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FXUS66 KMTR [Monterey CA] 281730 AFDMTR ∨

FXUS66 KMTR 281730
AFDMTR [HEAD]

National Weather Service San Francisco Bay Area 10:30am PDT Thu May 28 2020 ∨

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SAN FRANCISCO BAY AREA
1030 AM PDT THU MAY 28 2020 [HEAD]

Synopsis. Interior locations will remain very warm to hot on Thursday where a Heat Advisory remains in effect. Cooler temperatures return to the coast on Thursday and region-wide on Friday as onshore flow increases. There is a chance of rain showers late Friday night into Saturday when isolated thunderstorms will also be possible. ∨

.SYNOPSIS...INTERIOR LOCATIONS WILL REMAIN VERY WARM TO HOT ON
THURSDAY WHERE A HEAT ADVISORY REMAINS IN EFFECT. COOLER
TEMPERATURES RETURN TO THE COAST ON THURSDAY AND REGION-WIDE ON
FRIDAY AS ONSHORE FLOW INCREASES. THERE IS A CHANCE OF RAIN
SHOWERS LATE FRIDAY NIGHT INTO SATURDAY WHEN ISOLATED
THUNDERSTORMS WILL ALSO BE POSSIBLE. [DISC]

Discussion. As of 08:50am PDT Thursday… the high pressure ridge responsible for several days of hot, dry weather across the region will bring another hot day to inland areas away from the coast, though to a lesser extent than previous days. Areas nearer to the coast will remain much cooler today, particularly after a deep marine air mass intrusion from late yesterday through this morning. 12Z [5am PDT] Oakland upper air sounding shows two distinct air masses impacting surface conditions this morning, with the cool, moist, (and denser) marine air mass allowing most urban areas to hover in the 50s to mid 60s while elevations over 1000 feet never escaped the warmer, drier, (and less dense) air hovering aloft and remained in the upper 60s to around 80° overnight. ∨

.DISCUSSION...AS OF 08:50 AM PDT THURSDAY... THE HIGH PRESSURE
RIDGE RESPONSIBLE FOR SEVERAL DAYS OF HOT, DRY WEATHER ACROSS THE
REGION WILL BRING ANOTHER HOT DAY TO INLAND AREAS AWAY FROM THE
COAST, THOUGH TO A LESSER EXTENT THAN PREVIOUS DAYS. AREAS NEARER
TO THE COAST WILL REMAIN MUCH COOLER TODAY, PARTICULARLY AFTER A
DEEP MARINE AIR MASS INTRUSION FROM LATE YESTERDAY THROUGH THIS
MORNING. 12Z OAKLAND UPPER AIR SOUNDING SHOWS TWO DISTINCT AIR
MASSES IMPACTING SURFACE CONDITIONS THIS MORNING, WITH THE COOL,
MOIST, (AND DENSER) MARINE AIR MASS ALLOWING MOST URBAN AREAS TO
HOVER IN THE 50S TO MID 60S WHILE ELEVATIONS OVER 1000 FEET NEVER
ESCAPED THE WARMER, DRIER, (AND LESS DENSE) AIR HOVERING ALOFT AND
REMAINED IN THE UPPER 60S TO AROUND 80 DEGREES OVERNIGHT. [DISC]

Satellite imagery shows a 565 dm [decameters] 500 mb cutoff low [A closed upper-level low which has become completely displaced (cut off) from basic westerly current, and moves independently of that current. Cutoff lows may remain nearly stationary for days, or on occasion may move westward opposite to the prevailing flow aloft] 700 miles southwest of Point Conception [Santa Barbara CA] this morning. This feature has been fairly stagnant over the last week, initially riding on the southern flank of the aforementioned ridge prior to the ridge moving ashore over the Memorial Day weekend… then transitioning to the southwestern flank of the ridge after it moved ashore. Forecast models are in high agreement that this feature will finally approach the Pacific coastline and move ashore directly over the San Francisco Bay Area after looming offshore for roughly a week. As this storm system shifts closer to home on a northeastward trajectory, southerly winds will increase across the usual spots (coast, higher terrain) but also create locally breezy conditions inland as well. IR [Infra-Red] satellite imagery indicates an area of increased convective enhancement on the north/northeastern segment of the upper low. Lightning detection networks have also signaled a steady amount of lightning strikes in this outer band as well as in the core of the upper low. These two hotspots of moisture and instability will pass over the region from late Friday through midday Saturday and bring with them a chance of late season convective precipitation but also the potential for lightning. The best chance of perception and thunderstorm development will be throughout Saturday morning through early afternoon, primarily over the immediate San Francisco Bay Area and north/eastward as the upper low shifts inland and merges with a broader parent trough. Lightning may be offset from any of the sporadic showery precipitation from this storm system and could be dry lightning, however, fuels currently are not as dry as they could be, thus no fire weather products (watches/warnings) are planned at this time. That said, now is the time to begin preparing for the upcoming fire season. ∨

