March isn’t typically the busiest time of year for tornadoes. But the residents of Henryville, Ind., can take no solace in that fact, after a powerful EF-4 category storm with winds of 175 mph ripped through town last week. And, unfortunately, neither can the rest of the Midwest, as towns await the brunt of cyclone season.
Deadly tornadoes continue as a frightening reality in the U.S, especially throughout an area dubbed Tornado Alley (the majority of Texas, Oklahoma, Kansas, Nebraska and portions of other states, including southern Indiana). The threat of the wind-whipping funnels is most acute in May and June.
Harold Brooks, a meteorologist with the National Oceanic Atmospheric Association’s National Severe Storms Lab, says tornadoes occur when certain weather elements come together. The mix happens most often in May and June, with April a close third, and a few other points throughout the year during shifting seasons, such as March and September.
For a tornado to form, Brooks says a mixture of warm, moist air at low levels (strongest in the summer) must collide with dry, relatively cold air (most common in winter and spring). Meanwhile, wind shear, the difference in the wind speeds near the surface and about 20,000 feet above ground, slams the contrasting conditions together. Wind shear is often strongest throughout late winter into early summer.
“Spring is when the ingredients are most likely to all be in place,” Brooks says. “Essentially, things are not at the peak moisture, peak cold air aloft or wind shear, but in the in between season when they’re all pretty good.”
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To pinpoint it even further, the most common time of day for this type of collision—and a resulting tornado—is between 3 and 9 p.m., but that doesn’t restrict tornadoes from hitting at other times.
While southern states may peak sooner in the year than northern states, determining the severity of a tornado season is basically impossible, says Roger Edwards, a forecaster with the NOAA’s Storm Prediction Center. Even forecasting tornadoes just a day or two beforehand proves challenging enough.
Before the 1950s it was even illegal to forecast a tornado, because of the uncertainty involved in the process and the panic a forecast could cause. As weather data models, Doppler radar and hand-drawn analyses have become more intricate, forecasters can now at least give some warning of conditions that could potentially prove dangerous.
“When predicting severe weather, including tornadoes, a day or two in advance we look for the development of temperature and wind flow patterns in the atmosphere, which can cause enough moisture, instability, lift and wind shear for tornadic thunderstorms,” Edwards says. “But it is not as easy as it sounds. How much is enough of those is not a hard and fast number, but varies a lot from situation to situation, and sometimes is unknown.”
As real-time weather observations pick up from satellites, weather stations, balloon packages, airplanes, wind profilers and radar-derived winds, forecasters shift their focus from models to the incoming storm-specific data.
“To figure out where the thunderstorms will form, we must do some hard, short-fuse detective work,” Edwards says. That work includes finding the location, strength and movement of the fronts, drylines, outflows and other boundaries and then determining the amount of moisture and temperatures both near the ground and aloft. But all that information also needs supporting wind structure data to help forecasters understand when a thunderstorm could rotate as a “supercell” and produce tornadoes.
The increased use of Doppler radar helps tell meteorologists when strong winds form in an “intense circulation,” which could lead to a tornado. But the radar’s beam is too wide to pinpoint visuals on even the largest tornadoes, Edwards says, forcing forecasters to depend on spotters to enhance the in-house analyses.
Tornadoes inflict an average of 70 fatalities and 1,500 injuries per year, with their rotating winds reaching more than 250 mph and an average forward-moving speed of 30 mph. With something so powerful in the sky above, a little warning helps, even if it is just a few hours beforehand.
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