National Weather Service United States Department of Commerce

Heavy Rainfall Moves into the Mid-Atlantic and Northeast

A slow moving cold front will gradually progress through the Mid-Atlantic and Northeast through Friday with possible severe weather and heavy rainfall which may bring flash flooding. The trailing end of the cold front will bring daily rounds of strong thunderstorms capable of producing locally heavy downpours over South Florida. 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).

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