Findings · Original analysis · Oct 2026 – Sep 2027
The Tide Clock: low tide at dawn almost never happens here.
Six mornings a year, none of them between September and April. The lows at La Saladita fall at about 1–4 am and 1–4 pm in every month, because the tide here is mixed and its twice-daily part keeps solar time. From October to March a dawn session is on a high or rising tide, and the dropping mid tide the forecast engine wants for the inside section is a late-morning tide.
Harmonic tide model · UHSLC Zihuatanejo gauge 2011–13 · 365 days × 24 hourly labels · local time UTC−6 · 26 September 2026
Mornings with a low tide between 06:00 and 09:00
1.6%
6 of 365 days · all May–Aug · Sep–Apr: none
Mornings with a high tide in that window, Oct–Mar
71–100%
Oct 100 · Nov 90 · Dec 90 · Jan 87 · Feb 86 · Mar 71
Where the lows fall (share of all lows in the year)
01–04h & 13–16h
40% + 41% · two three-hour bands hold 81%
Form factor (K1+O1) / (M2+S2)
1.19
mixed · semidiurnal band 58% solar · S2 0.114 m > M2 0.066 m
Surfers carry a tide rule from other coasts: get up early, catch the low, ride the push. At La Saladita that rule does not fit the clock. We took the site's own tide model — the harmonic fit to the University of Hawaii Sea Level Center's Zihuatanejo gauge that has driven the forecast since 24 September 2026 — and asked it one plain question for every day from 1 October 2026 to 30 September 2027: at what local hour do the highs and lows fall, and what does the engine call the tide at 5, 6, 7 … 13 o'clock?
The answer is a clock, not a calendar. Lows fall at about 1–4 in the morning and 1–4 in the afternoon in every month of the year. Highs fall at about 7–10 in the morning and 7–10 at night. The lunar part of the tide moves the heights around a lot — which of the two lows is the lower one, whether there is one low or two, how big the day's range is — but it barely moves the hours, because at this gauge the twice-daily tide is mostly solar and the solar tide keeps clock time.
The practical consequences are simple. A dawn session between October and March is on a high tide on 71–100% of days and on a rising tide on almost all of the rest. A low tide between 6 and 9 am happens on six mornings in the year, all of them from May to August. And the engine's rule that the inside section “connects on a dropping mid tide” can only be satisfied from mid-morning on — 10 to 12 o'clock in the dry season — which is exactly when the wind turns (see the wind clock). The same year run one year earlier (October 2025 – September 2026) gives the same answers within a few points; the differences are listed in the tables.
The clock · 365 days · local time UTC−6
Two low bands, two high bands, in every month.
Every high and low the model predicts for the year, binned by local hour. The tide's lows sit in two three-hour bands and its highs in two more; the six hours from 05:00 to 11:00 hold 2.5% of the year's 681 lows.
Local hours
Lows (% of all lows)
Highs (% of all highs)
00–03h
24.1%
1.2%
03–06h
23.1%
1.5%
06–09h
0.9%
32.4%
09–12h
1.6%
14.2%
12–15h
28.5%
1.5%
15–18h
18.8%
2.1%
18–21h
0.6%
34.0%
21–24h
2.5%
13.2%
Probability that the tide is in its “high” phase, by local hour and month. Each cell is the share of the month's days on which the engine's own phase label (tidePhaseAt: the top 20% of the surrounding 14-hour rise-and-fall) is high at that hour. The rust box is the 06–09h dawn window. The dark band sits on dawn from September to March, slides to mid-morning in April and May, and thins out in June and July — the months when the day's single diurnal high drifts across the morning. Hover a cell for the full four-way split.
Key finding
A low tide between 06:00 and 09:00 falls on 6 of 365 mornings (1.6%). Between September and April it does not happen at all.
The six are 9 May, 7–8 June, 6–7 July and 5 August 2027 — the days around the summer lunar standstill when the day collapses to a single low and that low drifts through the morning. The check year (October 2025 – September 2026) gives five such mornings (20 April, 19 May, 17–18 June, 17 July 2026), 1.4%. Over the whole year the engine labels the tide dropping-mid at 07–09h on 3% of hour-slots and low on 3%; it labels it high on 58% of 07:00 slots, 77% of 08:00 slots and 77% of 09:00 slots, and rising-mid on most of the rest.
Dawn, month by month · primary year Oct 2026 – Sep 2027 · check year Oct 2025 – Sep 2026
October to March: high or rising. June and July: the only months with a real spread.