SATELLITE IMAGERY SHOWS A 565DM 500MB CUTOFF LOW 700 MILES
SOUTHWEST OF POINT CONCEPTION THIS MORNING. THIS FEATURE HAS BEEN
FAIRLY STAGNANT OVER THE LAST WEEK, INITIALLY RIDING ON THE
SOUTHERN FLANK OF THE AFOREMENTIONED RIDGE PRIOR TO THE RIDGE
MOVING ASHORE OVER THE MEMORIAL DAY WEEKEND... THEN TRANSITIONING
TO THE SOUTHWESTERN FLANK OF THE RIDGE AFTER IT MOVED ASHORE.
FORECAST MODELS ARE IN HIGH AGREEMENT THAT THIS FEATURE WILL
FINALLY APPROACH THE PACIFIC COASTLINE AND MOVE ASHORE DIRECTLY
OVER THE SAN FRANCISCO BAY AREA AFTER LOOMING OFFSHORE FOR
ROUGHLY A WEEK. AS THIS STORM SYSTEM SHIFTS CLOSER TO HOME ON A
NORTHEASTWARD TRAJECTORY, SOUTHERLY WINDS WILL INCREASE ACROSS THE
USUAL SPOTS (COAST, HIGHER TERRAIN) BUT ALSO CREATE LOCALLY BREEZY
CONDITIONS INLAND AS WELL. IR SATELLITE IMAGERY INDICATES AN AREA
OF INCREASED CONVECTIVE ENHANCEMENT ON THE NORTH/NORTHEASTERN
SEGMENT OF THE UPPER LOW. LIGHTNING DETECTION NETWORKS HAVE ALSO
SIGNALED A STEADY AMOUNT OF LIGHTNING STRIKES IN THIS OUTER BAND
AS WELL AS IN THE CORE OF THE UPPER LOW. THESE TWO HOTSPOTS OF
MOISTURE AND INSTABILITY WILL PASS OVER THE REGION FROM LATE
FRIDAY THROUGH MIDDAY SATURDAY AND BRING WITH THEM A CHANCE OF
LATE SEASON CONVECTIVE PRECIPITATION BUT ALSO THE POTENTIAL FOR
LIGHTNING. THE BEST CHANCE OF PERCEPTION AND THUNDERSTORM
DEVELOPMENT WILL BE THROUGHOUT SATURDAY MORNING THROUGH EARLY
AFTERNOON, PRIMARILY OVER THE IMMEDIATE SAN FRANCISCO BAY AREA AND
NORTH/EASTWARD AS THE UPPER LOW SHIFTS INLAND AND MERGES WITH A
BROADER PARENT TROUGH. LIGHTNING MAY BE OFFSET FROM ANY OF THE
SPORADIC SHOWERY PRECIPITATION FROM THIS STORM SYSTEM AND COULD BE
DRY LIGHTNING, HOWEVER, FUELS CURRENTLY ARE NOT AS DRY AS THEY
COULD BE, THUS NO FIRE WEATHER PRODUCTS (WATCHES/WARNINGS) ARE
PLANNED AT THIS TIME. THAT SAID, NOW IS THE TIME TO BEGIN
PREPARING FOR THE UPCOMING FIRE SEASON. [DISC]

The cut off low reintegrates with a broader parent trough that is anticipated to sag over the Pacific coastline from Sunday through potentially Friday of next week. See previous discussion for more details on the current forecast package. ∨

THE CUT OFF LOW REINTEGRATES WITH A BROADER PARENT TROUGH THAT IS
ANTICIPATED TO SAG OVER THE PACIFIC COASTLINE FROM SUNDAY THROUGH
POTENTIALLY FRIDAY OF NEXT WEEK. SEE PREVIOUS DISCUSSION FOR MORE
DETAILS ON THE CURRENT FORECAST PACKAGE. [DISC]

Previous discussion… as of 3:37am PDT Thursday. A change in the weather is underway across the Bay Area and Central Coast at coastal clouds return, temperatures begin finally cool and a storm system lurks off the coast. ∨

.PREVIOUS DISCUSSION...AS OF 3:37 AM PDT THURSDAY...A CHANGE IN
THE WEATHER IS UNDERWAY ACROSS THE BAY AREA AND CENTRAL COAST AT
COASTAL CLOUDS RETURN, TEMPERATURES BEGIN FINALLY COOL AND A STORM
SYSTEM LURKS OFF THE COAST. [DISC]

Satellite imagery shows a welcomed sight across the region with low clouds filling in over the coastal waters and moving locally inland overnight. Fort Ord profiler [instrument designed to measure horizontal winds directly above its location, and thus measure the vertical wind profile. Profilers operate on the same principles as Doppler radar.] puts the marine layer depth at nearly 1,000 feet. The increased onshore flow is also reflective in the 24 hour temperature trend with many locations 5–10° cooler than 24 hours ago. However, go up above 1,000 feet and temperatures are still rather mild and in the 60s and 70s as of 3am. ∨

SATELLITE IMAGERY SHOWS A WELCOMED SIGHT ACROSS THE REGION WITH
LOW CLOUDS FILLING IN OVER THE COASTAL WATERS AND MOVING LOCALLY
INLAND OVERNIGHT. FORT ORD PROFILER PUTS THE MARINE LAYER DEPTH AT
NEARLY 1,000 FEET. THE INCREASED ONSHORE FLOW IS ALSO REFLECTIVE
IN THE 24 HOUR TEMPERATURE TREND WITH MANY LOCATIONS 5-10 DEGREES
COOLER THAN 24 HOURS AGO. HOWEVER, GO UP ABOVE 1,000 FEET AND
TEMPERATURES ARE STILL RATHER MILD AND IN THE 60S AND 70S AS OF 3
AM. [DISC]

The near term weather challenge will be the existing Heat Advisory and what to do with it: keep it or expire it. The return of the marine layer and increased onshore flow will definitely lead to cooler temperatures near the coast and locally inland with highs in the upper 60s to lower 80s. However, go inland a few miles more and temperatures will quickly rise to the upper 80s to near 100. The night shift, after coordination, decided to keep the Heat Advisory in place through 7pm. Heat risk impacts will be less today, but given that today is the fourth day in a row of hot weather away from coast societal impacts will remain for heat sensitive groups. ∨

