Extreme heat returns to eastern US ahead of July Fourth

The National Weather Service office in Mount Holly, New Jersey, issued an Extreme Heat Watch for all of its New Jersey and Pennsylvania counties, plus northern Delaware, from Wednesday afternoon through Saturday evening. Multiple days of near-record heat were expected to hit the region, with temperatures expected to rise into the mid and upper 90s on Wednesday and then surge to 100 to 105 on Thursday and Friday. High humidity was expected to make it feel even hotter.

The eastern half of the United States was headed toward another sharp warmup ahead of the July Fourth holiday, after a brief stretch of cooler air and some rain in parts of the country. National Weather Service offices described the setup as a dangerous heat wave and, in some places, a major pattern change after storms and cooler conditions.

The driver was a strong ridge of high pressure over the Great Lakes and Ohio Valley, which was expanding into the Northeast and Mid-Atlantic. That pattern would help lock in the heat dome responsible for some of the most widespread heat of the summer so far across the East, while other regions dealt with storms, flooding, or even unseasonably cold weather.

The Weather Prediction Center's experimental NWS HeatRisk index uses color and numbers to forecast the potential level of heat-related health impacts over a 24-hour period, with forecasts available out through seven days. Heat risk was extreme in some areas, and there may be little to no relief at night.

The Weather Prediction Center also flagged a Slight Risk of excessive rainfall, level 2 of 4, over parts of the Mid-Atlantic, Tennessee Valley and Northern High Plains through Monday morning.