This has been an incredibly active severe weather season. In May alone, there have been more than 300 tornadoes reported. This is also the most active season in eight years after several years of relative calm. The cyclical nature of severe weather is normal.
We’ve seen a lot of wild weather this week, and it looks to continue for a few more days. Until the contrast between the cool air to the north and hot air to the south relaxes, this will continue.
Tornado season generally starts later in February and continues through the early part of the summer, with the most active month being May. This is because mid-spring is the time of the greatest temperature contrast and change across the country.
The transition from winter to summer brings about this weather. Interestingly, there is another peak, not as large, for similar reasons in November.
Why the sudden increase?
Why has this year become particularly active once again? The answer lies in understanding the multiple layers of the atmosphere and how they interact. One of the key components of severe weather is in the higher reaches of this meteorological layer cake.
All weather, everything from clouds to hurricanes to thunderstorms and tornadoes, is caused by atmospheric lift. This is the concept that when the air is lifted by something it rises, cools, and condenses and forms clouds and eventually precipitation. The stronger the lift, the bigger the storms.
Other global factors such as El Niño and other ocean atmospheric interactions have caused the uptick in severe activity.
This spring, a particularly strong subtropical jet stream has helped elevate the level of lift across the middle part of the country into Pennsylvania and the mid-Atlantic and created a lot of severe weather. The jet is ever-present but is not always quite so active or long lasting over the United States.
A lot of rain, too
In addition to severe weather, it’s been wet. Because the jet stream has also remained relatively stable, we’ve seen a lot of rain in the same areas for several weeks. This has created near historic flooding as well. One of the greatest flooding disasters ever recorded was in 1927, and this year’s flooding across the Mississippi has rivaled that.
Check out this composite below of more than 9,000 river gauges across the country. Purple dots represent those gauges that indicate major flooding, and you can clearly see the flooding is all along the Mississippi River and its tributaries.
A closer look at where severe weather has occured this year also follows a similar pattern, with much of the severe weather happening just to the south of the subtropical jet.
One of the reasons why we have not seen a lot of extended heat here in New England or severe weather is because the upper winds have cut off the heat and allowed it to remain south.
Although May has been a particularly wet and stormy month, April was also very wet across a large portion of the country. When you combine this with snow melt, you end up with the flooding situation we have seen.
Amazing to think that several years ago some of the same areas that were experiencing extreme drought now had one of the wettest springs on record. Needless to say, this year there is very little drought across the United States.
Follow Dave Epstein on Twitter @growingwisdom.