Besides a few stand-alone hot days, the East has generally escaped the heat that has plagued the Central and southwestern US since May. Summer will no longer be so benign by Friday.
Two upper-level ridges of high pressure will combine this week into one large dome of high pressure over the Eastern US late this week. Both have already resulted in scathing 100 degree temperatures in the southern third of the continental US, primarily in the desert Southwest and the southern Plains. This broad region of high pressure will gradually expand late this week into early next week, sparking a dangerous heatwave that may last through the July 4 holiday for some areas.
The thermostat will first crack 90° across the Upper Mississippi River Valley eastward to the coastal plain of Southern New England on Friday. The immediate coast, the Ohio River Valley, and higher elevations of the Mid-Atlantic and New England will have to wait for Saturday or Sunday for their first 90 degree day.
The heat and the humidity will turn up a notch over the holiday weekend. The dome of high pressure will ubiquitously increase heat indices to between 100 and 110° or greater everywhere east of the Mississippi River by Sunday, with the exception of the highest elevations and south facing shorelines. The Heat Index is the temperature that the air really feels when incorporating relative humidity. The heat will even spread into Canada, with 90° temperatures reaching as far north as the Hudson Bay in northern Ontario and Quebec.
Urban areas of the Mid-Atlantic and Northeast will face the brunt of the heat. Actual temperatures along the I-95 Corridor, from Washington, DC to Portland, ME, may approach or even exceed the infamous 100° Sunday and Monday. Some numerical guidance, including the 12 UTC June 25 GFS model, depict widespread temperatures of 100-105 along this corridor. The GFS only represents one possible. Other global models have depicted a similar extent to the heat with slightly cooler temperatures.
Ocean temperatures are still in the 60s in New England and 70s in the lower Mid-Atlantic, so the beaches will be the coolest places as opposed to the mountains. Temperatures on eastern Long Island’s south shore and Massachusetts’ Cape Cod may not even exceed 90° throughout the duration of the heat wave.
With high pressure dominating the weather, pollutants will build in the air throughout the weekend, especially in and to the northeast of urban areas. As smog buildup worsens the air quality, the elderly, young children, and those with respiratory problems are encouraged to stay inside an air-conditioned building. For those who do go outside, drink plenty of water and avoid laborious activities to avoid heat exhaustion and heatstroke.
Since the heat is expected to affect so many people, the high demand for electricity may result in rolling blackouts or brownouts. Rolling blackouts are intentionally engineered complete interruptions of service by power companies to prevent an overload of the electrical grid. An overload would result in grid-wide shock and fire-hazard risks in addition to damaging critical grid components. Conversely, brownouts are temporary drops in voltage across an electrical system and can be either intentional or unintentional.
The heatwave will end for many areas along and west of the Appalachians on Monday, though urban areas may still confront temperatures in the low 90s. The coastal plains of the Mid-Atlantic and New England will likely hold onto the heat through July 4 with the passage of a cold front.