/Excessive heat watch this weekend as D.C. faces 100-degree temperatures

Excessive heat watch this weekend as D.C. faces 100-degree temperatures

Forecast highs for Saturday from the National Weather Service.

With temperatures expected to rise near 100 degrees, and high humidity helping to push heat index values to 105 to 115, an excessive heat watch has been issued for the D.C. region this weekend. A heat advisory is in effect Friday, when the heat index is forecast to reach 105 to 109.

If temperatures hit 100 degrees during this stretch, it’ll be the first time since 2016, and there’s some chance it could happen more than once.

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

Hunkering down near air conditioning might be the best plan. Along with instant-sweat conditions during the day, overnight lows stay near and above 80 degrees in urban areas, with heat index values only settling near 90.

Heat index forecast for 2 a.m. Saturday shows much of the area still feeling like 90 or greater.

Detailed forecast

Temperatures soar to near 100 degrees Friday through Sunday before a front breaks the heat Monday. Relief is minimal at night, with temperatures only dipping to the mid-70s to low 80s in most spots. If you’re looking for good news, forecast temperatures for Monday have come down some since Wednesday, as the cooling front may arrive a bit earlier. Fingers crossed.

  • Friday — High: 98; peak heat index: 109
  • Saturday — High: 100; peak heat index: 112
  • Sunday — High: 98; peak heat index: 113
  • Monday — High: 93; peak heat index: 92

Heat index values are likely to be higher than shown below in some areas, perhaps even exceeding 115 in a few spots. That’s as hot as the forecast high temperature in Death Valley (where it’s a dry heat) over the weekend.

Heat index forecasts at 2 p.m. Friday, Saturday and Sunday via the high resolution NAM weather model.

The combination of heat and humidity is near our maximum historically, hence the heat watches, which will probably become warnings.

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.

Just how sultry is it?

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.

Maximum heat index by year in Washington, per hourly observations. (Ian Livingston/The Washington Post)

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.

It’s somewhat rare to experience extremely high temperatures and humidity simultaneously. Dry air heats up quickly while humid air is harder to warm. In other words, it’s easier for Washington to top 100 degrees when the air isn’t so humid. On the other hand, humid air is harder to cool, which makes humid nights particularly oppressive.

Dew points are a measure of how much humidity is in the air. Once the dew point exceeds 70 degrees, you’re into muggy or “gross” humidity levels. Through the weekend, our dew point is predicted to hover around 75 degrees, or even higher than that. The city saw a 78 degree dew point Wednesday, which is already rarefied air.

(The Washington Post)

Washington has seen dew points reach 80 degrees as recently as 2017. Such extreme humidity is a very unusual occurrence, more common in the coastal South, although seemingly showing up here more often in the past few decades.

Dew points tend to be highest near bodies of water, so locations near the Potomac, with water temperatures in the mid-80s to near 90, may be particularly oppressive.

What records are in jeopardy?

Records could be tough to come by in large quantities given this heat wave is coming during the hottest time of year historically. 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.

Forecast lows from the National Weather Service for Saturday morning. The urban heat islands stick out.

The most likely records to fall in the District are the maximum low temperature numbers this weekend. Sunday’s 82 degree forecast low would tie the record for the date. Sunday’s predicted 83 degrees would be a record, and only a degree off the all-time warm low for the city.

Washington has been on a tear when it comes to 90-degree days this year. It has surprised us how many considering our summer outlook projected a below-normal number.

The 27 days of 90 degrees or greater through Wednesday is among the top 10 most on record year-to-date. By the time this heat spell ends, it should close in on the top five most year-to-date.

Many of our days hitting 90 or higher have only been in the low 90s though. We’ve only had three days at or above 95, which is slightly below normal year-to-date. In the summers of 2010, 2011 and 2012, the hottest three on record, the city had already seen 12, 10 and 17 days of 95 degrees or higher to this point.

With July running almost two degrees above normal through Wednesday, odds are very good the month finishes above normal. It would mark the eighth straight warmer-than-normal month.

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