You can use Fodor's, Frommer's, etc to see what they say about traveling certain times of the year. But, I prefer using Weather.com.
You can easily use this site to see most cities (some smaller cities do not have data) on current weather, upcoming 14 day forecasts, and historic high and lows/averages.
This will allow you plan clothing and accessories (umbrella?).
You can keep checking weather for more current forecasts as you trip gets closer as well. Assuming that most trips are less than 14 days, you can see you forecast for about your entire trips before even leaving.