THE NEAR TERM WEATHER CHALLENGE WILL BE THE EXISTING HEAT
ADVISORY AND WHAT TO DO WITH IT: KEEP IT OR EXPIRE IT. THE RETURN
OF THE MARINE LAYER AND INCREASED ONSHORE FLOW WILL DEFINITELY
LEAD TO COOLER TEMPERATURES NEAR THE COAST AND LOCALLY INLAND WITH
HIGHS IN THE UPPER 60S TO LOWER 80S. HOWEVER, GO INLAND A FEW
MILES MORE AND TEMPERATURES WILL QUICKLY RISE TO THE UPPER 80S TO
NEAR 100. THE NIGHT SHIFT, AFTER COORDINATION, DECIDED TO KEEP
THE HEAT ADVISORY IN PLACE THROUGH 7 PM. HEAT RISK IMPACTS WILL BE
LESS TODAY, BUT GIVEN THAT TODAY IS THE FOURTH DAY IN A ROW OF
HOT WEATHER AWAY FROM COAST SOCIETAL IMPACTS WILL REMAIN FOR HEAT
SENSITIVE GROUPS. [DISC]

Widespread cooling is still expected early Thursday night and moreso on Friday. The ridge that brought the hot weather moves eastward and is quickly replaced by an upstream upper low. Satellite imagery shows the low spinning roughly 600 mile SW of Point Conception [Santa Barbara CA]. This well advertised low will heavily influence Friday's weather with a deeper marine layer and a region wide cooldown. The marine layer will deepen enough that some patchy drizzle is possible Friday morning. Temperatures on Friday will be 5–15° cooler — 60s/70s coast and 70s/80s inland. ∨

WIDESPREAD COOLING IS STILL EXPECTED EARLY THURSDAY NIGHT AND
MORESO ON FRIDAY. THE RIDGE THAT BROUGHT THE HOT WEATHER MOVES
EASTWARD AND IS QUICKLY REPLACED BY AN UPSTREAM UPPER LOW.
SATELLITE IMAGERY SHOWS THE LOW SPINNING ROUGHLY 600 MILE SW OF
POINT CONCEPTION. THIS WELL ADVERTISED LOW WILL HEAVILY INFLUENCE
FRIDAY`S WEATHER WITH A DEEPER MARINE LAYER AND A REGION WIDE
COOLDOWN. THE MARINE LAYER WILL DEEPEN ENOUGH THAT SOME PATCHY
DRIZZLE IS POSSIBLE FRIDAY MORNING. TEMPERATURES ON FRIDAY WILL BE
5-15 DEGREES COOLER -- 60S/70S COAST AND 70S/80S INLAND. [DISC]

Precipitation still appears to be be possible Friday night into Saturday night as the low approaches and eventually tracks over the North Bay. Early Friday night could be interesting as some model guidance still suggest some high-based convection over the coastal waters and North Bay. For what it's worth, latest lightning detection networks are already picking up some lightning near the upper low. Thunderstorm chances look more likely on Saturday as the colder core upper low moves through. Best chance for thunderstorms will be over the North and East Bay Saturday and early Saturday night. Regardless of thunderstorm chances, locations around the region have a good shot as seeing some scattered showers. ∨

PRECIP STILL APPEARS TO BE BE POSSIBLE FRIDAY NIGHT INTO SATURDAY
NIGHT AS THE LOW APPROACHES AND EVENTUALLY TRACKS OVER THE NORTH
BAY. EARLY FRIDAY NIGHT COULD BE INTERESTING AS SOME MODEL
GUIDANCE STILL SUGGEST SOME HIGH-BASED CONVECTION OVER THE COASTAL
WATERS AND NORTH BAY. FOR WHAT IT`S WORTH, LATEST LIGHTNING
DETECTION NETWORKS ARE ALREADY PICKING UP SOME LIGHTNING NEAR THE
UPPER LOW. THUNDERSTORM CHANCES LOOK MORE LIKELY ON SATURDAY AS
THE COLDER CORE UPPER LOW MOVES THROUGH. BEST CHANCE FOR
THUNDERSTORMS WILL BE OVER THE NORTH AND EAST BAY SATURDAY AND
EARLY SATURDAY NIGHT. REGARDLESS OF THUNDERSTORM CHANCES, LOCATIONS
AROUND THE REGION HAVE A GOOD SHOT AS SEEING SOME SCATTERED
SHOWERS. [DISC]

Precipitation chances end by Sunday, but cooler weather continues. Medium range models keep flip-flopping on precipitation chances early next week as another upper low develops off the coast. Dry forecast for now, but temperatures will remain cooler through Wednesday. ∨

PRECIP CHANCES END BY SUNDAY, BUT COOLER WEATHER CONTINUES. MEDIUM
RANGE MODELS KEEP FLIP-FLOPPING ON PRECIP CHANCES EARLY NEXT WEEK
AS ANOTHER UPPER LOW DEVELOPS OFF THE COAST. DRY FORECAST FOR
NOW, BUT TEMPERATURES WILL REMAIN COOLER THROUGH WEDNESDAY. [DISC]

Aviation. As of 10:30am PDT Thursday… for 18Z [11am PDT] TAFs [Terminal Aerodrome Forecasts]. VFR [Visual Flight Rules] conditions the rest of the afternoon and into this evening as stratus has returned to the coast. Expecting an earlier return of low clouds this evening compared to yesterday as the marine layer deepens ahead of an approaching low pressure system. MVFR [Marginal Visual Flight Rules]/IFR [Instrument Flight Rules] ceilings expected overnight with patchy fog along the coast and in the North Bay. Onshore winds this afternoon 10–15 kt [12–17 mph] with locally higher gusts possible at KSFO [San Francisco CA]. ∨

