTO SUNDAY. COLD MORNING TEMPERATURES ARE
EXPECTED AT THE START OF NEXT WEEK. [DISC]

Discussion. As of 02:33pm PST Wednesday… satellite imagery shows clearing skies over the Bay Area and the Central Coast, with isolated fair-weather cumulus over hill tops that have bubbled up from the recent rains. Meanwhile, a long stretch of lower clouds of southwest to northeast orientation is about 100 miles west of Eureka and right along the Oregon coastline. Much colder clouds higher up in altitude are moving eastward toward the Pacific coastline. While these high clouds are associated with the leading edge of the next front, their first impact to the area will be to help insulate surface temperatures on Thursday morning. While cooler northwest advection is still assisting in making for temperatures in the 30s tomorrow morning, chances are increasing of keeping those temperatures in the mid to upper 30s, with only isolated, sheltered areas with the chance to approach freezing. Patchy frost may still develop, but confidence is low. ∨

.DISCUSSION...AS OF 02:33 PM PST WEDNESDAY...SATELLITE IMAGERY
SHOWS CLEARING SKIES OVER THE BAY AREA AND THE CENTRAL COAST, WITH
ISOLATED FAIR-WEATHER CUMULUS OVER HILL TOPS THAT HAVE BUBBLED UP
FROM THE RECENT RAINS. MEANWHILE, A LONG STRETCH OF LOWER CLOUDS
OF SOUTHWEST TO NORTHEAST ORIENTATION IS ABOUT 100 MILES WEST OF
EUREKA AND RIGHT ALONG THE OREGON COASTLINE. MUCH COLDER CLOUDS
HIGHER UP IN ALTITUDE ARE MOVING EASTWARD TOWARD THE PACIFIC
COASTLINE. WHILE THESE HIGH CLOUDS ARE ASSOCIATED WITH THE LEADING
EDGE OF THE NEXT FRONT, THEIR FIRST IMPACT TO THE AREA WILL BE TO
HELP INSULATE SURFACE TEMPERATURES ON THURSDAY MORNING. WHILE
COOLER NORTHWEST ADVECTION IS STILL ASSISTING IN MAKING FOR
TEMPERATURES IN THE 30S TOMORROW MORNING, CHANCES ARE INCREASING
OF KEEPING THOSE TEMPERATURES IN THE MID TO UPPER 30S, WITH ONLY
ISOLATED, SHELTERED AREAS WITH THE CHANCE TO APPROACH FREEZING.
PATCHY FROST MAY STILL DEVELOP, BUT CONFIDENCE IS LOW. [DISC]

By Thursday afternoon, the next frontal system will move southeastward through through the Bay Area and down the Central Coast. The Euro [European Center for Medium-Range Weather Forecasts model] and GFS [Global Forecast System: global spectral model used primarily for aviation weather forecasts; provides guidance out to 384 hours at 00, 06, 12 and 18UTC.] ensembles are not projecting the system to arrive until at or after 00Z [4pm PST], the most recent NAM [North American Meso (formerly Eta) model] model has sped it up so that it around just before 21Z [1pm PST] (1 PM local) on Thursday afternoon. Its no surprise that the numerical blend of models is leaning closer to just before 00Z [4pm PST]. Drivers should be prepared for this rain to take place during the evening commute. The Sonoma coastal ranges have the best chance for receiving the most rainfall where just under an inch as a 30% chance of occurring, where the majority of North Bay higher terrain has a 35–50% chance of receiving at least a half an inch. Meanwhile, the lower valleys on have a 30% chance for around a quarter of an inch. The San Mateo coastline has a 40% chance for a quarter of an inch, but most ensemble models really have the front start to disintegrate after passing over the Golden Gate Bridge as it interacts with drier air aloft. A tenth of an inch to quarter of an inch has a 20–30% chance of occurring along the Central Coast, places like the Santa Clara and Salinas Valleys may encounter the rain shadow effect that inhibits totals. All that being said, there is high confidence for rain Thursday night into Friday. Most models are trying to resolve the rain being done by the morning commute on Friday. ∨

