Original post from Wednesday afternoon
Hundred-degree weather is in the forecast for Washington for the first time since 2016, as a punishing, long-duration heat wave peaks over the area Friday into the weekend.
Today’s brutal combination of heat and humidity, in which heat index values have exceeded 105, is just a preview of even more oppressive conditions on the way. The heat index, which is a measure of how hot the air feels factoring in the humidity, could near or top 110 Friday through Sunday.
The exceptionally high humidity means temperatures will have a hard time cooling off at night, especially in urban areas, where temperatures may not drop below the low 80s while heat index values settle near 90.
“All of these factors point to a potentially dangerous heat event Friday through the upcoming weekend,” wrote the National Weather Service office serving the Washington region.
The heat “will become a significant threat to anyone exposed to the heat for an extended period of time,” the Weather Service warned. It is likely to issue an excessive-heat warning Friday through the weekend, the most severe kind of heat alert.
This episode of extreme heat is driven by a powerful heat dome that has overtaken much of the Lower 48 and will peak in the Mid-Atlantic and Northeast on Friday and Saturday.
Forecast details and heat safety
The forecast is a scorcher, with high temperatures around 100 Friday through Sunday. Factoring in the humidity, it will feel 10 to 15 degrees hotter:
- Thursday — High: 93; peak heat index: 103
- Friday — High: 98; peak heat index: 110
- Saturday — High: 100; peak heat index: 112
- Sunday — High: 99; peak heat index: 111
- Monday — High: 94; peak heat index: 103
As the heat began to swell over the Washington region Tuesday, the U.S. Office of Personnel Management urged federal agencies to “take all available steps” to protect employees, including providing access to drinking water and enabling flexible work schedules and telework options.
Most heat-related illnesses and deaths are preventable, and the Weather Service offers several tips for staying safe and staying cool:
- 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 the elderly, sick, or those without access to air conditioning.
- Never leave a person or pet in an unattended vehicle.
- Recognize the symptoms of heat illness and take action.
Possible records and the heat wave in historical context
The heat wave is coming at the hottest time of the year in the Washington, so even though it will be excessively warm, the intensity of the heat may fall shy of daytime records. For instance, the forecast high of 100 on Saturday is unlikely to challenge the record for the date of 106 degrees from 1930, which is also the highest temperature ever recorded in the city for any day of the year.
Friday’s record high of 102 (also from 1930) and Sunday’s of 104 (from 1926) also may prove hard to beat.
But because the air will be so humid, some record-warm overnight lows are possible. The Weather Service is predicting a low of 82 degrees Saturday, which would tie the highest mark for that date, set in 2015. The forecast low of 83 degrees Sunday would break the old daily record of 82 degrees in 1987 and come close to the warmest overnight low for any day of the year of 84 degrees, set in July 2011.
At Baltimore-Washington International Marshall Airport, where Baltimore records are kept, the odds of breaking some daytime records are higher, but all readings there are 100 degrees or greater during the day and 80 degrees or higher at night.
Records are more likely to fall at Dulles, which has a much shorter period of record (dating to 1963).
Washington is in the midst of its fourth heat wave (defined as events with 90-degree weather on at least three straight days) this year, compared with a yearly average of five, with half of the summer remaining. The longest heat wave each year averages around nine days, and this one is already a week long and should extend at least five more days.
Occurrences of 12 straight days at and above 90 degrees have happened in less than 15 percent of the years since Washington started keeping records, which date to the 1870s. If this heat wave reaches that length, it would be the sixth instance since 2006. As recently as 2016, the city had a streak of 13 days. The longest streak on record is 21 days, which happened in 1980 and in 1988.
If Washington strings a set of 100-degree days together, two in a row has happened 24 times in the modern record, but there are only six streaks of three or more in a row, and the maximum is four days, set most recently in 2012.
Jason Samenow contributed to this article.