“Low between 06–09h” and “high between 06–09h” count days on which an extreme of that kind falls inside the window. The phase columns pool the engine's hourly label over the 07:00, 08:00 and 09:00 slots (and 10:00–12:00 for the late-morning column). The last two columns repeat the first two for the check year.
Month
Low between 06–09h
High between 06–09h
High at 07–09h
Rising at 07–09h
Dropping at 07–09h
Dropping at 10–12h
Low 06–09h (check year)
High 06–09h (check year)
Jan
0%
87%
94%
3%
3%
73%
0%
94%
Feb
0%
86%
82%
14%
4%
63%
0%
79%
Mar
0%
71%
74%
17%
9%
50%
0%
65%
Apr
0%
60%
53%
32%
11%
33%
3%
50%
May
3%
32%
43%
40%
9%
26%
3%
36%
Jun
7%
10%
36%
50%
3%
16%
7%
23%
Jul
7%
10%
39%
52%
2%
16%
3%
13%
Aug
3%
32%
61%
34%
1%
28%
0%
36%
Sep
0%
60%
83%
16%
1%
61%
0%
63%
Oct
0%
100%
96%
1%
3%
79%
0%
100%
Nov
0%
90%
93%
0%
7%
89%
0%
93%
Dec
0%
90%
95%
0%
5%
85%
0%
90%
Year
1.6%
61%
71%
22%
5%
51%
1.4%
62%
Reading the table
The dropping mid tide is a 10-to-12 o'clock tide from September to March, and hardly a morning tide at all in June and July.
In November the engine calls the 10–12h slots dropping-mid 89% of the time; in December 85%, October 79%, January 73%. Over the same months it calls the 07–09h slots dropping-mid on 3–7% of slots. In June and July the high itself has slid to late morning (the 10–12h slots are high 50–61% of the time), so the dropping tide moves to early afternoon. The check year moves individual cells by up to 13 points (March high-at-dawn 65% against 71%, June 23% against 10%) but never moves a month across the line between “dawn is high” and “dawn is not”.
Why · constituent amplitudes from the harmonic fit
A mixed tide whose twice-daily half keeps solar time.
A tide is a sum of cosines with fixed periods. The four that matter here: K1 (23.93 h, luni-solar declinational) 0.123 m, O1 (25.82 h, lunar) 0.092 m, S2 (12.00 h, solar) 0.114 m, M2 (12.42 h, lunar) 0.066 m. The conventional form factor F = (K1+O1)/(M2+S2) is 1.19: on the Courtier scale that is “mixed, mainly semidiurnal” (the boundary to “mainly diurnal” is 1.5), and the two bands are in fact about equal — 0.27 m of once-a-day amplitude (K1+O1+P1+Q1) against 0.24 m of twice-a-day (M2+S2+N2+K2). Fourteen per cent of days have only one low.
What sets the clock is the unusual make-up of the twice-daily band. At most ocean coasts M2 is two to three times S2. Here S2 is nearly twice M2: 58% of the semidiurnal amplitude is solar. A solar cosine repeats every 12.00 hours exactly, so its lows fall at the same local hours on every day of the year — from the fitted phase, 02:34 and 14:34. The lunar M2 drifts 50 minutes a day around the clock, but at 0.066 m it can only pull the solar lows an hour or two either way, not relocate them. That is the 01–04h and 13–16h band in the histogram above.
The once-a-day band is what varies. K1 loses four minutes a day against the clock, so its high circles the 24 hours once a year; O1 loses 1.8 hours a day, so it circles once a fortnight. Together they decide which of the two solar lows is deeper and how big the day is (large near the moon's maximum declination, small near zero), and in May–July their combined high lands in the late morning, which is when the day sometimes collapses to a single low and one of those lows can land at dawn. In the dry season the combined diurnal high sits on the morning solar high, and dawn is a high tide almost without exception.
The clean way to say it: the moon decides how much the tide moves here; the sun decides when.
Finding
Compared with Acapulco, the unusual part of the Zihuatanejo tide is a weak Moon tide (M2), not a strong Sun tide.
The Zihuatanejo gauge (2011–2013) and the Acapulco gauge (2007–2011) were each fitted with the 18.6-year lunar corrections applied, so the two records can be compared across their different years. The once-daily tides agree between the gauges, and their clocks match (r = 0.992 over the 53 days the records overlap). In the twice-daily band, Zihuatanejo's M2 is only 0.35 of Acapulco's (95% range 0.34–0.37), while its neighbours are much closer to Acapulco's: N2 0.67, and the small NU2 and L2 about 0.9. The Sun's tide S2 and its close partner K2 are both larger at Zihuatanejo (1.43 and 1.29). The cause of the M2 deficit is still open: local ocean tide structure fits best, but an effect of the harbour gauge has not been ruled out. A month of the planned pressure logger at the break will show whether the open coast shares it; in every 30-day window of the gauge record, S2 exceeds M2.
