Here is some cloud info.
2008 eclipse
http://home.cc.umanitoba.ca/%7Ejande...tse08intro.htm
http://home.cc.umanitoba.ca/%7Ejande...8/08cloud1.png
SW Mongolia has the least cloud but totality is only about 2 minutes.
2009 eclipse
http://home.cc.umanitoba.ca/%7Ejande...tse09intro.htm
http://home.cc.umanitoba.ca/%7Ejande...09cloudmap.png
The least cloud is south of Japan and totality is more than 6 minutes.
At the end of the eclipse the cloud is also low but the time is shorter.