A forum in Webmaster World informs of Change in Yahoo's SERPs yesterday.
pgrote wrote, "Has anyone else noticed a change in the SERPS for Yahoo yesterday? We're seeing more traffic from Yahoo to our sites."
"Yes, I had a drop from #7 to #18 for a 1 word search that had been stable for a while. No other changes seen on that, or any other sites though."
The last Yahoo! weather update was on the 23rd of August.