Until this week, the hot weather this summer in the Washington region has been rather tame. We have yet to see the mercury climb higher than 97 degrees, and summer temperatures, overall, have been close to average.

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       But the most oppressive combination of heat and humidity of 2021 is on the way, prompting the National Weather Service to issue a heat advisory for Wednesday and an excessive heat watch for Thursday, when it could feel as hot as 110 degrees.

       Wednesday’s heat advisory is second of the season for the District and signifies it will feel as hot as 105 degrees between noon and 8 p.m. Last summer saw three such heat advisories issued.

       The excessive heat watch, in effect Thursday afternoon and evening, means it will be even hotter, with the combination of heat and humidity producing heat index values up to 110 degrees. The area hasn’t seen such a watch in effect since last July and it’s the second latest such watch issued during the summer since 2016.

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       “Extreme heat and humidity will significantly increase the potential for heat related illnesses, particularly for those working or participating in outdoor activities,” the Weather Service stated. Groups most vulnerable to extreme heat include older adults, the homeless and outdoor workers, or anyone without access to air conditioning.

       The heat will activate the District’s heat emergency plan, which operates whenever the heat index surpasses 95 degrees. Under the plan, the city makes cooling centers available.

       Tuesday offered a sneak preview of the incoming heat. Afternoon temperatures swelled into the low-to-mid 90s, pushing the heat index over 100. Dew points, a measure of humidity, surpassed 70 degrees; at such levels it’s downright humid and uncomfortable.

       How hot and when

       Computer models suggest highs on Wednesday will reach the mid-90s, with mid-to-upper 90s possible Thursday. With dew points hovering in the low 70s, peak heat index values will range from 100 to 105 Wednesday and from 105 to 110 Thursday.

       While the Weather Service has only issued heat alerts through Thursday, Friday may be just about as hot and muggy.

       Overnight temperatures will also be very warm, offering limited relief, only dipping into the low to mid-70s in most places, with upper 70s in urban areas.

       These temperatures are only about five to 10 degrees above average and not record-setting, but notable in a summer without much extreme heat.

       While we’ve had more than the normal number of days at or above 90 degrees to this point (34 such instances), most of the time it’s only surpassed 90 by a couple of degrees or so. During a typical summer, the average highest temperature is 99 degrees.

       While the District sees the highs hit at least 100 degrees on average every other summer, it hasn’t reached that threshold since 2016.

       The ongoing heat wave is the result of a zone of high pressure off the Mid-Atlantic coast that is pumping in hot, humid air from the south and southwest.

       It is predicted to break after Saturday, when a cold front sweeps through the region.