On September 12, 2001 ECMWF changed the manner in which they archive this data
They changed from 6 hourly forecast fields based on the  00,06,12 and 18z model
runs to 6 and 12 hourly fields from the 00 and 12Z model runs.  Please see the
<a href="http://dss.ucar.edu/datasets/ds111.3/docs/changes.html">changes</a>
document for more information