In fact, the year-to-date count of “intense tornadoes” — EF-3, 4 or 5 (on the Enhanced Fujita scale) — has reached a record low for this late in the season, with just three. This is right on the heels of the longest drought between such tornadoes, which started in May 2017 and ended earlier this year. It has also now been five years since the last EF-5.
With wind speeds of 136 mph or stronger, intense tornadoes can do a lot of damage. Tornadoes that are at least EF-3 on the Enhanced Fujita scale make up only 5 percent of all twisters, but they are responsible for more than 85 percent of all tornado deaths.
Although it’s true that weaker tornadoes can be just as deadly, it’s also true that intense tornadoes are responsible for almost all of the major tornado tragedies in history.
In an average year, about 20 intense tornadoes will have formed by this time in May. By the end of the average year, that number is about 35. Thus far in 2018, according to data gathered by the National Weather Service, there have been just three.
Why so few tornadoes of note?
It started with the winter of 2017-18. It was rather harsh in February (when some tornadoes did happen!), and the cold weather hung around longer than it should have. The patterns for March and April, when the tornado season tends to pick up steam, were generally not conducive for tornadoes.
Tornadoes need moisture streaming out of the Gulf of Mexico and abundant jet stream energy rushing in to meet it. In both months, on average, a blocking high-pressure system sent the jet stream well north before it came crashing back down into an East Coast low-pressure area.
This May, tornado activity has been low because of all of the high pressure. There have been several Northeast severe weather episodes this month, and the pattern looks more like summer than spring. Some storminess in the Gulf of Mexico has also tended to keep moisture flow into the Plains on the low end.
As of May 22, this year is tied with 1987 for the fewest EF-3, 4 or 5 tornadoes in a season to date. On May 25, 1987, two intense tornadoes were reported, which means that if there are none in the next few days, 2018 is going to hold the record.
Looking at the graph above, the historical trend of intense tornadoes is perhaps not what one would expect. It’s difficult to pick out a trend, but there appears to be a downward shift in intense tornadoes over time — especially since the extremely active 1974 season.
Important, though, it’s probably impossible to pick out any statistically significant trend when our data goes back only to 1950. Also, measuring and reporting techniques have changed markedly since recording began.
Speaking in generalities, though, tornado trends on the whole are extremely difficult to identify and attribute. Intense tornadoes tend to be clustered in outbreaks. It is normal to have quiet and intense years next to each other.
Those caveats in mind, the intense tornado “droughts” seem to be happening more often.
The three EF-3 tornadoes this year are well below normal, but they also come at the end of the longest drought of tornadoes of this intensity from 2017 into 2018.
The final EF-3 tornado of 2017 came on May 17 in Kansas. That was an unheard-of early end to the intense tornado season, which ran only 77 days. Although 19 intense tornadoes touched down during the early part of 2017, the end date was the earliest in the modern record and a major anomaly overall.
If we finish May with no more intense tornadoes — which seems like a good bet — the two-year total will be only 22. That’s what’s expected in an entire average season. It’s the lowest two-season tally through May since 2004 and 2005 combined, for 21 by this point. Both of those years ended up with more active late seasons, although they still finished below average in the intense tornado department.
The 1987 zone still beats this stretch in some ways. As mentioned, the country had only five through May of that year, but it also had only nine through May of the next year, 1988. Although our current stretch of tornadic drought is testing all-time lows, it is arguably not yet on par with the longevity of inactive years in the 1980s.
Although we do tend to see about 70 percent of a year’s intense tornadoes by the end of May in an average season, there are plenty of examples with a lot more than that in the months to come.
The previous record holder of 1987 racked up an additional 10 intense tornadoes before the end of the year to finish with a record low 15. There’s still plenty of time. As such, we can’t yet judge this year on the whole, but we can certainly say it is a very odd bird at this point.