Outflow Boundaries (Florida and Beyond)

Many of you may know about rising air currents in thunderstorms. These currents, often referred to as convection, signify rising, less dense warm air. There was no dearth of such rising air currents (updrafts) yesterday as the remnants of Tropical Storm Emily passed by east of Florida (Fig. 1).

But, what goes up must come down and the other side of convection is its sinking component, known as downdrafts. Downdrafts typically contain denser, rain-cooled air.

Intense downdrafts are called downbursts or, if small enough, microbursts. Such intense sinking currents have been responsible for some significant plane crashes (including the Delta crash at DFW Airport on August 2, 1985), long-lived high wind events (derechoes) and desert dust storms (haboobs). The outflows often take on a curved arc appearance in satellite and radar imagery, with the arc moving away from where the outflow strikes the Earth's surface (Figs 2 and 3). Visually, the outflow may look like a curved rain shaft (Fig. 4). More intense localized outflow boundaries that pick up dust may appear to look like a foot or a wall of dust.

Closely related to these thunderstorm-related downdrafts are sea and lake breezes. Sinking air over coastal waters and large lakes rushes toward land (where compensating rising air motion then occurs).

On Saturday, August 6, 2011, most of these sinking air currents permeated the weather across Florida and the Bahamas, except that none were severe.

First, let's look at the coastal regions of the peninsula and near Lake Okeechobee using a visible satellite imagery animation and visible still satellite imagery (Fig. 5). You can easily see the clear zone along the coast and to the west of Lake Okeechobee where cooler air from over water had spread inland. Notice the clear zone along the coastal waters and over the lake. You can also see some thunderstorms and larger, cottony-looking, cumulus clouds forming along these boundaries.

Next take a look at the upper left. There are some thunderstorms offshore and onshore near the Tampa area. You can see several curved arcs that signify cool air outflow boundaries well to their east. These have moved away from old age or dying thunderstorms.

Another group of outflow boundaries can be seen at the upper right. There was also a much larger scale outflow boundary moving to the east and northeast from the eastern Bahamas.

Finally, look at Miami radar imagery at 4:18 pm EDT and associated animation to see both sea breeze and thunderstorm outflow boundaries.