Weather Models: an explanation

Thanks to Paul Barber of the Net-weather team for providing this guide to the mathematical models now invariably used and referred to by weather forecasters.

This guide is extracted from the Net-weather discussion forum pages.

One of the most common features used by amateur forecasters such as ourselves is the studying of the NWP (Numerical Weather Prediction) charts. Basically these charts are the result of the processes that make up the atmospheric system can be represented by fundamental equations, which can be solved in discrete time steps to achieve a numerical forecast - in other words, physics.

The Models:

Those of you that read the 12Z Model Summary every evening will see that the main models used are:

ECMWF: European Centre for Medium-Range Weather Forecasts.

UKMO: United Kingdom Meteorological Office.

GFS: Global Forecast System by the National Centre for Enviromental Prediction (US).

DWD: The German Model - Deutscher Wetterdienst.

NOGAPS: Navy Operational Global Atmospheric Prediction System by the US Navy.

JMA: Japanese Meteorological Agency Model.

GME: Global Environmental Multiscale Model by The Candian Meteorological Centre.

The 12Z run from the ECMWF model has recently become available much earlier in the evening than before. It used to be released at around 11pm, but is now available from around 7pm (GMT).

There are many other NWP models available for viewing at the excellent website Westwind: but generally the ones listed above are what we use on a regular basis.

With regards to accuracy, then it is always best to remember NOT to become reliant on the models. There are so many other factors that need to be taken into account.

However, when looking at the NWP, the most respected models generally are the UKMO, ECMWF and the GFS.

Times of issue and forecasting distance:

Each model is usually run at least twice a day, at 00Z and 12Z. However, some models, such as the GFS are run four times a day. Below is a brief summary of what each model covers;

ECMWF: Two runs a day: 00Z (available at approx. 7am GMT) and 12Z (available at approx. 7pm GMT). Runs to 7 days (T+168 hours)

UKMO: Runs twice a day: 00Z (available at approx. 5:30am GMT) and 12Z (available at approx. 5:30pm GMT). Runs to 6 days out (T+144 hours)

GFS: Run 4 times a day: 00Z (available at approx. 5am GMT), 06Z (available at approx. 11am GMT), 12Z (available at approx. 5pm GMT), 18Z (available at approx. 11pm GMT). The model is run to 16 days out (T+384 hours).

DWD: Runs twice a day: 00Z (available at approx. 6am GMT) and 12Z (available at approx. 6pm GMT). Runs to 7 days out (T+168 hours)

NOGAPS: Runs twice a day: 00Z (available at approx. 7am GMT) and 12Z (available at approx. 7pm GMT) (although 06Z and 18Z charts are becoming available). Runs to 6 days out (T+144 hours).

JMA: One run a day: 12Z (available at approx. 6pm GMT). Runs to 8 days out (T+192).

GME: Two runs a day: 00Z (available at approx. 4am GMT) and 12Z (available at approx. 4pm GMT). Runs to 3 days out (T+72 hours)
Paul Barber - 04 December 2003
UPDATED: 22 August 2004