Peak Tornado Season: Active or Quiet?

Mid to late May is considered to be near the peak of the United States Plains tornado season. This is a time in which most storm “chasecationers” will venture out to the Plains in hopes for at least a few days of tornado chasing. In the past, some of the most well-known tornado chases have taken place during the second half of May across the Plains.

A recent such event that comes to mind would be the Dodge City tornadoes of May 24th, 2016.

While some events have happened earlier in the spring, areas such as Dixie Alley are favored and the chase terrain/road networks there are not always conducive for storm chasing. Later season can light up as well, particularly June, but events in late spring/early summer are more hit and miss. Plus you may have to venture toward the Upper Midwest or Great Lakes, areas that are not the best for chasing either.

All things considered, mid to especially late May is prime time for tornado chasing. I would argue this spills into early June as well, but for this discussion, we’ll keep focused on May.

Peak Season in the Plains

I decided to take a look at the number of tornadoes that have been reported across the Plains between May 16-31 over the past 25 years, from 1993 to 2017. Note that data for 2018 is not complete yet, so that year was not used. When considering the Plains states (Colorado, Kansas, Nebraska, New Mexico, North Dakota, Oklahoma, South Dakota, Texas and Wyoming), the region averaged 87 tornadoes during the May 16-31 time period. The most was 204 in 2004, while the least was just nine in both 2006 and 2009.

Since weather patterns shift and change, I wanted to isolate stretches that saw the most and least tornado activity in mid to late May. I chose to consider five to 15 day blocks, as that would help to smooth out any data that was skewed by, let’s say, one or two significant outbreaks that were preceded/proceeded by a lack of tornado activity.

The most active stretch was May 22-29, 2008, when an average of 23 tornadoes occurred each day for eight days across the Plains. Note that the climatological average using 1993-2017 is about five per day, so this was more than 300% above average. The least active stretch was May 16-28, 2006, when only three tornadoes occurred during the period. The average rounded down to zero per day for that case.

Here are the top five stretches:

1. May 22-29, 2008 – 23 tornadoes/day
2. May 18-25, 2010 – 17 tornadoes/day
3. May 23-28, 2015 – 16 tornadoes/day
4. May 16-30, 2004 – 14 tornadoes/day
5. May 21-30, 2016 – 12 tornadoes/day

Bottom five stretches:

1. May 16-28, 2006 – <1 tornado/day
2. May 16-31, 2009 – 1 tornado/day
3. May 17-31, 2003 – 1 tornado/day
4. May 16-27, 2005 – 1 tornado/day
5. May 16-25, 2014 – 2 tornadoes/day

What weather patterns are associated with long stretches of well above average and well below average tornado activity over the Plains?

As one might expect, when there is troughing across the western United States and ridging over the Southeast, tornado activity is favored across the Plains. In this scenario, the jet stream takes on a classic Four-corners region to central Plains trajectory. With ridging downstream, warm, moist air is often co-located on the Plains as large scale forcing approaches from the west.

Another aspect of the pattern that favors an extended stretch of tornado activity is the presence of low pressure “blocking” across eastern Canada. Think of this as a roadblock that works to keep the continental U.S. pattern going. With troughiness across the West, ridging over the East and downstream blocking over eastern Canada, the ridge, more or less, stays put. This allows for trough after trough to impinge on the Plains before the pattern eventually breaks down. The breakdown is often associated with the ridge migrating northwest, or the troughing moving toward the Midwest/Great Lakes.

In extreme cases, like late May 2008, the pattern across North America was extremely amplified. Not only was troughing stagnant across the western U.S., but the trough was unusually deep and strong. This mean pattern was able to remain in place for over a week as an anomalously deep trough was also evident downstream, across southeastern Canada and even into New England. With a ridge in between the two features, the Plains saw day after day of noteworthy tornado events, resulting in one of the most memorable chase periods in modern times.

Likewise, when ridging is prevalent across the central/western states, tornado activity is typically lessened. In this case, the jet stream is usually displaced to the north, west and/or east, keeping more favorable large scale forcing and deep layer wind shear away from the Plains. Other aspects of the average unfavorable tornado pattern for the Plains include troughing being displaced west into the Pacific Ocean, troughing over the eastern U.S. and instead of low pressure blocking in eastern Canada, an anomalous ridge of high pressure is usually evident there.

Another way to look at mid to late May is to consider what the climatological average pattern looks like. The mean 500mb pattern from 1993-2017 appears to feature a ridge across the central U.S. Notice that the ridge axis is just east of the High Plains and there is some troughing along the West Coast. With minimal blocking downstream, the mid to late May upper level pattern is usually fast-paced, meaning that troughs move from west to east, breaking down the ridge, but then the pattern reloads over the course of a few days to a week.

How do active/quiet patterns differentiate themselves from the average pattern for mid to late May? In active years, downstream blocking and/or ridging keeps the pattern going, where troughs repeatedly impinge on the High Plains. On the flip side, when ridging migrates west and north, often “prematurely,” the pattern over the Plains becomes quieter. Storm chasers often refer to this as a “death ridge” pattern, when a large ridge centers itself over the Plains. While such a ridge is common during the summer, usually it does not establish itself earlier in the spring. When it does, like in 2006, tornado activity grinds to a relative halt across the central states.

Summary

Tornado activity is favored across the Plains when the weather pattern features troughiness across the western half of the U.S. and ridging dominates over the eastern part of the country. The opposite is true when there is an anomalous ridge across the Plains and troughing is displaced to the Pacific Ocean and/or eastern U.S. While these patterns usually break down and recycle over the course of a few days to a week, sometimes the pattern is stagnant, favoring an extended period of above or below average tornado activity.

There are other considerations than the large scale pattern when it comes to tornado occurrence across the Plains, but it are larger, synoptic patterns, particularly over the course of a 1-2 week period, that favor extended stretches of active tornado production.