NCEP Ensemble Forecasts (Info)

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NCEP (NMC) Ensemble Forecast Information

NCEP produces a set of 17 global forecasts each day from nearly identical initial conditions. The differences between the initial conditions are assumed to be within the range of analysis error. The set of 17 forecasts is referred to as the "ensemble forecast". The mean of the ensemble should, on average, have higher skill than any individual member. Plots of the ensemble mean 500 mb height, Mean Sea-Level Pressure and 500-1000 mb thickness for each forecast time are included, along with "spaghetti" plots which consist of a single contour of 500 mb height for each member of the ensemble. Since the spread among ensemble members is often inversely proportional to skill, the "spaghetti" plots are useful in assessing the utility of the ensemble mean. The data used to generate these plots is obtained from NCEP's ftp server (

For more information, please see the following:

Toth, Z., and E. Kalnay, 1993: Ensemble forecasting at NMC: The generation of perturbations.
Bull. Amer. Meteor. Soc.,74, 2317-2330

Traction, M. S., and E. Kalnay, 1993: Ensemble forecasting at NMC: Operation implementation.
Weather and Forecasting,8, 379-398.