Our sweltering July was one of the hottest months Washington has ever experienced. The average temperature of 83.9 degrees was 4.1 degrees above normal and just a hair below the two hottest Julys on record, in 2011 and 2012.

July accumulated the most 90-degree days of any month on record and was the first month to never fall below 71 degrees since record-keeping began in 1871. While the city didn’t hit 100 degrees, the very high humidity propelled heat index values above the century mark several times.

Rain, often falling in intense thunderstorms, was sufficient to help fuel that humidity, with the monthly total of 6.51 inches running 2.78 inches wetter than normal and marking the overall wettest month since October of last year. It was the 21st wettest July on record.

Extremes and notable numbers

Our “coolest” day was on the month’s final day, Friday, when we barely topped 80.

The 28 days at or above 90 degrees blew past the old monthly record of 25 such days in July 2011. A heat wave that began in late June and extended through the first half of July became the second-longest on record in Washington. Then, after a one-day break, heat was back in a big way.

The five days in a row at or above 96 during the second heat wave in July was the seventh-longest streak on record with temperatures so high.

Five of Washington’s seven hottest Julys on record have occurred since 2010 and all since 1990, a strong indicator of climate warming.

The nights were also excessively warm.

Washington’s lowest temperature in July, 71 degrees, is the warmest minimum temperature of any month on record. The prior warmest monthly minimum temperature was 67 degrees, set during July 2011 and August 2016.

Low temperatures have remained at or above 70 degrees for a record 36 straight days and counting.

According to the Southeast Regional Climate Center, Washington’s temperatures during July best resembled typical conditions in cities such as New Orleans, Houston and Fort Myers, Fla.

Baltimore, where it hit 100 twice, notched its hottest month in records dating to 1872. Its average temperature of 82.6 degrees topped the old record of 81.7 degrees in July 2011. The 25 days at or above 90 during the month there also ended up being the most of any month on record, surpassing 24 in 2011.

At Dulles, where the period of records begins in 1963, the average temperature of 81 degrees tied for the hottest month on record with July 2011.

More broadly, it was the hottest July on record in places like Harrisburg, Pa.; Richmond; and Elkins, W.Va. Most long-period stations in the Mid-Atlantic and New England posted a top-five hottest July.

In terms of daily records, Washington had its wettest July 7 as thunderstorms unleashed 2.04 inches of rain, surpassing the record of 0.93 inches from 1958. Dulles tied a record of 98 degrees on July 19, matching 1977. Dulles also snagged a daily rainfall record on July 24, when its 0.86 inches topped the 0.78 inches observed in 1992.

How it got so hot

The prevailing pattern over the entire country was a hot one, too, with a preliminary estimate placing it as the third-hottest behind only 2011 and 2012.

A low-pressure zone at high altitudes near Alaska helped set up sprawling zones of hot high pressure downwind, which affected large parts of the Lower 48.

Slower-than-normal high-altitude winds in the Northern Hemisphere associated with a developing La Niña pattern also helped fuel this persistently hot weather pattern.

Year to date standings

The hot and wet July helped keep D.C.'s overall 2020 climate running warmer and wetter than normal.

Temperatures are tracking as the third-hottest of the decade and fourth-hottest since the late 1800s.

2020 is running as the fifth-wettest of the decade.

Forecast review

On June 30, Capital Weather Gang had predicted the following:

We predict July 2020 to run about 2 to 4 degrees hotter than the 30-year average (79.8 degrees) and for rainfall to be about 0.5 to 1.5 inches below average (3.73 inches). If our forecast is correct, July 2020 would be similar to July 2019, which was 2.3 degrees warmer than normal and ranked as the ninth hottest on record. And, if the high end of our forecast is right, the month would rank among the top five hottest Julys on record.
https://www.washingtonpost.com/weather/2020/06/30/july-weather-outlook-dc/

We correctly forecast a Top 5 hot July; however, we completely flunked the rain forecast. With the temperature success and rain failure, we grade our outlook for the month a B-.