* Excessive heat warning 11 a.m. to 9 p.m. Friday and Saturday, excessive heat watch 11 a.m. to 9 p.m. Sunday *

5:15 p.m. update: The high temperature in Washington on Friday hit 96, tying the hottest day of the year so far. Factoring in the humidity, it felt as hot as 106. Temperatures and heat index values should head a bit higher on Saturday and possibly Sunday, as well. Our updated forecast is available here: Excessive heat alert extending into Saturday as the region roasts through the weekend

Original post from midday

Washington has already endured nine straight days of 90-degree weather, and the heat rises to another level through the weekend when high temperatures are expected to near 100 degrees and feel like 105 to 115 degrees factoring in the humidity.

The National Weather Service has issued an excessive heat warning for the Washington region on Friday and Saturday, the most severe kind of heat alert.

“Dangerously high temperatures and humidity could quickly cause heat stress or heat stroke if precautions are not taken,” the Weather Service warned.

An excessive heat watch is in effect Sunday, and is likely to be upgraded to a warning.

If the air temperature hits 100 at any point Friday through Sunday, it will be the first such instance since 2016.

The forecast

Whether you’re interested in the outlook for Friday, Saturday or Sunday, the answer is the same: It’s going to be beastly hot. Here are the predicted numbers, noting the heat index is a measure of how hot it feels, factoring in the humidity:

  • Friday — High: 98; peak heat index: 109
  • Saturday — High: 100; peak heat index: 112
  • Sunday — High: 99; peak heat index: 108

Due to suffocating humidity levels that prevent the air from cooling off, extremely warm nighttime temperatures will give this heat wave added punch. Overnight low temperatures may only dip to the low-to-mid 80s in downtown Washington and just 75 to 80 even in outlying areas.

Inside the Beltway, the heat index may never fall below 85 to 90 degrees, even in overnight and predawn hours when it’s typically coolest.

A bulging zone of high pressure known as a heat dome is responsible for these punishing temperatures, some 10 to 20 degrees above normal. This heat dome is sprawled over the eastern half of the nation, resulting in oppressive heat and humidity from the Central Plains to the Northeast.

Heat safety

“It is vitally important to remember heat safety during a prolonged excessive heat event such as this,” wrote the Weather Service. Most heat-related illnesses and deaths are preventable by following simple tips:

  • Stay hydrated and take breaks in the shade.
  • Limit activity, especially during peak heat from roughly 10 a.m. through 4 p.m.
  • Check in with older adults and sick individuals — especially those living alone, and with anyone without access to air conditioning, including the homeless
  • Never leave a person or pet in an unattended vehicle.
  • Recognize the symptoms of heat illness and take action.

Outdoors workers and athletes, and their supervisors and coaches, should take extra precautions to prevent overexertion.

The District has declared a heat emergency in response to the high temperatures and opened cooling centers where residents can find relief.

What records could fall?

The most likely records to fall in the District are for the warmest low (overnight) temperatures this weekend. Saturday’s forecast low of 82 degrees would tie the record for the date. Sunday’s predicted low of 83 would set a record, just a degree off the warmest low for any month of the year in the city.

Afternoon (daytime) high temperature records may be tougher to come by given this heat wave coincides with the hottest time of year. Saturday’s forecast of 100 degrees, while intense, is still well shy of the all-time high of 106 degrees, which was set on the date in 1930.

Most existing record (daytime) high temperatures for the region should hold, except perhaps at Dulles where records have only been maintained since 1963 (in Washington and Baltimore, records date back to the 1870s).

How rare is this heat?

The forecast maximum heat index between 110 and 115 is not terribly uncommon, at least in recent years. Over the past few decades, Washington averages a maximum heat index of 114 degrees a little less than once a year, and 116 degrees about every other year. The city has seen a heat index as high as 120 degrees as recently as 2011. Values that extreme happen about once a decade on average.

A 122-degree heat index is believed to be the modern maximum for the District. On July 16, 1980, the 4 p.m. observation was 103 degrees with a dew point of 78 degrees to reach that high point.

This heat wave, which began on July 11, is helping Washington’s count of 90-degree days so far this year mount.

Through Friday, Washington has hit 90 degrees or higher on 28 days. Only 10 other years (7 percent) dating back to 1871 have had so many. By the time this heat wave ends Monday, the count of 90-degree days should close in on the top five most year-to-date.

When will relief arrive?

Early next week, a cold front will take an edge off the heat. Highs drop to near 90 on Monday, when showers and thunderstorms are likely. And between Tuesday and Thursday, highs should hold in the 80s.

Here’s how the U.S. is dealing with extreme heat

July 21, 2019 | Overheated visitors run through a sprinkler on the Mall in Washington, DC. (Bill O'Leary/The Washington Post)