.AVIATION...AS OF 10:30 AM PDT THURSDAY...FOR 18Z TAFS. VFR
CONDITIONS THE REST OF THE AFTERNOON AND INTO THIS EVENING AS
STRATUS HAS RETURNED TO THE COAST. EXPECTING AN EARLIER RETURN OF
LOW CLOUDS THIS EVENING COMPARED TO YESTERDAY AS THE MARINE LAYER
DEEPENS AHEAD OF AN APPROACHING LOW PRESSURE SYSTEM. MVFR/IFR CIGS
EXPECTED OVERNIGHT WITH PATCHY FOG ALONG THE COAST AND IN THE
NORTH BAY. ONSHORE WINDS THIS AFTERNOON 10-15 KT WITH LOCALLY
HIGHER GUSTS POSSIBLE AT KSFO. [AVIA]

Vicinity of KSFO [San Francisco CA]… VFR [Visual Flight Rules] through this evening. Southerly flow expected overnight and tomorrow morning may prevent broken IFR [Instrument Flight Rules]/MVFR [Marginal Visual Flight Rules] ceilings from reaching the terminal, though it is possible. Onshore winds this afternoon around 15 kt [17 mph] with gusts near 20 kt [23 mph] possible. ∨

VICINITY OF KSFO...VFR THROUGH THIS EVENING. SOUTHERLY FLOW
EXPECTED OVERNIGHT AND TOMORROW MORNING MAY PREVENT BKN IFR/MVFR
CIGS FROM REACHING THE TERMINAL, THOUGH IT IS POSSIBLE. ONSHORE
WINDS THIS AFTERNOON AROUND 15 KT WITH GUSTS NEAR 20 KT POSSIBLE. [AVI2]

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

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

Monterey Bay terminals… VFR [Visual Flight Rules] conditions into this evening before IFR [Instrument Flight Rules] ceilings expected to return and prevail overnight. Patchy fog possible late tonight/early tomorrow morning. Onshore winds this afternoon 10–15 kt [12–17 mph]. ∨

MONTEREY BAY TERMINALS...VFR CONDITIONS INTO THIS EVENING BEFORE
IFR CIGS EXPECTED TO RETURN AND PREVAIL OVERNIGHT. PATCHY FOG
POSSIBLE LATE TONIGHT/EARLY TOMORROW MORNING. ONSHORE WINDS THIS
AFTERNOON 10-15 KT. [AVI2]

Marine. As of 8:23am PDT Thursday… generally light to locally moderate northwest winds for today before gradually turning southerly tomorrow ahead of a low pressure system approaching from the southwest. This system will bring a slight chance of thunderstorms over the northern waters Friday night into Saturday as well as locally gusty winds for portions of the inner coastal waters and bays. Mixed seas will persist with a short period northwest swell and a longer period southwest swell. A longer period northwest swell will arrive tomorrow. ∨

.MARINE...AS OF 8:23 AM PDT THURSDAY...GENERALLY LIGHT TO LOCALLY
MODERATE NORTHWEST WINDS FOR TODAY BEFORE GRADUALLY TURNING
SOUTHERLY TOMORROW AHEAD OF A LOW PRESSURE SYSTEM APPROACHING FROM
THE SOUTHWEST. THIS SYSTEM WILL BRING A SLIGHT CHANCE OF
THUNDERSTORMS OVER THE NORTHERN WATERS FRIDAY NIGHT INTO SATURDAY
AS WELL AS LOCALLY GUSTY WINDS FOR PORTIONS OF THE INNER COASTAL
WATERS AND BAYS. MIXED SEAS WILL PERSIST WITH A SHORT PERIOD
NORTHWEST SWELL AND A LONGER PERIOD SOUTHWEST SWELL. A LONGER
PERIOD NORTHWEST SWELL WILL ARRIVE TOMORROW. [AVIA]

MTR Watches/Warnings/Advisories

Today: Heat Advisory…CAZ506–507–510>513–516>518–528 ∨

.MTR WATCHES/WARNINGS/ADVISORIES...
.TDAY...HEAT ADVISORY...CAZ506-507-510>513-516>518-528 [WARN]

Public Forecast: DRP/MM; Aviation: AS; Marine: AS ∨

PUBLIC FORECAST: DRP/MM
AVIATION: AS
MARINE: AS [CRED]

