Derechos are an interesting weather event that don’t get seen very often here in the United States with many people not knowing what they are until they experience one. They resemble a squall line on radar, as they are typically embedded in them when they do occur. A derecho is a type of severe thunderstorm that is a long-lived, widespread wind event of which is associated with a line of thunderstorms. In order to be classified as such, the event must span more than 240 miles and acquire wind speeds of 58 mph or greater throughout the entirety of the event. There must be consistent wind damage reports that span along the specified minimum length of track to verify that a derecho occurred. They can produce tornado-like damage; however, the damage will be aligned all in the same direction.
Derechos are rare because for them to thrive for such a long time with such intensity, they need consistent fuel (instability) and enough shear throughout the environment to support constant wind speeds of that nature. When a line of thunderstorms form along a front, it isn’t unlikely that storms will pop-up ahead of it; the issue with this is that those storms can eat up the fuel that the approaching line needs due to the strong downdrafts that the pop-up storms can produce. Also, timing has a significant influence on the development of derechos.
Fronts moving through at night have a harder time to keep the fuel going as the differential heating used to promote lift during the day is no longer there. So the environment typically becomes more stable and less likely to have the energy to propagate the storms with as much intensity as they would if it was a hot, sunny summer afternoon in the south. Derechos are a very interesting weather event that we are constantly researching to try and begin forecasting them ahead of time.
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©2019 Weather Forecaster Ashley Lennard
Tornado Alley May Be On The Move: Where it is Going and What You Need to Know (Credit: Nature Journal)
Recent tornado outbreaks in the Southeast U.S. have spawned talk of extreme severe weather that is occuring in an area of the country that is not typically used to such extreme tornado outbreaks. An early March outbreak in eastern Alabama ended with 23 fatalities and over 90 people injured after an EF4 tornado tore through over 60 miles of the state. By mid-March, another outbreak yielded 15 tornadoes in the same state, albeit no fatalities or injuries occurred during this outbreak.
The outbreaks may come as a precursor for what the 2019 Tornado season, which usually is beginning in February and March, could bring to the Central and Southeast U.S. Several factors go into creating a record-breaking tornado season: above-average temperatures, increased moisture from the Gulf of Mexico, and even the development of El Nio. However, it isn’t the frequency or magnitude of tornadoes that is catching most scientists’ intentions (although these are important nuances that should be noted). It is the movement of tornado outbreaks that has alerted scientists and prompted new tornado research. In 2018, two researchers, Vittorio A. Gensini of Northern Illinois University, and Harold E. Brooks of the National Severe Storm Laboratory in Oklahoma, studied the spatial trends of tornado outbreaks.
The results were clear. Tornado outbreaks are on the rise in the Southeast U.S. in states like Arkansas, Mississippi, Alabama and Kentucky. This is in comparison with the Great Plains, historically known as “Tornado Alley”, where tornado outbreaks are actually decreasing. The study used STP, the Significant Tornado Parameter index, and U.S. climatological tornado data to analyze where tornadoes are occurring and in what severity. The STP measures the magnitude of a tornado of EF2 strength or greater, and how likely strong tornadoes will occur in a given area. The scientists in this study were able to use STP to explain the changes in regional tornado formation in recent decades.
You may be wondering why this is a big deal. Brooks and Gensini noted increased physical risk and social vulnerability for people in the Mid-South as compared to the Central U.S. While most of the country’s tornadoes occur in the Plains, most tornado casualties occur in the Southeast. This may have to do with a lack of proper tornado-resistant infrastructure. In the Plains, most infrastructure dating back decades have been equipped with cellars in the case of a tornadic supercell. In the Southeast, many more people are impoverished, living in structures such as mobile homes and trailers which are not equipped with proper tornado shelters. This could spell disaster for an area with an increasing frequency of tornadoes, and it may only get worse.
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© 2019 Weather Forecaster Jacob Dolinger