One of the reasons often given for the persistence of above average precipitation over the past two decades was that wet begets wet.  In other words, the wet soils allowed for more evaporation, which in turn caused it to rain more and that cycle continued year after year.

This is referred to as a positive feedback loop where a change in one variable, soil moisture, reinforces the original process, rainfall.  Although there is some truth to this, it is or was a minor player.  An example would be the summer of 2011.  It was a very wet summer until August 6, but then the rain stopped and never returned.  What changed was the low-level wind pattern.  Almost all our summer moisture comes from the Gulf of Mexico and subtle changes in moisture advection from the south can radically alter how much moisture this area receives.

Although there has been a couple of exceptions, this summer has for now continued that pattern that started last summer with precipitation being near or below normal. If this continues through the entire summer will come down to variables we still can not forecast reliably for much beyond a week.