Gusts, thunderstorms and downdraughts: what the forecast does not show

Most scary moments on the water do not come from the mean wind in the forecast but from what happens in between: a gust under a shower cloud, a thunderstorm that arrives sooner than expected, a downdraught behind an island. A weather model can only hint at such things. Here is how to read the hints.

Mean wind and gusts

The wind in the forecast is an average: over a few minutes, at 10 m height and over the model's whole grid cell. The gust is the brief peak above it. In steady air over the open sea, gusts are not far above the mean; under showers, along steep coasts and in the lee of land they can be far higher. For reefing, the gust counts, not the mean.

How likely are strong gusts?

A model gives you a number, an ensemble a distribution. Sailseer takes the 51 members of the ECMWF ensemble and the 20 of ICON-D2 and shows what share of them produce gusts above 20, 25 or 30 kn. 30 % for gusts above 25 kn means almost one run in three. For the rating, Sailseer uses the upper end of the model gusts and, when the models disagree, the highest value from the models and the ensemble.

Thunderstorms: potential is not an event

The model expresses whether thunderstorms are possible through the instability of the air, measured as CAPE, the energy available for updraughts. Exactly where and when a cell forms is something it can hardly say: a thunderstorm is often smaller than a few grid cells and frequently lives for only an hour. Thundery conditions in the model therefore mean that a storm could break out somewhere in the area. Sailseer treats CAPE from 800 J/kg together with a chance of rain of at least 40 % as a reason not to sail. Only observations tell you where a thunderstorm actually is: lightning detection and radar.

Downdraughts

A thunderstorm cell sends cold air downwards, which spreads out in all directions when it hits the sea surface. Within minutes this downburst can bring storm-force wind, even from a direction that had barely any wind before. The sinking of the Bayesian off Sicily in August 2024 showed this; the interim report of the UK Marine Accident Investigation Branch (MAIB) cites winds of over 70 kn. No model forecasts something like that for a single location.

Downdraughts also occur without thunderstorms. Behind high islands and along steep coasts, air plunges down on the lee side. With the Meltemi this often hits the south sides of the Cyclades, with the Bora the channels below the Velebit. Precisely where the model shows little wind in the wind shadow, it can suddenly blow harder than out at sea.

Local winds

In the Mediterranean, local winds shape everyday sailing: the Bora in the Adriatic with gusts far above the mean wind, the Mistral in the Gulf of Lion, the Meltemi in the Aegean from May to September. Sailseer recognises such situations from the consensus and names them in the rating. More in the guide Mediterranean local winds.

What Sailseer shows

• Gusts next to every wind value, plus the probability from the ensemble.
• Lightning as strike locations from DMI and FMI in the north, elsewhere from the satellite (EUMETSAT MTG) with a delay of around a quarter of an hour, and in the Caribbean west of 68 degrees West from NOAA's ground-based networks.
• Radar from the DWD with a two-hour forecast and, for Europe, the OPERA composite.
• The next hour: in the point analysis, a box sums up lightning and rain at the point, with distance, direction and trend, in three levels. Red means, for example, lightning within 15 km or an approaching thunderstorm within 30 km, with the advice: take down the sails, seek shelter in harbour or at anchor, keep away from the mast. The thunderstorm watch checks every five minutes while the page is open.
• Warnings from the weather services along the route and as a warning alert by email or push.

Where observations end

Where no source measures for a location, the page says so explicitly instead of showing a green light that knows nothing. Likewise, in countries without retrievable warnings, such as Turkey, it says that no warning there does not mean all-clear.

What you can do on board

• Read the marine forecast and warnings before casting off and follow them under way on VHF or NAVTEX.
• In thundery conditions, reef early and plan to be in harbour by the afternoon, when thunderstorms build over land.
• Watch the sky: towering clouds, a dark shelf cloud, suddenly cooler, shifting wind.
• In the lee of high islands, expect downdraughts and keep the sail area small.

Sailseer does not replace a marine forecast or an official warning and is not intended for navigation. It brings the signs together; you have to decide with the official reports and a look at the sky. The basics of the models are explained in the guide to weather models.