BY THURSDAY AFTERNOON, THE NEXT FRONTAL SYSTEM WILL MOVE
SOUTHEASTWARD THROUGH THROUGH THE BAY AREA AND DOWN THE CENTRAL
COAST. THE EURO AND GFS ENSEMBLES ARE NOT PROJECTING THE SYSTEM TO
ARRIVE UNTIL AT OR AFTER 00Z, THE MOST RECENT NAM MODEL HAS SPED IT
UP SO THAT IT AROUND JUST BEFORE 21Z (1 PM LOCAL) ON THURSDAY
AFTERNOON. ITS NO SURPRISE THAT THE NUMERICAL BLEND OF MODELS IS
LEANING CLOSER TO JUST BEFORE 00Z. DRIVERS SHOULD BE PREPARED FOR
THIS RAIN TO TAKE PLACE DURING THE EVENING COMMUTE. THE SONOMA
COASTAL RANGES HAVE THE BEST CHANCE FOR RECEIVING THE MOST
RAINFALL WHERE JUST UNDER AN INCH AS A 30 PERCENT CHANCE OF
OCCURRING, WHERE THE MAJORITY OF NORTH BAY HIGHER TERRAIN HAS A 35
TO 50 PERCENT CHANCE OF RECEIVING AT LEAST A HALF AN INCH.
MEANWHILE, THE LOWER VALLEYS ON HAVE A 30 PERCENT CHANCE FOR
AROUND A QUARTER OF AN INCH. THE SAN MATEO COASTLINE HAS A 40
PERCENT CHANCE FOR A QUARTER OF AN INCH, BUT MOST ENSEMBLE MODELS
REALLY HAVE THE FRONT START TO DISINTEGRATE AFTER PASSING OVER THE
GOLDEN GATE BRIDGE AS IT INTERACTS WITH DRIER AIR ALOFT. A TENTH
OF AN INCH TO QUARTER OF AN INCH HAS A 20 TO 30 PERCENT CHANCE OF
OCCURRING ALONG THE CENTRAL COAST, PLACES LIKE THE SANTA CLARA AND
SALINAS VALLEYS MAY ENCOUNTER THE RAIN SHADOW EFFECT THAT
INHIBITS TOTALS. ALL THAT BEING SAID, THERE IS HIGH CONFIDENCE FOR
RAIN THURSDAY NIGHT INTO FRIDAY. MOST MODELS ARE TRYING TO
RESOLVE THE RAIN BEING DONE BY THE MORNING COMMUTE ON FRIDAY. [DISC]

Friday will have a brief respite with temperatures around seasonal normals before the next event over the weekend. Saturday returns the unsettled weather pattern as the next upper level low descends southward down the Pacific coastline featuring a 50–60% chance for tapping into precipitable water amounts greater than one inch. This translates to the Sonoma coastal mountains receiving a 40% chance for rainfall by Sunday morning of around two inches and most of the North Bay with a 40–80% chance of receiving at least an inch of rain by Sunday morning. While valley areas will experience some rain shadow lower amounts, confidence is increasing that most of the CWA [County Warning Area; see the LWP home page (link at bottom of page) for a CWA map] will get a healthy dose of rain. While a lot of this moisture extends out over the Pacific, deterministic and ensemble models as well as cluster analysis agrees that it will act as frontal in nature. Periods of moderate to heavy rain is possible, but it does not at this time act to slow down and become stationary. ∨

FRIDAY WILL HAVE A BRIEF RESPITE WITH TEMPERATURES AROUND SEASONAL
NORMALS BEFORE THE NEXT EVENT OVER THE WEEKEND. SATURDAY RETURNS THE
UNSETTLED WEATHER PATTERN AS THE NEXT UPPER LEVEL LOW DESCENDS
SOUTHWARD DOWN THE PACIFIC COASTLINE FEATURING A 50 TO 60 PERCENT
CHANCE FOR TAPPING INTO PRECIPITABLE WATER AMOUNTS GREATER THAN ONE
INCH. THIS TRANSLATES TO THE SONOMA COASTAL MOUNTAINS RECEIVING A 40
PERCENT CHANCE FOR RAINFALL BY SUNDAY MORNING OF AROUND TWO INCHES
AND MOST OF THE NORTH BAY WITH A 40 TO 80 PERCENT CHANCE OF
RECEIVING AT LEAST AN INCH OF RAIN BY SUNDAY MORNING. WHILE VALLEY
AREAS WILL EXPERIENCE SOME RAIN SHADOW LOWER AMOUNTS, CONFIDENCE IS
INCREASING THAT MOST OF THE CWA WILL GET A HEALTHY DOSE OF RAIN.
WHILE A LOT OF THIS MOISTURE EXTENDS OUT OVER THE PACIFIC,
DETERMINISTIC AND ENSEMBLE MODELS AS WELL AS CLUSTER ANALYSIS AGREES
THAT IT WILL ACT AS FRONTAL IN NATURE. PERIODS OF MODERATE TO HEAVY
RAIN IS POSSIBLE, BUT IT DOES NOT AT THIS TIME ACT TO SLOW DOWN AND
BECOME STATIONARY. [DISC]

This Saturday system does not just look to provide rain alone. Winds will also increase along the surface boundary. Higher elevations have a chance for gusts between 35–40 mph. The lower confidence is weather or not those winds will mix down to the surface to necessitate a Wind Advisory. For now, gusts between 15–20 mph is expected. Lastly, there is increasing confidence for elevated levels of CAPE [Convective Available Potential Energy. A measure of the amount of energy available for convection. Related to the maximum potential vertical speed within an updraft; thus, higher values indicate greater potential for severe weather. Observed values in thunderstorm environments often may exceed 1000 joules per kilogram (J/kg), and in extreme cases may exceed 5000 J/kg.] along the boundary as it moves southward. While the probability is only maxing out around 15%, the official forecast has added the slight chance for a thunderstorm out over the ocean waters west of Sonoma County as well as the immediate Sonoma coastline. So re-cap: all-in-all a dynamic system with the chance for strong winds, thunderstorms to the north, but plenty of rainfall for the Bay Area and the Central Coast. ∨

