The month got off to a wet start around Oklahoma City, which ended the May 6 calendar day with an incredible 7.1 inches of rain, a total that blew away the old record for the date and set a new record for the wettest day in May. The torrential downpours quickly saturated the ground, causing widespread flash flooding that even washed out some roadways. The National Weather Service in Norman, Okla., was forced to issue a flash-flood emergency for the first time in the history of the office, which is reserved for the most dangerous and life-threatening situations.
After the rain tapered off, some of the tornado country residents found their storm shelters floating up out of the ground. Sadly, one Oklahoma City woman lost her life during the severe weather outbreak when she became trapped in her flooded storm shelter.
The deluge may also have broken Oklahoma City’s all-time 24-hour rainfall record. Between 10 p.m. that Tuesday and 10 p.m. that Wednesday, 7.98 inches of rain fell. According to reporting by Weather.com’s Nick Wiltgen, the previous 24-hour record was 7.62 inches, set in 2010. However, Weather Underground’s weather historian Christopher C. Burt documents the 24-hour record as 8.95 inches, set on Oct. 20, 1983. Unfortunately 24-hour rainfall records are not officially tracked by the National Weather Service.
Lest you think Oklahoma is the only sopping state in the central U.S., this month, record totals stretch far into Texas, south through the Panhandle to San Angelo.
With a month-to-date total of 7.2 inches, Lubbock, Tex., is inching up the list of all-time wettest Mays. If the Weather Prediction Center’s extreme rainfall forecast pans out, Lubbock could see an additional 5 to 7 inches of rain in the next seven days, which would push them into at least the top three, if not make this the wettest May on record.
Last week, over 10 inches of rain fell in the course of two days in the Houston metro area. Harris County, which includes the city of Houston, registered 8.22 inches in 48 hours. So far, Houston’s May rainfall total is up to 6.05 inches as of May 18, which is still about two inches away from breaking into the top 10 wettest Mays.
On Monday night, San Angelo, Tex., was flooded by about 3 inches of rain in 45 minutes, which is more than the city typically sees in the entire month. In addition to the flooded vehicles and water rescues, the Weather Channel reports that floodwater inundated San Angelo Regional Airport, and all flights had to be diverted to Abilene, Tex. Monday ended with 4.01 inches at San Angelo, which doubled the record for the date — the previous was 2.08 in 1955.
Out West, an unusually late, winter-like storm provided a touch of rain and snow for Southern California and Arizona late last week. Though it’s far from the kind of drought-busting rainfall that basically the entire state of California needs right now, San Diego set a new record for wettest day in May on the 15th, when 1.63 inches of rain fell. The old record was 1.49 inches on May 8, 1977. Needless to say, the 1.63 inches also eclipsed the record for the date, which was previously just 0.4 inches.
[Oddly late-season storm pushes San Diego to wettest May]
San Diego is not a wet city, especially in May. “Our long dry season usually begins in May, and the average number of days with measurable rain is 2, with an average monthly total of 0.2 inches,” says the National Weather Service. “Stormy weather is almost unknown during the month.” May rainfall has exceeded two inches only three other times in San Diego records.
Taken as a whole, these events appear to have at least two things in common — a burgeoning El Niño in the equatorial Pacific, and a well-documented upward trend in extreme rainfall events.
Though the pattern tends to be more obvious in the winter months, El Niño’s very warm sea surface temperatures in the tropics tend to fuel a wetter than normal pattern across the West and the South. At the very least, last week’s Southwest storm looks to have been enhanced by El Niño, which, according to recent model forecasts, is shaping up to be an intense one.
[Strengthening El Niño spurs Earth’s warmest Jan.-April on record]
Beyond the tropical Pacific Ocean, these events are new data points in a string of increasing extreme rainfall events in the United States over the past few decades, particularly east of the Rockies. According to the U.S. National Climate Assessment, days of heavy rainfall are becoming more frequent across the country, and the amount of rain that’s falling in the heaviest events is on the rise, as well.
[Study finds warmer oceans are fueling stronger hurricanes]
The connection between a warming world and an increase in extreme rainfall events — the kind you only see every 10 or more years — is rooted in rudimentary atmospheric science. As air temperature increases, it can “hold” more water vapor. This means that, even if global weather patterns and the driving force behind storms doesn’t change, it’s more likely we will see more heavy precipitation events, whether it be mid-latitude cyclones in the spring, hurricanes in the summer or even snowy winter storms.
“Nearly all studies to date published in the peer-reviewed literature agree that extreme precipitation event number and intensity have risen, when averaged over the United States,” the National Climate Assessment states. Though scientists are confident that this trend will continue, the only remaining uncertainty is in the forecast models’ ability to determine exactly how that change will play out over time, and what regions will be affected the most.