Tornado Alley: Lee Cyclogenesis!

One way to strengthen an area of low pressure is via lee cyclogenesis! The “lee” refers to the downwind slopes of mountains. The Great Plains region has the Rockies to the west, setting the perpetual stage for lee cyclogenesis. Looking at a globe you can also see Tornado Alley is found in the midlatitudes (bounded between approximately 30-50 degrees north latitude). This means the region is located in the general path of the prevailing upper tropospheric Westerlies (polar jet stream). The Westerlies is the main storm track by providing stronger upper-level steering winds for midlatitude cyclones of varying strength. When weather disturbances pass over the Rockies and out from the Front Range (the gateway into the central United States) the opportunity for lee cyclogenesis begins! The cause is related to the concepts of conservation of potential vorticity, conservation of mass, and horizontal convergence.

Positive vorticity is “spin” in the atmosphere. The greater the spin, the more intense a cyclone is. The potential vorticity of a rotating fluid (e.g., a large spinning area of low-pressure) is derived by its absolute vorticity (AV) in relation to the depth of air column the spin is occurring. Absolute vorticity is the combination of vorticity imparted by the Earth’s rotation (higher contributions over middle and higher latitudes) plus relative vorticity (RV), which is influenced by the environment the circulation interfaces with.

The key for lee cyclogenesis is that RV must respond to changes in depth for an air column experiencing spin due to the conservation of mass of a rotating fluid. For our weather, the available vertical depth is bounded between Earth's surface and the top of the troposphere. A cyclone reaching the leeward side of the Rockies experiences sudden vertical stretching as elevations at the Front Range drop dramatically over a short distance and then maintain a general downwards slope into the heart of Tornado Alley. Vertical stretching (as opposed to vertical shrinking) has the effect of boosting relative vorticity and intensifying a cyclone (think of the classic example of figure skaters bringing in their arms and suddenly speeding up rotation).

Anticipating lee cyclogenesis is critical to forecasting severe thunderstorm outbreaks since stronger areas of low pressure tend to allow basic severe weather ingredients (moisture, wind shear, and lift) to converge east of the Rockies!

Photo Credit: Jonny William Malloy