Weather guide

Understanding marine weather models

GFS, AROME, ICON, ECMWF… Behind every forecast lies a numerical model. Knowing what they're worth means making better decisions aboard.

~5 min read · Beginner

What is a weather model?

A numerical forecast model divides the atmosphere into a 3D grid and solves, time step after time step, the equations of fluid physics. The weather you look at is therefore not an "observation" but a simulation computed several times a day from the observations of the moment (the "run").

Global models vs regional models

There are two main families:

Global (GFS, ECMWF, ICON, GEM): cover the whole planet, with a coarse mesh (≈ 9 to 25 km) and a long range (up to 10–16 days). Ideal for the trend and long crossings.
Regional / mesoscale (AROME, ICON-D2, WRF): zoomed onto a limited domain, with a fine mesh (1 to 3 km) but a short range (1 to 4 days). Far better for coastal wind, thermal breezes and terrain.

	Global models	Regional models
Examples	GFS, ECMWF, ICON, GEM	AROME, ICON-D2, WRF
Mesh	~9 to 25 km	~1 to 3 km
Range	up to 10–16 days	1 to 4 days
Ideal for	the trend, offshore	coasts, breezes, terrain

Simple rule: for the next 48 hours near the coast, favour a fine model (AROME). Beyond 3–4 days or offshore, switch back to a global model (GFS, ECMWF).

Resolution, lead time, run

Three words to know:

Resolution: the size of a grid cell. The finer it is, the better the model "sees" a headland, a valley, a local effect.
Lead time: how many hours into the future. Reliability decreases with lead time.
Run: the time of the computation (00Z, 06Z, 12Z, 18Z). A recent run incorporates fresher observations.

With a fine mesh, the model "sees" a headland, a valley or a local effect far better (blue line = the coast).

Comparing several models: the key to confidence

No model is always right. Best practice: overlay 2–3 models. If they agree, confidence is high. If they diverge, that's the signal that a situation is uncertain — keep a margin and check again at the next run.

On SailMeteo, you can display several models on the same chart and compare wind, gusts, waves and currents with a single gesture.

Example of divergence GFS forecasts the mistral setting in at 2 p.m., AROME at 6 p.m.: a 4-hour gap. Don't rely on a precise time — keep a margin and check again at the next run.

What about gusts, waves, currents?

The mean wind isn't everything: watch the gusts (often 30–40% above the mean wind), the sea state (swell height and period) and, in the Mediterranean as elsewhere, the currents that alter the real sea felt under the hull.