THIS SATURDAY SYSTEM DOES NOT JUST LOOK TO PROVIDE RAIN ALONE. WINDS
WILL ALSO INCREASE ALONG THE SURFACE BOUNDARY. HIGHER ELEVATIONS
HAVE A CHANCE FOR GUSTS BETWEEN 35 TO 40 MPH. THE LOWER CONFIDENCE
IS WEATHER OR NOT THOSE WINDS WILL MIX DOWN TO THE SURFACE TO
NECESSITATE A WIND ADVISORY. FOR NOW, GUSTS BETWEEN 15 TO 20 MPH
IS EXPECTED. LASTLY, THERE IS INCREASING CONFIDENCE FOR ELEVATED
LEVELS OF CAPE ALONG THE BOUNDARY AS IT MOVES SOUTHWARD. WHILE THE
PROBABILITY IS ONLY MAXING OUT AROUND 15 PERCENT, THE OFFICIAL
FORECAST HAS ADDED THE SLIGHT CHANCE FOR A THUNDERSTORM OUT OVER
THE OCEAN WATERS WEST OF SONOMA COUNTY AS WELL AS THE IMMEDIATE
SONOMA COASTLINE. SO RE-CAP: ALL-IN-ALL A DYNAMIC SYSTEM WITH THE
CHANCE FOR STRONG WINDS, THUNDERSTORMS TO THE NORTH, BUT PLENTY OF
RAINFALL FOR THE BAY AREA AND THE CENTRAL COAST. [DISC]

Beyond Sunday looks to be a quieter pattern from a precipitation perspective. However, cold air advection behind the weather system will likely create chilly morning low temperatures for the early part of next week. Many ensemble and deterministic models are suggesting a return to the upper 20s for many interior valley locations. A lot is still be determined, but confidence is increasing for a trend of drier and cooler, if not chillier, conditions. ∨

BEYOND SUNDAY LOOKS TO BE A QUIETER PATTERN FROM A PRECIPITATION
PERSPECTIVE. HOWEVER, COLD AIR ADVECTION BEHIND THE WEATHER SYSTEM
WILL LIKELY CREATE CHILLY MORNING LOW TEMPERATURES FOR THE EARLY
PART OF NEXT WEEK. MANY ENSEMBLE AND DETERMINISTIC MODELS ARE
SUGGESTING A RETURN TO THE UPPER 20S FOR MANY INTERIOR VALLEY
LOCATIONS. A LOT IS STILL BE DETERMINED, BUT CONFIDENCE IS
INCREASING FOR A TREND OF DRIER AND COOLER, IF NOT CHILLIER,
CONDITIONS. [DISC]

Aviation. As of 10:13am Wednesday… for the 18Z [10am PST] TAFs [Terminal Aerodrome Forecasts]. Satellite shows mostly clear skies over the region. Therefore, VFR [Visual Flight Rules] for most terminals except North Bay and KLVK [Livermore CA], where low visibility and low ceilings is expected to linger for another hour, as diurnal heating continues to take into effect. Ceilings begin to build overnight, allowing for MVFR [Marginal Visual Flight Rules] conditions, starting late tonight into Thursday morning. Moderate confidence for the timing of when rain will start for the North Bay and San Francisco Bay terminals, but expect showers near mid to late Thursday morning. Winds will remain light to moderate most of the TAF [Terminal Aerodrome Forecast] period and will turn to a more southerly component near late tonight to early tomorrow morning. ∨

.AVIATION...AS OF 10:13 AM WEDNESDAY...FOR THE 18Z TAFS. SATELLITE
SHOWS MOSTLY CLEAR SKIES OVER THE REGION. THEREFORE, VFR FOR MOST
TERMINALS EXCEPT NORTH BAY AND KLVK, WHERE LOW VISIBILITY AND LOW
CIGS IS EXPECTED TO LINGER FOR ANOTHER HOUR, AS DIURNAL HEATING
CONTINUES TO TAKE INTO EFFECT. CEILINGS BEGIN TO BUILD OVERNIGHT,
ALLOWING FOR MVFR CONDITIONS, STARTING LATE TONIGHT INTO THURSDAY
MORNING. MODERATE CONFIDENCE FOR THE TIMING OF WHEN RAIN WILL
START FOR THE NORTH BAY AND SAN FRANCISCO BAY TERMINALS, BUT
EXPECT SHOWERS NEAR MID TO LATE THURSDAY MORNING. WINDS WILL
REMAIN LIGHT TO MODERATE MOST OF THE TAF PERIOD AND WILL TURN TO
A MORE SOUTHERLY COMPONENT NEAR LATE TONIGHT TO EARLY TOMORROW
MORNING. [AVIA]

Vicinity of SFO [San Francisco CA]… VFR [Visual Flight Rules] for most of the TAF [Terminal Aerodrome Forecast] period with variable directed winds. Winds are expected to turn southerly near early tomorrow morning, but moderate confidence on timing. Also, moderate confidence that ceilings will get to MVFR [Marginal Visual Flight Rules] levels, as there is a chance ceilings may stay VFR [Visual Flight Rules] through the entire period. ∨

VICINITY OF SFO...VFR FOR MOST OF THE TAF PERIOD WITH VARIABLE
DIRECTED WINDS. WINDS ARE EXPECTED TO TURN SOUTHERLY NEAR EARLY
TOMORROW MORNING, BUT MODERATE CONFIDENCE ON TIMING. ALSO, MODERATE
CONFIDENCE THAT CIGS WILL GET TO MVFR LEVELS, AS THERE IS A CHANCE
CIGS MAY STAY VFR THROUGH THE ENTIRE PERIOD. [AVI2]

