Monsoon Flash Flooding - A Force of Nature

Water is both beautiful and destructive. Gentle rains bring tranquility, drought conditions evolve by a long-term absence of precipitation, and a sudden downpour has the power to reshape the landscape. The climate of the southwestern United States inherently possesses a dichotomous tension between too little and too much water. In other words, the hydrology for this part of the world is quite sensitive to the weather happening at both macro and micro spatial scales occurring over wide temporal ranges. This hydrologic sensitivity keeps meteorologists, hydrologists, and others associated with flood preparedness and water resource conservation on their toes, especially during the summer North American Monsoon.

A mature monsoon environment brings excessive subtropical moisture loading in the atmosphere favorable for torrential rainfall rates exceeding 1 to 3 inches per hour at times. Such significant bursts of rainfall, even if hyper-localized, may lead to rapid and destructive runoff along normally dry washes or urban interfaces. The caveat is that many living in the region may not directly experience these isolated heavy rain events during the monsoon, but that doesn’t mean you shouldn’t be aware of the hazard as flash flooding effects can be experienced many miles downstream of where the heavier rain actually occurred! Water basins are often complex covering hundreds of square miles, meaning it can capture a single thunderstorm’s rainfall at one end of the basin and transfer the excess runoff to another portion of the basin in a relatively quick manner, too.

There are a few key overlapping geographic considerations magnifying the monsoon flash flooding risk in the Desert Southwest region of the United States. First, higher terrain tends to be focal points for thunderstorm initiation due to the “chimney effect” discussed in a previous article on why monsoon thunderstorms favor the mountains. Runoff from heavy rain occurring over steeper mountains and foothill elevations are assisted by complex terrain funneling and the force of gravity. The effect is exacerbated should nearly impermeable wildfire burn scar surfaces be present. Additionally, there are sprawling urbanized metropolitans having surfaces that don’t readily soak in heavy rainfall (asphalt, roofs, concrete, etc.). Furthermore, over the course of a monsoon, areal coverage of saturated soils gradually increase to yield faster and more efficient runoff with any additional thunderstorm rainfall.

Always remember the familiar National Weather Service's flood safety message, “turn around don’t drown”! It only takes about 6 inches of swift water to knock down an adult and around 12-18 inches to float a vehicle. During flash flooding (day or night), rapidly flowing water is often turbulent and dirty making it difficult to both estimate water depth and determine if any hidden debris was caught in the flowing water, such as large tree branches and boulders. 

To emphasize the potential power of flash flooding let's put some numbers behind flowing water. A common measurement of water discharge flow rate in streams and rivers is quantified in units of cubic feet per second or "cfs". For perspective, the volume of four basketballs is close to a cubic foot; however, a cubic foot of water would weigh about 60 pounds! Since flows can exceed well over 1,000 cfs during flash flooding events (visualize if you can 4,000 basketballs weighing collectively 60,000 pounds passing by per second), it quickly becomes apparent how destructive flowing water could be. For example, that familiar roadway crossing now under water you are debating to drive through could have significant damage or may actually no longer exist! Bottom line, don’t ever feel pressured to cross a flooded roadway. Find an alternative route or postpone travel. It's simply not worth putting yourself or others in danger, not to mention the imposed risk to emergency responders attempting a water rescue.

-JWM

Photo: Jonny William Malloy