By now, we’ve all been inundated (pardon the pun) with model graphics such as the one Eric shared earlier this morning:
As he mentioned, it is indeed a realistic portrayal of the rain our area could get over the next week. Twenty-plus inches of precipitation in such a short period of time is daunting, especially when you consider Houston averages around 50 inches a year.
We’ve had a number of questions about how the area’s bayous will manage the rain. Surely, they could handle four inches day, for five days, right? Unfortunately, for tropical systems like Hurricane Harvey, it isn’t the amount of rainfall that becomes a problem, as much as the rainfall rate. What is rainfall rate, and how will it and other factors influence how bad our flooding will be this week?
What is rainfall rate?
Rainfall rate describes how much rain falls over a period of time, and is measured in inches of rain per hour. A rainfall rate of 0.5″ per hour is considered heavy, while anything above 2.0″ per hour is intense. For context, Harris County experienced a maximum rate of 4.7″ per hour during the 2016 Tax Day Floods in a few isolated locations. It’s the difference between your sink faucet dripping for a week (a low rate), and your faucet breaking off for a half hour (a high rate). The same amount of water may come out of your pipes, but one will flood your kitchen much faster than the other.
Ditches and storm drains in Houston and Harris County can generally handle a rate of 1″ to 2″ per hour, for a few hours. Anything higher than that, over a longer period of time, leads to excessive runoff that can flood roads and fill the bayous faster than they can handle it.
Rainfall rates and tropical storms
Intense rainfall rates naturally come with tropical systems, due to the tons and tons of moisture they contain. In 2008, Hurricane Ike reached a maximum 1-hour rate of 4.1″, and a 12-hour maximum rate of 12.7″. At times in June 2001, Tropical Storm Allison’s rainfall rates varied between 4″ up to 6″ per hour.
However, tropical storms do not dump rain evenly over an area. Areas that get hit by more of a hurricane’s rain bands, or are in the path of the eyewall, will receive greater rainfall rates for longer periods of time. Moreover, rainfall rates are more intense on the right side of hurricanes–and Houston is forecast to remain on the right side of Harvey for several days. You can clearly see the rainbands and areas of heavier precipitation on today’s radar loop for Harvey, as it approaches the coast:
During a hurricane, even after landfall, the thunderstorms within it constantly develop, pour down rain, dissipate, and redevelop. The storm is constantly pulling moisture, and fuel, for rain from the warm Gulf waters. Additionally, unlike a small thunderstorm with an intense rainfall rate (something that causes localized flooding all the time here), tropical storms are hundreds of miles wide, pulsating with different waves of intensity that last for days and days. All of these factors can create a perfect storm (again, sorry for the pun), and explains why someone in downtown Houston can get 5 to 10 more inches of rain from a tropical system than their cousin over in Katy. Distribution of rainfall in storms such as these can vary wildly over a small area.
What does it mean?
Let’s look at that forecast for 20+” of rain again, but now in the context of rainfall rates. If 20+” of rain falls gradually, over the course of the next week, flooding will definitely occur—but it would be manageable.
However, we are in the path of a large tropical system, bursting with moisture and primed to dump rain at very high rainfall rates. Therefore, rainfall that overwhelms drainage systems and bayous seem likely. If you are interested in monitoring Harvey’s rainfall and impact on Harris County’s water channels, follow live data from the Harris County Flood Control District here.
Tropical systems can be erratic and unpredictable in their rainfall intensity. This is why, when you see feet of rain forecast for a storm such as this, it’s best to plan for heavy rains and extended periods of flooding. If your area is prone to flooding after storms with high rainfall rates, it’s likely to happen again this weekend.
We will be back with a comprehensive forecast for Harvey and Texas by around 2:30 or 3pm CT.
Posted at 1:15pm CT on Friday by Braniff