May Tornadoes Leave U.S. in a Whirlwind

Late May 2019 was a startlingly active period for severe convective storms (SCS) in the U.S., even after considering that May is typically one of the most active months of the year. Until about halfway through the month, the number of tornadoes being reported was around average, but after a major outbreak starting in mid-May this number shot up, bringing the year-to-date total to 1,017 tornado reports. This count is only surpassed by the extremely active years of 2008 and 2011 (Figure 1).

This year’s late May outbreak was also unusually long: by the end of Wednesday, May 29, at least eight tornadoes had been experienced each day across a record-breaking 13 consecutive days, according to preliminary data from the National Weather Service (NWS). The previous record was set in 1980, after 11 consecutive days with at least eight tornadoes.

The reason this latest outbreak lasted so long was due to a particularly persistent set of atmospheric conditions favourable for SCS. A combination of a stubborn high-pressure system over the Southeast U.S. and a stationary and unusually cold trough over the Rocky Mountains, saw warm, moist air drawn from the Gulf of Mexico into the central U.S., and a strong jet stream aided the formation of supercell thunderstorms, which can generate very powerful tornadoes.

The outbreak spawned tornadoes across a wide area of the U.S. (Figure 2), but most have occurred in the southern Great Plains (so-called “Tornado Alley”) and the Midwest. It has not only been tornadoes causing damage, the severe weather also generated large hail – with a 5.5 inch (14 centimeter) diameter hailstone (see above) reported in Texas, and strong straight-line winds.

SCS is a major contributor to annual insured losses in the United States. RMS modeling shows that average annual loss (AAL) from SCS roughly equals that from hurricanes, at around US$17 billion. Unlike losses from hurricanes, SCS losses (especially hail) accumulate over the course of a year; however, major outbreaks can make the difference between an “average” year and a significant impact to the insurance industry. Prolonged and damaging outbreaks also increase the likelihood that losses will flow through to reinsurance layers.

The most significant past tornado outbreaks, for example, the tornadoes that hit Tuscaloosa, Alabama and Joplin, Missouri in 2011, have cost single-digit billions of insured loss (USD). One particularly interesting characteristic of the late-May 2019 SCS outbreak, however, is that the damage could have been much worse. Meteorological records have been broken but – for the most part – the tornadoes avoided major cities.

That said, it will take some time before all of the claims (hail, tornado and wind) have been verified and processed, so it is too early to know what the cost of the outbreak to the insurance industry will be. It is worth noting that the SCS outbreak was accompanied by significant flooding in Arkansas, Oklahoma and Missouri, but since flood damage is not typically covered by private insurance in the U.S., if the properties were not covered by the National Flood Insurance Program (NFIP), these costs will be borne by individual home and business owners.

Below are shown some statistics and charts covering this remarkable outbreak. Unless otherwise stated, they cover the period 20-30 May 2019:

Number of Confirmed Tornadoes in the U.S. (May 16–30): ≥ 228

22 of these have not yet been given a strength rating by the National Weather Service (NWS). More tornadoes may be confirmed by the NWS as their surveys continue.

Number of EF5 Tornadoes: 0

There have been no tornadoes of the highest category on the Enhanced Fujita (EF) Scale.

Number of EF4 Tornadoes: 3

EF-4 in Dayton, Ohio

EF-4 in Linwood, Kansas

EF-4 hit Marshall, Oklahoma