Question on winter and severe thunderstorms

[Pete Kustra]

I've heard that this year's shortage of tornadoes and severe thunderstorms was caused mostly by a trough which caused below seasonal temperatures in the east and above-seasonal temperatures in the west. Was it this specific arrangement of air temperatures that suppressed the development of severe storms, or do cooler winter/early spring temperatures generally not favor severe weather later in the spring? If that's the case then can we expect more severe storms in the Midwest and less in the south in 2016 if the forecasted temperature patterns hold?

Once again forgive my ignorance; like I said, I'm pretty new to severe weather and there's not a lot of info online about connections between winter conditions and storms later in the spring.

 

[Jeff Duda]

To be technical, neither a single event (i.e., a trough) nor a trough in general causes a lack of tornadoes. No single synoptic scale event lasts long enough or is strong enough to impact a statistic like annual tornado occurrence. Also, tornadoes are not a phenomenon that occurs out of a frequently occurring equilibrium or something like that. Said differently, tornadoes require an extreme set of conditions to exist. I would say that a lack of troughs is better language to use when discussing low tornado numbers since synoptic scale troughs typically contribute to the extreme set of conditions needed for tornadoes.

If you look at the 2015 time series of tornado counts by day (since this is linked to the source, it will probably no longer be relevant after December 31st)


You should notice that this year's low tornado counts are coming from the winter and fall seasons. The spring and summer actually were more active than usual. The lack of winter activity probably had to do with the persistent western US ridge/eastern US trough, likely a response to the very warm SST anomalies in the eastern North Pacific (i.e., strong positive PDO event), which has finally calmed down a bit. The western US ridge/eastern US trough pattern is not very conducive to tornadoes in the US since it results in frequent surges of cold air very far south across the central and eastern US.The low numbers in the fall (especially Aug-Oct) is probably related to the inactive Atlantic tropical season. Many tornadoes that occur in the US during that time are caused by landfalling tropical cyclones; approximately zero occurred this year. However, consider the fact that November may have been close to a record for tornadoes.

Finally, consider that, with the exception of the anomalous years of 2011 and 2008, 2015 is going to fall pretty close to the center of the distribution of tornado counts over the last 10 years: