What Caused December’s Weather Havoc?

The last 10 days of 2015 saw some of the deadliest December weather on record for the US Southeast and Midwest. 55 tornadoes were recorded from Texas to Alabama from December 20 through 29. An EF4 tornado with wind speeds approaching 200mph (320km/h) took 9 lives in Mississippi. A rash of tornadoes near Dallas, Texas, including a second EF4, took 11 lives.

The tornadoes were followed by unusually heavy rain events from Louisiana to Ohio that resulted in the worst December flooding in over 60 years. Rainfall totals in one 36-hour period included 5.44″ in Des Moines, Iowa, 10.81″ in Ft. Smith, Arkansas, and 11.74″ in St. Louis.

The National Weather Service said that more than 400 river gauges reported river flooding from Texas to Ohio, and Mississippi to Virginia. The Mississippi River at St. Louis crested at 42.58 ft., third highest level on record. Tributaries to the Mississippi also crested at record levels. The Illinois, Meramac, Bourbeuse, Osage, and Gasconade all topped previous cresting records, one by more than 4 ft. As of this writing, the high water continues to surge down the Mississippi toward New Orleans and the Gulf of Mexico. Thousands of structures and thousands of acres of farmland bordering the Mississippi have been impacted by the flooding.

Historically, this kind of tornado and flood activity happens in the spring, from March through June. So what happened to bring spring in December? The weather pattern that triggered these outbreaks featured 2 very important components. The Bermuda High is a massive high pressure system that normally lingers off the US east coast during the winter months. In this case the Bermuda High drifted westward, bringing a mass of unusually moist and warm air into the southeastern states and the Mississippi Valley. At the same time, an intense dip in the jet stream brought a core of cold, arctic air and rapidly spinning winds into the same area. The extreme air mass temperature contrast destabilized the atmosphere, formed low pressure systems, and provided a critical source of energy for developing cyclones and severe rain.

The December tornadoes and floods topped off a weird weather year. 2015 was the world’s warmest year on record. The drought in California resulted in the lowest snowpack in the Sierra Nevada in 500 years. In June, the Pacific Northwest experienced its hottest day on record, with temperatures topping 110°F at many Oregon and Washington reporting stations. In early October, South Carolina was hit by a rainstorm that broke dams, washed out roads, and resulted in what was termed a thousand-year flood. Later in October, Hurricane Patricia, an EF5, the strongest hurricane ever to hit North America with wind speeds of 200 mph, formed off the coast of Baja California, raced eastward across Northern Mexico and brought record rainfall and massive flooding to parts of Texas. And finally, in December, the US northeast had no snow.

Did global warming play a part in 2015’s strange weather? Although not everyone agrees, a number of recent scientific studies have shown a definite link between global warming and the increased frequency and severity of extreme weather events