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.