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March-April 2018

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Retrospect: April 11, 1965: Palm Sunday Tornado Outbreak

In the course of weather history, there have been several tornado outbreaks in the United States that occurred on Palm Sunday. This is not altogether coincidental, as Palm Sunday—which precedes Easter by one week—is a movable feast in the Christian calendar, which, by ecclesiastical rules, can fall anywhere from March 15 through April 18, a period that typically coincides with the beginning of severe weather season. None of these outbreaks, however, was as prolific or deadly as the one that occurred on Palm Sunday, April 11, 1965.

What began as a warm day—“the first really pleasant day [the] year,” according to the U.S. Weather Bureau—ended in tragedy, as 47 confirmed tornadoes touched down across six Midwestern states, stretching from Iowa to Ohio, resulting in 271 deaths and some 3,400 injuries. At the time, it ranked as the worst tornado outbreak the United States had seen in 40 years, since the devastating Tri-State outbreak of 1925.

It was a classic setup for a springtime severe weather event. “Three meteorological conditions, necessary for tornado formation, converged over the upper Mississippi valley,” according to a Weather Bureau analysis, which also appeared in the June 1965 issue of Weatherwise. These included warm, moist air that “streamed northeastward from the Gulf of Mexico, ahead of a fast moving low pressure area.” This combined with a relatively dry mass of cooler air that approached “equally fast from the west.” The clash of these contrasting air masses allowed thunderstorms to develop along the frontal boundary that separated them. The third ingredient, a “much stronger than normal jet stream dominated the central United States and flowed over the area where thunderstorms were beginning to form.” Each of these ingredients served as a component of a strong mid-latitude cyclone, with an area of low pressure centered over the eastern Dakotas early on the morning of April 11, as depicted on the U.S. Weather Bureau's Daily Weather Map. Together, they “worked synergistically to produce multiple violent and destructive tornadoes,” according to an analysis by the National Weather Service Northern Indiana forecast office.

U.S. Weather Bureau Daily Weather Map for 1:00 a.m. EST, April 11, 1965. A strong low pressure system centered over the eastern Dakotas set the stage for a massive tornado outbreak that would unfold later that day across the Midwest. The outbreak would become known as the Palm Sunday tornado outbreak of 1965.

At 1:00 p.m. CST, the Weather Bureau's Severe Local Storms Center in Kansas City issued the following Severe Weather Forecast, which prompted local Weather Bureau offices to issue local statements prior to tornado warnings, which were, in turn, issued upon actual sightings of tornadoes:

SEVERE WEATHER FORECAST NUMBER 68

ISSUED 100 PM CST APRIL 11 1965

U.S. WEATEHR BUREAU TORNADO FORECAST FOR….

EXTREME SOUTHERN WISCONSIN

EXTREME EASTERN IOWA

PORTIONS OF NORTHERN ILLINOIS

A FEW SEVERE THUNDERSTORMS WITH LARGE HAIL DAMAGING WINDS AND ONE OR TWO TORNADOES ARE EXPECTED FROM 1 PM UNTIL 6 PM CST THIS SUNDAY AFTERNOON AND EVENING IN AN AREA BOUNDED BY THE POINTS 40 MILES SOUTH OF BURLINGTON IOWA TO 50 MILES WST OF LONE ROCK WISCONSIN TO MILWAUKEE WISCONSIN TO 40 MILES SOUTH EAST OF CHICAGO ILLINOIS BACK TO THE POINT 40 MILES SOUTH OF BURLINGTON IOWA.

WOOD ….1906Z

 

A post-storm analysis conducted by members of a Weather Bureau survey team early on April 12 indicated that the bureau's tornado forecasts issued the previous day “were excellent.” The staggering death toll, however, which at that time was already approaching 200, left the authors of the survey team's report to ask the simple question, “Why?” One reason given was the fact that, due to the “balmy” weather that afternoon “many were away from radio and TV and did not hear the subsequent warnings.” The team also concluded that, again because of the balmy weather that day, “many considered that this was just another instance of crying ‘wolf.’” The report also attempted to use statistics to explain the apparent lack of response to official warnings:

Many had heard tornado forecasts before and hadn't experienced any tornadoes. This is not surprising. The area included in a tornado forecast generally encompasses some 20,000 to 30,000 square miles. The area traversed by an individual tornado averages on the order of four square miles. An average of about two tornadoes occurs in a tornado forecast area. This means that only about 1/3,000 of the area will, on the average, actually experience a tornado. Considering a tornado path on the order of 16 miles in length, on the average only about 1/50 of the area would be within even five miles of a tornado.

 

As a result, one of the recommendations of the survey team was to increase public awareness of the threat of tornadoes and the bureau's forecast and warning program. “Saturate the public before and during the season with explanatory material on tornadoes, tornado forecasts, tornado warnings and the appropriate action to be taken in each instance with the assistance of press, radio and TV, schools and community groups,” the team wrote.

After extensive post-storm analysis based on aerial photographic surveys of the damage, famed tornado researcher T. Theodore “Ted” Fujita discovered several new “important characteristics of tornado circulation,” that led to understanding of a feature that has become known as “suction vortices,” defined as smaller-scale secondary vortices within a tornado core that orbit around a central axis. This feature, which usually occurs near the base of a tornado, explains why destructive damage can occur to houses on one side of a street, while the homes on the other side of the street remain untouched.

“The Palm Sunday tornado outbreak is an example of how good can come from something bad,” according to John Curran, retired meteorologist-in-charge of the NWS Indianapolis forecast office, who worked the Palm Sunday tornado outbreak as an entry-level observer/briefer at the Weather Bureau's airport station in Fort Wayne, Indiana. Sharing his personal story on the NWS Northern Indiana forecast office's website, Curran says the Weather Bureau, now the National Weather Service, “has, with strong taxpayer support, done a magnificent job of upgrading the entire warning process, from equipment, to manpower, to training and strengthening relationships with the media, law enforcement, and the general public.”

 

Contributing Editor SEAN POTTER is a Certified Consulting Meteorologist (CCM), Certified Broadcast Meteorologist (CBM), and science writer with an interest in weather history.

 

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