Tonight will be the best time for viewing the annual Perseid meteor shower (see diagram above). The peak will arrive at approximately 1:00 Universal Time. That is at about 7 PM CDT here in Winnipeg. To work out what that is in your own time zone see the Time and Date website. That also also gives useful information about such things as sunset and moonset. Tonight the very thin waxing crescent Moon will set early and provide little interference with meteor viewing.
The weather report from the Weather Network predicts overcast skies tonight here in Winnipeg. There is, however, a much better sight for forecasts of sky conditions. The Clear Dark Sky site also gives viewing conditions for the entire North American continent if you plug your location in. It lists not just cloud cover but other conditions that affect visibility. According to that sight there will be a window of opportunity here in Winnipeg for viewing between 9PM and midnight. That is exactly what it looks like to me observing the sky shortly before sunset. The best time for viewing meteor showers is usually after midnight, but you take what you can get. If tonight fails for you don't panic. While predictions for the Perseids this year say anywhere from 50 to 80 meteors per hour you will be able to see a diminishing number in the next few nights.
Molly has blogged extensively about the Perseids in previous years. For 2009 see here. In 2008 there was this, this and this. For 2007 see this and this. So, for all sorts of fun facts about this event and meteor showers in general consult these previous posts.
Here are a few other fun sites where you can read about the Perseids.
Shadow and Substance (an animation of the shower)