Here is a summary of the latest NWP output for noon (GMT) on Saturday.
Issued 0503z, 18th August 2009.

The weekend will see low pressure approach the UK from the west, with a
frontal system moving slowly eastwards. The north and west will bear the
brunt of the rain, but even the south and southeast will see some by Sunday.

ECMWF: http://www.wzkarten.de/pics/Recm1201.gif
A weak ridge covers the UK, bringing SW'lies to Scotland and SE'lies
elsewhere. On day 6 a low deepens to the WNW, leading to strong southerlies
and SE'lies. There's little change on day 7, with further southerlies.

MetO: http://www.wzkarten.de/pics/Rukm1201.gif
The UK lies under southerlies, this time with a high to the east and a ridge
over SE England.

GFS: http://www.wzkarten.de/pics/Rtavn1201.png
Southerlies cover the UK, with a deep low to the NW and a high to the east.
A weak trough moves eastwards on day 6, with southerlies in advance and
cooler SW'lies following behind. Cooler southerlies cover much of the UK on
day 7, as a secondary low crosses Ireland.

GEM: http://www.wzkarten.de/pics/Rgem1201.gif
The Canadian run brings a southerly flow and a ridge over SE England.

JMA: http://www.wzkarten.de/pics/Rjma1201.gif
The Japanese run has a weak SW'ly flow over the UK and a secondary low over
Cornwall.