The Washington PostDemocracy Dies in Darkness

‘Dangerous’ heat: Washington could near 100 degrees Friday and Saturday and feel like 110

Overnight low temperatures may remain above 80 in the city.

Heat indexes are forecast to rise to around 110 or higher this weekend.

(This article, first published Monday, was updated with the latest forecast information on Tuesday.)

Hot weather has settled into the Mid-Atlantic, and it’s not going anywhere. Late this week, the mercury could flirt with 100 degrees and, factoring in the humidity, it may feel like 110 degrees or higher Friday and Saturday.

Practically every day in the next week is assured to hit at least 90 degrees, with the highest temperatures potentially coming Friday through Sunday. If it hits 100 degrees or higher, it will mark the first instance since August 2016.

How many heat waves does Washington see annually?

The heat index, which is how hot the air feels factoring in the humidity, is expected to be 10 to 15 degrees higher than the actual temperature.

“Dangerous heat and humidity are likely Friday through Sunday across the entire area,” the National Weather Service wrote in a hazardous weather statement. “Heat indices of 110 to 115 degrees are possible during the afternoon and evening hours each day, and heat indices may hold in the 80s and 90s at night. If the prolonged heat and humidity is realized, it will become a significant threat to anyone exposed to the heat for an extended period of time.”

Here is the forecast high and peak heat index from the National Weather Service from Wednesday through Sunday:

  • Wednesday: High temperature: 95, Peak heat index: 102
  • Thursday: High temperature: 91, Peak heat index: 101
  • Friday: High temperature: 97, Peak heat index: 109
  • Saturday: High temperature: 98, Peak heat index: 112
  • Sunday: High temperature: 98, Peak heat index: 101

The cause is a massive heat dome that is setting up shop and expanding over the region. Although it may not set records for its intensity, it comes during the annual peak in temperatures. In other words, it’s well suited to cause trouble.

The heat dome reaches its maximum intensity late in the week, when temperatures measured by weather balloons a few thousand feet off the ground may approach all-time highs, according to model forecasts.

Given D.C. managed to hit 90 degrees Monday for a fifth day in a row, this heat wave has the potential to extend over a long duration. At least another seven days seem a decent bet. Twelve days would mark the longest heat wave since 13 days in a row of 90-degree weather in 2016. The longest heat wave on record in Washington is 21 days, which happened in 1980 and 1988.

The suffocating humidity levels, with dew points expected to hover in the 70s through Monday, may hold back temperatures some. Abundant moisture in the air makes it harder to heat up.

In many ways, getting to 100 degrees or beyond in Washington is as tricky as getting a snowstorm, with many ingredients having to come together just right.

However, many of the factors needed to hit the century mark this weekend do seem to be coming together.

Whether the air temperature hits and tops 100, it will feel at least that hot given the heat index forecast.

The predicted heat indices of 105 to 115 degrees should trigger heat advisories, if not heat warnings. An excessive heat watch has already been issued for Philadelphia.

The high humidity levels mean it won’t cool off much after dark, resulting in outrageously warm overnight lows.

Washington and other urban centers should stay above 75 degrees Tuesday through Saturday. At least a night or two with lows of 80 ⁠degrees or higher seems likely. These are uncomfortable, even dangerous, conditions for those without air-conditioning.

If Washington stays at or above 80 any night during this stretch, it would be for a record fifth year in a row. Once rare, lows of 80 degrees or higher have occurred 32 times since 2010. There were only 31 from 1872 to 2009. Eight of the past nine years have seen them, two (2011, 2016) witnessed the most on record with seven each.

Although the peak of this heat wave is generally in the neighborhood of the District’s expected highest temperature of the year, or about 99 degrees, reaching such values multiple days in a row is less common, as is reaching 100 degrees.

While we average about one 100-degree day per year, they often come in bunches or don’t happen at all. During Washington’s hottest three summers on record, in 2010, 2011 and 2012, the city recorded 17 days of 100 degrees or higher. Since then, there have been four, all in 2016.

It is true that humidity may put somewhat of a brake on temperatures, but the chance for 100-degree weather is helped by the fact that it’s the hottest time of year. At least a few model runs have predicted high temperatures around 105 degrees in the city, with record-threatening warm overnight lows as well.

The highest temperature ever recorded in Washington is 106 (in August 1918 and July 1930), while the warmest overnight low is 84 (in July 1983 and July 2011). The door is opening a bit for some truly impressive values, but it’s hard to say all-time records will be tested. Some daily records should be broken regardless.

Any relief from the heat is at least a week away. There are conflicting signals given the distance in the future, but there are some signs we could be in for a near normal or even below normal temperature run that lasts a while by late month.

Jason Samenow contributed to this article.