SFO [San Francisco CA] bridge approach… similar to SFO [San Francisco CA]. ∨

SFO BRIDGE APPROACH...SIMILAR TO SFO. [AVI2]

Monterey Bay terminals… VFR [Visual Flight Rules] for most of the TAF [Terminal Aerodrome Forecast] period with moderate confidence that it will become MVFR [Marginal Visual Flight Rules] tomorrow morning. High confidence that winds will flip to a southerly component overnight into tomorrow morning. ∨

MONTEREY BAY TERMINALS...VFR FOR MOST OF THE TAF PERIOD WITH
MODERATE CONFIDENCE THAT IT WILL BECOME MVFR TOMORROW MORNING. HIGH
CONFIDENCE THAT WINDS WILL FLIP TO A SOUTHERLY COMPONENT OVERNIGHT
INTO TOMORROW MORNING. [AVI2]

Marine. As of 02:29pm PST Wednesday… rain returns to the coastal waters and bays Thursday into early Friday morning. Another system will impact the coastal waters again Friday night through the weekend. Strong and gusty winds expected. Light northwest swell will continue through Thursday, followed by a larger, moderate period swell on Friday. ∨

.MARINE...AS OF 02:29 PM PST WEDNESDAY...RAIN RETURNS TO THE
COASTAL WATERS AND BAYS THURSDAY INTO EARLY FRIDAY MORNING.
ANOTHER SYSTEM WILL IMPACT THE COASTAL WATERS AGAIN FRIDAY NIGHT
THROUGH THE WEEKEND. STRONG AND GUSTY WINDS EXPECTED. LIGHT
NORTHWEST SWELL WILL CONTINUE THROUGH THURSDAY, FOLLOWED BY A
LARGER, MODERATE PERIOD SWELL ON FRIDAY. [AVIA]

MTR Watches/Warnings/Advisories

Tonight: Small Craft Advisory…Pt Arena [Mendocino CA] to Pigeon Pt [Pescadero CA] 10–60 nm [69 miles] from 3am ∨

.MTR WATCHES/WARNINGS/ADVISORIES...
.TNGT...SCA...PT ARENA TO PIGEON PT 10-60 NM FROM 3 AM [WARN]

Public Forecast: DK; Aviation: SO; Marine: CW ∨

PUBLIC FORECAST: DK
AVIATION: SO
MARINE: CW [CRED]

VISIT US AT WWW.WEATHER.GOV/SANFRANCISCO [LINK]

Follow us on Facebook, Twitter, and YouTube at: www.facebook.com/nwsbayarea www.twitter.com/nwsbayarea www.youtube.com/nwsbayarea ∨

FOLLOW US ON FACEBOOK, TWITTER, AND YOUTUBE AT:
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WWW.YOUTUBE.COM/NWSBAYAREA [DISC]

