There's something almost as good as a Clear Sky Alarm and that is the Clear Sky Chart! It lets you look about 36 hours into the future to see if tonight or tomorrow night will be clear enough for observing.
http://cleardarksky.com/csk/
From there, click the Find A Chart link on the left and select a state, or enter your coordinates (if you know them) and then choose a site near you. Bookmark this site to see how the seeing conditions will be for tonight!