Extreme Precipitation Statistics for APL

The data in the table below express extreme precipitation amounts corresponding to "average return periods" for 15-minute, 30-minute, 1-hour, 2-hour, 3-hour, 6-hour, 12-hour, 18-hour, 24-hour, and 48-hour precipitation totals. Based upon statistical analyses of historical precipitation data, it is estimated that precipitation events as large or larger than the magnitudes shown in the table will be separated, on average, by the number of years given by the return period. It is important to note that over the course of centuries, the average of the separation times between pairs of these events should be close to the specified return period. Thus, the proverbial "100-year storm event" might not occur in a given century but could occur more than once in another century. On average, however, it will occur approximately once every 100 years over many centuries.

As an example on how to read the table, suppose APL receives 3 inches of rain in a 1-hour period. How frequently is that expected to occur in a statistical sense? Moving down the 1-hour column (rainfall duration), we find a value of 3.0 inches corresponds to a row value (return period) of 25 years. Thus, one would expect that a 3-inch rainfall in a 1-hour period would occur at APL, on average, only about once every 25 years. If the same amount of rain fell instead in a 30-minute period, the frequency of this event occurs, on average, slightly more than once every 50 years.

These statistics were derived from the following reference: Wilks, D.S. and R.P. Cember, Atlas of Precipitation Extremes for the Northeastern United States and Southeastern Canada, Publication No. RR 93-5, Northeastern Regional Climate Center, Cornell University, Ithaca, New York, 40 pp., September 1993.



        Extreme Precipitation Statistics for APL

        (Rainfall Amounts are in Inches)

15 minutes
30 minutes
1 hour 
2 hours
3 hours
6 hours
12 hours
18 hours
24 hours
48 hours
2 years
0.9
1.2
1.5
1.8
2.0
2.3
2.7
2.9
3.1
3.6
5 years
1.1
1.5
1.9
2.4
2.6
3.1
3.5
3.8
4.1
4.6
10 years
1.4
1.9
2.3
2.9
3.2
3.7
4.3
4.7
5.0
5.6
25 years
1.8
2.4
3.0
3.8
4.1
4.8
5.6
6.0
6.4
7.4
50 years
2.2
2.9
3.7
4.6
5.0
6.0
6.9
7.4
7.9
8.8
100 years
2.6
3.6
4.5
5.6
6.1
7.2
8.3
9.0
9.6
10.7







Questions or comments should be sent to R.Giannola