Time in Forecasts

Weather forecasts are organized around time concepts that determine when a model ran, what data it used, and what future moments it predicts. All times are in UTC, often written with a "z" suffix (e.g. 18z = 18:00 UTC).

Key Concepts

Term	Definition
Initialization time	The reference time that identifies a specific model run. For example, "the 12z ECMWF run" means the run initialized at 12:00 UTC. See the Initialization Times Endpoint.
Forecast zero time	The starting clock for a forecast's time axis. In traditional models, forecast zero equals the initialization time. In WeatherMesh, multiple runs can share the same forecast zero (see below).
Valid time	The future moment a forecast describes. If a forecast is valid at 18:00 UTC next Tuesday, it represents the predicted weather at that exact moment.
Forecast hour	The offset between forecast zero and valid time. Simply: valid time = forecast zero + forecast hour.
Generation time	When a forecast run finishes computing and becomes available for download.
Data assimilation	The process of combining recent atmospheric observations into an initial state. The model then propagates that state forward to produce predictions.

Traditional Forecasting

Traditional models like ECMWF run on a fixed schedule (4 times per day at 0z, 6z, 12z, 18z). Each run assimilates observations from a 6-hour window centered on its initialization time. For example, the 0z run uses data from 21z the previous day to 03z.

The forecasting process starts after the assimilation window closes and takes several hours to complete. The 0z ECMWF run typically becomes available around 05:40z, nearly six hours after its initialization time.

Continuous Forecasting (WeatherMesh)

WeatherMesh-5c runs every 20 minutes. Forecast generation takes about 16 minutes, so the latest forecast always reflects observations from just 20 to 40 minutes ago. This is a fundamental difference from traditional models that lag hours behind their initialization time.

Property	ECMWF	WeatherMesh-5c
Run frequency	4 per day (0z, 6z, 12z, 18z)	Every 20 minutes
Forecast zeros	Same as initialization times	Every 3 hours (0z, 3z, 6z, ... 21z)
Forecasts per run	Varies by product	120 (at 3-hour intervals, out to 360 hours / 15 days)
Data freshness	~6 hours behind initialization time	20 to 40 minutes behind real time

Multiple WeatherMesh runs can share the same forecast zero. For example, the 03:00 and 03:20 runs both use a forecast zero of 03:00. For system timing details, see Run Information.