Daily range · highest high minus lowest low, per local day · the engine flags ≥ 0.6 m
A small tide that is probably a fifth bigger right now than the model says.
The model's daily range averages 0.47 m and tops out at 0.75 m. The engine's “large tidal range” flag (≥ 0.6 m, −1 point for current at the point) fires on 18% of days in the primary year and 14.5% in the check year, most of them from January to March and July to September. Two adjusted columns follow, for the nodal reason explained below.
Month
Mean range (m)
Median (m)
p90 (m)
Days ≥ 0.6 m
≥ 0.6 m if ×1.2
≥ 0.6 m nodal-adjusted
One-low days
Mean range (check year)
Jan
0.49
0.49
0.67
32%
48%
42%
10%
0.50
Feb
0.48
0.48
0.69
32%
50%
39%
0%
0.47
Mar
0.48
0.49
0.64
26%
48%
32%
3%
0.46
Apr
0.45
0.45
0.56
10%
23%
13%
17%
0.46
May
0.45
0.46
0.53
0%
32%
10%
26%
0.47
Jun
0.47
0.47
0.58
3%
40%
27%
30%
0.48
Jul
0.48
0.50
0.68
32%
52%
39%
13%
0.46
Aug
0.46
0.49
0.66
29%
45%
42%
3%
0.42
Sep
0.43
0.42
0.63
23%
40%
30%
3%
0.44
Oct
0.47
0.45
0.60
13%
32%
19%
16%
0.45
Nov
0.46
0.46
0.56
0%
23%
10%
27%
0.46
Dec
0.47
0.44
0.62
16%
36%
19%
19%
0.49
Year
0.47
0.46
0.64
18%
39%
27%
14%
0.46
Nodal caveat · medium confidence
The fit was made near the bottom of the 18.6-year nodal cycle; 2026–27 is near the top. Ranges are probably underestimated by about 15–25%.
The moon's orbit precesses over 18.61 years and modulates the diurnal constituents strongly. The gauge record behind the model (March 2011 – July 2013) sat at a node longitude of about 246°, where Schureman's factors give fK1 = 0.96 and fO1 = 0.93; the model carries no nodal correction, so it froze those depressed amplitudes in. The 2024–25 major lunar standstill (N ≈ 0) has just passed; for the middle of the primary year N ≈ 318°, fK1 = 1.09 and fO1 = 1.15. Relative to the fit, that is K1 × 1.14, O1 × 1.23 and M2 × 0.96. Re-running the year with those factors lifts the mean range from 0.47 to 0.50 m and the ≥ 0.6 m share from 18% to 27%; a flat × 1.2 on every day's range (the crude version) takes it to 39%. The clock does not move: the adjusted year still has no dawn low from September to April, because the factors scale amplitudes, not phases. Medium confidence because the fit's two years cannot resolve the node and the factors are being applied to a fitted rather than a true constituent set.
What it means · for the engine and for a surfer
The engine's best inside-section tide is a late-morning tide.
Engine rule
inside_section: optimal swell + dropping-mid → “connects”. From October to March that combination is essentially only available between 10 and 12 o'clock.
The rule is right about the water; it is the clock that is unforgiving. The hourly label is dropping-mid at 07–09h on 3% of slots year-round, and at 10–12h on 50–89% of slots from September to March. Those are also the hours in which the morning land breeze gives way to the onshore west or south-west wind (the pattern the score grades against, and the subject of the wind clock). So on this coast the “connects” inside and the clean wind rarely overlap in the dry season; the honest dawn call is a high or rising tide on the outside point, and the inside is a mid-morning gamble against the wind. In June and July the high slides to late morning and the dropping tide to early afternoon, after the wind has already turned.
For a surfer
Do not plan around a dawn low here. Plan around the wind, and take the tide as it comes: high at first light from October to March, low around lunchtime, high again at dinner.
Note that the model has not yet been tied to how the wave breaks — the engine's own note on the range flag says “effect on current at the point is not yet measured” — so this page describes the water level's clock, not the wave's. Until 27 September 2026 the forecast read the hourly tide label six hours early (a UTC/local mistake), which put “dropping-mid” on dawn slots that were in fact high; that is fixed, and none of the numbers here inherit it.
