Looking ahead: 2017 Chase/Tornado Season

As March begins, the Plains chase season is just around the corner. I haven’t posted anything in a while, mostly because there hasn’t been much weather of interest around here. Since we are getting closer to the Plains tornado season, I thought I would share my thoughts on what has already been a big 2017 tornado season and my chase prospects for the spring/summer.

2017 has got off to a big start. The U.S. has been hit by multiple significant tornado events in January and February. This may sound odd, but the U.S. gets plenty of wintertime tornado events. Outbreaks like February 5, 2008 (Super Tuesday) and the Enigma Outbreak of 1884 are examples of major winter outbreaks. However, the volume of tornadoes these past two months has been impressive. According to the SPC Inflation Adjusted Annual Tornado Trend graph, there have been 209 inflation-adjusted tornado reports this year. This is fairly close to the record-level of 275 for March 3rd. The events of January 21-23rd and the recent February 28-March 1st outbreak were particularly impressive, and unfortunately, killed 24 people combined. Georgia in particular was hard hit on January 22nd, with 16 killed in that state on from two tornadoes. The Adel, Georgia area tornado was rated EF3 as it struck a mobile home park around 2 am, killing 11 through its path. Hattiesburg, Mississippi was hit hard in the early morning of January 21st, and Albany, Georgia suffered a similar fate on January 22md. February 28th saw multiple strong and long-track tornadoes across Illinois, Missouri and Indiana especially, with four killed. That day also produced Kansas’ first two tornadoes of the year.

Outbreaks aside, perhaps the most impressive detail of the 2017 season thus far has been the volume of events. There have been strong tornadoes and modest outbreaks almost weekly to this point. We have been locked into an active pattern for basically the entire winter (counting November and December 2016, which saw some events) and it doesn’t look like there is a major pattern change coming, either. But what does this all mean for the rest of the tornado season, and the Plains chase season in particular?

I should point out that by no means am I an expert in seasonal forecasting. It is a very difficult and relatively new area of research, and I don’t envy those who try to forecast seasonally. However, it appears to me that there are some factors that one can look at to try to determine seasonal tornado activity. ENSO state (El Nino/La Nina) is one, as well as the PDO, AMO, MJO phase, and various others. Many people seem to think that ENSO is the most important; El Nino results in generally less activity and La Nina more activity. It is true that strong La Nina events tend to produce large outbreaks in the Midwest and Southeast especially, but diving into some past events, it appears to me that El Nino, La Nina and Neutral states seem to produce similarly in the Plains. With my limited knowledge, it seems that the PDO seems to be more important. The PDO is the temperature of ocean waters in some areas of the North Pacific ocean; cooler waters (-PDO) appear to help tornado activity. While there have been big chase years with a +PDO, in general it appears that tornado activity is somewhat lessened.

Since 2012, we have been in a record +PDO. It’s not much of a surprise, then, that tornado numbers and significance have been pretty low since then. In fact, the last major, synoptically-evident (obvious for days that there would be an outbreak) was on April 14th, 2012. There have been good chase periods since then, such as May 15-31st, 2013, or May 21st-25th, 2016, but in general, the southern and central Plains has been in a lull period. The PDO has been gradually returning towards a neutral state, and may be heading towards a negative state. We shall see if this helps the chase season or not.

Analogs are an important forecasting technique. Look at the years similar to this one, and try to see if those years had a lot of tornadoes or not. Looking at ENSO, PDO, and other indices and patterns, there are some years that come up. Here are six that I think are good matches for 2017:

1999, 2004, 2006, 2008, 2009, 2012

There are some stark differences in terms of chase seasons with these. Three were all-time great chase seasons, one was decent, and two were pretty terrible. Time to look at each in-depth.

1999, 2004 and 2008 were great tornado seasons featuring multiple significant Plains events. These three years went down as some of the best chase seasons ever. All had La Nina to some degree, and 1999 and 2008 contained very active January-February periods. If 2017 is anything like one of these years, buckle up for a lot of chasing.

2012 was an OK year. Early on, there were several events to keep chasers occupied in late March and early April. April 14th, 2012 is one of the biggest tornado outbreaks on record in the Plains, with ~90 tornadoes in one day, and many strong/violent long track tornadoes across Kansas and Oklahoma. However, after April 14th, there was very little of significance. May 19th and May 25th were good chase days, but overall, Plains activity was pretty low. If 2017 is like 2012, expect there to be some unhappy chasers, but there will be at least some chases to keep me occupied.

2006 and 2009 were bad. Both years were fairly similar to this year with a weak La Nina or near Neutral. Both years also had some early-season events (March 9-13, 2006 and February 10, 2009) that would have been good chase days, but overall, there was very little to chase. May 2009 was plagued by lines of storms rather than individual supercells, and 2006 only had one tornado of note in the Plains (Westminster, Texas on May 9th). If 2017 is like 2006 or 2009, I will be very bored.

Now, what do I think will happen? I suspect that we won’t see a terrible year like 2006. It’s hard to tell if our chase season will be great or just decent. I could easily see a 2012-like season with early activity shutting off in mid-April. I could also see a 2008 type year with numerous significant Plains events. Perhaps most likely solution is a 2004-type year, with more numbers than significance (which I will gladly take). Either way, I’m really looking forward to this year.

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