Weather Window (by Weatherbug) is probably one of the best online Weather apps out there. The catch is that you have to be using the Google Chrome browser to take advantage of this app by installing it through the Chrome Web Store.
When you first load Weather Window and put in your location, you are given a virtual look of the weather through a window. So if it is raining, you will see rain; if it sunny, you will see sunshine. The window interface is only the eye candy to Weather Window. All-in-All, Weather Window is a great browser based weather application. The real power lies in the weather features that are readily available.
In summary, with weather window you can bring up the following:
Alerts are currently available for the United States, Canada, United Kingdom and Spain.
The alert icon will only appear if an alert is active. The number within the icon displays the number of active alerts.