This D.C. heat wave may be the hottest and most dangerous of the summer

The heat and humidity “will significantly increase the potential for heat-related illnesses, particularly for those working or participating in outdoor activities,” the Weather Service said. “Drink plenty of fluids, stay in an air-conditioned room, stay out of the sun, and check up on relatives and neighbors. Young children and pets should never be left unattended in vehicles under any circumstances.”

No alerts are posted for Wednesday yet, when the heat index could reach near 105.

Here are the predicted highs through Wednesday and the records that could be threatened:

• Monday: Forecast high of 101; record is 100 from 1988.
• Tuesday: Forecast high of 101; record is 104 from 1988.
• Wednesday: Forecast high of 97; record is 102 from 1980.

Nighttime lows only dropping to around 80 degrees could also challenge records.

While this heat wave looks to be shorter than previous ones this summer, it could very well end up as the hottest and most dangerous. Much of the region is forecast to reach a Level 4 on the National Weather Service’s HeatRisk scale Monday and Tuesday, which rates the danger of the heat to human health. That is the highest level, described as “rare and/or long-duration extreme heat with little to no overnight relief.”

Climate Central’s Climate Shift Index for Monday indicated that human-caused climate change has made such heat five times as likely.

Portions of the region could still be at a Level 4 HeatRisk on Wednesday while others drop to a Level 3, described as “major” heat that “affects anyone without effective cooling and/or adequate hydration.”

If D.C. hits 100 degrees or higher Monday, Tuesday and Wednesday, it will have reached triple digits on four consecutive days. That would tie the longest such streak on record, matching heat waves in 1930 and 2012. There’s a chance, though, that more clouds keep Wednesday’s temperature below 100.

A cold front coming through late Wednesday, bringing the week’s best chance of showers and thunderstorms, should drop highs back to the 80s to near 90 Thursday through Saturday. Highs could climb back into the 90s on Sunday through the middle of next week, but as of now it looks like the more tolerable low to mid-90s, rather than the oppressive mid-90s to near 100.

The first, and longest, heat wave of the summer spanned 11 days, reaching 90 degrees or higher from June 17 to 27. The average high temperature during the hottest portion of that stretch, June 21 to 26, was 96.3.

During a seven-day stretch from July 5 to last Thursday, each day’s high temperature reached 95 or higher, with an average high of 97.4.

After a one-day break in the heat, with a high of 83 on Friday, D.C. started its most recent streak of highs at or above 90 on Saturday. Here are some of the scorching statistics from this sizzling D.C. summer:

• With an average daily temperature of 81.1, this summer is the hottest on record to date, surpassing 80.9 in 2010.
• The 27 days at or above 90 degrees thus far is 10 above average and more like the number typically expected by the first week of August.

• D.C. has reached 98 or higher on nine days. The most on record in a summer was 13 days in 1930, followed by 11 in 2012.

• D.C. set calendar-day record highs of 101 on Sunday and 99 on June 23.
• The six excessive-heat warnings issued by the Weather Service so far this July are the most on record in a month since at least 2006, according statistics from the Iowa Environmental Mesonet. An excessive-heat warning is issued when heat indexes near or above 110 are expected.
• D.C.’s highest heat index so far this summer is 111.
• Five days have had lows of 80 or higher, tied for the second-most on record in a summer, and Monday is likely to be the sixth. The maximum in a year was seven, in both 2016 and 2011.

Jason Samenow contributed to this report.