Time in Forecasts

In forecasting, it’s common to describe everything within the context of an individual run, which is identified by its initialization time. Take ECMWF as an example: it runs four times a day, labeled 0z, 6z, 12z, and 18z. Here, z is a stand‑in for UTC, so 18z = 18:00 UTC; everything generally operates in the UTC timezone.

People commonly refer to runs by their initialization time, e.g., “the 12z ECMWF run”. In other words, the initialization time tells you which run of the forecasting model you’re looking at. See the Initialization Times Endpoint for details.

  • Initialization time: the reference time that acts as the “name” of a given run.
  • ECMWF: 4 per day at around 0z, 6z, 12z, and 18z.
  • WM‑5c: Every 20 minutes at 00:00z, 00:20z, 00:40z, and so on.

Data assimilation

A traditional forecast run assimilates observations from the 6‑hour window centered around its initialization time. For example, the 0z ECMWF forecast assimilates data from 21z the day before to 3z on the day of. The actual forecasting process therefore starts sometime after 3z. For traditional models, the forecasting process then takes several hours to run. For this example 0z run, the final forecast won’t come out until ~5:40z. In other words, traditional forecasts don’t become available until nearly six hours after the initialization time.

Data assimilation is the process of combining recent atmospheric observations into an initial state. The forecasting model then propagates that initial state forward in time to create a prediction of the future.

Traditional vs Continuous

By contrast, Continuous Forecasting operates much faster. Forecast generation takes around 16 minutes, so the latest available forecast always reflects observations from just 20–40 minutes ago. For system timing, see Run Information.

Forecast zero time

The process of forecast generation creates individual forecasts, each of which represents the atmosphere at a different moment in time. Each run, WeatherMesh will create 120 individual forecasts. The first of these represents the atmosphere at what we call the forecast zero time. The second will represent the atmosphere three hours later, the third three hours after that, etc.

  • Forecast zero time: the “starting clock” for a forecast.
  • In traditional forecasting, the forecast zero is always precisely equal to the initialization time. In ECMWF, there are 4 per day: 0z, 6z, 12z, and 18z.
  • In WeatherMesh, multiple runs can share the same forecast zero. For example, the 03:00 and the 03:20 WM‑5c runs will both have a forecast zero of 03:00. These come every 3 hours, at 0z, 3z, 6z, 9z, … 21z.

Other terms

Valid Time

The moment in the future a forecast is describing. For example, if a forecast is valid at 18:00 UTC next Tuesday, it represents the weather at that exact moment.

Forecast Hour

The time difference between forecast zero and valid time. Simply: valid time = forecast zero + forecast hour.

Generation Time

When a forecast finishes.