How to read seasonal forecast maps

What the colours, units and legends on a seasonal forecast map mean, and how to use point inspection to read a value across several models.

Every panel shares one scale

Each map on this site shows one seasonal model, one initialization and one valid period. When several panels are open, they always share the same field, the same forecast type, the same valid period and the same colour scale. A stronger colour in one panel therefore means a stronger signal in that model, not a different legend.

Temperature and sea-surface temperature anomalies are in °C, precipitation in mm/day and mean sea-level pressure in hPa. The colour bands are fixed for each field: temperature, SST and precipitation change colour at ±0.25, ±0.75, ±1.25 and ±1.75 in their units, and pressure at ±1, ±3, ±5 and ±7 hPa. The outer bands are open-ended, so the darkest colour means “beyond the last boundary”, not a specific value.

Warm and cool, wet and dry

Anomaly maps are diverging: one hue for below the model climate, the other for above, and a neutral tone in the middle. For temperature and SST, blue is cooler and orange is warmer. For precipitation, brown is drier and teal is wetter. For pressure, purple is lower and green is higher. A near-neutral cell is a forecast close to the model’s own average for that month, not a forecast of “no weather”.

Probability maps use a different legend

When the forecast type is a tercile or quintile probability, the map shows the percentage of ensemble members that fall into that category, from 0 to 100%. The legend marks the climatological baseline, which is 33% for a tercile, 20% for a quintile and 50% for exceeding the median. A value at the baseline means the model has no opinion; values well above it mean the members lean one way. See tercile and quintile probabilities for how the categories are defined.

Reading a single location

Click anywhere on a map to inspect that location in every panel. The value shown is the nearest node on the original 1° grid, not an interpolated pixel colour, so it is the number the model actually produced for that cell. Cells that the source marks as missing, such as land cells in an SST field, are reported as unavailable rather than filled in.

Where the maps stop

Maps are drawn between 85°S and 85°N and wrap across the antimeridian. Colours between grid nodes are bilinear interpolation of the four surrounding 1° values, which keeps gradients smooth but means a colour boundary on the map is not a sharp boundary in the forecast. Country outlines come from Natural Earth and are for orientation only.