There are two types of weather that are generally considered the worst: tornadoes and hurricanes. This year, the United States has experienced very low levels of both tornado and hurricane activity. This is great news, of course! But it still begs the question... why? In this 2-part series, I'll break down the numbers, put them in historical context, and present a theory or two for the lack of bad weather. This is the second part of the series, where the focus is on the 2013 severe weather season.
For tornadoes, it has been a very unusual year. While we did have the double-whammy of deadly tornadoes in the Oklahoma City area in May, the rest of the year was fairly quiet for the entire country. As of November 12th, 814 tornadoes have been confirmed in preliminary reports. This might sound like a lot, but the 20-year average is 1,278.6 tornadoes per year! The year is not over, of course, but we are entering the least-tornadic time of year right now, so it's highly unlikely we'll have hundreds more tornadoes touching down in between now and year's end.
If the current tornado trend continues, we will finish the year with the fewest tornadoes since 2002. It will also be the first time since 2002 that fewer than 1,000 tornadoes have been reported in a calendar year.
|Graphic courtesy of NOAA's Storm Prediction Center
I think it's tough to compare severe weather seasons, apples to apples, because there are a few factors that have recently helped to elevate the number of tornado reports we receive. One factor is that there are simply more people living in suburban areas, which means there are more people who could potentially spot a tornado. I do not think it's a coincidence that 1973 is the first year in history with more than 1,000 tornadoes were confirmed. This was a time when people were leaving the cities for suburbs in droves, and the interstate highway system was quickly growing, making rural locations more accessible. Since then, a number greater than 1,000 has become the norm! In this article I wrote in September, I discuss some of those tornado-spotting factors in greater detail.
In this day and age, if a tornado is happening, chances are good that either a person or a Doppler radar saw it. So the low tornado count in 2013 is a definite phenomenon. So, the question begs asking... why such a low number? Unlike the hurricane season, which I talked about earlier this week, the low tornado year is likely due to several factors. But, just like the hurricanes, dry air is a factor in the lack of tornadic activity. A tornado draws its energy from atmospheric clashes- clashes between warm and cold air, moist and dry air, changes in wind speed and direction. This year, it seems that the moisture plume from the Gulf of Mexico that usually feeds severe storms was not in place very often. Without this moisture from the Gulf, there wasn't a potential clash with dry air descending from Canada or the Rocky Mountains. So, why wasn't this moisture feed as strong as usual? I'm pointing my finger at the Bermuda/Azores High, which is normally parked off the East Coast of the USA during a good portion of the summer. The clockwise windflow around the high brings southerly breezes inland after moving across the Gulf waters. This year, the center of the high pressure was further east, so that Gulf influence wasn't as strong.
Another factor was in the upper atmosphere. The upper-air pattern was unfavorable for tornadic development for much of the summer. A huge ridge was consistently parked over the Rocky Mountains and the western portions of the Plains this summer. This led to record-breaking heat in places like Boise and Salt Lake City, and a bubble of hot, dry air over the Plains. Again, dry air was a dominant feature. The ridge over the Rockies also means that, obviously, there wasn't a trough over the Rockies. A typical severe weather setup includes this upper air feature. See my rudimentary drawing below:
In the late spring and early summer months, a pattern like this, with plenty of surface moisture from the Gulf and a trough of cooler air digging in, is a slam-dunk for severe storms. Tornadoes are highly likely to form in this environment, too. The deadly tornadoes that formed on May 20th and May 31st of this year were spawned in an atmosphere very similar to the one diagrammed above. However, as I mentioned earlier, this was not the setup in the United States for the vast majority of the summer. Here's rudimentary drawing #2, depicting a more typical setup for us this year:
Big difference, right? By the time the pattern shifted, the peak of severe weather season was long gone. This pattern is not unusual during the summer months, but what is unusual is how long the pattern lasted. It's the reason for the record heat in the Rockies, and our relatively cooler and wetter summer here in the DC Metro area as well.
The atmospheric elements collaborated to make for a very calm year for bad weather in the United States, both in the case of hurricanes and tornadoes. If you missed the first part of this series, follow this link for the full article.