How Seaweed Radar works

Seaweed Radar reads one public map a day and shows what it says about the coast around twenty beaches. This page explains that map, and where it stops being useful.

The map

Every day NOAA publishes a Sargassum Inundation Risk map for the Caribbean, the Gulf and Florida. It is made from satellite images of floating sargassum, collected over the previous week, and it colours the shoreline by how much of that seaweed is nearby and could wash in. NOAA calls the map experimental and says plainly that it is not a forecast.

There are four levels: low, warning, medium and high. On this site each level always appears with its letter (L, W, M, H) as well as its colour.

What a level covers

NOAA scores the shoreline in long runs, usually 20 to 190 km each, and gives a whole run one level. Playa Delfines and Playa Tortugas in Cancún are 9 km apart, face different ways, and sit in the same run, so they always get the same level here. That is why every page says "the coast around" a beach.

The level is about seaweed in the water offshore. It says nothing about whether a hotel raked the sand at six this morning, or whether the wind pushed yesterday's mat past the point.

How old it is

NOAA's map runs one to two days behind the calendar. Seaweed Radar checks for a new one twice each afternoon (UTC) and rewrites every page when it arrives. Each page shows the date of the map it is quoting. If NOAA skips a day, the page keeps the last map and says how old it is.

The history

Each beach page shows the last fourteen days and a month-by-month count that goes back as far as NOAA still serves its daily maps. A day NOAA did not publish is counted as missing, never as low.

Which way a beach faces

Each page states the direction its beach faces and the water it looks onto. Sargassum drifts in from the open sea, so a beach that faces away from it often stays cleaner than one a few kilometres off that faces into it. This is a rule of thumb. It is the reason some pages point you to a sheltered beach nearby.

What is coming

A morning report for one beach during your trip, sent at 7 AM, starts in spring 2027 and can be ordered ahead from any beach page. Reports from people standing on the beach are planned after that. Until then, this site shows NOAA's risk and nothing it cannot back up.

Credit

Data courtesy of NOAA/AOML, NOAA CoastWatch, and USF. Seaweed Radar is not affiliated with NOAA, NASA or the University of South Florida, and none of them endorses it. Beach positions were checked against OpenStreetMap (© OpenStreetMap contributors).