National Weather Service Weather Forecast Office

An Impressive Looking, Mid-morning Gust Front, 26 August, 2002

mouse click the figures for a larger view

1. Event overview

During the pre-dawn hours of 26 August 2002, a cluster of thunderstorms developed in north central Nebraska and by mid-morning had moved southeast into central Nebraska. The heaviest activity became situated over portions of York and Polk counties. Often long-lived thunderstorm complexes produce what are known as outflow boundaries. Outflow boundaries are of particular interest to the forecast and research communities as the boundaries can initiate future thunderstorm development, and also locally enhance the threat of tornadoes associated with strong thunderstorms that travel along the boundaries. The boundaries themselves can produce minor structural wind damage. At times, impressive cloud structures can develop along the leading edge of outflow boundaries. One such outflow boundary passed over the National Weather Service Forecast Office in Hastings, Nebraska during the mid-morning hours of 26 August.

2. Details

Outflow boundaries are produced in the following manner. When rain falls from a thunderstorm, some of the raindrops evaporate as they fall. The evaporation cools the airmass underneath the storm, near the ground. If the rainfall is particularly heavy, as is often the case with a thunderstorm complex, a large area of cold air will develop underneath the complex and surge out away from the storms. The reason the cold air moves out away from the storm is due to the fact that cold air is more dense than warm air (relatively warmer air surrounds the thunderstorm complex). Because of the density difference between the cold air and warm air, the cooler, denser air flows toward a region of less dense air. The leading edge of the surging outflow is called a gust front. A gust front separates the cold air produced by the thunderstorm, from the warmer and moister airmass out ahead of the outflow.

The radar image in figure 1 occurred at 9:17 AM, local time. The thin line of reflectivity extending south of the main thunderstorm complex was the leading edge of the outflow; the gust front. Note the surface observations (colored white). North of the boundary, temperatures are generally 6-7 degrees Fahrenheit cooler than the warmer temperatures south of the gust front. Also of interest are the opposing surface winds where northerly winds gusting up to 35 mph existed north of the boundary, with weaker, 5-10 mph, southerly winds south of the boundary. The opposing winds created strong convergence along the leading edge of the gust front lofting insects and dust particles high into the air. The National Weather Service Doppler radar can detect the airborne agents; hence the fine-line denoting the location of the gust front. Additionally, the converging winds induce strong lift along the gust front.

Fig. 1. Radar reflectivity and surface observations valid for 9:17 AM local, 26 August 2002. Relevant features depicted..

When strong lift occurs, the upward motion of the air can produce large and ominous looking clouds along the gust front, as is evident in figure 2. The low cloud hugging the ground is called an arcus cloud. However, on this day, the airmass that enveloped the gust front was very stable. Because of the stable airmass, the clouds appeared very smooth (conversely, clouds that develop in an unstable airmass will usually appear billowy, similar to that of cotton balls or cauliflower, i.e., a developing thunderstorm).

Fig. 2. Photo of the leading edge of the gust front. The view is to the east from the National Weather Service Office, at 9:15 AM local, 26 August 2002.

Figure 3 is the same photo as in figure 2 but with lines depicting the low-level, air-stream along the leading edge of the gust front. Note the strong convergence that existed along the gust front. The convergence can produce tremendous wind shear along and underneath the arcus cloud presenting a hazard to aviation. Sometimes the wind shear will produce funnel clouds. Note the cone shaped funnel cloud in figure 4. The funnel cloud looked impressive but given the lack of thunderstorm development above the funnel cloud, due to the stable environment, the funnel did not strengthen to produce a tornado.

Fig. 3. As in Fig. 2 but with relevant features depicted.

Fig. 4. Photo of the leading edge of the gust front. The view is to the northeast from the National Weather Service Office, at 9:16 AM local, 26 August 2002. Note the cone shaped funnel cloud.

Figure 5 is a view toward the northwest.

The gust front would eventually travel as far south as the Nebraska-Kansas border before it became stationary and eventually dissipated.

Web page, photos, and graphics produced by Albert Pietrycha