I've never subscribed to the theory, and TBH though the 6z and 18z *are* a little less reliable (due to the lower amount of surface observation data entered into 6z and 18z vs 0z and 12z runs), I've never thought it enough of an effect to be noticed (unlike at Christmas due to the marked decrease in aircraft data).
It's interesting, though, that the Christmas data loss tends to manifest as increased blocking and a weaker jet in our part of the world. Perhaps the mild data loss of the 6z and 18z runs has a similar effect?
Originally Posted by: Retron