Take a deep breath: yesterday's excessive heat was a flash in the pan. Cooler and drier air is on the way. Starting now...
Today
Decreasing humidity, not as hot. Behind yesterday's cold front, much fresher air will trickle in. Expect plenty of sunshine with high temperatures about 10 degrees cooler. Humidity levels will drop, so it will feel 15 degrees cooler. High temperatures should be in the mid 80s with a refreshing northerly breeze around 15mph. Overnight, it will be clear and pleasant, with lows 56-61 (suburbs-city).
Sunday
About as good as it gets. Cool high pressure to the north will keep it comfortable. Look for mostly sunny skies, low humidity and high temperatures in low 80s. Winds will be northerly at around 10mph. Wear sunscreen, as the UV Index will be very high (around 9).
Yesterday's Weather
For extreme weather fans, yesterday was an exciting day. First a heat advisory, then a severe thunderstorm watch. High temperatures reached 93, 97, and 95 at Reagan National, Dulles and BWI Airports, respectively. Dulles Airport's 97 set a record high for the date. Next thunderstorms, some severe, caused power outages in 30,000 homes, mostly in Montgomery County. For a recap, see this Washington Post story:
Storms Offset Record Heat But Cut Power To ThousandsWeekend Reading