The East Coast is bracing for excessive heat to kick off the workweek, with triple-digit heat indexes and hazardous conditions from Maine to Florida. Heat advisories and excessive heat warnings are in effect as the combination of heat and humidity causes heat indexes to soar as high as 110 degrees, or even above that in some locations.

The big cities of the East Coast, including the Acela corridor from Washington to Boston, will suffer their hottest day so far this summer, with heat advisories and excessive heat warnings in effect across the region.

Meanwhile, a second wave of sweltering heat was building over the West Coast on Monday, set to expand east across the Lower 48 during the upcoming week.

The sizzling forecast comes on the heels of a toasty weekend that featured temperatures flirting with or climbing into the triple digits. Baltimore hit 100 on Sunday, D.C. managed 99 degrees, and New York City rocketed up to 94. And at 95 degrees, Boston saw its hottest day of the year. Monday was expected to be even hotter.

  • Highs on Monday were expected to hit 97 degrees in Boston, heralding temperatures in the 90s that will make it all the way to the Canadian border.
  • New York is looking at a forecast high of 95, while Philadelphia could hit 98 for the fourth time in five years.
  • In the Mid-Atlantic, temperatures could reach their hottest levels since 2016.
  • D.C. is forecast to hit 99 on Monday and had already hit the 90-degree mark as of 9 a.m. The nation’s capital could squeak out a 100-degree reading. Washington’s morning low on Monday was 83 degrees, which — if it holds until midnight Monday night — would tie for the city’s second-warmest night on record.
  • Richmond is likely to hit 101 degrees, while Baltimore may lag just a hair with a predicted high of 98 degrees.
  • Nashville and Charlotte were both in line for a high near 97 on Monday, along with sultry levels of humidity.

Monday features a decent chance of a few isolated thundershowers for the D.C. and Philadelphia metro areas, but these storms will bring only a brief, localized cool-down.

The hot, steamy weather and elevated overnight low temperatures could prove dangerous for vulnerable populations, such as those with health conditions and the elderly. These populations are also most at risk of contracting and being adversely affected by covid-19, the illness caused by the novel coronavirus.

Urban residents who lack air conditioning are at significant risk since overnight lows have not fallen enough to provide the body with relief, and this can hasten the development of potentially deadly heat illness.

Authorities in many areas have had to alter how cooling shelters are operated to comply with social distancing guidance.

A pair of steamy weather systems

The toasty temperatures stem from a pair of high-pressure systems, also known as heat domes since they essentially form protective bubbles over large areas, preventing significant precipitation from moving in, that have built across the continent. One such dome is anchored just off the East Coast, which is amplifying already hot weather and providing a flow of tropical air from the South into the Northeast.

Another significant heat dome is entering the Pacific Northwest, with the potential to expand into the midsection of the country during the coming days.

Reinforcing heat arrives in the West

The heat that’s percolating in the Pacific Northwest is the first sign of a looming wave of heat parked just offshore. Medford, Ore., is forecast to hit 103 degrees on Monday, while Portland is set to climb into the lower 90s.

Much of that heat extends down into the Intermountain West and to the Four Corners region.

Even Boise, Idaho, could hit 94 or 95 degrees on Monday, a touch above its average high. Salt Lake City was expected to hit 98 degrees.

In the coming days, that axis of heat will shift east, into the Plains. Billings, Mont., may make it to 100 degrees on Wednesday.

That heat then looks to linger over the central United States for the remainder of the upcoming week. Thanks to these two reinforcing weather systems, the National Weather Service’s Climate Prediction Center is highlighting odds of above-average temperatures over the entire contiguous United States for the foreseeable future.

How the heat could be hazardous for some

The combination of hot temperatures and high humidity will prove potentially dangerous for many Americans who may not have access to a cool respite. That’s especially concerning by the pandemic, which has forced tens of millions of residents to remain at home when they may otherwise be at an air-conditioned workplace or have access to air-conditioned recreation, such as shopping malls and movie theaters.

The Centers for Disease Control and Prevention has released guidance to local officials on how to mitigate the threat of covid-19 transmission in cooling shelters, recommending mask-wearing and social distancing, for example.

While heat waves have always been a part of summer in the United States and elsewhere, climate change is shifting the odds in favor of more frequent and intense heat extremes. Data shows overnight low temperatures, which play a crucial role in making heat waves deadlier, are increasing faster than daytime highs, as is expected in a climate that is warming because of greenhouse gas emissions.

Recent studies have found evidence showing some heat waves occurring today could not have taken place without the influence of human-caused global warming.