I think yahoo has noticed this bug and might be working on it.
I tried playing around with their developer network api trying to pull the various condition codes and had no luck. Then I finally found this which is where they store the gifs for the weather condition codes and on there it is incorrect as well. Yet when you go to their main page it says that "Yahoo! Will be right back...Our Engineers are working quickly to resolve the issue.
http://l.yimg.com/
http://l.yimg.com/a/i/us/we/52/40.gif
http://l.yimg.com/a/i/us/we/52/39.gif - Both give scattered rainshowers
http://l.yimg.com/a/i/us/we/52/37.gif
http://l.yimg.com/a/i/us/we/52/38.gif - Both give isolated thunderstorms