The most likely reason people do it server-side is to avoid cross site requests. Weather Underground however provides a JSONP capable service API, so I would use that to implement it client-side.
http://www.wunderground.com/weather/api/d/documentation.html
http://eggsylife.co.uk/2010/04/22/gwt-2-jsonp-and-javascript-overlays-with-jsonprequestbuilder/
solved GWT show weather [closed]