Modern aircraft can fly through almost any weather conditions and pilots are trained to land with limited visibility using only their instruments. So why are weather delays so common?
One reason is the danger to ground personnel. The people who service the plane, put on and take off the luggage, catering and cleaning crews are all exposed to the elements. Because of this most airports have strict safety measures in place that requires these workers to remain inside during inclement weather (such as lightning strikes).
Another reason is that these storms can produce a lot of turbulence and make flying uncomfortable for passengers. Pilots and Air Traffic Control (ATC) will route traffic around these large storm cells but that causes another problem, traffic. Too many planes in any one area can cause a safety problem so ATC will often slow down or delay flights at their origin to reduce the number of planes in the air. This means that even though the weather might be nice at your departure airport you will have to wait to get permission to fly past a busy storm area.
To see if your airport is experiencing weather delays (or any other delays for that matter) you can go to the FAA’s website https://www.fly.faa.gov/flyfaa/usmap.jsp .
One thing to remember is that thunderstorms are most active in the afternoon in the summertime. This leads to more delays in the afternoon. If it is possible you should try to book your flight for the morning when these storms are less likely.