National Weather Service United States Department of Commerce

Northern Tier Snow and Heavy Rain in the South; Blizzard Conditions in Alaska

A storm system originating from the Pacific Northwest will spread snow from the Upper Midwest to the Northeast into Tuesday. Locally heavy rain will be possible Tuesday in the Deep South. In Alaska, a storm will rapidly strengthen over the Bering Sea Monday where it will sweep strong winds and heavy snow across the state through midweek. Localized Blizzard conditions are forecast around Bethel. 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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