The extreme flooding that devastated portions of Texas and Oklahoma during May 2015 made national headlines (for more see our article on this record breaking event) as numerous locations ended up with rainfall totals approaching 30 inches! Considering the huge swath of the central US spanning from Colorado and Wyoming through Louisana and Arkansas that received at least 10 inches of rain, May 2015 has taken top honors as the wettest month ever for the Continental US.
Overall, the average rainfall across the “Lower 48” during May 2015 was 4.36 inches, exceeding the previous record wet month of 4.28 inches from June 1928. In addition to Texas and Oklahoma, Colorado also had its wettest month of all time. Five other states (Nevada, Wyoming, South Dakota, Kansas and Arkansas) finished the month with top 5 rankings. While these are certainly impressive statistics on the mainland, Hawaii has actually been the wettest state, with a few spots topping over 40 inches since April 1st!
US Rainfall Anomalies for May 2015. Courtesy of the National Centers for Environmental Information
Interestingly, May 2015 became the wettest month on record for the contiguous US primarily on the backs of the southcentral states. As many of us on the East Coast can testify, May was not exactly soggy. In fact, multiple states in the Northeast (Massachusetts, Rhode Island, Connecticut, and New Jersey) ranked May among the top 5 driest. Even still, the extreme rainfall from the Great Plains, Rockies and Southcentral US more than compensated for the dry trend over the Northeast and Pacific Northwest.
Statewide precipitation rankings for May 2015 across the Continental US. A value of 121 indicates the wettest May on record (since 1895), while 1 means the driest. Courtesy of the National Centers for Environmental Information.
So what led to this historic rainfall? The chief culprit for these frequent downpours in Oklahoma and Texas was an enhanced subtropical jet stream (see image below) which ushers in many more rain events to the region. These faster than average winds in upper levels of the atmosphere are a classic signature of a strengthening El Niño event (for a more in depth explanation about El Niño see our article). While El Niño’s impacts are most significant during the winter, it also plays a role during the warmer months (decreased tropical activity in the Atlantic Ocean being another effect).
The reds, oranges and yellows represent positive wind speed anomalies, or faster than average wind speeds in the upper levels of the atmosphere. Courtesy of NCEP/ESRL.
With the expectation that El Niño will continue for the rest of year, the biggest question is “how strong will it get?”. While there is still a good deal of uncertainty, a “strong” El Niño is looking more plausible with each passing day. This type of event would not only keep wetter than normal conditions across the Great Plains and Rockies throughout the summer, but would also have very important implications for the upcoming winter in the Midwest, Northeast and Mid-Atlantic.
