Glidemet

While this isn’t enough for doing a detailed forecast or covering every eventuality, it will get you started.

Basics

All forecasts (BBC, weather.com etc.) are generated by computer models of the atmosphere. All models start with the same data (taken from air land and sea observations and satellite data), but each has slightly different physics, so they can sometimes come out with slightly different answers.

The main models are:

• UKMET: The UK’s Met Office’s global model
• The Met Office’s UK mesoscale model, which gives today and next-day forecasts.
• GFS: the U.S. National Weather Service global model.
• ECMWF: The European Centre for Medium Range Weather Forecasting global model.

Models work by dividing the atmosphere into thousands of individual cells in a three dimensional grid. Typically a global model will have a grid spacing of around 40 km, while a “mesoscale” model can have a grid spacing of 4 to 12 km. All models usually have around 70 vertical levels, reaching from the surface to as far as the ionosphere.

The models produce a forecast by calculating how the atmosphere in each grid cell changes every few minutes at a time (called a “time step”). The output (e.g. sea level pressure, 10 m wind etc.) is plotted onto maps of every 3 to 12 hours from the “model run time”. The model run time is the time that the model is initialised: 0000, 0600, 1200, and 1800z for the GFS, and 0000 and 1200z for other models.

Models will produce forecasts for time periods ranging from 36 hours ahead for the mesoscale models to two weeks or more for the global models. Mesoscale models have short ranges as not only do the smaller grids use much more computing power, the small grid cells tend to rapidly destabilise, giving useful forecasts for only a day or two ahead.

For more information on models and how they work, read this excellent overview.

The forecasts you see in the media are produced either by taking output directly from the GFS model (weather.com, metcheck.com, accuweather.com etc.), the UKMET model (bbc.co.uk), or by a weather forecaster looking at the output themselves and interpreting it (BBC tv/radio forecasts, newspapers, and of course here on Glidemet). You can produce forecasts by looking at the model output yourself and learning to interpret the information.

Confidence in the forecast can be estimated by comparing the output of different models: if they agree, then the forecast will be probably be right; if each model says something different, it’s difficult to trust any one model over another.

Making your own forecast

All the information you need is available from the links in the sidebar on the left.

1. When you are several days out from the day you want to forecast for, compare the sea level pressure charts of the GFS, ECMWF and UKMET via Wetterzentrale. If they show roughly the same pattern of isobars, you can reasonably assume that the rest of the model output (e.g. rainfall, wind) will be accurate. If they differ, then there’s a large degree of uncertainty in the forecast.
2. Closer to the day in question, look at the UKMET fax charts to give yourself a general idea of where fronts will lie.
3. Look at the GFS precip charts on Wetterzentrale. Dots indicate convective rain; no dots indicates frontal rain.
4. Look at the 10m wind charts.
5. Compare these with the charts on bbc.co.uk/weather (the UK meso).
6. When the day concerned is tomorrow, the BBC weather charts are very good.

Will it be a soaring day?

For this you need to look at the RASPviewer. This displays the results of a special WRF model run to provide soaring-specific parameters such as Cu base, thermal strength, and thermal height on blue days. The parameters are ordered in rough order of utility.

The WRF model in RASP is “nested” inside GFS output, so it’s worth noting that if the GFS is wrong, RASP will likely be too, so be cautious when looking at forecast data for more than about three days ahead. For more information on the parameters provided in the viewer, look at this page.

Will it be foggy/what will the (non-cumulus) cloud base be?

Finding answers to these questions requires the study of soundings. Read this excellent guide from Weatherjack, then use the “GFS soundings” link in the sidebar.

Where the blue and red lines come close or meet is where cloud will form. If that happens on the surface, and windspeed is low (below 10 knots), fog is likely. As a rough guide to cloud cover, if the lines meet expect 8/8s cover, if they’re two degrees apart expect 6/8s, four degrees apart 4/8s etc.