But D.C. did break a different record early this morning when it didn’t cool down past 70 degrees. That’s never happened in the month of April, and in a lot of ways that’s more significant than a record high. We’re seeing more overnight heat as the climate warms, which is also a significant health risk in the form of more heat stress on the body.

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As marchers demand climate action on the Mall on Saturday, they will sweat through one of the most sweltering April days ever recorded in Washington.

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The heat index — what it feels like outside factoring in both the temperature and humidity — could peak in the mid- to upper 90s during the afternoon.

The actual air temperature is predicted to soar to 89 to 93 degrees, in the range of Saturday’s record high of 91 degrees, set in 1974.

A tropical wind riding up the Potomac River is the one weather factor that may prevent a record high. As Washington’s official temperature is measured at Reagan National Airport, adjacent to the river, the wind flowing over its cool waters may take a slight edge off the heat.

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But it won’t just be the temperature contributing to uncomfortably warm conditions. Dew points, a measure of humidity, are predicted to range from 65 to 70 degrees. Such levels, characteristic of midsummer, would rank among the highest ever recorded during April:

Taking the temperature and humidity together, incredibly, the high resolution NAM model predicts a 5 p.m. heat index of 97 degrees — which is almost unheard of in April in Washington.

Anyone spending long hours outside in the heat Saturday should be sure to dress accordingly (light, loose-fitting clothes) and remain hydrated. People are most vulnerable to heat-related illness during early season heat waves when they are not yet acclimated to hot weather.

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The weather this Saturday, for the People’s Climate March, will represent a remarkable contrast from rainy, cool conditions during the March for Science a week before.

Saturday’s forecast high temperature of around 91 degrees is 20 degrees above normal for the date and around 30 degrees warmer than the previous Saturday. It would be hotter than a typical afternoon in mid-July, when the average high is 89 degrees.

Should Washington hit 90 Saturday, it will be 19 days ahead of the average first 90-degree day (May 17) and the earliest first 90-degree day since 2013 (on April 10).

While Saturday afternoon’s temperature will be abnormally warm, the overnight and early morning temperatures may well be unprecedented.

Forecast low temperatures both Saturday and Sunday morning are near 70, which would break the warmest levels ever recorded on those dates of 68 (from 1956) and 67 (from 1973).

As Capital Weather Gang’s Ian Livingston pointed out Thursday, if the low temperature remains 70 or higher either weekend day, it would mark the first such instance in April and the earliest on record. The existing earliest date with a low temperature of 70 degrees or higher is May 2 (from 2010).

The predicted heat is the result from a sprawling area of high pressure off the Southeast coast pumping steamy air toward the Mid-Atlantic.

The hot weather this weekend will put an exclamation mark on what is destined to become the warmest April on record — more than 7 degrees above normal.

It will mark Washington’s second record-warm month in the past three (February was also record warm) and seventh record-warm month since 2010.

The coincidence of a record-hot day in a record-warm month during a climate march is fitting. Over the past several years, Washington has set an astonishing number of heat records as greenhouse gas concentrations have soared to their highest levels in over 800,000 years because of human activity.

Part of the warming trend in Washington is due to urbanization, but the accumulating greenhouse gases are stacking the deck for such extremely warm weather. They are like the atmosphere’s version of steroids, increasing the intensity and frequency of exactly the kind of hot weather predicted on Saturday — much like performance-enhancing drugs inflated home run totals in major league baseball during the 1990s.

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Model forecast high temperatures on Saturday in Washington: