During the monsoon months of June, July, August and September, we receive an average of 2.71 inches of rain at Phoenix Sky Harbor. That is nearly one third of our yearly rainfall total.
Monsoon rain comes so fast that our dry desert soil can’t take all of it in and it runs off causing flash flooding. Streets, washes and riverbeds all fill up very fast and sometimes without much warning.
Our wettest monsoon occurred in 1984, when we picked up 9.38 inches of rain. The driest monsoon was in 1924 when we saw only .35 inches of rainfall.
Sometimes, our monsoon storms can also bring hail.
If the air is cold enough, water droplets in the clouds can begin to freeze and collect on each other. When they begin sticking together, giant pieces of ice form.
These hail stones get bigger and bigger as they get carried up and down through the cloud until they eventually fall to the ground.
As they fall, the air temperature warms them up causing the ice to melt.
If the ice melts completely, it comes down as rain. If not, we get hail.
Typically, during the monsoon, the air is so warm that most hail melts before it reaches the ground.
The two key elements of flash flooding are rainfall intensity and how long the rain lasts. Topography, soil conditions and ground cover play critical roles, as well.
Because our land is so dry, there is a hard crust over it which makes it difficult for the ground to absorb moisture. When water cannot seep into the ground, it runs off and collects in our low lying roads and washes leading to flooding.
The Federal Emergency Management Agency, FEMA, compiles detailed mapping of areas that have "a one percent chance to be equaled or exceeded in any year," which they deem the 100-year floodplains map. In other words, this map depicts areas that see flooding and have a chance for excessive flooding.
Take a look at the FEMA floodplain map below. The red lines comprise the currently defined floodplains areas in the Valley.