Statement on the Use of GFS Output Beyond Day 7

There is some concern about NCEP model variability from run to run, particularly at extended ranges beyond 7-8 days. The American Meteorological Society Statement adopted on January 14, 2001, titled "Statement on Seasonal to Interannual Climate Prediction" (Bull. Amer. Met. Soc., 82, 701), states, in part, the limit of predictability for day-to-day weather changes for deterministic forecasts is "in the range of 1-2 weeks." NCEP agrees with this statement and notes that for current state-of-the-art models, the limit of "useful skill" is about 7-8 days. Beyond that range, deterministic numerical weather predictions are characterized by little or no skill and suffer from large run to run variability. However, ensemble or probability forecasts may offer improved predictive skill.

Another AMS statement adopted January 13, 2002, titled "Enhancing Weather Information with Probability Forecasts" (Bull. Amer. Met. Soc., 83, 450), praises the value of probability forecasts and endorses ensemble model forecasts as a means for improving skill levels in the extended ranges while quantifying the level of certainty (uncertainty) in any forecast. NCEP agrees with this statement. NCEP currently runs a global and regional ensemble of forecasts twice per day which present information on the predictability of a forecast out to day 16 in a more quantitative way. Global Ensemble Products can be accessed at this research website: http://wwwt.emc.ncep.noaa.gov/gmb/ens/

NCEP is working to making these ensemble forecasts products more readily available. In the meantime, we recommend using the GFS, or any global forecast system, with care beyond day 7 and recognizing the inherent variability in long range deterministic forecasts. Also recognize that probability forecasts, while providing much more flexibility and value to the user, require the user be trained to properly understand and use the new product suites based on ensemble model output. Some relevant training material can be found at http://www.hpc.ncep.noaa.gov/ensembletraining/

For ensemble output on shorter time scales with finer resolution see this research web site: http://wwwt.emc.ncep.noaa.gov/mmb/SREF/SREF.html .

The NWS is seeking comments about this web page and access to NCEP's ensemble products. Please send comments to: NCEP Public Liaison