The instability of the atmosphere places an upper bound on the predictability of instantaneous weather patterns. The skill with which current operational forecasting procedures are observed to perform determines a lower bound. Estimates of both bounds are obtained by comparing the ECMWF operational forecast for each day of a 100-day sequence at one range with the operational forecast for the same day at another range, and with the analysis for that day. The estimated bounds are reasonably close together.
Predictions at least ten days ahead as skilful as predictions now made seven days ahead appear to be possible. Additional improvements at extended range may be realized if the one-day forecast is capable of being improved significantly.