May 2015: All-Time Wettest Month on Record for the U.S.

It wasn’t a soggy May for the whole country, but where it did rain last month, it poured--enough to give the 48 U.S. states as a whole their wettest single month since records began in 1895, according to the monthly report from NOAA’s National Centers for Environmental Information (formerly called the NCDC). With a 48-state average of 4.36”, last month beat out the 4.28” observed in June 1928 and the 4.24” from May 1957. The main reason for the record was the unusually prolonged and intense sequence of heavy thunderstorms centered on the south-central states, a spell that conveniently extended from just a couple of days after the first of May until right before the month ended. Colorado had its wettest May on record, while Texas and Oklahoma had their wettest single month since records began. For a large swath of the country, from the southern Great Basin throughout the Great Plains, this month ranked among the ten wettest Mays on record (see Figure 2).


Figure 1. Motorists commute across Interstate 30 (right) over a swollen Trinity River west of downtown Dallas on May 29, as weeks of torrential rain were drawing to a close. Image credit: AP Photo/Brandon Wade.


Despite the Plains deluge, many of the nation’s biggest population centers were actually on the dry side. Eastern states got the short end of the precipitation stick last month, with much-below-average rainfall from the Appalachians to the coastline. Six states had a top-ten dry May: South Carolina, Delaware, New Jersey, Connecticut, Rhode Island, and Massachusetts. Since there is little irrigation in this part of the country, it only takes three or four weeks of scant rainfall to begin causing problems for agriculture and ecosystems. Fortunately, a series of wet frontal systems over the last few days has put a dent in this short-term dry spell. For the nation as a whole, May was a colossal drought-buster. The fraction of the contiguous U.S. categorized as being in drought by the weekly U.S. Drought Monitor plummeted from 37.4% in late April to 24.6% on June 2, hitting its lowest percentage since February 2011.


Figure 2. Precipitation (top) and temperature (bottom) rankings for May 2015 in each of the 48 contiguous U.S. states. In the top image, a value of 121 denotes the wettest May in records that go back to 1895, while 1 indicates the driest. For the bottom image, 121 corresponds to the hottest May on record, and 1 denotes the coolest. Image credit: NOAA National Centers for Environmental Information.


Temperatures for the month largely reflected where it was persistently wet or dry. Frequent clouds and storms blocked enough late-spring sunlight to bring the monthly temperature well below average from Arizona, New Mexico, and Texas to Wyoming and South Dakota. For the East Coast, and especially the Northeast, it was weather whiplash, temperature-style. Not long after each state saw its second-coldest February on record, the residents of New Hampshire, Massachusetts, Connecticut, and Rhode Island all sailed into their warmest May in more than a century of record keeping. Seven other adjoining states had a top-ten warmest May, as did Florida and Washington state. It was also the warmest May on record in Alaska--a full degree beyond than the previous record-holder, May 2005.

Even with temperatures now in the 70s and 80s, people in the Boston area have a stubborn, unsightly reminder of winter: several enormous piles of trash-bearing snow that are proving excruciatingly slow to get rid of. The multiple reasons for the leisurely melt are nicely outlined in this Boston Globe article. The snow/trash pile at the Seaport District was still three stories tall as of late May, and it may take until July to completely disappear.


Figure 3. Canada geese swim beneath a debris-covered snow pile on May 28 in Boston’s Seaport District. A snow mound that once towered 75 feet high was by this point a three-story pile of dirt and trash, including bicycles, traffic cones and even half a $5 bill, that remains encrusted in solid ice. Crews have been working for weeks to clean away the trash as it breaks free from the mound. As of late May, they had pulled 85 tons of debris from the pile. Image credit: AP Photo/Elise Amendola.

El Niño making its presence known
The El Niño event that’s been steadily strengthening in recent weeks likely played a key role in fostering the downpours across the south-central U.S. As the event brings warmer-than-average waters over the eastern tropical Pacific Ocean, showers and thunderstorms moves eastward as well. This change in atmospheric circulation triggers a chain of reverberating effects, typically bringing wetter-than-usual conditions to the southern U.S. and drier-than-usual weather across parts of the northern tier of states. Part of the adjustment involves a strengthening of the subtropical jet stream, which was unusually strong across Mexico and the southwest U.S. last month. At the California Weather Blog, Daniel Swain provides an excellent description of how the balance between the subtropical and polar jet streams is affected by El Niño events of various intensities. If the current event remains potent, it substantially raises the odds of at least some drought relief for California this fall and winter. Another factor in the storminess across the central states during May was moisture-rich low-level air coursing from the Caribbean through the Gulf of Mexico and into Texas. Warm sea-surface temperatures helped boost the moisture content in this persistent surface flow.


Figure 4. Observed decadal rate of change in global annual precipitation over land areas, 1951 - 2010. The cumulative change in annual precipitation at any spot, in mm/yr, can be calculated by multiplying the value shown by 6 (for six decades]. Dots represent areas where the change is significant, including the central United States. Areas in white either have little or no trend, or too little data to calculate a robust estimate. Image credit: IPCC AR5 Synthesis Report, Figure 1.1, page 41 (PDF link).

Record-wet month is in line with a warming climate
The sodden swath across the Great Plains last month is intriguingly similar to what shows up in annual precipitation trends over the last 50 years, as analyzed by the Intergovernmental Panel on Climate Change, or IPCC, in its most recent synthesis report (see Figure 4). Although it’s a challenge to measure on a global scale, the average annual precipitation for the entire planet appears to be on the increase, and the 48-state U.S. average has increased from just over 29” per year in the late 1890s to nearly 31” today (see Figure 5). Droughts are still with us, of course—and drought impacts are greatly exacerbated by hotter temperatures—but at the same time, wet periods are increasingly wetter, as more water vapor enters the atmosphere from steadily warming oceans. Not all of the world is getting wetter, though. IPCC projections for the late 21st century, based on multiple climate models, show that much of the global increase in precipitation will occur at middle and higher latitudes, with subtropical areas tending to dry out, a pattern already showing up in recent years. Grinding drought has plagued the Southwest U.S. for much of the last 15 years, a period that corresponds to a predominantly negative phase of the Pacific Decadal Oscillation. Now that the PDO has shifted into a positive phase, which tends to favor El Niño, it will be interesting to see if the Southwest--including California--manages to get a few years of rainfall more in line with the 20th-century average. Even if it does, the region still has major challenges ahead, as warming temperatures and increasing populations will add to the impact when drought does, inevitably, recur.

Bob Henson


Figure 5. Average precipitation for the contiguous 48 U.S. states, 1895 - 2014. The annual average increased by about 5% during the 20th century. Image credit: NOAA National Centers for Environmental Information.