GFS MOS Sky Cover
Changes from Total to Opaque Sky
On June 5, 2007, 12Z, the Sky Cover (SKY) guidance
in the short-range GFS MAV MOS was changed from Total Sky Coverage to
Opaque Sky Coverage (this change will be made to the extended-range
GFS MEX MOS at a later date). The MOS Sky Cover guidance predicts
an estimate of the total percentage of the sky covered by
clouds in terms of the five categories reported in the METAR
observations: clear (CLR), few (FEW), scattered (SCT),
broken (BKN), or overcast (OVR). The old Total Sky Cover
guidance did not distinguish between opaque and translucent
cloud cover. The new Opaque Sky Cover guidance produces an
estimate of only clouds judged to be opaque. The anticipated
effect of this change is to decrease the number of broken
or overcast forecasts and to increase the number of clear or few
forecasts when only cirrus clouds are expected.
This change will make the MOS guidance consistent with the
NWS requirement for opaque sky cover information. Moreover,
verifications of the two Sky Cover guidance systems indicate the
new Opaque Sky Cover guidance is more skillful than the old Total
Sky Cover guidance system.
Note: We do not have GOES Satellite Cloud Product (SCP) coverage
over Alaska, so our new guidance for Alaska is not truly an opaque
sky cover product, but rather a total sky cover.