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This Day in History: The Palm Sunday 1965 Tornado Outbreak – Breaking Down the Disaster

Image Courtesy WTHR-TV, Indianapolis:

Fifty-nine years ago today, in a similarly rainy time for Muncie (the NWS Applied Climate Information System states 14 inches had fallen by this point in 1965), a powerful storm system whipped up supercells that spawned 55 tornadoes in 6 Midwestern states in 16 hours – 27 of them significant (F/EF-2 or stronger) & 18 of them violent (F/EF4 or more potent). It was the fourth-deadliest outbreak in United States history, with 137 Hoosiers and 258 total fatalities. What environment could have made this outbreak so powerful, and why was it so deadly? Let’s get into the ingredients. Four are crucial to creating a severe weather event, and they are brought together in an easy-to-remember acronym called SLIM: Shear, Lift, Instability, and Moisture. In this post, we will discuss the adequate level of each and determine how far above those benchmarks these parameters climbed to produce such a violent evening. Many of these images will also have the tornado paths/intensities overlaid to easily visualize where all the parameters in S.L.I.M came together to develop them.

Shear (S) & Instability (I)

Instability is the energy in the atmosphere (typically airmasses that have more moisture to work with) that comes from getting the sun to heat the air up and/or while a drier/cooler airmass rolls over it. Too many clouds and/or morning storms can dampen that, as that activity cools the earth’s surface while leaving a relatively warmer layer (or cap) where the clouds are. At 7 pm Central/8 pm Eastern, tornadoes were in progress in an environment that possessed 1,000-1,500 joules/kg (j/kg) of a severe weather instability parameter known as Convective Available Potential Energy. You might think that an outbreak of this caliber might need values of 3,000 J/kg, but even the values on this day were more than adequate on a day where shear – the changing of wind direction (directional shear) and speed (speed shear) with height. How does this relate to the bullseye & curve graphs on the map? We will get to this in a bit, but to start us off on shear & severe weather events like these, we need to know how this shear stuff works to understand those graphs better.

Screenshot of 0-6km wind shear analysis from the SPC Violent Tornado Webpage (Chris Broyles)

Speed shear is crucial to the longevity of a storm as it separates the updraft of a storm (where it ingests warm/moist air) from the downdraft (cooler/drier air), keeping the storm from dissipating quickly by starving itself of the former. During the storms’ formation and rampaging through the northern half of Indiana, the speed shear in the atmosphere’s first 6 kilometers (3.7 miles) was clocked at 90 knots (or 103.57 mph). Typically, meteorologists at the National Weather Service state that only 35-40 knots is adequate in terms of severe weather. But what about the difference in wind velocity in the lowest kilometer (about 2/3 miles up)?

Screenshot of 0-1km wind shear analysis from the SPC Violent Tornado Webpage (Chris Broyles)

According to the National Weather Service, greater than 15-20 knots of 1km shear structure the lower parts of storms such that they become supercells capable of producing tornadoes (in addition to the other severe weather hazards they happen to be making happen at the time). April 11th exceeded that benchmark, with values greater than 25 knots spreading from SE Iowa through most of Illinois and Indiana and then into Ohio. Now that we’ve covered shear when it comes to speed, how about directional shear?

Screenshot of 0-6km wind shear vector analysis from the SPC Violent Tornado Webpage (Chris Broyles)
Screenshot of 0-1km wind shear vector analysis from the SPC Violent Tornado Webpage (Chris Broyles)

These two images show a top-to-bottom view of the direction our storm event’s wind shear was moving in the 0-6km (top image) and 0-1km level (bottom image). Viewing these images from the bottom up, the winds turn clockwise with height.

Screenshot of 0-3km storm-relative helicity analysis from the SPC Violent Tornado Webpage (Chris Broyles)

Combining this shear with instability gives us helicity or the tendency for a storm’s updraft to rotate around an axis rather like a DNA helix. That is a vital part of the twisting motion and vertical updraft we know a tornado to be. The map above shows us how this parameter looked over the first 3 km of the atmosphere. Typically, it would be best to have this value at least 250 meters-squared/seconds-squared (m2/s2). However, *gasps* we had way more than this, as analysis from that day says it was over *checks notes* 1000 m2/s2(!)

You might have read me mentioning curves on a bullseye-shaped thing throughout that instability map (second image in this blog). That’s a graph of how those winds turn with height and how fast they do. In the case of a supercell thunderstorm – especially ones that produce tornadoes – you would need that stretched-out curve that shoots out from the center of that bullseye and curves at a near right angle to the right after leaving that point. That tells us that both speed and directional shear are going to – or in this case, have already – created the shape of a supercell that means business amid that 1,000+ J/kg of instability. The shear is aided by a healthy low-level jet (shown in the map below) rushing into our power-packed low at 50-60 knots (60-70 mph). That’s also helping to deliver another crucial ingredient to this severe stew: Moisture.

Moisture (M)

Typically, you need about 60-degree dewpoints for the adequate moisture you need to create supercell thunderstorms, and especially to deepen the low bases of the storms as a part of the process of creating tornadoes in the supercells you get during an evening like April 11th, 1965. 60-63-degree dewpoints were in place over nearly all of Indiana. But how do you get all that moisture to rise and capitalize on all the atmosphere has to offer? Enter the kicker mechanism of lift to get things going, offered up by sources such as jet streaks, shortwaves, and surface fronts.


Screenshot of 500 MB wind analysis above 60 kts from the SPC Violent Tornado Webpage (Chris Broyles)

Standing out in the middle of the country is one stout jet streak 18,000 feet above the ground with a large area of 90 knot+ winds stretching through Missouri into the northern half of Indiana. The core of that area had winds as strong as 120 knots! When you are at the left front or right rear of that jet stream, you get spreading out of winds in the upper levels, or divergence. This creates a space where air from the surface is forced up due to the vacuum effect that divergence/opening creates. That acted as one source of lift while a potent cold front from a deep low, which, at the surface, brought barometers down to 986 MB at its center(!)

So here we have the anatomy of a classic and devastating tornado outbreak and what made it so exceptional. There was a LOT to cover here, and I hope this provides a great resource to look “under the hood” of events like these. So, when meteorologists take a look at their forecasts and start speaking in severe tones & terms about an upcoming severe weather event, this & what they have learned from the past through education and experience is why they do so!

-Cardinal Weather Service Forecaster Ryan Hill


Critical Values of Severe Weather Parameters:

Violent Tornado Outbreak Database:

Tornado Archive:;1965-04-12T12:00Z&map=-86.1029;40.5285;4.69&env_src=era5&env_type=td2m&sel_data_time=1965-04-12T00:00:00.000Z&domain=North%20America&filters=partition|PartitionFilter|f_scale|(E)F5,(E)F4,(E)FU,(E)F0,(E)F1,(E)F2,(E)F3

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