Weather NWS glossary About NWS point forecasts Hourly Weather Graph Long-range forecast Radar Satellite Max Temps Graphical Forecast	Area Forecast Discussion FXUS66 KMTR [Monterey CA] 072233 AFDMTR ∨ FXUS66 KMTR 072233 AFDMTR [HEAD] National Weather Service San Francisco Bay Area 2:33pm PST Wed Dec 7 2022 ∨ AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE SAN FRANCISCO BAY AREA 233 PM PST WED DEC 7 2022 [HEAD] Synopsis. Increasing high clouds ahead of the next front, but chilly morning temperatures expected Thursday morning. Chances for rain Thursday night into Friday morning, with highest amounts along the North Bay coastal ranges. Another system will bring even more rain again Saturday into Sunday. Cold morning temperatures are expected at the start of next week. ∨ .SYNOPSIS...INCREASING HIGH CLOUDS AHEAD OF THE NEXT FRONT, BUT CHILLY MORNING TEMPERATURES EXPECTED THURSDAY MORNING. CHANCES FOR RAIN THURSDAY NIGHT INTO FRIDAY MORNING, WITH HIGHEST AMOUNTS ALONG THE NORTH BAY COASTAL RANGES. ANOTHER SYSTEM WILL BRING EVEN MORE RAIN AGAIN SATURDAY INTO SUNDAY. COLD MORNING TEMPERATURES ARE EXPECTED AT THE START OF NEXT WEEK. [DISC] Discussion. As of 02:33pm PST Wednesday… satellite imagery shows clearing skies over the Bay Area and the Central Coast, with isolated fair-weather cumulus over hill tops that have bubbled up from the recent rains. Meanwhile, a long stretch of lower clouds of southwest to northeast orientation is about 100 miles west of Eureka and right along the Oregon coastline. Much colder clouds higher up in altitude are moving eastward toward the Pacific coastline. While these high clouds are associated with the leading edge of the next front, their first impact to the area will be to help insulate surface temperatures on Thursday morning. While cooler northwest advection is still assisting in making for temperatures in the 30s tomorrow morning, chances are increasing of keeping those temperatures in the mid to upper 30s, with only isolated, sheltered areas with the chance to approach freezing. Patchy frost may still develop, but confidence is low. ∨ .DISCUSSION...AS OF 02:33 PM PST WEDNESDAY...SATELLITE IMAGERY SHOWS CLEARING SKIES OVER THE BAY AREA AND THE CENTRAL COAST, WITH ISOLATED FAIR-WEATHER CUMULUS OVER HILL TOPS THAT HAVE BUBBLED UP FROM THE RECENT RAINS. MEANWHILE, A LONG STRETCH OF LOWER CLOUDS OF SOUTHWEST TO NORTHEAST ORIENTATION IS ABOUT 100 MILES WEST OF EUREKA AND RIGHT ALONG THE OREGON COASTLINE. MUCH COLDER CLOUDS HIGHER UP IN ALTITUDE ARE MOVING EASTWARD TOWARD THE PACIFIC COASTLINE. WHILE THESE HIGH CLOUDS ARE ASSOCIATED WITH THE LEADING EDGE OF THE NEXT FRONT, THEIR FIRST IMPACT TO THE AREA WILL BE TO HELP INSULATE SURFACE TEMPERATURES ON THURSDAY MORNING. WHILE COOLER NORTHWEST ADVECTION IS STILL ASSISTING IN MAKING FOR TEMPERATURES IN THE 30S TOMORROW MORNING, CHANCES ARE INCREASING OF KEEPING THOSE TEMPERATURES IN THE MID TO UPPER 30S, WITH ONLY ISOLATED, SHELTERED AREAS WITH THE CHANCE TO APPROACH FREEZING. PATCHY FROST MAY STILL DEVELOP, BUT CONFIDENCE IS LOW. [DISC] By Thursday afternoon, the next frontal system will move southeastward through through the Bay Area and down the Central Coast. The Euro [European Center for Medium-Range Weather Forecasts model] and GFS [Global Forecast System: global spectral model used primarily for aviation weather forecasts; provides guidance out to 384 hours at 00, 06, 12 and 18UTC.] ensembles are not projecting the system to arrive until at or after 00Z [4pm PST], the most recent NAM [North American Meso (formerly Eta) model] model has sped it up so that it around just before 21Z [1pm PST] (1 PM local) on Thursday afternoon. Its no surprise that the numerical blend of models is leaning closer to just before 00Z [4pm PST]. Drivers should be prepared for this rain to take place during the evening commute. The Sonoma coastal ranges have the best chance for receiving the most rainfall where just under an inch as a 30% chance of occurring, where the majority of North Bay higher terrain has a 35–50% chance of receiving at least a half an inch. Meanwhile, the lower valleys on have a 30% chance for around a quarter of an inch. The San Mateo coastline has a 40% chance for a quarter of an inch, but most ensemble models really have the front start to disintegrate after passing over the Golden Gate Bridge as it interacts with drier air aloft. A tenth of an inch to quarter of an inch has a 20–30% chance of occurring along the Central Coast, places like the Santa Clara and Salinas Valleys may encounter the rain shadow effect that inhibits totals. All that being said, there is high confidence for rain Thursday night into Friday. Most models are trying to resolve