by William Perry and Jan Jackson
For the Blacksburg Hydrologic Service Area, flooding can occur at anytime of year. However, looking at the more historic events, those which produced the highest river levels at our forecast points, the fall through early spring (September-March), is the normal time frame. In this article we will be discussing historical river floods, and not localized flash flooding.
First, let us take a look at why the period of September through March, normally sees more flooding in our area of responsibility. Well the two main factors leading to flooding are rainfall intensity and rainfall duration. This can occur anytime of year. However, in the warm season, April-August, as the jet stream heads further north, the more organized fronts and storm systems stay to our north as well. We get thunderstorms which can produce heavy amounts of rain, but thunderstorms are small and do not cover a large area, such as the river basins. Whereas, from fall through early spring, the jet stream dips further south bringing more frontal systems to the area with organized areas of rainfall. Also during the fall, we can occasionally get remnants of tropical systems, which can bring a great amount of rain. These factors are important, but other parameters can lead to more flooding especially in the winter. These include:
The image below has all our river forecast points. To view historical crests, move the cursor over the site.
NOTE: Some historical crests, especially at Altavista, Brookneal, and Wilkesboro occurred before a dam was built