Welcome to the Rainfall Predictor
What is the average annual rainfall at my house?
We plotted your address on an annual rainfall map and from our extrapolation (estimation) your average annual rainfall is expected to be 35.7 inches per year.
How do we know?
We used the Mean Annual Precipitation and Precipitation Depth Duration Data (Rantz 1971) as our source and we extrapolated the annual rainfall from the isohyetal (or lines of constant rainfall amounts) maps of Marin County. Figure 1 is a map of annual average rainfall amounts for Marin.
What is the 100-year 24-hour event for my address?
8.52 inches in 24 hours
What is the 100year 1 hour event for my address?
1.48 inches in 1 hour
How do we know?
There isnt a rain gauge near your town, so we used published data of nearby stations to estimate these events so we used the State of California, Department of Water Resources, Summary of Short-Duration Precipitation Frequency in the San Francisco Bay Area, the 100 year 1 hour and 24 hour events for Kentfield and extrapolated for Greenbrae (with the factor that the rainfall at Greenbrae is 77 percent of of the rainfall at Kentfield).
What exactly is a 100-year or a 1000-year event?
100 year events are examples of statistical jargon that engineers and meteorologists use to describe rainfall intensity and to help design storm water management programs.
When all of the 24 hour totals of rainfall were analyzed statistically for locations in the Bay Area, the results were plotted, and the likelihood (chance) of certain amounts of rainfall were calculated for different time periods (hour, day, year). A 100-year 24-hour event (also called a return period) means that there is a 1/100 chance that this amount of rainfall can occur over a 24-hour period during a season.
The idea of a return period is somewhat misleading Just because there is a 1/100 chance of an event in a season does not mean that if it happens this season that youd have to wait another 100 years for the event to occur again. Next season you start over and youd have a 1/100 chance again. (until the engineers and meteorologists recalculate the statistics!!).
Figure 1 Isohyetal Map