The Evansville, IN Tornado of November 6, 2005

On the early morning hours of November 6, 2005, a deadly tornado tore through several Kentucky and Indiana Communities killing 25. The combination of climatologically unusually ripe atmospheric conditions for tornadoes and sleeping communities made for disaster. In this story map, we will cover the life cycle of the tornado as well as the meteorology that made this monster happen.

At the end of the tornado's 45 minute life, a 41 mile long path of devastation laid in its wake. 25 people had been killed and 230 people suffered varying injuries from the tornado, 24 out of the 25 were killed in mobile homes. A minimum of 500 buildings were destroyed or severely damaged costing tens of millions of dollars. The National Weather Service surveyed the damage of the tornado and concluded the tornado was an F3 with winds peaking at 200mph and a maximum width of 500 yards.

To analyze the meteorology that caused this tornado, we need to start at a nationwide scale at 500mb height (roughly 18,000ft). Here, we are primarily looking at wind speeds and directions. We see a trough propagate through the US and pass over the Ohio River Valley.

A trough is a weather pattern that houses midlatitude cyclones, the phenomena that is responsible for the majority of our impactful weather. These troughs go through different tilts/orientations throughout their life cycle that give us clues on their overall strength and stage of life.

Our trough in question digs into a negatively tilted orientation which tells us that the midlatitude cyclone is in its mature stage and is quite strong.

The other item to look at with these maps are the wind speeds, these winds represent the jet stream in the atmosphere. An area with higher wind speeds is referred to as a jet streak, these jet streaks are an important ingredient with severe weather.

Moving down in the atmosphere to 850mb (roughly 5,000ft) we see the same trough pattern that is providing the structure and necessary energy for storms to develop. We also see winds out of the south. These winds transport, or advect, the air from one place to another. In this case, southerly winds from the Gulf of Mexico are advecting very warm and moist air into the Ohio River Valley. This warm moist air will provide an unstable atmosphere for developing storms to strengthen and potentially become severe.

It is also important to note that the wind direction from 500mb down to 850mb are different and are at different speeds. This is called wind shear, a vital component to helping severe storms maintain their strength and structure. Additionally, winds changing direction like this throughout the atmosphere can aide in tornado potential.

Continuing to move down in the atmosphere, we arrive down to the surface to put the final pieces of the puzzle together. First, we look at temperatures and dewpoints. This map represents observations very close to the time of the tornado (1:00am), temperatures are in red and dewpoints are in green. The Evansville airport recorded a temperature of 71 and dewpoint of 63 which represents an incredibly anomalous atmosphere that is ripe for severe weather. Temperatures and dewpoints, at 1:00am in November, should be nowhere near this level.

Remaining at the surface, we can now look at exactly how much instability, or CAPE (Convective Available Potential Energy), existed that night. This variable can be thought of as fuel for the storms. During the fall and winter times, as little as 500 J/kg of CAPE can be enough to fuel severe weather. In this instance, the tornado had up to 1,000 J/kg to work with.

I mentioned earlier the term wind shear, we can examine the amount of wind shear available with this parameter. When looking at this effective shear parameter, the higher the number, the greater the likelihood of storms becoming severe and sustaining themselves. Values of 25-40kts or greater is when supercells, like the one that spawned our tornado, become more likely. During the time of the tornado, southwestern Indiana had between 50-60kts which is more than enough to promote supercell development.

Another way to look at wind shear is that you have winds going much faster at higher levels of the atmosphere than lower levels. This produces columns of horizontally spinning air, similar to when you roll a pencil between your hands. When these rotating columns of air encounter a thunderstorm updraft, the horizontal spinning air can be pulled vertical if the updraft is strong enough. The now vertical column of spinning air is one of the building blocks for a mesocyclone from which tornadoes can develop. The likelihood for these horizontal columns to be pulled vertical is called storm relative helicity, a necessary parameter when discussing tornado potential. When looking at the storm relative helicity from the night of the tornado, we had values as high as 400m^2/s^2. Values over 150 are considered to be conducive for increased tornado potential. On that fateful night, we had almost triple that amount.

All of these discussed components, or ingredients, came together that night to create a volatile atmosphere. That atmosphere produced a severe squall line with embedded supercells stretching over several states and produced our tornado. Squall lines are not typically associated with tornadoes, let alone violent and long tracked tornadoes, but nothing about that night was normal.