As of July 11, 2018, we are having an incredible tornado season. An incredibly low number of tornadoes, that is! Let’s review where we are, why we are, and where do we go from here.
First off, the spring pattern exhibited a “La Nina”-ish type pattern whereby the jet stream would crash into the lower 48 states from the northern Plains, to the Carolinas, and then offshore. This caused a couple of interesting things.
First, it caused a trough in the jet stream in the far western U.S., giving very welcome rains….until the flooding started. Cities literally got partially covered with mud, causing fatalities. The eastern Rockies remained dry, helping to set the stage for wildfires now. But in the Plains and Great Lakes, it was unseasonably cool to cold.
Northwest flow from Canada shut down the Gulf of Mexico, and kept many systems from developing and coming into the Plains. And when they managed to do so, there was so much cold and dry air in place, and with the Gulf scoured of deep moisture, even intense cyclones produced amazingly few tornadoes.
As Sam Lillo noted on Twitter, as of July 8, we only had 63 tornado watches issued by the Storm Prediction Center. Most of the time, that number is reached in March or April, and in some cases, early to mid-May. But not July 8! But this year, it took us that long to get to that point.
And then, in Late May and early June, it happened. The jet stream roared northward to the Canadian border, and flow across the central U.s. was weak. With weak shear and marginal instability profiles, only a few tornadoes were big and photogenic, like Bennington, KS on May 2 (really? Really?). You had to be at the right place at the right time, and many storm chasers missed that one.
So, how are we doing compared to, say, last year? Let’s check out a couple of benchmarks.
Number of days with at least one tornado across the U.S. (preliminary data for 2018) in:
January 10 4
February 5 6
March 14 13
April 23 10
May 27 27
June 22 27
Now, by looking at this, you would think that April was pretty “dry”, but May and June were active, and that the primary tornado season was pretty typical, with a high number of tornado days in June. But the total *numbers* of tornadoes compared to, as an example, last year are staggeringly low:
Number of tornadoes each month (preliminary data for April 2018 onward):
January 142 15
February 115 48
March 176 55
April 218 129
May 299 89
June 168 30
And here’s where 2018 went off the rails and into the ditch for severe weather enthusiasts. Thanks to persistent cold air intrusions from Canada, Alaska and, a few times, from the North Pole and Siberia through the spring, the necessary ingredients for tornadoes didn’t show up often, and when they did, they were marginal or very marginal situations at best, resulting in a handful (or less) of tornadoes, and very weak at that.
So does that mean that storm chasers and spotters have nothing to do for the rest of the year? Hardly. We’re entering derecho season. A derecho is a long-lived, large line of severe thunderstorms that produces widespread, significant wind damage, with 70-90 MPH gusts or even higher. For those, we need to see decent moisture in the ground for the crops to “sweat” it back into the air for these lines of thunderstorms to use. And, while there is modest drought in some areas, many areas have adequate moisture, or even too much. And, as of this typing, we have our third named tropical system, Chris, spinning inbetween North Carolina and Bermuda, has 105 MPH winds. While it won’t hit the U.S., hurricane season has started early this year, and it will be very interesting to watch as time goes on. You bet I’ll be watching, and you should be as well.