August 1, 2018 at 10:44 AM
It rained, and it rained, and it rained. A river in the sky pointed at the Mid-Atlantic during the second half of July, resulting in historic rainfall tallies in the Washington and Baltimore regions.
Nearly 10 inches of rain fell in Washington last month, making it the fourth-wettest July on record and rainiest since 1945. It was the most rain to fall in any month since June 2015.
Remarkably, the first half of the month saw no measurable precipitation, setting a record for the least rainfall, while the second half was the rainiest on record.
The 9.73 inches that fell in July made 2018 the second straight year with at least nine inches during the month:
Aside from 2017, all the other top five wettest Julys occurred before 1950. It has been a while!
Baltimore’s amazing 16.73 inches of rain smashed its July record, blowing out 1889’s 11.03 inches. Dulles also set a record for its wettest July with 11.21 inches, topping last year’s 8.8 inches.
Rainfall totals in 2018 (see more below) of 35 to 40 inches are now closing in on what we expect in an entire year, with five months left to go.
It was exceptionally wet in July, and it was warm. The hottest month of the year posted an average temperature of 80.7 in Washington, coming in 0.9 degrees warmer than normal, the 26th hottest of all time though the coolest since 2014.
All of the clouds and rain tempered afternoon high temperatures. Washington reached or exceeded 90 degrees only three times in the second half of the month.
The highest temperature — and the hottest of this summer to date — was 98 degrees on July 16. Our coolest high temperature occurred five days later at 75.
Our wettest day was that super-soggy Saturday, July 21, when four inches came down, the fifth-wettest July day on record.
Most of the records that happened in July were rain-related and are summarized below:
The upper level pattern in July featured two high-pressure heat ridges on the Pacific and Atlantic side that connected across much of the United States frequently. Washington’s position south of the northwest Atlantic high-pressure zone helped place it in the sweet spot for moisture riding right up the East Coast. Shown here is the upper-level pattern difference from normal where you can see all of the areas of higher-than-normal pressure (in red):
And here is the temperature difference from normal:
The widespread warm conditions, which resulted from these persistent hot high-pressure systems, is a summer impact from lingering atmospheric La Niña conditions. They were especially strong in the first part of July but weakened in the final third of the month.
A month ago, just ahead of July’s start, we made the following predictions:
Here is what happened:
Given our correct temperature prediction and the right direction for precipitation, even if we missed the intensity, equates to a grade of “B” for July.
The super -wet July moves 2018 to the second wettest position of the 2000s now, just barely behind 2003. August 2003 picked up 4.65 inches of rain, so it will be interesting to see if we can make up more ground against that year in the month ahead given our wet start.
Our middle-of-the-road temperature result is keeping 2018 in the middle of the pack for the 2000s, too.