Ik vond nog een aardige bijdrage.
Those maps on weekdays are compiled by forecasters at NOAA in the USA after all the 12hrs outputs. So they would have seen both the GFS and ECM operational runs. It comes down to one of two options, energy over the top or underneath, those maps suggest the latter.
te vinden hier
http://www.cpc.ncep.noaa.gov/products/predictions/814day/500mb.php
http://www.cpc.ncep.noaa.gov/products/predictions/610day/500mb.php
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