Marine Weather and Tides
3/30/2021 -- ANZ330 has been split into ANZ331 and ANZ332. Click EDIT in Marine Forecast and select your new zone.
1/26/2021 -- The West Coast Satellite images havd been updated. They now use GEOS-17.
1/1/2021 -- The 7 day forecast are now working well. Thank you NOAA for your support.
|Sunrise 6:50AM||Sunset 7:13PM||Friday September 17, 2021 3:36 PM PDT (22:36 UTC)||Moonrise 4:59PM||Moonset 2:19AM||Illumination 86%|
7 Day Forecast for Marine Location Near Carmel-by-the-Sea, CAHourly EDIT Help
Area Discussion for - San Francisco/Monterey Bay Area, CA  (on/off)  Help NOTE: mouseover dotted underlined text for definition
FXUS66 KMTR 171729 AFDMTR
Area Forecast Discussion National Weather Service San Francisco Bay Area 1029 AM PDT Fri Sep 17 2021
SYNOPSIS. Mostly cloudy this morning turning sunny by midday. Increasing clouds Saturday with a slight chance of North Bay showers. Frontal system passes over the Bay Area Saturday night into Sunday morning with some light rain showers North Bay down into the Santa Cruz mountains. Dry but continued warm Sunday afternoon. Offshore winds develop late Sunday night followed by rapid warming and drying Monday through much of next week.
DISCUSSION. as of 08:45 AM PDT Friday . Morning satellite imagery shows a blanket of 2400 foot marine stratus banked along the coast and inundating most urban locations this morning. The marine layer deepened over 2000 feet early yesterday morning in response to a passing weak shortwave trough and has since stabilized at its current level. 24 hour trends also indicate a rising pressure tendency (up 2.5-3.0 mb versus yesterday morning) and a slightly warmer, drier air mass in the subcloud level from 1000 feet and below to the surface. The morning synoptic pattern features a disorganized cutoff low lingering off the Central Coast this morning, with higher pressure towards the Four Corners and a much deeper low over the Gulf of Alaska. Today will be similar, if not slightly warmer across the interior versus yesterday and act as an intermediary day between yesterdays weaker, drier trough and tomorrows deeper, wetter trough. Expect a tangible shift in the weather pattern towards more fall like concerns including a chance of rain Saturday night into early Sunday and offshore wind concerns in the wake of the trough Sunday night into Tuesday.
The overnight discussion below covers the current forecast package in great detail, so please review it for more detail on both the weekend rain & SW wind and early next week offshore wind forecast.
PREVIOUS DISCUSSION. as of 3:30 AM PDT Friday . A fair amount of weather info to unpack this morning. In the short term the marine layer is 2500 feet deep with widespread low clouds covering much of the Bay Area, leaving only the higher peaks of the Central Coast cloud free. Low clouds will slowly mix out by midday leaving a sunny and pleasant Friday afternoon for the region. All eyes to our north where satellite shows impressive plume of moisture taking aim at the Pacific northwest. The origins of which were tropical activity off the coast of Asia that got entrained in the westerlies across the Pacific over the last week or so. TPW values in excess of 1.5 inches for Washington and Oregon which translates to values in excess of 200% of normal for mid Sept. Will be watching this boundary slip southward on Saturday in association with a well defined upper trough, jet dynamics and cold advection that will stay focused well to our north. However the moisture plume will slowly drift into Northern California, though most focused north of Cape Mendocino. Our pre-existing marine boundary layer may be influenced by the synoptic lift with some morning drizzle or even stray showers, especially near the coast Saturday morning. This is depicted in the model output and our forecast package this morning.
