An extremely dangerous heat wave is expected to build across Delmarva and the Mid-Atlantic beginning Wednesday and lasting through the Fourth of July weekend, with the worst of the heat likely Thursday and Friday.
A strong ridge of high pressure will expand eastward from the Great Lakes and Ohio Valley into the Northeast and Mid-Atlantic by midweek. This setup will allow temperatures to surge well above normal, with highs reaching the mid and upper 90s Wednesday before climbing into the 100 to 105 degree range Thursday and Friday.
The combination of heat and humidity will create dangerous conditions for anyone spending time outdoors. Heat index values are expected to reach around 100 to 110 degrees Wednesday, then climb into the 110 to 120 degree range Thursday and Friday. Some locations could see heat index values near or above 115 degrees, especially where dew points remain higher during the afternoon.
This pattern could challenge daily and even all-time record high temperatures across parts of the region. For Georgetown, Delaware, the all-time record high is 104 degrees, set on July 22 and July 23, 2011. Forecast highs late this week may come close to that level, especially inland where a west to northwest wind could promote down sloping off the Appalachians and additional warming.
The hottest stretch is expected Thursday into Friday as winds shift more westerly to northwesterly. That flow should allow dew points to mix out somewhat during the afternoon, but the tradeoff will be even hotter air temperatures. Even if humidity drops slightly inland, the heat will remain dangerous.
Coastal areas may not be spared from the impacts. A sea breeze could hold actual temperatures down slightly near the beaches, but higher humidity could keep heat index values just as dangerous, especially during the afternoon and early evening.
An Extreme Heat Watch remains in effect for northern Delaware, New Jersey, southeast Pennsylvania, the Delaware Valley and the southern Poconos. Future updates could expand the watch to include more of Delmarva as confidence increases in the duration and severity of the heat.
Saturday will still be dangerously hot, even as the upper-level ridge begins to weaken and shift back toward the south and west. Highs are still expected to reach the mid 90s to near 100 degrees, with heat index values generally between 100 and 105 degrees. Locally higher values near 110 degrees remain possible where humidity does not mix out.
Relief will be slow to arrive. Temperatures should ease slightly by Sunday and Monday, but highs may still remain in the low to mid 90s with heat index values in the mid 90s to low 100s. That means heat-related impacts could continue beyond Saturday, especially for vulnerable groups and those without reliable cooling.
The pattern change may also bring increasing chances for showers and thunderstorms late Friday into the holiday weekend. Storm chances look higher Saturday and Sunday as the ridge breaks down and a trough tries to slide into the Northeast. Any storms that develop could briefly interrupt outdoor plans, but they may not provide widespread or lasting relief from the heat.
Residents should prepare now for a prolonged stretch of dangerous heat. Limit outdoor activity during the hottest part of the day, drink plenty of water, check on elderly neighbors and family members, never leave children or pets in vehicles and make sure pets have shade and water. Outdoor workers and anyone attending holiday events should take frequent breaks in air conditioning or shaded areas.
This type of heat can become dangerous quickly. Heat exhaustion and heat stroke are possible when the body cannot cool itself properly, especially during multi-day heat waves when overnight temperatures remain warm and recovery time is limited.
