Archive of past heavy precipitation events

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Analysis

Note: This page is a work in progress and new images and information (including updates) will be added in the near future!

Maps of all points exceeding the different thresholds, colored by month (click on an image to enlarge)

These maps illustrate the distribution of heavy rainfall events across the country. At the 24-hour timescale, events are distributed fairly evenly across the country, which we should expect if the return frequencies are correct. There are numerous "swaths" of points that represent individual extreme events, such as the remnants of Tropical Storm Nicole in the northeast in 2010 (the many purple points in the northeast); the flood in Nashville and surrounding areas in 2010; and the Minnesota and Wisconsin heavy rainfall from 2004 and 2007. At the 6-hour timescale, there are fewer of these "swaths", and at the 1-hour timescale most of the points are fairly isolated.

Monthly distributions of points exceeding the different thresholds, 2002-2011. (click on an image to enlarge)

These show when extreme rainfall tends to occur throughout the year. In general, it occurs most often in the summer (when there is more moisture available in the atmosphere), and least often in the winter. This analysis shows that points exceeded the 24-hour threshold most often in September during this period, but the maximum for 6-hourly and 1-hourly points fell in the summer.

Hourly distributions of points exceeding the different thresholds, 2002-2011. (click on an image to enlarge)

For the hourly data, these graphs show the hours when extreme rain occurred most often. There is a maximum in the overnight hours, peaking between 0300-0500 UTC (10 pm to 1 am Central daylight time), and a minimum in the morning (0700-1700 UTC).