Tonight at 8pm EDT, TWC will be airing a special program on the recent historic tornado outbreak, called "Twisters: Trail of Destruction." Here are some additional reflections on the three-day onslaught, and we'd be interested in yours via comments to this entry. Our thoughts are with those who have experienced the wrath of the atmosphere.
The outbreak on Thursday, Friday and Saturday, April 14-16, 2011 from Tushka, Oklahoma to Askewville, North Carolina left many memorable images. Here are a couple: this photo of schoolchildren looking out the window at a tornado in Clinton, Mississippi (why were they not taking shelter?), and a man sitting in his car and calmly observing as a vicious tornado races in his direction (those are not leaves as he says, they're pieces of buildings!).
Source: http://yfrog.com/hsvctwhj
This outbreak didn't match the most intense one on record, the Superoutbreak of April 3-4, 1974. That produced numbers which no outbreak in recorded history before or since has come close to rivaling: of the 148 tornadoes within 24 hours, 30 were of at least F4 on the Fujita Scale (now known as the Enhanced Fujita Scale, with EF instead of F ratings), including six F5s.
What made the recent one so notable was the way it was so potent and tragic three days in a row, culminating in one of the deadliest tornado outbreaks in the history of North Carolina along with destruction that was extraordinarily widespread for that part of the country.
And what occurred meteorologically to result in that was the nature of a sharp dip in a strong jet stream, which took the shape of what meteorologists call a "negatively tilted trough," and how that plowed east like a machine, producing spinning, deadly tornadic supercells day after day.
This set of maps illustrates what I'm talking about. The way the dashed line representing the trough axis extends from northwest to southeast is the "negative tilt," as opposed to a positive tilt, which would be from northeast to southwest, or a neutral one, which is due north-south. All kinds of troughs can be associated with thunderstorm outbreaks, snowstorms, and other hazardous weather, but on average the negative tilts tend to have a little extra oomph, and this was an exceptionally strong and persistent one.
Those upper-level jet stream maps, by the way, have some similarities to the pattern present for big outbreaks last May in Oklahoma, April 1998 in the Deep South, and March 1984 in the Carolinas ... except this time it happened on three consecutive days.
With this kind of dip, the jet stream energy tends to really punch over the warm, humid, unstable air ahead of it, conducive to severe storms. Also, the air ahead of the trough spreads widely apart in upper levels of the atmosphere (a particularly vivid example of which is above, as I illustrated on the Friday map with the arrows that follow along with the wind flags), which helps lead to thunderstorm development as air rises to take the place of the air evacuated above.
[There's a whole separate technical topic of "diffluence" vs. "divergence," but that's beyond the scope of this blog! Readers who are interested in learning more about that can find a quick tutorial here.]
Then, with the right combination of other ingredients such as winds at different levels of the atmosphere blowing in different directions, tornadoes can form. Those ingredients were also strongly present on Thursday, Friday, and Saturday.
It's not uncommon to get this kind of trough. What's unusual is the way it kept its form, and kept slamming nearly due east day after day. As a friend of a friend on Facebook (you can sign up for my weather posts on my TWC Facebook fan page) noted, these troughs often either lift quickly to the northeast and become somewhat disconnected to the most explosive instability, or they become "cutoff lows," in which, as the name sounds, they cut off from the main jet stream and grind to a halt, and can still produce very inclement weather but generally not bad tornado outbreaks.
There's no rest for the weary, with, per the graphics below, another such trough coming, which is going to bring about another collision with warm, moist, unstable air tomorrow (Tuesday) and Tuesday night. The main threat area will be in the Mid-Mississippi Valley into the Ohio Valley.
Source of images: wright-weather.com
This time, though, the system will swing northeast rather than crashing over the Carolinas and Virginia. It'll be a two-day outbreak, not three, and tornado-producing ingredients on Wednesday are not expected to be present to the degree they were on Saturday when that system came east. There will be a lot of wind energy, though, so a threat for damaging storms will exist in places such as PA, NY, and New England, and at least some risk of energetic thunderstorms into the Deep South.
It's been a wild month, starting with what was, by the standards of straight-line wind damage events rather than tornadoes, a superoutbreak on April 4; then a destructive evening in Iowa on April 9 and the following day an outbreak in Wisconsin which was record-setting for that state; then the most recent siege last Thursday, Friday, and Saturday, and now the upcoming system ... And looking ahead, the pattern will stay active this weekend and into next week with the potential for additional severe thunderstorms as well as flooding rains in the nation's midsection.
Stay safe, everyone!
