Why Summer's Lowest Tides Happen at Dawn on the Pacific Coast

On the US West Coast the two daily low tides are unequal, and Earth's tilt decides which one goes deep: in summer the deeper low lands in daylight, in winter it lands at night. The 2026 predictions agree. Across the 11 Pacific stations Tidewindow tracks, 6 AM is the busiest hour for daylight minus tides (56 of 535), and Seattle's deepest daylight low of the year, -3.8 ft at NOAA station 9447130, arrives at 11:20 AM on July 14.

If you keep one explanation, keep this one:

Pacific coast tides are mixed semidiurnal: two low tides a day, one much lower than the other, and how far the moon and sun sit north or south of the equator decides which one that is. In summer the geometry parks the deep low in daylight; in winter the same deep low happens at night.

The rest of this page is the supporting evidence, straight from NOAA's harmonic predictions.

Why does the West Coast get two different low tides a day?

The Pacific coast runs on what NOAA calls a mixed semidiurnal tidal cycle: "two high and two low tides of different size every lunar day." A lunar day is 24 hours and 50 minutes, the time it takes a point on Earth to rotate back under the moon, which is why everything in a tide table drifts about 50 minutes later each day.

The "different size" clause is the part that matters. On most West Coast days one low is unremarkable and the other is much lower, and only the lower one is a candidate for a minus tide (a low below 0.0 ft MLLW). So "when is low tide?" is really two questions on this coast, and only one of the two answers can put bare reef in front of you.

How does declination decide which low goes deep, and when?

Declination is the angle of the moon or sun north or south of Earth's equator. When the moon stands off the equator, NOAA's tide-physics primer explains, "the tidal force envelope produced by the moon is canted." Tilt the envelope and the two daily tides stop matching: as Earth rotates, your beach sweeps through a deep part of one tidal bulge and a shallow part of the other. That mismatch is the unequal-lows pattern above. The moon crosses the equator twice each month, so the inequality swells and shrinks on a monthly rhythm.

The sun runs the same geometry on a yearly clock. At the solstices, June 21 and December 22, the sun reaches "its maximum declination, i.e., its largest angle to the equator" — a line NOAA's declination page quotes from Sumich's 1996 marine-science text. And around new and full moon, NOAA notes, the solar tide "has an additive effect on the lunar tide" — the spring tides that push lows below zero roughly twice a month.

Put the monthly clock and the yearly clock together and you get the seasonal schedule. The University of Washington's LiveOcean tide primer states the result plainly: on this coast the lower low water comes in daytime in summer and at night in winter, because "the reason has to do with the tilt of the Earth's axis relative to the sun - the same tilt that causes Summer and Winter." One tilt, two jobs: it makes July warm, and it parks July's deep low in the light.

What does the 2026 data actually show?

If declination is really steering the deep low toward summer daylight, the year's schedule should be lopsided by hour. It is. Of the 940 minus tides NOAA predictions give Tidewindow's 11 West Coast stations in 2026, 535 land in daylight, and here is when their lows strike, by local hour:

Computed 2026-07-03 from NOAA predictions at Tidewindow's 11 West Coast stations (WA, OR, CA).

The shape is a two-humped day with a crater at lunch. The dawn hump peaks at 6 AM with 56 minus lows, five times the 11 apiece at 11 AM and noon, and 269 of the 535 (50%) land between 3 and 9 AM. A second, slightly smaller hump rises in the late afternoon, topping out at 51 events at 5 PM. Which months feed which hump is easy to see on the year heatmap, which draws all twelve months for any station at once.

What does a summer run look like at one station?

Seattle, NOAA station 9447130, makes a clean specimen. Its best stretch of 2026 is six consecutive July days, every low in daylight:

Computed 2026-07-03 from NOAA station 9447130 (Seattle) predictions. The full month is on the July 2026 calendar.

Read down the time column: 8:49, 9:40, 10:31, 11:20, 12:08, 12:55. Each low arrives 47 to 51 minutes after the previous day's — the lunar day's 50-minute lag, visible in a tide table. Over six days the low walks from mid-morning to just past lunch while deepening to -3.80 ft on July 14, the deepest daylight low of Seattle's year. This is what "summer minus tides land near dawn and midday" means in practice: the run starts early and the lunar lag walks it through noon. The Tide Window Finder turns any such run into an arrive-by time.

Where do the deep lows go in winter?

Nowhere. They just stop being visible. Seattle's monthly supply of genuinely low lows barely budges through the fall; what collapses is the daylight overlap:

Computed 2026-07-03 from NOAA station 9447130 (Seattle) predictions.

October has 22 lows below +1 ft MLLW, exactly as many as July, and not one daylight minus tide. Two things did that: the deep low crossed to the night half of the clock as the declination geometry reversed, and daylight itself contracted around what remained. The daytime water in winter runs high instead — that is king tide season, the same tilt showing its other face. You can watch the collapse month by month, for every station, on the daylight-minus-tide index.

The practical summary is unromantic. On the Pacific coast, a summer minus tide is a morning appointment, and the schedule was set by a 23.5-degree tilt long before you bought boots. The methodology page documents exactly how each window in these counts is defined and scored. Set the alarm; the moon will not move it for you.