Heavy Rain Events More Common During the Summer Months

Severe weather and flooding rain can occur anytime during the Spring and Summer months, but we tend to see more heavy rain events during the summer months of July and August. 

It's important to be prepared for severe weather anytime of the year.  Tornadoes have occurred during the fall and winter season, and even heavy rain has occurred when the precipitation was supposed to be snow.  So, what's happening in the atmosphere that causes heavier rain to occur during the summer?

To answer that we need to look at the jet stream.  The reason the jet stream exists is because of the temperature difference throughout the world.  The atmosphere is trying to balance itself out.  When there is an imbalance, storm systems develop.  Jet stream winds are fastest during the winter months because that's when the greatest temperature difference occurs from north to south.  During the summer months, jet stream winds are usually weak because the temperature is more uniform. 

Most severe weather outbreaks in the United States occur during the months of April and May, but can also happen any time of year.  During the spring the jet stream winds are still relatively strong because seasons are changing and the warm air trying to overcome the cold air.  Because the air during the spring hasn't reached its peak heating, the air mass isn't able to hold as much moisture as it would be able to during the summer.  The overall pattern in the atmosphere tends to be a little more progressive, pushing storm systems along rather than developing into a pattern that favors continued rainfall for many hours, or many days. 

During the summer, the winds within the jet stream are much weaker.  Climatologically July is our hottest month of the year.  By this time, the air throughout the atmosphere has warmed not only at the surface, but aloft as well.  This means the air is now capable of holding a lot more moisture than during the spring.  Storm systems (low pressure systems) don't tend to be as strong as they are in the spring and fall.  However, as hot domes of air settle into a particular area of the country, a 'ring of fire' pattern tends to develop.  This is where the sinking, hot air underneath the high pressure system doesn't allow thunderstorms to develop, but on the outer edges great amounts of instability occur.  This tends to set up boundaries that not only low pressure systems travel along, but also allows moisture to build up and produce significant amounts of rainfall.  This is also why during the summer months we tend to hear more about 'Derecho' type thunderstorms - or long lasting complex of thunderstorms that move over several hundred miles from its starting point, producing damaging winds along its path.

Jet stream winds will then begin to increase towards the fall months as the season begin to change and cooler air starts to move back south.