Today marks the 17th anniversary of the F-5 tornado that tore through the Central Texas town of Jarrell taking 27 lives. Aside from being a rare F-5 this tornado was unique for other reasons.
On the day of the storm the atmosphere across Central Texas was extremely unstable. The image below shows the thermodynamic diagram from a sampling of the atmosphere taken around noon in Temple, Texas, less than 30 miles north of Jarrell.
The area shaded in red represents the Convective Available Potential Energy (CAPE) in the atmosphere. CAPE is essentially an indication of atmosphere's instability, or the ability of air to rise through the atmosphere. A CAPE value of about 500 j/kg is enough for thunderstorms to form and a value of 2,000 j/kg is enough to lead to the formation of super cells. On the day of the Jarrell Tornado the CAPE was measured at 6,840 j/kg, more than 3x the amount of CAPE needed for a tornadic storm to form. To say the atmosphere was very unstable would be an understatement, it could be more accurately described as explosively unstable. However, similarly to a pile of TNT all of that CAPE means nothing if there is not a triggering mechanism. Unfortunately for those in the path of the impending storm there were multiple triggers.
Earlier that day a dry line had stalled across northern and central parts of the state. This allowed moisture streaming in from the Gulf of Mexico to effectively pool across the state east of the dry line. On an ordinary day a dry line is more than capable of setting off thunderstorms but on that day the dry line was only part of the equation. The night before a large area of thunderstorms was moving across Oklahoma and when those storms began to collapse and die they sent out ripples of energy through the atmosphere. These ripples are known as gravity waves. These gravity waves acted as another focus for lift in the atmosphere and when they collided with the dry line in an already juicy and volatile atmosphere there was an epic convective explosion. The satellite images of that meeting of the two triggers is breath taking as you can very clearly make out all of the players involved and the end result.
At the beginning of the loop the dry line is visible as the thin line of clouds which snakes from the northeastern corner of the state down to the southern border. You can also see the gravity waves at the beginning of the loop as they make their way southwest across the eastern portion of the state. Although the gravity wave and dry line appear to meet before the burst of storms, it is not until they meet in the area of the highest instability where that 6,840 j/kg CAPE was located, that things really begin to pop.
Perhaps the most unique and most deadly aspect of this storm was the fact that it moved in the opposite direction that most tornadic super cells move. The vast majority of the time super cells move in a general north-easterly direction. This time however the storm moved to the southwest. Looking at the above satellite image it is not difficult to see why. Once storms initiated they blew up in a chain reaction along the dry line aided by the southwesterly movement of the gravity waves.
The tornado itself grew to 3/4 of a mile wide and swallowed up the Double Creek Estates subdivision of Jarrell. It was there that entire families perished in their homes. The extent of the devastation was due in part to the fact that the tornado was a slow mover, subjecting areas to a prolonged period of winds likely close to 300 MPH. The tornado not only swept foundations completely clean of the houses that once stood on them, but also ripped all vegetation and even pavement from the ground leaving nothing but barren mud in it its wake.
In recent years bigger and even stronger EF-5 tornadoes have occurred, but the Jarrell tornado should not be forgotten as it was truly unique.