  To evaluate the capability of numerical cloud forecast as a meteorological
reference for astronomical observation, we compare the cloud forecast from NCEP
Global Forecast System (GFS) model for total, layer and convective cloud with
normalized satellite observation from the International Satellite Cloud
Climatology Project (ISCCP), for the period of July 2005 to June 2008. In
general, the model forecast is consistent with the ISCCP observation. For total
cloud cover, our result shows the goodness of the GFS model forecast with a
mean error within +/-15% in most areas. The global mean probability of < 30%
forecast error (polar regions excluded) declines from 73% to 58% throughout the
180h forecast period, and is more skilled than ISCCP-based climatology forecast
up to tau~120h. The comparison on layer clouds reveals a distinct negative
regional tendency for low cloud forecast and a questionable positive global
tendency for high cloud forecast. Fractional and binary comparisons are
performed on convective cloud forecast and revealed the GFS model can only
identify less than half of such cloud. In short, our result suggests that the
GFS model can provide satisfactory worldwide total cloud forecast up to a week
ahead for observation scheduling purpose, but layer and convective cloud
forecast is less reliable than the total cloud forecast.
