I'd like to use a custom icon set, but ran into trouble trying to do that. I'm using
[forecast iconset="my_custom_icon_set_name"] so that the default icon directory URLs aren't prefixed to $icon_url, and then using the
wp_wunderground_forecast_icon filter to rebuild the img markup, but the filter doesn't pass in the $icon_url or any of the other variables needed.
So, instead of being able to cleanly rebuild it, I have to use a regex to strip out the icon URL and other parameters (yuck), or modify your code to pass in the parameters, and then block the plugin from updating so my changes won't be overridden.
It'd be really helpful if you'd pass those variables to the filter callback, like this:
apply_filters('wp_wunderground_forecast_icon',$icon, $icon_url, $icon_size, $conditions, $date['pretty']);