National Weather Service forecast high temperatures for Saturday. (WeatherBell.com)

On the second-to-last-day of what’s destined to be Washington’s warmest April on record, tens of thousands of marchers will congregate on the Mall to call for climate action. And, perhaps as a symbolic point of exclamation, the mercury is predicted to soar to near-record levels.

The high temperature forecast for Saturday is 90 degrees — just one degree shy of the record of 91 for the date, set in 1974. If Washington hits 90 on 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).

Unless the forecast changes, visitors attending the People’s Climate March should wear summer attire and be sure to hydrate. A high of 90 on April 29 is 19 degrees above normal and near the average maximum temperature in mid-July.

The warmth will result from a sprawling area of high pressure off the Southeast coast that will pump hot air toward the Mid-Atlantic.


GFS model simulation showing high pressure at high altitudes off the Southeast coast on Saturday. (WeatherBell.com)

There is some chance that if the cloud cover is more pervasive than forecast, temperatures will not be quite as warm. Some models are predicting highs more in the 80- to 85-degree range rather than near 90.

It is likely to be not only unseasonably warm but also unusually humid. The dew point, a measure of humidity, is expected to reach the mid- to upper-60s.


GFS model forecast dew points Saturday evening.

Any dew point over 65 is considered quite humid and more typical of July than April. Factoring in Saturday’s predicted humidity, the air could feel as hot as 92 to 95 degrees in the mid- to late afternoon.


The hot-weather forecast to end April in Washington is almost certain to catapult the month to its warmest average temperature on record, more than seven 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.

Saturday’s climate march comes about 29 years after the famous congressional climate change hearing, in which NASA scientist James Hansen, considered the father of global warming awareness, said he was 99 percent certain that a global warming trend was the result of human activity. The hearing was held June 23, 1988, in Washington, and the temperature surged to a record high of 98 degrees.