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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] 281730 AFDMTR ∨ FXUS66 KMTR 281730 AFDMTR [HEAD] National Weather Service San Francisco Bay Area 10:30am PDT Thu May 28 2020 ∨ AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE SAN FRANCISCO BAY AREA 1030 AM PDT THU MAY 28 2020 [HEAD] Synopsis. Interior locations will remain very warm to hot on Thursday where a Heat Advisory remains in effect. Cooler temperatures return to the coast on Thursday and region-wide on Friday as onshore flow increases. There is a chance of rain showers late Friday night into Saturday when isolated thunderstorms will also be possible. ∨ .SYNOPSIS...INTERIOR LOCATIONS WILL REMAIN VERY WARM TO HOT ON THURSDAY WHERE A HEAT ADVISORY REMAINS IN EFFECT. COOLER TEMPERATURES RETURN TO THE COAST ON THURSDAY AND REGION-WIDE ON FRIDAY AS ONSHORE FLOW INCREASES. THERE IS A CHANCE OF RAIN SHOWERS LATE FRIDAY NIGHT INTO SATURDAY WHEN ISOLATED THUNDERSTORMS WILL ALSO BE POSSIBLE. [DISC] Discussion. As of 08:50am PDT Thursday… the high pressure ridge responsible for several days of hot, dry weather across the region will bring another hot day to inland areas away from the coast, though to a lesser extent than previous days. Areas nearer to the coast will remain much cooler today, particularly after a deep marine air mass intrusion from late yesterday through this morning. 12Z [5am PDT] Oakland upper air sounding shows two distinct air masses impacting surface conditions this morning, with the cool, moist, (and denser) marine air mass allowing most urban areas to hover in the 50s to mid 60s while elevations over 1000 feet never escaped the warmer, drier, (and less dense) air hovering aloft and remained in the upper 60s to around 80° overnight. ∨ .DISCUSSION...AS OF 08:50 AM PDT THURSDAY... THE HIGH PRESSURE RIDGE RESPONSIBLE FOR SEVERAL DAYS OF HOT, DRY WEATHER ACROSS THE REGION WILL BRING ANOTHER HOT DAY TO INLAND AREAS AWAY FROM THE COAST, THOUGH TO A LESSER EXTENT THAN PREVIOUS DAYS. AREAS NEARER TO THE COAST WILL REMAIN MUCH COOLER TODAY, PARTICULARLY AFTER A DEEP MARINE AIR MASS INTRUSION FROM LATE YESTERDAY THROUGH THIS MORNING. 12Z OAKLAND UPPER AIR SOUNDING SHOWS TWO DISTINCT AIR MASSES IMPACTING SURFACE CONDITIONS THIS MORNING, WITH THE COOL, MOIST, (AND DENSER) MARINE AIR MASS ALLOWING MOST URBAN AREAS TO HOVER IN THE 50S TO MID 60S WHILE ELEVATIONS OVER 1000 FEET NEVER ESCAPED THE WARMER, DRIER, (AND LESS DENSE) AIR HOVERING ALOFT AND REMAINED IN THE UPPER 60S TO AROUND 80 DEGREES OVERNIGHT. [DISC] Satellite imagery shows a 565 dm [decameters] 500 mb cutoff low [A closed upper-level low which has become completely displaced (cut off) from basic westerly current, and moves independently of that current. Cutoff lows may remain nearly stationary for days, or on occasion may move westward opposite to the prevailing flow aloft] 700 miles southwest of Point Conception [Santa Barbara CA] this morning. This feature has been fairly stagnant over the last week, initially riding on the southern flank of the aforementioned ridge prior to the ridge moving ashore over the Memorial Day weekend… then transitioning to the southwestern flank of the ridge after it moved ashore. Forecast models are in high agreement that this feature will finally approach the Pacific coastline and move ashore directly over the San Francisco Bay Area after looming offshore for roughly a week. As this storm system shifts closer to home on a northeastward trajectory, southerly winds will increase across the usual spots (coast, higher terrain) but also create locally breezy conditions inland as well. IR [Infra-Red] satellite imagery indicates an area of increased convective enhancement on the north/northeastern segment of the upper low. Lightning detection networks have also signaled a steady amount of lightning strikes in this outer band as well as in the core of the upper low. These two hotspots of moisture and instability will pass over the region from late Friday through midday Saturday and bring with them a chance of late season convective precipitation but also the potential for lightning. The best chance of perception and thunderstorm development will be throughout Saturday morning through early afternoon, primarily over the immediate San Francisco Bay Area and north/eastward as the upper low shifts inland and merges with a broader parent trough. Lightning may be offset from any of the sporadic showery precipitation from this storm system and could be dry lightning, however, fuels currently are not as dry as they could be, thus no fire weather products (watches/warnings) are planned at this time. That said, now is the time to begin preparing for the upcoming fire season. ∨ SATELLITE IMAGERY SHOWS A 565DM 500MB CUTOFF LOW 700 MILES SOUTHWEST OF POINT CONCEPTION THIS MORNING. THIS FEATURE HAS BEEN FAIRLY STAGNANT OVER THE LAST WEEK, INITIALLY RIDING ON THE SOUTHERN FLANK OF THE AFOREMENTIONED RIDGE PRIOR TO THE RIDGE MOVING ASHORE OVER THE MEMORIAL DAY WEEKEND... THEN TRANSITIONING TO THE SOUTHWESTERN FLANK OF THE RIDGE AFTER IT MOVED ASHORE. FORECAST MODELS ARE IN HIGH AGREEMENT THAT THIS FEATURE WILL FINALLY APPROACH THE PACIFIC COASTLINE AND MOVE ASHORE DIRECTLY OVER THE SAN FRANCISCO BAY AREA AFTER LOOMING OFFSHORE FOR ROUGHLY A WEEK. AS THIS STORM SYSTEM SHIFTS CLOSER TO HOME ON A NORTHEASTWARD TRAJECTORY, SOUTHERLY WINDS WILL INCREASE ACROSS THE USUAL SPOTS (COAST, HIGHER TERRAIN) BUT ALSO CREATE LOCALLY BREEZY CONDITIONS INLAND AS WELL. IR SATELLITE IMAGERY INDICATES AN AREA OF INCREASED CONVECTIVE ENHANCEMENT ON THE NORTH/NORTHEASTERN SEGMENT OF THE UPPER LOW. LIGHTNING DETECTION NETWORKS HAVE ALSO SIGNALED A STEADY AMOUNT OF LIGHTNING STRIKES IN THIS OUTER BAND AS WELL AS IN THE CORE OF THE UPPER LOW. THESE TWO HOTSPOTS OF MOISTURE AND INSTABILITY WILL PASS OVER THE REGION FROM LATE FRIDAY THROUGH MIDDAY SATURDAY AND BRING WITH THEM A CHANCE OF LATE SEASON CONVECTIVE PRECIPITATION BUT ALSO THE POTENTIAL FOR LIGHTNING. THE BEST CHANCE OF PERCEPTION AND THUNDERSTORM DEVELOPMENT WILL BE THROUGHOUT SATURDAY MORNING THROUGH EARLY AFTERNOON, PRIMARILY OVER THE IMMEDIATE SAN FRANCISCO BAY AREA AND NORTH/EASTWARD AS THE UPPER LOW SHIFTS INLAND AND MERGES WITH A BROADER PARENT TROUGH. LIGHTNING MAY BE OFFSET FROM ANY OF THE SPORADIC SHOWERY PRECIPITATION FROM THIS STORM SYSTEM AND COULD BE DRY LIGHTNING, HOWEVER, FUELS CURRENTLY ARE NOT AS DRY AS THEY COULD BE, THUS NO FIRE WEATHER PRODUCTS (WATCHES/WARNINGS) ARE PLANNED AT THIS TIME. THAT SAID, NOW IS THE TIME TO BEGIN PREPARING FOR THE UPCOMING FIRE SEASON. [DISC] The cut off low reintegrates with a broader parent trough that is anticipated to sag over the Pacific coastline from Sunday through potentially Friday of next week. See previous discussion for more details on the current forecast package. ∨ THE CUT OFF LOW REINTEGRATES WITH A BROADER PARENT TROUGH THAT IS ANTICIPATED TO SAG OVER THE PACIFIC COASTLINE FROM SUNDAY THROUGH POTENTIALLY FRIDAY OF NEXT WEEK. SEE PREVIOUS DISCUSSION FOR MORE DETAILS ON THE CURRENT FORECAST PACKAGE. [DISC] Previous discussion… as of 3:37am PDT Thursday. A change in the weather is underway across the Bay Area and Central Coast at coastal clouds return, temperatures begin finally cool and a storm system lurks off the coast. ∨ .PREVIOUS DISCUSSION...AS OF 3:37 AM PDT THURSDAY...A CHANGE IN THE WEATHER IS UNDERWAY ACROSS THE BAY AREA AND CENTRAL COAST AT COASTAL CLOUDS RETURN, TEMPERATURES BEGIN FINALLY COOL AND A STORM SYSTEM LURKS OFF THE COAST. [DISC] Satellite imagery shows a welcomed sight across the region with low clouds filling in over the coastal waters and moving locally inland overnight. Fort Ord profiler [instrument designed to measure horizontal winds directly above its location, and thus measure the vertical wind profile. Profilers operate on the same principles as Doppler radar.] puts the marine layer depth at nearly 1,000 feet. The increased onshore flow is also reflective in the 24 hour temperature trend with many locations 5–10° cooler than 24 hours ago. However, go up above 1,000 feet and temperatures are still rather mild and in the 60s and 70s as of 3am. ∨ SATELLITE IMAGERY SHOWS A WELCOMED SIGHT ACROSS THE REGION WITH LOW CLOUDS FILLING IN OVER THE COASTAL WATERS AND MOVING LOCALLY INLAND OVERNIGHT. FORT ORD PROFILER PUTS THE MARINE LAYER DEPTH AT NEARLY 1,000 FEET. THE INCREASED ONSHORE FLOW IS ALSO REFLECTIVE IN THE 24 HOUR TEMPERATURE TREND WITH MANY LOCATIONS 5-10 DEGREES COOLER THAN 24 HOURS AGO. HOWEVER, GO UP ABOVE 1,000 FEET AND TEMPERATURES ARE STILL RATHER MILD AND IN THE 60S AND 70S AS OF 3 AM. [DISC] The near term weather challenge will be the existing Heat Advisory and what to do with it: keep it or expire it. The return of the marine layer and increased onshore flow will definitely lead to cooler temperatures near the coast and locally inland with highs in the upper 60s to lower 80s. However, go inland a few miles more and temperatures will quickly rise to the upper 80s to near 100. The night shift, after coordination, decided to keep the Heat Advisory in place through 7pm. Heat risk impacts will be less today, but given that today is the fourth day in a row of hot weather away from coast societal impacts will remain for heat sensitive groups. ∨ THE NEAR TERM WEATHER CHALLENGE WILL BE THE EXISTING HEAT ADVISORY AND WHAT TO DO WITH IT: KEEP IT OR EXPIRE IT. THE RETURN OF THE MARINE LAYER AND INCREASED ONSHORE FLOW WILL DEFINITELY LEAD TO COOLER TEMPERATURES NEAR THE COAST AND LOCALLY INLAND WITH HIGHS IN THE UPPER 60S TO LOWER 