America's worst severe weather outbreak so far in 2015 was Thursday, when a preliminary total of sixteen tornadoes touched down in Illinois, Iowa, and Missouri. Many of the tornado reports came from a single long-lived supercell in northern Illinois, about 80 miles northwest of Chicago, that spawned a large wedge-shaped tornado around 7 pm local time. Significant damage was reported near the town of Rochelle, and every structure in the tiny town of Fairdale was reportedly damaged or destroyed
. Two people were killed in Fairdale, and twenty were injured. Figure 1.
Rochelle, Illinois tornado near Highways 64 & 251 on April 9, 2015. Image credit: Angie Medernach-Harris.Figure 2.
Radar reflectivity image of the supercell thunderstorm (circled in white) that spawned the Rochelle, Illinois, tornado at 6:55 pm CDT on April 9, 2015. The hook echo of the storm lies just northwest of the town, which is marked by a black circle. Image credit: Storm by Weather Underground
Impressive video of the large wedge tornado that hit Rochelle, Illinois on April 9, 2015.
The long-lived supercell that spawned the tornado formed in a nearly ideal location, ahead of a strong cold front plowing across Illinois and just south of a warm front that had moved to the WI/IL border. Some of the worst tornadoes on record have formed near the low pressure found at the intersection of these fronts, known as the “triple point”
. Upper-level winds on Thursday were much stronger than on Wednesday, exceeding 100 mph in the midst of a compact jet-stream impulse that was heading into Illinois as the most damaging tornado developed (see Figure 3). Figure 3.
Winds at the jet-stream level (300 mb, or about 30,000 feet) at 0100 GMT Thursday, with icons denoting preliminary reports of tornadoes, severe hail, and severe wind as of late Wednesday night. Image credit: WunderMap
Preliminary damage assessments put the tornado at EF4 strength with 180 - 200 mph winds, which would make it the first U.S. tornado of 2015 to be rated EF3 or stronger. Even if some homes were swept clean from their foundations, it would not automatically rank the tornado as an EF5 on the Enhanced Fujita Tornado Damage Scale
, though. Unlike the original Fujita scale, EF rankings take into account the strength of a building’s construction. A violent tornado that struck Vilonia, Arkansas, on April 27, 2014, was rated as EF4 despite ripping homes from their foundations. NWS meteorologist John Robinson pointed out in an interview
: “. . . If a house that has a foundation built with nuts, bolts, and appropriate-sized washers is swept away, leaving only the concrete foundation, the ‘expected value’ of wind in the Enhanced Fujita Scale is 200 mph, which puts it at the top of the EF4 category, not an EF5. It would have to be an exceptionally well-built house to go over 200 mph and thus achieve an EF5.”
While Thursday’s outbreak of tornadoes arrived relatively early in the season for northern Illinois, the Midwest is no stranger to early-spring twisters. It was on the same date (April 9) in 1953 that a radar in Champaign, IL, observed the first documented hook echo
, a feature that soon became a hallmark of tornado detection. And this weekend (April 11-12) marks the 50th anniversary of the Midwest’s extremely violent 1965 Palm Sunday tornado outbreak
. A total of 47 twisters killed 261 people, making it the fourth-deadliest outbreak in U.S. records behind only the Tri-State Tornado of 1925
and the Super Outbreaks of 1974
.This week’s WunderPoster: The Brocken spectre
Avid mountain climbers might recognize the phenomenon featured in this week’s WunderPoster
, right): the ghostly Brocken spectre. You might see it when standing atop a high point, looking down into fog or mist, with the early- or late-day sun behind you. Under these conditions, your shadow may appear against the mist, directly opposite the sun, surrounded by a faint rainbow-colored halo. It can be a startling sight, given that it’s hard to judge your shadow’s size if any nearby landmarks are obscured by the fog or mist. The spectre got its name from a peak named Brocken in Germany’s Harz Mountains. All WunderPosters can be downloaded in formats suitable for posters or postcards.
Bob Henson and Jeff Masters