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A long-duration heat wave is beginning in Washington when it will frequently feel as if it’s 100

NAM model predicts a heat index of 100 Sunday at 2 p.m.

How do nine straight days of 90s with suffocating humidity levels sound?

Starting as soon as Thursday and continuing well into next week, high temperatures are forecast to hit at least 90 in Washington. The hottest stretch is predicted between Saturday and Tuesday, when maximum temperatures in the mid-90s or higher are likely.

But temperatures will tell only part of the story. Especially Sunday through next week, humidity levels become oppressive, adding 5 to 10 degrees to how hot it feels. The combination of heat and humidity will produce heat index values near and above 100 for days on end.

Dew-point temperatures, an indicator of humidity, are forecast to climb to 70 degrees or warmer starting Sunday and are not predicted to ease next week. At times, they could even exceed 75, which we consider practically unbearable. Pretty much every day next week will be brutally humid with no sign of meaningful relief.

The culprit for the heat wave is an area of high pressure at high altitudes, sometimes called a heat dome. It will be centered over the Northeast this weekend before it is forecast to expand and cover much of the United States by the middle of next week. There’s some chance that by the end of next week, or in about 10 days, it will weaken some and retreat toward the West Coast, but our confidence in the specifics of its evolution decreases with time.

Summer’s first heat wave arrives in central U.S., will shift to Northeast this weekend

In the shorter term, the main wrinkle in the forecast is a weak cool front forecast to slip into the region during the middle of next week. It may introduce enough cloudiness to keep highs below 90 around Wednesday, but we wouldn’t bet on it.

The Commodity Weather Group, which averages simulations from the American, European  and Canadian modeling systems, suggests highs have the potential to reach at least 90 in Washington for nine straight days between Friday, June 29, and Saturday, July 7.

The heat dome parked over the region will tend to quash thunderstorm activity, with the possible exception of the period between Tuesday and Thursday, when the weak front in our vicinity could act as the focus for widely scattered late-day storms (20-30 percent chance of measurable rain on those days).

For planning purposes, here’s a summary of the heat and humidity forecast over the next week (I’ve tweaked a few of the high temperatures where I think the model average may be a bit off):

  • Thursday: High: 90. Max heat index: 95. Humidity: High
  • Friday: High: 91. Max heat index: 94. Humidity: Moderately high
  • Saturday: High: 94. Max heat index: 98. Humidity: Moderately high
  • Sunday: High: 96. Max heat index: 104. Humidity: High
  • Monday: High: 97. Max heat index: 106. Humidity: Very high
  • Tuesday: High: 96. Max heat index: 106. Humidity: Very high
  • Wednesday: High: 91. Max heat index: 100. Humidity: Very high

None of these predicted temperatures is record-challenging, and it’s fair to say Washington is accustomed to temperatures in the 90s in late June and early July. But the duration of the heat and the very high humidity is certainly notable.

Capital Weather Gang contributor Greg Porter tweeted that if Dulles Airport hits 90 for nine straight days, it would be the longest streak since 2012.

In Washington, the longest streak of 90-degree days is 21, set in the summers of 1980 and 1988.

During a long heat wave such as this, it’s important to remain hydrated and, when possible, delay strenuous exercise outside peak heating hours. Also, remember to keep your pets cool and hydrated, and never leave a child in a parked car for any length of time.