GFS model forecast of heat index at 2 p.m. Wednesday in Washington.

Every May, it’s only a matter of time before the Washington region is confronted by its first heat wave. 2017 may well deliver heat wave version 1.0 between Wednesday and Friday this week.

Forecast models agree that highs each day will be around 90 degrees, possibly even a little hotter.

“Daytime highs and nighttime lows during the long term will be August-like, with temperatures 10-15 degrees above normal Wednesday through Friday,” the National Weather Service office serving Washington said early Monday.


GFS model ensemble forecast system (GEFS) with average high and low temperatures for the next 15 days in Washington. (WeatherBell.com)

High humidity will complement the heat. Dew point temperatures, a measure of humidity, are forecast to be in the 65-to-70-degree range during this span. Any time the dew point exceeds 65 degrees, it feels sticky.


Forecast dew points Wednesday afternoon from GFS model.

The combination of heat and humidity will make afternoon temperatures feel as high as the mid-90s throughout the three-day stretch.

The late week weather pattern shows the classic features of a typical heat wave in the Mid-Atlantic and Northeast.

At high altitudes, a big dip in the jet stream will set up over the Rockies, resulting in unusually chilly air and snow in the Rockies. On the flip side, a large ridge of high pressure will develop over the eastern United States. Underneath the ridge, an expanding heat dome will form.


GFS model high altitude weather pattern on Thursday, showing a big dip in the jet stream and a pool of cold air in the West, and a bump in the jet stream over the East with a heat dome building underneath it. (WeatherBell.com)

At lower altitudes near the surface, a high pressure cell will develop around Bermuda, which will act like a pump circulating warm, humid air up the East Coast.


GFS model of surface weather map on Thursday showing the development of high pressure off the Southeast coast, which will pump warm, humid air north. (WeatherBell.com)

The Weather Service noted that temperatures about a mile up in the atmosphere are forecast to reach the 98th percentile, conveying the intensity of the predicted hot air mass.

By the weekend, somewhat cooler air is forecast with highs of 75 to 80 degrees.

High temperatures of at least 90 on three consecutive days this week in Washington would satisfy official criteria for a heat wave.

Meeting such criteria is fairly common in Washington from late June to late August, but much more rare in May. Since records began in 1872, Washington has only officially posted 17 May heat waves, or about one every eight or nine years.

Washington’s most recent May heat wave, which lasted the minimum three days, occurred in 2013. The longest May heat wave on record occurred in 1991, lasting seven days — a week’s worth of 90-degree weather.