Although Yahoo Weather has a metric mode, it is, unfortunately, incomplete. The textual forecast still uses things like "29F" and "15 to 25 mph". Since this is not very useful for the rest of us, :-) I wrote this script, which fixes the problem. It even tries to do something sensible when encountering things like "in the upper 30s".
A reasonably good URL to test this script would be: http://weather.yahoo.com/forecast/DAXX0009_c.html .
This script currently has version 1.4.