When a pattern or block like this sets up, if often stay in place for several days - creating a traffic jam in the atmosphere. The website Haby’s Hints explains:
The high pressure covers such a broad latitude that the west to east air flow has difficulty going around the high. The region under the omega block experiences dry weather and light wind for an extended period of time while rain and clouds are common in association with the two troughs on either side of the omega block. Omega blocks make forecasting easier since you can pinpoint areas that will be dominated by dry or rainy weather for several days.
In this particular circumstance, the D.C. metro region auspicously sits on the east side of the block where winds come from the north (as a result of the clockwise circulation from the high pressure center to the southwest). The north wind has kept it from getting too hot or humid. But on the west side of the block, there has been record heat in the Midwest due to opposite flow from the south.
Chicago set a record high of 90 yesterday, its earliest 90 degree reading in 31 years. On Monday, many heat records were set in Kansas, Missouri, Oklahoma, Nebraska and Iowa. Wichita had its earliest 100 degree reading on record.