the rain being done by the morning commute on Friday. ∨ BY THURSDAY AFTERNOON, THE NEXT FRONTAL SYSTEM WILL MOVE SOUTHEASTWARD THROUGH THROUGH THE BAY AREA AND DOWN THE CENTRAL COAST. THE EURO AND GFS ENSEMBLES ARE NOT PROJECTING THE SYSTEM TO ARRIVE UNTIL AT OR AFTER 00Z, THE MOST RECENT NAM MODEL HAS SPED IT UP SO THAT IT AROUND JUST BEFORE 21Z (1 PM LOCAL) ON THURSDAY AFTERNOON. ITS NO SURPRISE THAT THE NUMERICAL BLEND OF MODELS IS LEANING CLOSER TO JUST BEFORE 00Z. DRIVERS SHOULD BE PREPARED FOR THIS RAIN TO TAKE PLACE DURING THE EVENING COMMUTE. THE SONOMA COASTAL RANGES HAVE THE BEST CHANCE FOR RECEIVING THE MOST RAINFALL WHERE JUST UNDER AN INCH AS A 30 PERCENT CHANCE OF OCCURRING, WHERE THE MAJORITY OF NORTH BAY HIGHER TERRAIN HAS A 35 TO 50 PERCENT CHANCE OF RECEIVING AT LEAST A HALF AN INCH. MEANWHILE, THE LOWER VALLEYS ON HAVE A 30 PERCENT CHANCE FOR AROUND A QUARTER OF AN INCH. THE SAN MATEO COASTLINE HAS A 40 PERCENT CHANCE FOR A QUARTER OF AN INCH, BUT MOST ENSEMBLE MODELS REALLY HAVE THE FRONT START TO DISINTEGRATE AFTER PASSING OVER THE GOLDEN GATE BRIDGE AS IT INTERACTS WITH DRIER AIR ALOFT. A TENTH OF AN INCH TO QUARTER OF AN INCH HAS A 20 TO 30 PERCENT CHANCE OF OCCURRING ALONG THE CENTRAL COAST, PLACES LIKE THE SANTA CLARA AND SALINAS VALLEYS MAY ENCOUNTER THE RAIN SHADOW EFFECT THAT INHIBITS TOTALS. ALL THAT BEING SAID, THERE IS HIGH CONFIDENCE FOR RAIN THURSDAY NIGHT INTO FRIDAY. MOST MODELS ARE TRYING TO RESOLVE THE RAIN BEING DONE BY THE MORNING COMMUTE ON FRIDAY. [DISC] Friday will have a brief respite with temperatures around seasonal normals before the next event over the weekend. Saturday returns the unsettled weather pattern as the next upper level low descends southward down the Pacific coastline featuring a 50–60% chance for tapping into precipitable water amounts greater than one inch. This translates to the Sonoma coastal mountains receiving a 40% chance for rainfall by Sunday morning of around two inches and most of the North Bay with a 40–80% chance of receiving at least an inch of rain by Sunday morning. While valley areas will experience some rain shadow lower amounts, confidence is increasing that most of the CWA [County Warning Area; see the LWP home page (link at bottom of page) for a CWA map] will get a healthy dose of rain. While a lot of this moisture extends out over the Pacific, deterministic and ensemble models as well as cluster analysis agrees that it will act as frontal in nature. Periods of moderate to heavy rain is possible, but it does not at this time act to slow down and become stationary. ∨ FRIDAY WILL HAVE A BRIEF RESPITE WITH TEMPERATURES AROUND SEASONAL NORMALS BEFORE THE NEXT EVENT OVER THE WEEKEND. SATURDAY RETURNS THE UNSETTLED WEATHER PATTERN AS THE NEXT UPPER LEVEL LOW DESCENDS SOUTHWARD DOWN THE PACIFIC COASTLINE FEATURING A 50 TO 60 PERCENT CHANCE FOR TAPPING INTO PRECIPITABLE WATER AMOUNTS GREATER THAN ONE INCH. THIS TRANSLATES TO THE SONOMA COASTAL MOUNTAINS RECEIVING A 40 PERCENT CHANCE FOR RAINFALL BY SUNDAY MORNING OF AROUND TWO INCHES AND MOST OF THE NORTH BAY WITH A 40 TO 80 PERCENT CHANCE OF RECEIVING AT LEAST AN INCH OF RAIN BY SUNDAY MORNING. WHILE VALLEY AREAS WILL EXPERIENCE SOME RAIN SHADOW LOWER AMOUNTS, CONFIDENCE IS INCREASING THAT MOST OF THE CWA WILL GET A HEALTHY DOSE OF RAIN. WHILE A LOT OF THIS MOISTURE EXTENDS OUT OVER THE PACIFIC, DETERMINISTIC AND ENSEMBLE MODELS AS WELL AS CLUSTER ANALYSIS AGREES THAT IT WILL ACT AS FRONTAL IN NATURE. PERIODS OF MODERATE TO HEAVY RAIN IS POSSIBLE, BUT IT DOES NOT AT THIS TIME ACT TO SLOW DOWN AND BECOME STATIONARY. [DISC] This Saturday system does not just look to provide rain alone. Winds will also increase along the surface boundary. Higher elevations have a chance for gusts between 35–40 mph. The lower confidence is weather or not those winds will mix down to the surface to necessitate a Wind Advisory. For now, gusts between 15–20 mph is expected. Lastly, there is increasing confidence for elevated levels of CAPE [Convective Available Potential Energy. A measure of the amount of energy available for convection. Related to the maximum potential vertical speed within an updraft; thus, higher values indicate greater potential for severe weather. Observed values in thunderstorm environments often may exceed 1000 joules per kilogram (J/kg), and in extreme cases may exceed 5000 J/kg.] along the boundary as it moves southward. While the probability is only maxing out around 15%, the official forecast has added the slight chance for a thunderstorm out over the ocean waters west of Sonoma County as well as the immediate Sonoma coastline. So re-cap: all-in-all a dynamic system with the chance for strong winds, thunderstorms