Maui's Weak High Tides in Winter

 Here is a plot of NOAA high tide predictions for Kahului Harbor, Maui, in 2023:

Watersport folks know that the shallow reefs in Kanaha Beach Park become problematic during winter, and this plot shows clearly how different winter is from summer in Maui in the mid-day.

Terminology: "Diurnal" means a once daily tide.  Semi-diurnal means twice per day.  One might think that a word that begins "di-" would mean twice per day, since the Greek root prefix "di-" implies twice.  However, the etymology for diurnal is Latin, from dies (day), diurnus (daily).

Many US ports have mainly semi-diurnal tides.  That means there will usually be two unequal high tides and two low tides.  The highest high tide is known as the "high high water (HHW)", and the lower high tide is the "low high water (LHW)".  Maui has been classified as "mixed, mainly semi-diurnal."  In 2024 Maui will have 286 days with two high tides.  On semi-diurnal days, there is a high tide associated with the Moon's zenith transit overhead, and also with the Moon's zenith transit on the far side of the Earth.

The tidal range at Maui is relatively small, with the lowest tide in 2024 predicted at -.74 ft on Jan 12, and the highest tide at 2.99 ft on July 22 and Aug 20.  The mean water level for the year is 1.1 ft.  Why not 0.0?  In the US tides are measured from MLLW, or mean lower low water.  That is why you see fewer negative than positive tides.  All the predictions are offset upwards from mean sea level by +1.12 feet.

The highest high tides in the figure above are the blue dots.  These are above the yearly average HHW.  A green dot denotes an above average lower high water for the day.  Orange and red dots signify below average HHW and LHW.  A solid dot means the moon was above the horizon at the time, and a hollow dot means the moon was below the horizon.  A high tide with the moon above the horizon implies that the high tide is associated with an "upper transit" of the moon, where the moon passes through the local zenith meridian.  A hollow dot denotes a high tide associated with a "lower transit", the moon passing through the lower meridian on the opposite side of the planet.

In the figure above we can see that in the first half of the year, high tides seem to avoid the 6am to noon timeframe, and the tides in the early afternoon tend to be weak, red dots.  Here is another look at the issue, this time showing red where the tide is under 1.0 ft:

And zooming in on the first half of the year:

Is Maui Unusual?

For US ports, yes.  Have a look at San Francisco, below:

Very regular,  no holes.  You might object that San Francisco is at a very different latitude than Maui, 37N vs 21N.  There is no other US port at 20N, but here is Fiji at 20S:

However, take a look at this figure for Puerto Vallarta, 2024:

Puerto Vallarta is at the same latitude as Maui and shows a similar set of "holes", although they seem to be inverted in local time.  The closest US port would be San Diego, and here are the high tides for La Jolla:

La Jolla is significantly north of Puerto Vallarta, latitude 33N vs 21N, but you can almost see the "holes" developing.  What is it about the 20th parallel in the Pacific?