Frequently asked questions - Forecasts
Forecasts are issued by the Met Office throughout the day. Their forecasters are constantly monitoring the latest observed weather from the observing network and weather satellites and amending the forecast accordingly. Each forecast that is issued is the best that is available at the time of issue, but can of course become out of date fairly quickly. Forecasts are supplied to different web sites and customers at different times during the day according to their own requirements.
Both forecasts should be treated as being accurate at the time that they are updated on the respective web sites.
Traditionally weather forecasts are issued based upon a single best solution forecast. For businesses to make planning and operational decisions some idea of the uncertainty of the forecast is a more suitable guide. To this end probabilistic forecasts derived from multiple forecast model runs, provide more meaningful advice as it can provide the range of likely forecast coupled with the degree of uncertainty or likelihood of outcome.
Forecast models work on a grid system, predicting values for fixed, regularly spaced points throughout the atmosphere. Where a forecast is presented as being for a named location, for example a town or city, then the forecast data must first be interpolated from the grid to the intermediate location. This is done by applying weightings to the adjacent four grid points to produce a single value for the site. Corrections for varying orography and land type are taken into account in this process. E.g. where a site has forecast grid points adjacent that are located over the ocean, the weighting will be biased towards those remaining two points that lie over land. Such downscaled forecasts are a vital tool when trading site specific weather Indices such as those traded through the CME.
Forecasts can be made days, weeks and even months ahead, however the techniques used will change the further ahead you go. Our short term forecasts, that is out to 10 days ahead are based upon a the ECMWF 51 member ensemble forecast. Forecasts out to a month or a season use a coupled atmospheric/ocean model and ensemble techniques which introduce elements of uncertainty into the forecast, and also probability of events - see ensemble forecasts
The forecast models that are used to produce the weatherXchange products are capable of producing forecasts for any location around the world, providing that there is an observing site at or close to the location.
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