80S. HOWEVER, GO INLAND A FEW MILES MORE AND TEMPERATURES WILL QUICKLY RISE TO THE UPPER 80S TO NEAR 100. THE NIGHT SHIFT, AFTER COORDINATION, DECIDED TO KEEP THE HEAT ADVISORY IN PLACE THROUGH 7 PM. HEAT RISK IMPACTS WILL BE LESS TODAY, BUT GIVEN THAT TODAY IS THE FOURTH DAY IN A ROW OF HOT WEATHER AWAY FROM COAST SOCIETAL IMPACTS WILL REMAIN FOR HEAT SENSITIVE GROUPS. [DISC] Widespread cooling is still expected early Thursday night and moreso on Friday. The ridge that brought the hot weather moves eastward and is quickly replaced by an upstream upper low. Satellite imagery shows the low spinning roughly 600 mile SW of Point Conception [Santa Barbara CA]. This well advertised low will heavily influence Friday's weather with a deeper marine layer and a region wide cooldown. The marine layer will deepen enough that some patchy drizzle is possible Friday morning. Temperatures on Friday will be 5–15° cooler — 60s/70s coast and 70s/80s inland. ∨ WIDESPREAD COOLING IS STILL EXPECTED EARLY THURSDAY NIGHT AND MORESO ON FRIDAY. THE RIDGE THAT BROUGHT THE HOT WEATHER MOVES EASTWARD AND IS QUICKLY REPLACED BY AN UPSTREAM UPPER LOW. SATELLITE IMAGERY SHOWS THE LOW SPINNING ROUGHLY 600 MILE SW OF POINT CONCEPTION. THIS WELL ADVERTISED LOW WILL HEAVILY INFLUENCE FRIDAY`S WEATHER WITH A DEEPER MARINE LAYER AND A REGION WIDE COOLDOWN. THE MARINE LAYER WILL DEEPEN ENOUGH THAT SOME PATCHY DRIZZLE IS POSSIBLE FRIDAY MORNING. TEMPERATURES ON FRIDAY WILL BE 5-15 DEGREES COOLER -- 60S/70S COAST AND 70S/80S INLAND. [DISC] Precipitation still appears to be be possible Friday night into Saturday night as the low approaches and eventually tracks over the North Bay. Early Friday night could be interesting as some model guidance still suggest some high-based convection over the coastal waters and North Bay. For what it's worth, latest lightning detection networks are already picking up some lightning near the upper low. Thunderstorm chances look more likely on Saturday as the colder core upper low moves through. Best chance for thunderstorms will be over the North and East Bay Saturday and early Saturday night. Regardless of thunderstorm chances, locations around the region have a good shot as seeing some scattered showers. ∨ PRECIP STILL APPEARS TO BE BE POSSIBLE FRIDAY NIGHT INTO SATURDAY NIGHT AS THE LOW APPROACHES AND EVENTUALLY TRACKS OVER THE NORTH BAY. EARLY FRIDAY NIGHT COULD BE INTERESTING AS SOME MODEL GUIDANCE STILL SUGGEST SOME HIGH-BASED CONVECTION OVER THE COASTAL WATERS AND NORTH BAY. FOR WHAT IT`S WORTH, LATEST LIGHTNING DETECTION NETWORKS ARE ALREADY PICKING UP SOME LIGHTNING NEAR THE UPPER LOW. THUNDERSTORM CHANCES LOOK MORE LIKELY ON SATURDAY AS THE COLDER CORE UPPER LOW MOVES THROUGH. BEST CHANCE FOR THUNDERSTORMS WILL BE OVER THE NORTH AND EAST BAY SATURDAY AND EARLY SATURDAY NIGHT. REGARDLESS OF THUNDERSTORM CHANCES, LOCATIONS AROUND THE REGION HAVE A GOOD SHOT AS SEEING SOME SCATTERED SHOWERS. [DISC] Precipitation chances end by Sunday, but cooler weather continues. Medium range models keep flip-flopping on precipitation chances early next week as another upper low develops off the coast. Dry forecast for now, but temperatures will remain cooler through Wednesday. ∨ PRECIP CHANCES END BY SUNDAY, BUT COOLER WEATHER CONTINUES. MEDIUM RANGE MODELS KEEP FLIP-FLOPPING ON PRECIP CHANCES EARLY NEXT WEEK AS ANOTHER UPPER LOW DEVELOPS OFF THE COAST. DRY FORECAST FOR NOW, BUT TEMPERATURES WILL REMAIN COOLER THROUGH WEDNESDAY. [DISC] Aviation. As of 10:30am PDT Thursday… for 18Z [11am PDT] TAFs [Terminal Aerodrome Forecasts]. VFR [Visual Flight Rules] conditions the rest of the afternoon and into this evening as stratus has returned to the coast. Expecting an earlier return of low clouds this evening compared to yesterday as the marine layer deepens ahead of an approaching low pressure system. MVFR [Marginal Visual Flight Rules]/IFR [Instrument Flight Rules] ceilings expected overnight with patchy fog along the coast and in the North Bay. Onshore winds this afternoon 10–15 kt [12–17 mph] with locally higher gusts possible at KSFO [San Francisco CA]. ∨ .AVIATION...AS OF 10:30 AM PDT THURSDAY...FOR 18Z TAFS. VFR CONDITIONS THE REST OF THE AFTERNOON AND INTO THIS EVENING AS STRATUS HAS RETURNED TO THE COAST. EXPECTING AN EARLIER RETURN OF LOW CLOUDS THIS EVENING COMPARED TO YESTERDAY AS THE MARINE LAYER DEEPENS AHEAD OF AN APPROACHING LOW PRESSURE SYSTEM. MVFR/IFR CIGS EXPECTED OVERNIGHT WITH PATCHY FOG ALONG THE COAST AND IN THE NORTH BAY. ONSHORE WINDS THIS AFTERNOON 10-15 KT WITH LOCALLY HIGHER GUSTS POSSIBLE AT KSFO. [AVIA] Vicinity of KSFO [San Francisco CA]… VFR [Visual Flight Rules] through this evening. Southerly flow expected overnight and tomorrow morning may prevent broken IFR [Instrument Flight Rules]/MVFR [Marginal Visual Flight