FORECASTSEASON.COMInterstate 90 · the long haulModelsDriversAbout
SEASON90Forecast SeasonSeasonal Outlooks · U.S. · Europe · Global
Interstate 90 · Seattle to Boston · 3,020 miles

The season ahead,
read three ways.

Seasonal forecasting is the long haul: one month out to seven, where the weather has stopped being weather and become odds. Forecast Season takes the systems that do it — ECMWF's SEAS5, the North American Multi-Model Ensemble, and the Copernicus multi-system — and reads them for the United States, for Europe, and for the world.

The data engines are being built. Everything that does not need one is open now: the model garage, the drivers, and the methods that decide whether any of it is worth believing.

The Model GarageDriversWhat this is

Three lanes

United States · Europe · Global
U.S.90
United States
U.S. route
CPC's monthly and seasonal outlooks, the NMME behind them, and the drought outlook — read against the models rather than instead of them.
Take the exit→
E 40
Europe
European route
The C3S multi-system and SEAS5 over the continent, and the honest story about winter: a low signal-to-noise ratio, and what you can still say through it.
Take the exit→
Global
World route
Where seasonal forecasting actually works — the tropics, the monsoons, and the ENSO teleconnections that reach out of them into everyone else's winter.
Take the exit→

The route board

What is open, what is being paved

Every product on this site is a numbered route. A route is only ever signed open once it is genuinely here — there are no placeholder maps and no invented numbers anywhere on this site.

Route 01Paving
SEAS5 Explorer
ECMWF's seasonal system, all 51 members, seven monthly leads — anomaly maps you can pan across nine regions, the ensemble spread behind any point on Earth, and the way the outlook for one target month has changed from one initialisation to the next.
ECMWF · 51 members · 7 months
Route 02Paving
NMME Engine
The North American Multi-Model Ensemble: eight coupled models, roughly a hundred members between them, pooled the way the NMME is meant to be read — as a multi-model probability rather than any one centre's best guess.
8 models · ~100 members · 12 months
Route 03Surveyed
Copernicus Multi-System
The C3S seasonal multi-system, which puts ECMWF, the Met Office, Météo-France, DWD, CMCC, NCEP, ECCC and JMA on one grid with one calendar. The place to see whether an outlook is a signal or one model's opinion.
8 centres · one grid · monthly
Route 04Open
The Model Garage
Every operational long-range system, what it actually is under the hood, how many members it carries, how far it runs and when it publishes. The reference you want open in the next tab while you read an outlook.
ReferenceOpen →
Route 05Open
Drivers
ENSO, the Indian Ocean Dipole, the NAO and the polar vortex, the QBO, the MJO, and the slow ocean modes. What each one is, how it is measured, and what it does — and does not do — to a season.
ReferenceOpen →
Route 06Surveyed
Verification & Skill
The uncomfortable half. Hindcast skill by region, season and lead — where a seasonal forecast is worth acting on, where it is worth reading, and where the honest answer is climatology.
ROC · RPSS · reliability
Route 07Surveyed
The Season Ahead
City briefings: the full ensemble distribution for one place, month by month, against that model's own climatology — the fan, the terciles, and the probability of a warmer or wetter season than normal.
Point outlooks · distributions
Route 08Surveyed
The Wire
A machine-readable feed of every outlook the site publishes, for broadcast, models and agents. The network's other sites carry one; this one will too.
JSON · text · feeds

Next departures

Published issue schedules

The major systems run to a timetable. These are their routine publication times — schedules, not forecasts.

T-14DAYS
ECMWF SEAS5
Sat 5 Sep 12:00 UTC
5th of the month, 12 UTC
T-14DAYS
Copernicus C3S multi-system
Sat 5 Sep 12:00 UTC
5th of the month, 12 UTC
T-17DAYS
NMME
Tue 8 Sep 12:00 UTC
around the 8th
T-26DAYS
NOAA CPC outlooks
Thu 17 Sep 20:00 UTC
third Thursday

How to read a seasonal forecast

Four things worth knowing
01

Ensembles, not forecasts

A seasonal model is run many times from slightly different starting points. No single member is the forecast; the spread between them is the forecast. Read the distribution — where it sits, and how wide it is.

02

Anomalies against the model's own past

A coupled model drifts. The fix is to re-run it over decades of past years — the hindcast — and express the live run as a departure from its own climatology. That is why a SEAS5 anomaly is not an ERA5 anomaly, and why the two should never be quietly swapped.

03

Terciles and the odds

Seasonal outlooks are framed as probabilities of the coming season landing in the coldest, middle or warmest third of the record. A 40 % chance of the warm tercile is a real, useful shift from the 33 % you would say knowing nothing — and it is still more likely than not that something else happens.

04

Skill has a geography

Seasonal forecasts are good in the tropics, useful in ENSO-teleconnected regions in winter, and close to worthless for mid-latitude summer rainfall. A site that shows you a map without telling you where the map is any good is selling you something.

The one rule this site is built on. A seasonal forecast is a statement about the odds over three months, not about a day. If a page here ever shows you a number that looks like a weather forecast, it is doing something wrong — and if it shows you a map without telling you what the skill is under it, so is that.

Where the data comes from

Every source, named
ECMWF SEAS5Copernicus C3SNMMENOAA CPCERA5IRI