to the north, but plenty of rainfall for the Bay Area and the Central Coast. ∨ THIS SATURDAY SYSTEM DOES NOT JUST LOOK TO PROVIDE RAIN ALONE. WINDS WILL ALSO INCREASE ALONG THE SURFACE BOUNDARY. HIGHER ELEVATIONS HAVE A CHANCE FOR GUSTS BETWEEN 35 TO 40 MPH. THE LOWER CONFIDENCE IS WEATHER OR NOT THOSE WINDS WILL MIX DOWN TO THE SURFACE TO NECESSITATE A WIND ADVISORY. FOR NOW, GUSTS BETWEEN 15 TO 20 MPH IS EXPECTED. LASTLY, THERE IS INCREASING CONFIDENCE FOR ELEVATED LEVELS OF CAPE ALONG THE BOUNDARY AS IT MOVES SOUTHWARD. WHILE THE PROBABILITY IS ONLY MAXING OUT AROUND 15 PERCENT, THE OFFICIAL FORECAST HAS ADDED THE SLIGHT CHANCE FOR A THUNDERSTORM OUT OVER THE OCEAN WATERS WEST OF SONOMA COUNTY AS WELL AS THE IMMEDIATE SONOMA COASTLINE. SO RE-CAP: ALL-IN-ALL A DYNAMIC SYSTEM WITH THE CHANCE FOR STRONG WINDS, THUNDERSTORMS TO THE NORTH, BUT PLENTY OF RAINFALL FOR THE BAY AREA AND THE CENTRAL COAST. [DISC] Beyond Sunday looks to be a quieter pattern from a precipitation perspective. However, cold air advection behind the weather system will likely create chilly morning low temperatures for the early part of next week. Many ensemble and deterministic models are suggesting a return to the upper 20s for many interior valley locations. A lot is still be determined, but confidence is increasing for a trend of drier and cooler, if not chillier, conditions. ∨ BEYOND SUNDAY LOOKS TO BE A QUIETER PATTERN FROM A PRECIPITATION PERSPECTIVE. HOWEVER, COLD AIR ADVECTION BEHIND THE WEATHER SYSTEM WILL LIKELY CREATE CHILLY MORNING LOW TEMPERATURES FOR THE EARLY PART OF NEXT WEEK. MANY ENSEMBLE AND DETERMINISTIC MODELS ARE SUGGESTING A RETURN TO THE UPPER 20S FOR MANY INTERIOR VALLEY LOCATIONS. A LOT IS STILL BE DETERMINED, BUT CONFIDENCE IS INCREASING FOR A TREND OF DRIER AND COOLER, IF NOT CHILLIER, CONDITIONS. [DISC] Aviation. As of 10:13am Wednesday… for the 18Z [10am PST] TAFs [Terminal Aerodrome Forecasts]. Satellite shows mostly clear skies over the region. Therefore, VFR [Visual Flight Rules] for most terminals except North Bay and KLVK [Livermore CA], where low visibility and low ceilings is expected to linger for another hour, as diurnal heating continues to take into effect. Ceilings begin to build overnight, allowing for MVFR [Marginal Visual Flight Rules] conditions, starting late tonight into Thursday morning. Moderate confidence for the timing of when rain will start for the North Bay and San Francisco Bay terminals, but expect showers near mid to late Thursday morning. Winds will remain light to moderate most of the TAF [Terminal Aerodrome Forecast] period and will turn to a more southerly component near late tonight to early tomorrow morning. ∨ .AVIATION...AS OF 10:13 AM WEDNESDAY...FOR THE 18Z TAFS. SATELLITE SHOWS MOSTLY CLEAR SKIES OVER THE REGION. THEREFORE, VFR FOR MOST TERMINALS EXCEPT NORTH BAY AND KLVK, WHERE LOW VISIBILITY AND LOW CIGS IS EXPECTED TO LINGER FOR ANOTHER HOUR, AS DIURNAL HEATING CONTINUES TO TAKE INTO EFFECT. CEILINGS BEGIN TO BUILD OVERNIGHT, ALLOWING FOR MVFR CONDITIONS, STARTING LATE TONIGHT INTO THURSDAY MORNING. MODERATE CONFIDENCE FOR THE TIMING OF WHEN RAIN WILL START FOR THE NORTH BAY AND SAN FRANCISCO BAY TERMINALS, BUT EXPECT SHOWERS NEAR MID TO LATE THURSDAY MORNING. WINDS WILL REMAIN LIGHT TO MODERATE MOST OF THE TAF PERIOD AND WILL TURN TO A MORE SOUTHERLY COMPONENT NEAR LATE TONIGHT TO EARLY TOMORROW MORNING. [AVIA] Vicinity of SFO [San Francisco CA]… VFR [Visual Flight Rules] for most of the TAF [Terminal Aerodrome Forecast] period with variable directed winds. Winds are expected to turn southerly near early tomorrow morning, but moderate confidence on timing. Also, moderate confidence that ceilings will get to MVFR [Marginal Visual Flight Rules] levels, as there is a chance ceilings may stay VFR [Visual Flight Rules] through the entire period. ∨ VICINITY OF SFO...VFR FOR MOST OF THE TAF PERIOD WITH VARIABLE DIRECTED WINDS. WINDS ARE EXPECTED TO TURN SOUTHERLY NEAR EARLY TOMORROW MORNING, BUT MODERATE CONFIDENCE ON TIMING. ALSO, MODERATE CONFIDENCE THAT CIGS WILL GET TO MVFR LEVELS, AS THERE IS A CHANCE CIGS MAY STAY VFR THROUGH THE ENTIRE PERIOD. [AVI2] SFO [San Francisco CA] bridge approach… similar to SFO [San Francisco CA]. ∨ SFO BRIDGE APPROACH...SIMILAR TO SFO. [AVI2] Monterey Bay terminals… VFR [Visual Flight Rules] for most of the TAF [Terminal Aerodrome Forecast] period with moderate confidence that it will become MVFR [Marginal Visual Flight Rules] tomorrow morning. High confidence that winds will flip to a southerly component overnight into tomorrow morning. ∨ MONTEREY BAY TERMINALS...VFR FOR MOST OF THE TAF PERIOD WITH MODERATE CONFIDENCE THAT IT WILL BECOME MVFR TOMORROW