Clouds will increase across the North Bay by Saturday afternoon with some light rain showers possible lasting overnight. GFS shows the moisture plume narrowing and focusing as TPW values increase to around 1.80 inches over Cape Mendocino and then to around 1.50 inches over the Bay Area by Saturday evening. This simply means there will be plenty of moisture in the atmosphere but forcing will be the question. One thing nearly everyone will notice is much more humid or muggy conditions by Saturday afternoon as dewpoints will rise into the upper 50s and even lower 60s across the North Bay due to closer proximity to the moisture plume. Model cross sections suggest the moisture will be shallow and thus forcing will be most efficient against the topography of the North Bay coastal mountains of Sonoma county and perhaps around Mt Tamalpais. Some of our usual wet spots around Venado and Cazadero could overproduce in terms of rainfall forecast under this type of setup by Sunday morning. Official forecasts will keep values around 0.20-0.30 for northwest Sonoma county but local amounts of up to 0.50 inches would not be out of the question for these locations given the available moisture, long residence time and good orographics. That being said much of the Bay Area proper will see little or no rain from this event. Much drier for say Napa county with just some light rain showers forecast and a few showers for SF down the coastline towards Santa Cruz. Little or no precip for the East/South Bay and Central Coast with perhaps a hundredth around the Monterey peninsula Sunday morning as the boundary washes out.
The front will quickly wash out into a warm frontal boundary with a rapid rebound in temperatures by Sunday afternoon as high pressure quickly builds.
The question for Sunday night will turn to fire weather concerns as our first offshore wind pattern of the season looks to set up. (see fire weather section below for more specifics) Latest model trends suggest this wont be an archetype strong Diablo wind event but still have 72 hours to fine tune details. Current model simulations suggest moderate northeast winds will indeed develop Sunday night into Monday morning. Given the juicy airmass that will be in place it will take awhile to dry things out and humidity values look moist much of Sunday night with the strongest winds likely staying above 2500 feet.
By Monday we should see rapid warming and drying as north winds work down the Sacramento valley. Temperatures should jump well into the 80s to mid 90s with rapidly lowering humidity during the day Monday. Light offshore flow should continue Monday night into Tuesday with little or no humidity recovery but offshore winds look to remain pretty light at this time. Will continue to monitor closely for fire weather concerns as we balance rainfall potential over the weekend, a current jump in 10 hour fuel moisture and the misalignment of strongest offshore winds with lowest humidity values.
Long range ensembles and cluster analysis as well as strongly negative PNA give above average confidence for unseasonably warm and dry weather for much of next week.
AVIATION. as of 10:31 AM PDT Friday . For the 18z TAFs. The latest coastal profilers continue to pick up a 2000-2500ft marine boundary layer along our coast that has allowed for stratus to intrude all of the Bay Area and Monterey Bay terminals this morning. Nonetheless, are seeing a rapid deterioration of the stratus regionwide as the layer mixes out. Are on track for VFR across the terminals after 18Z, with coastal terminals (e.g. MRY/SNS/OAR) not clearing until 201-21Z. A typical sea breeze will bring onshore winds 10-15kts to most of the coastal and bay terminals, while winds will remain lighter inland (e.g. LVK/CCR). The stratus deck is set to return to the terminals starting this evening, with some slight uncertainty as to how early it will exit tomorrow morning. This is due to an approaching cold front that will begin moving in to the region by 15-18Z tomorrow, coinciding with the timing of the usual departure time of the stratus deck. For now, HREF members and the latest hi-res/short- range deterministic solutions continue trending towards a similar departure time for the marine stratus tomorrow morning.
Vicinity of KSFO . MVFR cigs have quickly mixed out, with SCT skies along the bridge approach that will continue to clear through the remainder of the morning (~18Z). VFR cigs will dominate the SFO/OAK vicinities for much of the early to mid- afternoon with 10-15kt onshore winds. Given the high relative humidity values and deep marine boundary layer, this VFR environment will be short-lived. The stratus deck is set to advect past the Golden Gate and funnel into the SF Bay starting around 3-4Z tonight before spreading into SFO by 6-8Z. Given the timing of the approaching cold front, may need to adjust the departure time of the stratus deck tomorrow morning. Will update as newer runs come in.
