Why so hot though? Well there's two major things affecting the airflow leading into the Eastern seaboard. Below is the Jet Stream:
By Sunday however, the Jet will begin to take over the High, and the frontal system along the Eastern edge of the Jet will begin to move East once again, bringing a chance of showers and thunderstorms while also cooling temperatures down 5-10° as the front finally gathers the strength to push into the Atlantic once again. Remember that the rain many of us received in the beginning of the week (and some of NY state, VT, Eastern MA, and the areas west as well are still receiving today) is from this same system and front, and it'll be the same system and front bringing us a chance of rain Sunday into Tuesday. This blockage is what we say last year as well, though it blocked a large area of high pressure rather than precipitation.
So take care when you're outside, keep the AC on for your pets if you keep them in the car (at 90° even with the windows cracked temperatures inside the car can reach 100° just 10 minutes, 120° in a half hour), bring water with you, and keep cool until Sunday when the temperatures drop a few degrees and we have clouds to help us out.