Ensemble forecast for Sheffield

Instead of using one computer model run such as the Global Forecast System (GFS) the forecast here adds in data from the Global Ensemble Forecast System (GEFS). The GEFS is basically the GFS model run an additional 20 times using slightly different starting conditions in an attempt to account for uncertainty.

On the charts look to see whether the lines are clustered together or spread out. Tight clustering suggests a higher degree of confidence whereas a wide spread usually indicates a range of weather outcomes are possible.

850hPa temperatures (°C), rainfall (mm) and the snow row

850hPa temperatures, precipitation and the snow row

The top half of the graph shows 850hPa temperatures. These are forecast temperatures at approximately 1500m above sea level. As they aren't subject to diurnal (day to night) variation they provide an easy way of checking how cold or warm the air mass above the given location is forecast to be.

The lower half of the graphs shows forecast rain or to be more technically correct precipitation.

The snow row in the footer shows the number of computer model runs forecasting snow at any give time in the selected location. This can range from 0 to 23. It is a forecast so can be wrong, but 0 suggests snow isn't at all likely and 23 means it is.

Ground level temperatures (°C)

These are the temperatures we experience at ground level and are what commonly appear in weather forecasts such as the place or postcode ones on the site. The thick white lines show the average of the maximum and minimum forecasts from all of the runs.

The forecast charts on this page usually update 4 times every day. The updates in chronological order are labelled GEFS 00z, GEFS 06z, GEFS 12z, GEFS 18z.

UK locations

If your exact location isn't listed pick the nearest city. Ensemble forecasts are run at a lower resolution so the same data covers a bigger geographical area. This isn't a problem when using the forecast data to identify trends and probabilities.

Overseas locations

The UK's weather doesn't happen in isolation, so checking what's happening overseas can be very helpful.

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