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. The shared legend lets you compare anomaly magnitudes or category probabilities. A stronger colour does not establish greater forecast skill.
Temperature (2 m, sea surface and 850 hPa) anomalies are in °C, precipitation in mm/day, mean sea-level pressure in hPa, 500 hPa geopotential height in metres and 10 m wind speed in m/s. The colour bands in the C3S (default) scale 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. T850 uses the temperature bands; 10 m wind uses ±0.25, ±0.75, ±1.25 and ±1.75 m/s, and Z500 uses ±20, ±60, ±100 and ±140 m. The outer bands are open-ended, so the darkest colour means “beyond the last boundary”, not a specific value.
Use Colour scale in the sidebar controls to choose Extended (uneven). This optional scale keeps finer intervals near zero and adds wider intervals for stronger anomalies. Temperature boundaries are ±0.25, ±0.75, ±1.5, ±2.5, ±4 and ±6°C, with open-ended bands beyond ±6°C. Other anomaly fields use extended thresholds in their own units. The choice applies to every panel and is saved in shared links and image exports. Both presets stay fixed when changing models, runs, periods or map extent. For probability products, Extended adds bands at 95% and 99% within the 0–100% range. The extended tercile summary distinguishes outer-category probabilities at 80%, 90% and 95%, while keeping the same rule for neutral cells.
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, blue is lower and orange is higher. Z500 uses blue for lower heights and orange for higher heights; wind speed uses blue for weaker winds and orange for stronger winds. A near-neutral cell is a forecast close to the model’s own average for that valid period, 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 names the event and its nominal climatological probability: 33.3% for a tercile, 20% for a quintile and 50% for exceeding the median. Finite samples and tied hindcast thresholds can change the achieved category frequencies, especially for zero precipitation. 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 uses the nearest node on the C3S 1° grid; maps interpolate between nodes. Values are stored in increments of 0.01 display units; this is encoding precision, not forecast accuracy. A 1° grid value is not a city-scale forecast. 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. Products use bilinear interpolation of four surrounding 1° values. Tercile summaries interpolate the three tercile probabilities first and classify the result, so category codes are never blended. Forecast colours are opaque and match the legend over land and ocean. The grey near-zero anomaly band is an interval, not an exact zero; the dark missing-data swatch is separate. Band labels include their lower endpoint and exclude their upper endpoint unless stated otherwise. Country outlines come from Natural Earth and are for orientation only.
Sources and method
These references explain the underlying science and provider conventions. Descriptions of this site’s calculations, colours and coverage were checked against its processing code and imported data.