Thunderstorm Downdrafts
Introduction
When thunderstorms threaten the Great Lakes, forecasters at the National Weather Service (NWS) traditionally conclude their marine forecasts with the following phrase -- Winds and Waves Higher In And Near Thunderstorms. Both an observation and a warning, the statement is intended to alert mariners to the potential for localized areas of much stronger winds than those anticipated in a non-storm environment. Such warnings are justified, as even a garden-variety thunderstorm is capable of producing winds of 50 knots or more.
Downdrafts
The wind danger related to thunderstorms is associated with the downdraft, a storm-scale flow of air descending towards the ground from several thousand feet above the surface. Particularly strong downdrafts are referred to as downbursts and are further sub-categorized based upon the size of the affected area and the duration of the peak wind. Microbursts affect the smallest area (less than 2.5 miles in length) and have peak winds lasting less than five minutes. On a broader scale, macrobursts impact an area greater than 2.5 miles in length and contain peak winds that persist for as long as twenty minutes.
Two incidents which occurred at Andrews Air Force Base during the 1980s serve as an example of the remarkable winds that may be associated with a downburst. The first, containing a gust of 113 knots (130 mph), occurred on August 1, 1983 just minutes after Air Force One (and the President) had touched down. In 1986, the second, and much stronger, downburst peaked at 137.3 knots (158 mph). While downdrafts may occasionally be quite strong and produce a surprising amount of damage, they are a natural part of a thunderstorm's life cycle.
Thunderstorm Life Cycle
The images below show the three stages of development associated with a garden-variety single cell thunderstorm. Thunderstorms develop where the atmosphere is unstable, generally typified by a warm, moist layer of air near the surface and relatively colder and drier air aloft. It is the difference in both temperature and moisture that provides the energy for thunderstorm development.
The first stage of development -- Towering Cumulus -- is characterized by the upward motion of warm, moist air parcels. These air parcels cool during their ascent; eventually the water vapor in the parcels condenses into very small water droplets and a cloud begins to form. The process of condensation releases heat, which warms the air column in which the thunderstorm is developing and paves the way for the water vapor in subsequent air parcels to condense at ever higher altitudes. As this process is repeated, the cloud associated with the storm's updraft grows taller. Given an abundance of upward motion and low-level instability, a thunderstorm may reach the upper limit of the atmosphere. During this period of initial development, no precipitation (downdraft) is associated with the storm.
As the storm's updraft develops upward into colder air (below 32° F), atmospheric processes promote the growth of larger precipitation particles (water droplets, ice crystals, hail, etc.). These particles eventually become too heavy to be suspended by the updraft and they begin to fall. The storm's downdraft now begins to form, and the storm reaches the Mature Stage, as the falling precipitation drags some of the surrounding air along the journey to the surface. During descent, the melting of ice crystals and the evaporation of water droplets serve to cool, and thereby strengthen, the speed of the downdraft. Eventually, this descending column of cool, dense air reaches the surface and spreads out in all directions. The leading edge of cool air from the storm is referred to as a gust front or outflow boundary, and is frequently described as a small-scale cold front.
The falling precipitation, and associated cooling, diminishes the strength of the storm's updraft. In addition, the formation and growth of a cool, dense pool of air at the surface disrupts the ability of the storm's updraft to ingest warm, moist air. The persistence of the downdraft and the absence of an updraft marks the arrival of the Dissipating Stage. Although precipitation may persist for a while during this stage, the intensity will steadily diminish as the storm approaches the end of its lifecycle.
A single cell thunderstorm may transition from the Towering Cumulus to the Dissipating Stage in approximately sixty minutes. However, not all storms complete the process due to unfavorable atmospheric dynamics such as weak instability, a lack of moisture, or warm temperatures aloft. And not all thunderstorms are garden-variety single cell thunderstorms.
Supercell Thunderstorms
Based upon their appearance on Doppler radar and other observations, the thunderstorms which occured during the 2010 Ontario 300 were likely supercels. Supercells are a special long-lived type of single cell thunderstorm, characterized by a rotating updraft . Supercell thunderstorms may last for several hours and are responsible for producing nearly 100% of all strong tornadoes (greater than EF2) and most reports of large hail. Supercells are also capable of producing downdrafts of 100 mph or more. Fortunately, only a small portion (< 5%) of severe thunderstorms in the lower Great Lakes are supercells.