Rules] ceilings from reaching the terminal, though it is possible. Onshore winds this afternoon around 15 kt [17 mph] with gusts near 20 kt [23 mph] possible. ∨ VICINITY OF KSFO...VFR THROUGH THIS EVENING. SOUTHERLY FLOW EXPECTED OVERNIGHT AND TOMORROW MORNING MAY PREVENT BKN IFR/MVFR CIGS FROM REACHING THE TERMINAL, THOUGH IT IS POSSIBLE. ONSHORE WINDS THIS AFTERNOON AROUND 15 KT WITH GUSTS NEAR 20 KT POSSIBLE. [AVI2] SFO [San Francisco CA] bridge approach… similar to KSFO [San Francisco CA]. ∨ SFO BRIDGE APPROACH...SIMILAR TO KSFO. [AVI2] Monterey Bay terminals… VFR [Visual Flight Rules] conditions into this evening before IFR [Instrument Flight Rules] ceilings expected to return and prevail overnight. Patchy fog possible late tonight/early tomorrow morning. Onshore winds this afternoon 10–15 kt [12–17 mph]. ∨ MONTEREY BAY TERMINALS...VFR CONDITIONS INTO THIS EVENING BEFORE IFR CIGS EXPECTED TO RETURN AND PREVAIL OVERNIGHT. PATCHY FOG POSSIBLE LATE TONIGHT/EARLY TOMORROW MORNING. ONSHORE WINDS THIS AFTERNOON 10-15 KT. [AVI2] Marine. As of 8:23am PDT Thursday… generally light to locally moderate northwest winds for today before gradually turning southerly tomorrow ahead of a low pressure system approaching from the southwest. This system will bring a slight chance of thunderstorms over the northern waters Friday night into Saturday as well as locally gusty winds for portions of the inner coastal waters and bays. Mixed seas will persist with a short period northwest swell and a longer period southwest swell. A longer period northwest swell will arrive tomorrow. ∨ .MARINE...AS OF 8:23 AM PDT THURSDAY...GENERALLY LIGHT TO LOCALLY MODERATE NORTHWEST WINDS FOR TODAY BEFORE GRADUALLY TURNING SOUTHERLY TOMORROW AHEAD OF A LOW PRESSURE SYSTEM APPROACHING FROM THE SOUTHWEST. THIS SYSTEM WILL BRING A SLIGHT CHANCE OF THUNDERSTORMS OVER THE NORTHERN WATERS FRIDAY NIGHT INTO SATURDAY AS WELL AS LOCALLY GUSTY WINDS FOR PORTIONS OF THE INNER COASTAL WATERS AND BAYS. MIXED SEAS WILL PERSIST WITH A SHORT PERIOD NORTHWEST SWELL AND A LONGER PERIOD SOUTHWEST SWELL. A LONGER PERIOD NORTHWEST SWELL WILL ARRIVE TOMORROW. [AVIA] MTR Watches/Warnings/Advisories Today: Heat Advisory…CAZ506–507–510>513–516>518–528 ∨ .MTR WATCHES/WARNINGS/ADVISORIES... .TDAY...HEAT ADVISORY...CAZ506-507-510>513-516>518-528 [WARN] Public Forecast: DRP/MM; Aviation: AS; Marine: AS ∨ PUBLIC FORECAST: DRP/MM AVIATION: AS MARINE: AS [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 28 May 10:20am PDT Today Mostly sunny, with a high near 67°. West wind around 7 mph. Tonight Patchy drizzle and fog after 2am. Mostly cloudy, with a low around 53°. West wind 5 to 9 mph becoming south after midnight. Friday Patchy drizzle and fog before 8am. Mostly cloudy, with a high near 68°. South wind 16 to 21 mph, with gusts as high as 25 mph. Friday Night A 30% chance of showers after 11pm. Mostly cloudy, with a low around 54°. Breezy, with a south wind 20 to 23 mph, with gusts as high as 29 mph. New precipitation amounts of less than a tenth of an inch possible. Saturday A 40% chance of showers. Partly sunny, with a high near 68°. South wind 16 to 21 mph, with gusts as high as 24 mph. New precipitation amounts of less than a tenth of an inch possible. Saturday Night Mostly cloudy, with a low around 52°. Sunday Mostly cloudy, with a high near 66°. Sunday Night Mostly cloudy, with a low around 51°. Monday Partly sunny, with a high near 65°. Monday Night Partly cloudy, with a low around 51°. Tuesday Mostly sunny, with a high near 68°. Tuesday Night Mostly clear, with a low around 51°. Wednesday Sunny, with a high near 68°. Today Mostly sunny, with a high near 67. West wind around 7 mph. Tonight Patchy drizzle and fog after 2am. Mostly cloudy, with a low around 53. West wind 5 to 9 mph becoming south after midnight. Friday Patchy drizzle and fog before 8am. Mostly cloudy, with a high near 68. South wind 16 to 21 mph, with gusts as high as 25 mph. Friday Night A 30 percent chance of showers after 11pm. Mostly cloudy, with a low around 54. Breezy, with a south wind 20 to 23 mph, with gusts as high as 29 mph. New precipitation amounts of less than a tenth of an inch possible. Saturday A 40 percent chance of showers. Partly sunny, with a high near 68. South wind 16 to 21 mph, with gusts as high as 24 mph. New precipitation amounts of less than a tenth of an inch possible. Saturday Night Mostly cloudy, with a low around 52. Sunday Mostly cloudy, with a high near 66. Sunday Night Mostly cloudy, with a low around 51. Monday Partly sunny, with a high near 65. Monday Night Partly cloudy, with a low around 51. Tuesday Mostly sunny, with a high near 68. Tuesday Night Mostly clear, with a low around 51. Wednesday Sunny, with a high near 68.
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