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Meteorological Development Laboratory

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.

Verification Charts:

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.

National Weather Service
Office Of Science and Technology
Meteorological Development Laboratory
Statistical Modeling Branch
1325 East West Highway
Silver Spring, MD 20910
Page last modified: 07 June 2007
Page Author: MOS Web Team
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