Honest caveats
This is a model of the Zihuatanejo gauge, 25 km down the coast, not a gauge at La Saladita. Fit RMS 6.4 cm on a tide with 18.6 cm standard deviation; heights within about 20% and times within about 15 minutes of published tables (checked against two tide services for 24–30 September 2026, r = 0.956). Any systematic offset between the gauge and the point is unmeasured; it would shift the clock by minutes, not hours.
No nodal correction. Amplitudes are frozen at their 2011–13 values. The range table's last columns show the likely correction for 2026–27; treat the ≥ 0.6 m shares as a floor. Phases and therefore the clock are unaffected.
“Dawn” is a fixed 06:00–09:00 window (start inclusive, end exclusive). Sunrise here runs from about 06:10 in June to 07:15 in January, so the window covers first light to mid-morning in every month rather than tracking the sun. Widening it to 05–10h adds five May–August lows and one 05-o'clock low in each of December and January; 06–09h itself stays empty from September to April.
The phase label is the engine's, with its thresholds.tidePhaseAt calls a level “high” when it is in the top 20% of the range seen in the surrounding ±7 hours and “low” in the bottom 20%; the mid labels follow the sign of the next 30 minutes. On the small-range days of a mixed tide these labels can flip on a few centimetres; the extreme-time counts (the low/high-between-06–09 columns) do not depend on the thresholds.
One year, and one check year. The clock repeats annually because it is solar; the lunar modulation (which low is lower, the range) shifts by about 11 days a year and the nodal cycle over 18.6 years. Percentages for a 28–31 day month move by a few points from year to year, as the check-year columns show.
Here the moon decides how much the tide moves and the sun decides when. The lows keep to one to four, morning and afternoon, twelve months a year. Dawn is a high tide from October to March, a rising one most other mornings, and a low one six mornings a year. The dropping tide the inside wants comes at ten or eleven — with the wind.
Synthesis · 26 September 2026
Sources and method
Tide model:functions/api/_tide_model.js — 33 harmonic constituents fitted by least squares to the UHSLC research-quality hourly record for Zihuatanejo (station 687, 17.637°N 101.558°W, 3 March 2011 – 10 July 2013, 13,206 valid hours), heights above MLLW, no nodal correction. The same tideExtremes() and tidePhaseAt() the forecast engine calls.
Windows: primary 1 October 2026 – 30 September 2027 (365 days); check 1 October 2025 – 30 September 2026. Local calendar days in America/Mexico_City (UTC−6, no daylight time). Every instant passed to the model is a true UTC millisecond.
Extremes: highs and lows per local day from a one-minute scan; “low/high between 06–09h” = at least one extreme of that type with 06:00 ≤ t < 09:00.
Hourly phase:tidePhaseAt at each of the 24 local hours, tallied per month (28–31 days × 24 slots); pooled shares reported for 07–09h and 10–12h.
Hour histograms: all lows and highs in the year by local hour; “clusters” are runs of hours each holding ≥ 5% of the year's extremes.
Daily range: highest minus lowest extreme within the local day (the engine's tideSwingsFromModel definition); share ≥ 0.6 m, share if every range is scaled by 1.2, and share from a re-prediction with K1, O1, Q1 and M2 rescaled by the ratio of Schureman node factors (fK1 = 1.006 + 0.115 cos N, fO1 = 1.009 + 0.187 cos N, fM2 = 1.0004 − 0.0373 cos N; N from the Meeus mean-node polynomial, evaluated at the fit midpoint and the window midpoint).
Form factor: F = (K1+O1)/(M2+S2) from the fitted amplitudes; classification on the Courtier scale (<0.25 semidiurnal, 0.25–1.5 mixed mainly semidiurnal, 1.5–3 mixed mainly diurnal, >3 diurnal). With the 2026–27 node factors applied F = 1.43, still under 1.5.
Script and artifact:scripts/analyze_tide_clock.mjs → functions/api/_findings_tide_clock.js (export FINDINGS_TIDE_CLOCK: every table on this page, the 24 × 12 phase strips for all four labels, the six dawn-low dates, and generated_at). Rerun with node scripts/analyze_tide_clock.mjs; it takes about a second.
Analysis run: 26 September 2026.
Cite as: “The tide clock: why low tide at dawn almost never happens at La Saladita”, La Saladita Field Guide, lasaladita.com/findings/the-tide-clock/, 2026-09-26.