Where the tide predictions come from
Every number on a tide page is NOAA's, fetched live when the page loads. This is which NOAA product, what the numbers mean — and, most importantly, what they leave out.
The source
Tide data comes from NOAA CO-OPS, the Center for Operational Oceanographic Products and Services — the official United States tide authority. The page asks its predictions API for one thing: the high and low list for that station.
Only the extremes are requested, because only the extremes are published as measurements. The smooth curve drawn between them on the chart is interpolation for readability, not additional data, and should not be read as a prediction of the level at some in-between minute.
Nothing is stored between visits. There is no copy of NOAA's tables on this site to go stale, and no nightly job that could quietly fail and leave last month's numbers up.
What "astronomical prediction" excludes — read this one
A tide prediction models the gravitational pull of the moon and the sun on the water at that station, worked out from decades of observation at that specific place. It is genuinely good at that. It knows nothing whatsoever about the weather.
Wind. A sustained onshore wind piles water against the coast; an offshore wind pushes it away. Either can be worth a foot or more.
Barometric pressure. Low pressure lets sea level rise; high pressure presses it down.
Storm surge. During a storm, the difference between predicted and actual is the entire point of a surge warning.
Rain and river discharge. In an estuary, upstream rainfall changes the level days later.
So: do not use these pages for navigation or for any decision where being wrong is dangerous. Not for clearing a bar, not for anchoring, not for judging whether a sandbar walk is safe on the way back. For those, go to NOAA directly and check current conditions with the U.S. Coast Guard. These pages are for knowing roughly when the water will be high on Saturday.
Reading the numbers
Most stations see two highs and two lows a day, of noticeably unequal size — the diurnal inequality. Some places on the Gulf coast get one of each. Both are in the data as NOAA publishes it; neither is an error.
How a city gets matched to a station
Sun and moon pages exist for cities; tide stations exist where NOAA has instruments, which is rarely the middle of a city. The two are matched once, when the site is built: each station is paired with the nearest city that has a sun page, within 35 miles and in the same state. Outside that radius, no link is offered.
That is why most city pages have no tide link and are not missing one. The United States has coasts; Kansas does not. A matching rule that reached further would produce a link that technically resolved and practically misled.
The links are also checked for reciprocity at build time: if a station links to a city, that city must link back, and the build fails if it does not. A one-way link renders perfectly and silently dead-ends anyone following it.