Lt. Island Tides

When is the road to Lieutenant Island (Wellfleet, MA) underwater? Live at https://ltislandtides.com. The old address, https://aliabuckner1.github.io/lieutenant-island/, and the short link https://ltisland.netlify.app both forward there.

Residents of Lieutenant Island use printed tide calendars, built from NOAA's predictions for Wellfleet, to know when the road on and off the island will be flooded. Those predictions have grown less reliable. They're pinned to 1983–2001 sea level, and the sea at Boston is about 6½ inches higher now. They leave out wind and air pressure. And Wellfleet has no tide gauge of its own, so its numbers are Boston's, scaled and shifted. Today 94% of high tides come in above the prediction, by about 8 inches on average.

This site corrects for that. It adds today's sea level and a weather model (National Weather Service wind, Open-Meteo pressure) built from more than 3,000 past high tides, leans on NOAA's live Boston tide gauge for the next day or so, and compares the result with the road's height from USGS airborne lidar. Islanders can also report what they see at the road. Reports are recorded in a Google Sheet and can help pin down the road's real height and check the forecast against what actually happens at Wellfleet, so the model can be improved over time.

How well it works

Replaying about 900 high tides from 2024–26 with the weather forecasts available at the time, against what Boston's tide gauge measured:

Forecast	Typical error
NOAA's prediction (what the calendars use)	about 9 in
This site, a day ahead	about 4 in
This site, a week ahead	about 5 in

How it works

The page runs entirely in the browser. On every load it fetches NOAA tide predictions, the live Boston tide gauge, the NWS wind forecast and Open-Meteo pressure, then compares the result with the road's low point: 9.9 ft above MLLW, measured from the road on 15 Sep 2026 (nine tape-measure depths through one tide, data/road_measurements.csv). It had been 10.4 ft from lidar, good to about ±6 in. No server and no build step. Between NOAA's high and low times the curve is Boston's, stretched to fit, with a quicker fall off the top: depths measured at the road every minute or two (data/road_measurements.csv) fit Boston's shape on the rising tide but run about 14 min ahead of it on the falling tide, so the curve after high water is pulled 14 min earlier. The USGS harbor gauge two miles away (2017–22, scripts/wellfleet_shape.py, data/wellfleet_shape/results.txt) shows the same quicker fall, at about 8 min there.

What's where

Folder	What it is
apps-script/	The Google Apps Script that receives road reports and adds each one to a Google Sheet. Setup steps are at the top of the file.
site/	The live site. index.html, tide-data.js (data fetching and the forecast model), base.css. Edit these directly.
scripts/	Python analysis: tide and weather history, the road profile, and backtest_fade.py, which replays 2024–26 tides to check the forecast.
data/	Downloaded data and results (see data/backtest/results.txt).
archive/	Not deployed. Earlier versions of the live page; design-options/, the mockups the current layout was chosen from; original-analysis/, the first one-page analysis; and field-card/, the September 2026 card for logging when water reached the road.

Deploying

GitHub Actions publishes the site/ folder to GitHub Pages on every push to main that changes it (.github/workflows/pages.yml). It can also be run by hand from the Actions tab.

Rerunning the forecast check

python3 scripts/fetch_backtest_data.py
python3 scripts/backtest_fade.py | tee data/backtest/results.txt