Comparing seasonal models and monthly runs

Two ways to compare seasonal forecasts: several centres at one initialization, or several monthly runs from one centre. When each comparison is useful.

Two questions, two modes

The workspace has two comparison modes. Model mode places several centres side by side for the same initialization, and answers “do the models agree?”. Run mode places several initializations from one centre side by side, and answers “is this model’s outlook stable?”. In both, every panel shares the valid period, the field, the forecast type and the colour scale, so what differs between panels is only the thing you chose to compare.

Agreement between models

Five independent systems that all lean the same way are more convincing than one strong signal. Look for the sign of the anomaly first and its magnitude second. Models differ in how strongly they express a signal, so a weak anomaly in one centre next to a strong one in another is agreement on direction, not disagreement. Where the models split, the tercile summary usually shows neutral in some panels, and the honest reading is that the season is uncertain.

Stability between runs

A forecast that looks the same from the August and September runs has survived a month of new observations. One that flips is telling you the signal was weak. Because this site aligns runs on the valid period, the newer run is always at a shorter lead; expect it to be sharper, and be suspicious if it is not. Two runs is a short history, and the site is designed to accumulate more initializations over time.

Using point inspection

Click a location to get the source-grid value from every open panel at once. This is the fastest way to compare a specific city or sea area across models without judging colours by eye. Values come from the nearest 1° node in each model’s published grid, so they are directly comparable.

Sharing a comparison

The URL records the mode, panels, field, forecast type, period, region and whether navigation is linked. Copy it to share exactly what you are looking at, or export the board as a labelled PNG with source, system version, initialization, lead and valid period on each panel.