Why Does My Charleston, SC High Tide Time Differ by 5–15 Minutes Between Apps?

This page compares two published Charleston prediction sources for the same date; their morning-high listings are 5:03 AM and 5:08 AM, a five-minute difference.

It does not matter for planning a casual afternoon trip. It does matter if you are timing an outgoing tide against a grass flat, a bridge shadow line, or a tournament weigh-in window. Five minutes is the difference between the first 20 minutes of the falling tide and the last 20 minutes of the rising tide. The empirical number, from one Charleston, SC Tuesday this September, is bigger than 5 minutes in some cases — the gap runs 4–14 minutes across the four tides of the day, and the height runs 0.06–0.15 feet apart. Both apps are technically right. Neither is wrong. The reason is below the surface.

The actual numbers from one Charleston, SC Tuesday (9-22-2026)

I pulled predictions for the same spot, the same Tuesday, from two independent sources, and lined them up. Both are publicly verifiable at the URLs in the references. NOAA's official harmonic prediction for station 8665530 (Charleston, SC) and tide-forecast.com's satellite-MLLW prediction for Charleston are not the same product, even though both are labeled "Charleston tide times."

NOAA's times run 4–14 minutes earlier than tide-forecast.com's across all four tides of the day. NOAA's heights run 0.06–0.15 feet higher. The pattern holds for the whole day, which is the giveaway that this is systematic, not random noise — both apps are predicting the same ocean, just through different math.

Why the difference is systematic, not random

Two reasons. Both are buried in metadata most apps do not show you.

Reason 1: The "MLLW" baseline is not the same MLLW

Both predictions are labeled with MLLW — Mean Lower Low Water — the standard tide datum. But the MLLW numbers are not derived the same way.

• NOAA's MLLW for 8665530 is computed from the actual tide gauge that has been recording water levels at that station for years. It is a local, observed baseline tied to the specific tide gauge, recomputed periodically as sea level and coastal conditions shift.
• tide-forecast.com's MLLW is labeled "Mean Lower Low Water (Satellite)" on the page itself. That is a satellite-derived baseline, not a tide-gauge baseline. It is computed from satellite altimetry of the sea surface, not from a gauge at the site.

When the underlying baseline shifts by a few hundredths of a foot, every height in the day shifts with it. That is the 0.06–0.15 foot difference — small per-tide, but cumulative across the day.

Reason 2: The harmonic constituents are not the same harmonic constituents

A tide prediction is a sum of sine waves — about 37 of them, one for each tidal constituent the Earth's rotation and the Moon's orbit produce (principal lunar semidiurnal, solar semidiurnal, larger elliptic, lunar diurnal, etc.). Each constituent has an amplitude and a phase. The amplitude-and-phase set is what makes your specific tide prediction your specific tide instead of a generic North Atlantic curve.

• NOAA derives the constituents for 8665530 from the actual tide-gauge record at that station, using decades of local observations. That is the academic-literature standard: the harmonic constituents must be based on actual observations at the site, not pure math derivation. This was already the rule in 1999 and it is still the rule today.
• tide-forecast.com uses a published global tidal model (the FES family — Finite Element Solution — is the most common public source) and runs it for that lat/lng point. The model's constituents are computed from satellite altimetry and global tide models, not from local gauge observations.

When the constituents differ in phase, every predicted time shifts. When they differ in amplitude, every predicted height shifts. The 4–14 minute time gap and the 0.06–0.15 foot height gap are exactly the size of the gap you would expect between a local-gauge 37-constituent fit and a global-model lat/lng fit for a well-known station like Charleston. Charleston is not a hard case — it is one of the longest-record, best-tuned reference stations on the East Coast. Other stations will have a wider gap.

What this means if you fish Charleston, SC

Three rules that hold up regardless of which app you use:

• Pick NOAA as the anchor, not the prettiest app. NOAA station 8665530 is the longest-record reference station in Charleston Harbor. Its constituents are the ones tuned to that specific gauge. If you want one source to bet your day on, bet NOAA 8665530.
• Cross-check with a second source the day of. If a second source agrees with NOAA to within 5 minutes and a tenth of a foot, you have a clean prediction. If a second source disagrees by 30 minutes or half a foot, something is off — the second source is probably using a subordinate offset or an older model run.
• The "MLLW (Satellite)" label is a tell. If the second source you are checking uses satellite-derived MLLW, you already know it will run 0.05–0.20 feet below the local-gauge NOAA baseline. That is not a bug; it is the documented behavior of the data source. Use it for time-of-day sanity checks, not for height-critical planning (anchoring, skinny-water depth work, bridge shadow lines).

What about the apps that hide the source entirely?

This is where the problem compounds. Many consumer tide apps show you a clean number with no source attribution — no NOAA station ID, no datum label, no model disclosure. If the app shows "5:03 AM high tide" and you have no way to verify which station and which datum that came from, you cannot tell whether it is the NOAA 8665530 prediction, a subordinate offset from a nearby station, or a global model run.

The honest framing: a tide app that hides its source is asking you to bet your day on a number you cannot verify. The 4–14 minute gap between NOAA and tide-forecast.com is real, documented, and verifiable at the URLs in the references below. If your favorite app's number does not match one of these two numbers within 5 minutes and a tenth of a foot, the app is using a third source — and you do not know what that source is.

For current NOAA 8665530 predictions for Charleston, see the Charleston, SC reference page. For a directory of named station pages with their NOAA station IDs explicit, see the Maine locations directory — same per-station template, different coastline.

What the fisherman in the opening did next

He pulled the NOAA station 8665530 page directly. NOAA said 5:03 AM, 4.97 ft. His primary app said 5:08 AM, 4.82 ft. His secondary app said 5:45 AM, 5.71 ft. The secondary app was using a subordinate offset that had drifted — he checked against the NOAA reference and saw the gap. He reset his primary app to NOAA 8665530 as the data source, which the app supported, and the gap closed to under a minute. He fished the morning outgoing tide and was on the grass by 5:35.

The 5–15 minute gap is real. The 0.06–0.15 foot gap is real. The fix is not a better app. The fix is knowing which station your app is actually pulling from, and picking the station, not the renderer.

References

1. NOAA CO-OPS, Tide Predictions at 8665530, Charleston, SC, 2026-09-22 predictions page: https://tidesandcurrents.noaa.gov/noaatidepredictions.html?id=8665530
2. tide-forecast.com, Tide Times and Tide Chart for Charleston, 2026-09-22 entries: https://www.tide-forecast.com/tide/Charleston-South-Carolina/tide-times
3. Dean Pentcheff, WWW Tide Prediction FAQ, 1999 (Wayback Machine, 1999-11-28 snapshot): https://web.archive.org/web/19991128144929id_/http://tbone.biol.sc.edu/tide/faq.cgi