The highest tide of the fall arrives at 9:50 on a Tuesday morning in late October

The highest tide NOAA predicts for St. Augustine this fall arrives at 9:50 on the morning of Tuesday, Oct. 27.

At the city dock the prediction is 5.83 feet above mean lower low water. The average daily high tide at that same dock is 4.98 feet. So the water is expected to stand about ten inches higher than it stands on an ordinary morning, at an hour when most people are at work and every school in the county is in session.

Wednesday, Oct. 28 is predicted within a hundredth of a foot of the same height, at 10:43 a.m. Monday the 26th and Thursday the 29th are within a tenth of a foot of it. The whole week of Oct. 25 through Oct. 30 is elevated.

This is arithmetic about the moon, and it is worth being precise about what it is not. A tide prediction is a harmonic calculation from the positions of the sun and the moon. It contains no wind. It contains no rain. It contains no storm. What it gives you is the floor the water starts from, and everything else that happens that week gets added on top of it.

What the numbers mean at ground level

Tide predictions are published above mean lower low water, which is an average of the lower of each day’s two low tides over the years 1983 through 2001. It is a useful reference for a boat and a useless one for a street, because it is measured from the bottom of the tidal range rather than from anything you can stand on.

The datum that matches ground elevation is NAVD88, the vertical datum surveyors and floodplain maps use. At the city dock NOAA publishes both, and the conversion is a subtraction of 2.93 feet.

So the Oct. 27 prediction of 5.83 feet MLLW is 2.90 feet NAVD88. If you have a survey, an elevation certificate or a flood map for a property near the water in St. Augustine, that is the number to hold it against.

Three of the four nearby stations tell the same story in their own datums:

Four stations, four different ranges, one week. That agreement is the point. It is not a local quirk of one gauge.

The other dates

Late October is the peak, but it is not the only elevated period between now and the middle of November. Three earlier clusters run high, all of them peaking in late morning at the city dock.

• Sept. 27 to Oct. 1. Peaks Tuesday, Sept. 29 at 10:59 a.m., 5.58 feet MLLW, 2.65 NAVD88, about seven inches over an ordinary high.
• Oct. 9 to 12. Peaks Saturday, Oct. 10 at 8:58 a.m., 5.61 feet MLLW, 2.68 NAVD88.
• Nov. 8 to 9. Peaks Sunday, Nov. 8 at 7:33 a.m., 5.48 feet MLLW, 2.55 NAVD88.

Each of these runs is the same phenomenon as the October one and a few inches shy of it. In every case the morning tide is the higher of the day, sometimes by more than half a foot, which is why a neighborhood that stays dry at 10 p.m. can have water in it at 10 a.m.

What we cannot tell you, and why

There is no NOAA station in St. Augustine that records what the water actually did.

The four stations above are prediction stations. They publish a calculated tide and a set of datums, and the datums at the city dock were derived from a two-year analysis in 1977 and 1978, referenced to a control station at Mayport. They do not publish a measured water level, because there is no continuously recording gauge at any of them.

The nearest station in NOAA’s water level index is Mayport, at the mouth of the St. Johns River, about thirty-five miles north and in a different river system. Fernandina Beach is the next one up.

That is a real gap. It means the Tattler can tell you what the water is predicted to do in St. Augustine on Oct. 27, and can tell you afterward what it did at Mayport, but cannot tell you from a federal gauge what it did on Avenida Menendez. Anyone reporting a St. Augustine tide height as an observed fact is reporting it from somewhere other than NOAA, and should say where.

Predictions above were retrieved Wednesday, Sept. 9. They do not change, because the moon does not revise its schedule. Everything else about those mornings will be decided by the weather in the week before them.