Probabilistic forecasting
- Probabilistic forecasting
Probabilistic forecasting is a technique for weather forecasting which relies on different methods to establish an event occurrence/magnitude probability. This differs substantially from giving a definite information on the occurrence/magnitude (or not) of the same event, technique used in deterministic forecasting. Both techniques try to predict events but information on the uncertainty of the prediction is only present in the probabilistic forecast.
The probability information is typically derived by using several numerical model runs, with slightly varying initial conditions. This technique is usually referred to as Ensemble forecasting or EPS (ensemble prediction system).
If it were possible to run the model for every possible set of initial conditions, each one with an associated probability, then according to how many members (i.e. individual model runs) of the ensemble predict a certain event, one could compute the actual conditional probability of the given event. In practice, forecasters try to guess a small number of perturbations (usually around 20) which are deemed most likely to yield distinct weather outcomes. Two common techniques for this purpose are breeding vectors (BV) and singular vectors (SV). [Zoltan Toth and Eugenia Kalnay, Ensemble Forecasting at NCEP and the Breeding Method, 1997 AMS Monthly Weather Review volume 125 page 3298. ] This technique is not guaranteed to yield an ensemble distribution identical to the actual forecast distribution, but attaining such probabilistic information is one goal of the choice of initial perturbations. Other variants of ensemble forecasting systems that have no immediate probabilistic interpretation include those which assemble the forecasts produced by different numerical weather prediction sysyems.
Canada has been one of the first countries to broadcast their probabilistic forecast by giving chances of precipitation in percentages.
Assessing probabilistic forecasts
Jolliffe & Stephenson (2003) provide an overview of methods that can be used to assess the success, or otherwise, of probabilistic forecasts.
ee also
*Scientific determinism
*Forecasting
External links
* [http://www.wmo.ch/web/www/DPS/EPS-HOME/eps-home.htm Several EPS online (from the World Meteorological Organisation)]
References
Jolliffe, I.T., Stephenson, D.B. (2003) Forecast Verification: A Practitioner's Guide in Atmospheric Science. Wiley. ISBN 0-471-49759-2
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