"Wet begets wet and dry begets dry" (although I hear the latter referenced more 
).
When you go into summer with that dry ground, you have a higher chance of hotter temps (and heat waves with those higher temps) because the energy that is needed to evaporate any moisture, which would delay further temp increases, would not have to happen, and up up up go the temps starting earlier in the day.
I know last year here in the Philly area, we had a relatively rainy spring, coming off a high-moisture year, so although we had our heatwaves, we didn't get any persistent upper 90s or triple digit temps last year (and haven't had triple digits in awhile now). This year is a bit different, with less spring precip and even a couple weeks without any precip at all this month until a couple days ago, so will have to see how that translates as we get into summer.
One of things that does mitigate some of this here on the east coast, is the hurricane season and potential rain from tropical systems (and/or their remnants).
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