KSFO Bridge Approach . Similar to KSFO.
Monterey Bay . Brief VFR cigs this afternoon before clouds quickly return by the early evening. Onshore winds will remain light to moderate (5-15kts) along the immediate coast and northern Salinas Valley. Are on track for another evening/night of MVFR/IFR cigs along with reduced visibilities in the early morning hours. Skies are forecast to remain MVFR/IFR through the current TAF period.
FIRE WEATHER. as of 3:53 AM PDT Friday . Deep marine layer has brought high RH values overnight into Friday morning. Looking at 10 hour fuel sticks values are quite moist, so the marine layer over the last couple of days has mitigated some of the fine fuels though long term drought conditions continue to impact all fuel classes. For the weekend we'll first be watching precip chances and rainfall totals. Most likely location for wetting rains will be coastal Sonoma county and perhaps around Mt Tamalpais. Less than a tenth for Napa, San Mateo and Santa Cruz counties with little or no rain East/South Bay and Central Coast. As noted above the airmass will be very moist so RH values to remain very moist Sunday afternoon into Sunday night.
First shot of northeast winds to arrive just before midnight Sunday night across the Napa hills. At this time humidity values look too moist to support critical fire weather conditions as there will be a fairly long lag time to dry out the airmass. In addition the strongest winds will be at or above ridgetop level with of course stronger winds for Mt Saint Helena and Diablo.
Shortly after sunrise Monday winds could peak and humidity values start to plummet as the dry air comes down the Sac Valley. However we often will see widespread RH values in the teens and right now it looks more likely to be around ~30%. Do expect rapid warming during the day Monday with northeast offshore winds bringing widespread upper 80s and 90s to the valleys of the North and East Bay.
The rule of thumb is the first night (in this case Sunday night)is when winds are strongest and the second night (in this case Monday night) is when humidity values show no recovery but winds are lighter. This looks to be the case with RH values staying very dry, say below 30 percent in the hills Monday night into Tuesday morning. Initial indications suggest the northeast winds on Monday night will stay fairly light generally in the 15-20 mph for the hills. All this to say is for the moment we are not expecting the fuels, humidity and wind profiles to line up for critical offshore wind event at this time. However, we are still several days out and the large scale pattern suggests the potential is there. Will also need to monitor precip totals over the weekend. Right now the East Bay hills may be the most likely to receive no precip but still get breezy and dry weather Monday into Tuesday.
Longer range trends show above average confidence for unseasonably warm and dry weather for much of next week keeping fire weather concerns elevated through the extended forecast as we get deeper into peak fire season for the Bay Area.
MARINE. as of 08:48 AM PDT Friday . Locally breezy conditions persisting into mid morning over the northern outer waters. Winds will shift out of the south to southwest on Saturday as a cold front moves through the waters before shifting back out of the northwest behind the front. As an upper trough moves inland late this weekend, expect north to northwest winds to increase over the waters into early next week. Seas remain mixed with a short period northwest swell and a longer period weak southerly swell.
MTR WATCHES/WARNINGS/ADVISORIES. Tday. None.
PUBLIC FORECAST: DRP/RWW AVIATION: Diaz MARINE: Diaz FIRE WEATHER: RWW
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|Stations||Dist||Age||Wind||Air Temp||Water Temp||Waves||Pressure||DewPt|
|46028 - CAPE SAN MARTIN - 55NM West Northwest of Morro Bay, CA||97 mi||27 min||SSE 5.8 G 9.7||60°F||63°F||1016.7 hPa|
|46239 - Point Sur, CA (157)||99 mi||41 min||63°F||6 ft|
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|Monterey Regional Airport, CA||120 mi||43 min||SW 12 G 18||10.00 mi||Fair||68°F||53°F||59%||1017 hPa|
Link to 5 minute data for KMRY
Wind History from MRY (wind in knots)
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