National Weather Service United States Department of Commerce

Areas of Showers & Thunderstorms with Hot Temperatures for Many Locales During the Holiday Weekend

Showers and thunderstorms are expected along a stationary frontal boundary from the Mid-South to the eastern Gulf Coast region and northern Florida through Saturday. In the north-central U.S., locally severe thunderstorms and flash flooding will remain possible. Hot temperatures will persist through the weekend over much of the central and eastern U.S. with pleasant temperatures in the Northwest. Read More >

Slide 15

Wind Speed in Relation to a Highway Overpass

As stated in the caption for slide 11, to our knowledge, there have not been any specific modeling studies done about the airflow interaction between a tornado and an overpass. However, based on our current knowledge of airflow through and around obstacles, such as buildings and other man-made structures, it is possible to indicate the outcome of an interaction between a tornado and an overpass with a fairly high degree of confidence. In general, the wind speed decreases as we approach the surface, becoming zero right at the ground. This is why one of the first and foremost rules in general tornado safety is to get as low as possible, because that is where the wind speed is the lowest! By climbing up underneath the overpass, people are moving into a place where the wind speeds typically will be higher. In addition, under an overpass, it is possible in some situations that when air is forced through the narrow passage underneath the bridge, this might cause an increase in the wind speeds (as mentioned earlier). Further, under different circumstances, the area beneath and just downstream of an overpass might become a debris deposition zone, where piles of debris accumulate, as was the case at the Shields Boulevard overpass on 3 May 1999. (see slide 8 for more details).

Prev    Slide 15 of 25    Next