Is it worth getting out of bed?
The rules tell you what you may take. They say nothing about whether the water is worth the drive. This is the other half: a go/no-go for every spot on our maps over the next three mornings, and the reasoning behind each one.
Reading the forecast for every spot…
Go means wind under 8 kt, under 3 ft of swell reaching the spot and under 1.5 ft of surge at 30 ft, every hour from 7 to 11 am. Rules of thumb, not a safety call: how each spot is judged. Visibility is not in the verdict, because no feed measures it at the spot: the Vis column shows a diver’s report from the last 48 hours when there is one, and otherwise an estimate from satellite water clarity, surge and rain.
Alerts
Get told when it’s good
Pick your side and your bar. The evening before a morning that clears it, you get one message. Quiet the rest of the time.
Wind is the first thing to check
A freediver spends most of the day at the surface, breathing up. Chop slaps you in the face, pulls the float line around and makes a clean recovery breath harder than it should be. Under about 8 knots the surface stays friendly. Past 13 it is work. Most Southern California mornings start light and the west or northwest sea breeze fills in after lunch, which is why the read below only looks at 7 to 11 in the morning.
Which way it blows matters as much as how hard. A wind blowing off the land leaves flat water close to that shore. At Catalina the usual northwest breeze leaves the front side, Avalon to Two Harbors, in its lee. During a Santa Ana the wind comes from the northeast and the geometry flips: the front side gets the chop and the back side goes glassy.
Swell: the period matters more than you think
On a boat, long-period swell is the gentle kind. Underwater it is the opposite. The water under a passing wave moves back and forth, and that motion fades with depth far more slowly when the period is long. At thirty feet a fifteen second swell keeps most of its push; a six second wind swell keeps about a third. That back-and-forth is surge. It turns a shallow reef into a washing machine and lifts the sand that ruins visibility.
Summer south swells, including the ones Mexican hurricanes send north, are the classic case: modest on the forecast, long period, and hard on every south-facing shore.
Direction decides which side to dive
An island is a wall. Swell from the northwest wraps poorly around to the far side, and south swell does not reach a north-facing cove. So the same forecast can be rough on one side and flat on the other, and picking the side is most of the decision. That is why the live read scores each side separately.
Visibility: nobody measures it at your spot
It is the number every diver wants and no public feed reports it directly. Satellites come closest: NOAA’s VIIRS instruments measure how fast light fades in the top of the water column (a value called Kd490) and how much chlorophyll is in it, about two days behind and only where there were no clouds. The estimate here starts from that and then reads the things that change the water near the bottom. Rain washes runoff off the mainland and out of harbors for days afterward. Big or long-period swell lifts sediment off the bottom. Spring upwelling brings cold, green, plankton-rich water. Long calm spells, offshore wind and the late summer and fall months tend to bring the clearest water, especially at the islands. The best visibility report is still the person who dove there yesterday, which is why each live read carries the reports divers post, and why yours helps the next person.
Temperature, current and tide
Surface temperature sets your wetsuit, but a thermocline can sit a few metres down and drop it sharply. Tidal current barely matters in a cove and matters a great deal at a point or an offshore pinnacle such as Farnsworth Bank, where slack water is the time to be on the line.
Alerts end the conversation
A small craft advisory or a gale warning from the National Weather Service overrides everything above. The live read shows any active marine alert for the zone first, and says so plainly when it could not check.
How each spot is judged
These are rules of thumb, written down so you can disagree with them. They have not yet been calibrated against a season of real dive logs, and they are not a safety call. Every spot gets its own forecast and its own verdict for the worst hour between 7 and 11 in the morning. The verdict is the worst of three readings.
- Wind at the spot. Go under 8 kt, Marginal up to 14, No-go above. If land lies within 2 km upwind, the spot is in the lee and the wind counts for 45% of itself; within 8 km, 75%.
- Swell reaching the spot. Each swell in the forecast is followed from the direction it arrives. If open water lies that way, all of it reaches the spot. If the island is in the way, it fades to nothing over 20 degrees into the shadow, widening to 40 for long-period swell, which bends further round a headland. Go under 3 ft, Marginal up to 5.
- Surge at 30 ft. What you feel on the bottom. Wave motion fades with depth, slowly for long-period swell and fast for short, so the same swell height can be a gentle rock at depth or a washing machine. Go under 1.5 ft of motion left at 30 ft, Marginal up to 2.25.
- NWS alerts. A small craft advisory or anything stronger makes every spot in the zone No-go for the mornings it covers.
- Rain. 0.25 inches or more over three days flags likely runoff. It does not change the verdict, because nobody can say how much it costs in visibility.
Where the exposure comes from
Nobody typed in which way a spot faces. From each spot a line is run out every five degrees across the same coastline the maps draw, and the distance to land is recorded. The little compass beside each spot shows the result: shaded where open water runs out 25 km or more, blank where land is in the way, with an arrow for the biggest swell. Swell from a blank direction is blocked; wind from a direction where land is close is offshore.
Two limits. Land outside each map is not counted, so a far island’s shadow only shows up as far as the swell model itself carries it. And the forecast is a model of open water a few kilometres out, not of your cove.
Alerts work by side of the water. Each side takes the verdict of the best spot on it, so an alert never disagrees with the board.
Have a morning that scored wrong? That is exactly the data that tunes these numbers. Post a visibility report from the live read above and say what the water was doing in the note.