MORNING. HIGH CONFIDENCE THAT WINDS WILL FLIP TO A SOUTHERLY COMPONENT OVERNIGHT INTO TOMORROW MORNING. [AVI2] Marine. As of 02:29pm PST Wednesday… rain returns to the coastal waters and bays Thursday into early Friday morning. Another system will impact the coastal waters again Friday night through the weekend. Strong and gusty winds expected. Light northwest swell will continue through Thursday, followed by a larger, moderate period swell on Friday. ∨ .MARINE...AS OF 02:29 PM PST WEDNESDAY...RAIN RETURNS TO THE COASTAL WATERS AND BAYS THURSDAY INTO EARLY FRIDAY MORNING. ANOTHER SYSTEM WILL IMPACT THE COASTAL WATERS AGAIN FRIDAY NIGHT THROUGH THE WEEKEND. STRONG AND GUSTY WINDS EXPECTED. LIGHT NORTHWEST SWELL WILL CONTINUE THROUGH THURSDAY, FOLLOWED BY A LARGER, MODERATE PERIOD SWELL ON FRIDAY. [AVIA] MTR Watches/Warnings/Advisories Tonight: Small Craft Advisory…Pt Arena [Mendocino CA] to Pigeon Pt [Pescadero CA] 10–60 nm [69 miles] from 3am ∨ .MTR WATCHES/WARNINGS/ADVISORIES... .TNGT...SCA...PT ARENA TO PIGEON PT 10-60 NM FROM 3 AM [WARN] Public Forecast: DK; Aviation: SO; Marine: CW ∨ PUBLIC FORECAST: DK AVIATION: SO MARINE: CW [CRED] VISIT US AT WWW.WEATHER.GOV/SANFRANCISCO [LINK] Follow us on Facebook, Twitter, and YouTube at: www.facebook.com/nwsbayarea www.twitter.com/nwsbayarea www.youtube.com/nwsbayarea ∨ FOLLOW US ON FACEBOOK, TWITTER, AND YOUTUBE AT: WWW.FACEBOOK.COM/NWSBAYAREA WWW.TWITTER.COM/NWSBAYAREA WWW.YOUTUBE.COM/NWSBAYAREA [DISC]	Point Forecast Half Moon Bay Airport 07 Dec 05:55pm PST Tonight Patchy fog. Otherwise, increasing clouds, with a low around 40°. Calm wind. Thursday Rain, mainly after 10am. High near 54°. South wind 8 to 18 mph, with gusts as high as 25 mph. Chance of precipitation is 80%. New precipitation amounts between a tenth and quarter of an inch possible. Thursday Night Rain, mainly before 10pm. Low around 42°. SSW wind 5 to 10 mph becoming NW after midnight. Winds could gust as high as 18 mph. Chance of precipitation is 90%. New precipitation amounts between a quarter and half of an inch possible. Friday A 30% chance of rain after 4pm. Partly sunny, with a high near 54°. Calm wind becoming SSW around 6 mph in the afternoon. Friday Night Rain likely, mainly after 10pm. Cloudy, with a low around 46°. Breezy, with a south wind 9 to 14 mph increasing to 18 to 23 mph after midnight. Winds could gust as high as 28 mph. Chance of precipitation is 60%. New precipitation amounts between a quarter and half of an inch possible. Saturday Rain. High near 56°. Windy. Chance of precipitation is 100%. New precipitation amounts between 1 and 2 inches possible. Saturday Night Rain. Low around 44°. Chance of precipitation is 100%. Sunday Rain. Mostly cloudy, with a high near 51°. Sunday Night A chance of rain. Mostly cloudy, with a low around 39°. Monday Mostly sunny, with a high near 52°. Monday Night Mostly clear, with a low around 37°. Tuesday Sunny, with a high near 52°. Tuesday Night A slight chance of rain. Partly cloudy, with a low around 37°. Wednesday A chance of rain. Mostly sunny, with a high near 52°. Tonight Patchy fog. Otherwise, increasing clouds, with a low around 40. Calm wind. Thursday Rain, mainly after 10am. High near 54. South wind 8 to 18 mph, with gusts as high as 25 mph. Chance of precipitation is 80%. New precipitation amounts between a tenth and quarter of an inch possible. Thursday Night Rain, mainly before 10pm. Low around 42. South southwest wind 5 to 10 mph becoming northwest after midnight. Winds could gust as high as 18 mph. Chance of precipitation is 90%. New precipitation amounts between a quarter and half of an inch possible. Friday A 30 percent chance of rain after 4pm. Partly sunny, with a high near 54. Calm wind becoming south southwest around 6 mph in the afternoon. Friday Night Rain likely, mainly after 10pm. Cloudy, with a low around 46. Breezy, with a south wind 9 to 14 mph increasing to 18 to 23 mph after midnight. Winds could gust as high as 28 mph. Chance of precipitation is 60%. New precipitation amounts between a quarter and half of an inch possible. Saturday Rain. High near 56. Windy. Chance of precipitation is 100%. New precipitation amounts between 1 and 2 inches possible. Saturday Night Rain. Low around 44. Chance of precipitation is 100%. Sunday Rain. Mostly cloudy, with a high near 51. Sunday Night A chance of rain. Mostly cloudy, with a low around 39. Monday Mostly sunny, with a high near 52. Monday Night Mostly clear, with a low around 37. Tuesday Sunny, with a high near 52. Tuesday Night A slight chance of rain. Partly cloudy, with a low around 37. Wednesday A chance of rain. Mostly sunny, with a high near 52.
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