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What is a 100 year storm you ask?

What is a 100-year storm? Well, a 100-year storm/rainfall does not necessarily mean that it is a storm that occurs every 100 years. The term “100-year storm” is in reference to a rainfall event that has a one percent (one in one hundred) chance of occurring in a year. Different locations have an average/typical amount of rainfall in a year; when that is exceeded, the amount of time that the rainfall occurred, location, and the total amount of rain is calculated to determine the frequency in which the rain has fallen.

While it could be considered “rare” for these storms to happen, seeing as there is a one percent chance in any given year, this term is simply a means to express probability; this means that while unlikely, an area could experience a 100-year storm more than once in a year, or not at all . In fact, the probability of a 500-year storm occurring is 0.2 percent, but in 2011 Philadelphia alone had two 500-year storms that occurred a couple of weeks apart.

More recently, we have begun to see more large storm events, with the Philadelphia Gage showing an increase in the Intensity Duration Frequency (IDF) curves, as shown below.

In the Northeast United States, heavy rainfall has been increasing, with an increase in these “100-year” storms. In the past 67 years the number of storms has doubled, so much so that a 100-year 24/hour storm is equal to a thirty year storm, which has a higher chance of occurring at three percent in any given year. These storms increase the risks of extreme flooding, which was seen in 2011 . Global climate models have shown that annual rainfall is likely to increase, including extreme rainfall .

When the soil has absorbed all of the moisture that it can, excess water begins to pool, causing a flood. While every 100-year storm does not guarantee extreme flooding, it is likely that there will be some flash flooding. Stormwater runoff can cause damage including stream bank erosion, bank undercutting, channel widening, a decrease in the quality of water in streams, and increased sedimentation and deposition.

Flooding from these storms can cause a lot of money in damages, though it is not guaranteed to occur. While rain levels may be equivalent to a 100-year storm, this does not mean that flooding and damages will occur, this depends on soil quality, drainage systems, and spaces for stormwater runoff to collect.

Check out the resources below for more information on 100-year storms!


  1. Maimone M, Smith CDM. Projecting Rainfall Intensity and Storm Event Frequency: Applications for Urban Stormwater Planning in Southeastern Pennsylvania. Published September 20, 2019.

  2. Pennsylvania Stormwater Best Management Practices Manual: Section 2 Stormwater and Pennsylvania's Natural Systems. Published 2005.

  3. Saenger S. Hydrology 101: The 100-Year Rainfall Explained. Published July 13, 2018.

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