There are good theoretical reasons for saying that a 20-day forecast of
the positions of synoptic-scale systems is intrinsically impossible, regardless
of the accuracy of the initial observations and the degree of perfection of the
computer model. This is due to the way in which energy is transferred from the
sub-micro scale (inches or less) to the synoptic scale in the atmosphere. In
other words, chaos. Also, more pragmatically, no-one has ever achieved success
on the synoptic scale at this range, even if long-term forecasts of monthly
averages are occasionally useful. So why should anyone take you seriously?

Tudor Hughes, Warlingham, Surrey.