“Likely …. the most prolific five-day period of tornado activity on record for so early in the year“
NBC: “It’s as if a huge chunk of the country has suffered a deep, deep scar.”
National Weather Service Warnings for Past Week
The unexpectedly fierce and fast tornado outbreak so early in the season has folks asking again about a possible link to climate change. Climatologist Dr. Kevin Trenberth emailed me that, because of climate change, “there is every expectation that the [tornado] season will move up in time. The warm winter in the US is perhaps an indicator of the nature of the changes to be expected.”
The former head of the Climate Analysis Section of the National Center for Atmospheric Research stands by his 2011 statement, “It is irresponsible not to mention climate change in stories that presume to say something about why all these storms and tornadoes are happening.” Below is some clarification of the context of that quote that he added. Trenberth also said:
Joe, what we can say with confidence is that heavy and extreme precipitation events often associated with thunderstorms and convection are increasing and have been linked to human-induced changes in atmospheric composition.
Insured losses due to thunderstorms and tornadoes in the U.S. in 2011 dollars. Data and image from Property Claims Service, Munich Re.
After April 2011 saw records set for most tornadoes in a month and in 24 hours, I examined the link in great detail here, looking at the data, the literature, and expert analysis. That piece concluded:
- When discussing extreme weather and climate, tornadoes should not be conflated with the other extreme weather events for which the connection is considerably more straightforward and better documented, including deluges, droughts, and heat waves.
- Just because the tornado-warming link is more tenuous doesn’t mean that the subject of global warming should be avoided entirely when talking about tornadoes.
This post will run through the scientific literature along with some analyses from this year and last by leading experts.
First, though, some of the details on this week’s tornado outbreak.
MONDAY UPDATE: USA Today has a good piece, ”Warm winter helped fuel tornado outbreak,” which cites today’s post by Weather Underground meteorologist Dr. Jeff Masters
This year’s unusually mild winter has led to ocean temperatures across the Gulf of Mexico that are approximately 1°C above average–among the top ten warmest values on record for this time of year, going back to the 1800s. (Averaged over the month of February, the highest sea surface temperatures on record in the Gulf between 20 – 30°N, 85 – 95°W occurred in 2002, when the waters were 1.34°C above average). Friday’s tornado outbreak was fueled, in part, by high instability created by unusually warm, moist air flowing north from the Gulf of Mexico due to the high water temperatures there. This exceptionally warm air set record high temperatures at 28 airports in Louisiana, Arkansas, Mississippi, Alabama, Tennessee, Kentucky, and Georgia the afternoon of the tornado outbreak (March 2.) Cold, dry air from Canada moved over the outbreak region at high altitudes. This created a highly unstable atmosphere–warm, low-density air rising in thunderstorm updrafts was able to accelerate rapidly upwards to the top of the lower atmosphere, since the surrounding air was cooler and denser at high altitudes. These vigorous updrafts needed some twisting motion to get them spinning and create tornadoes. Very strong twisting forces were present Friday over the tornado outbreak area, thanks to upper-level jet stream winds that blew in excess of 115 mph. These winds changed speed and direction sharply with height,imparting a shearing motion on the atmosphere (wind shear), causing the air to spin. High instability and a high wind shear are the two key ingredients for tornado formation.
Here’s more from Masters on the record-setting storms: