More than six inches of rain are in the forecast for the Houston region through early next week, prompting concerns about flooding. Unlike the catastrophic flash flooding that occurred week after week in parts of the Mid-Atlantic and Southeast in May, which was triggered by a constant deluge and supersaturated soil, Houston’s problem is the exact opposite. The ground in Southeast Texas is too dry.
Initially, the near-drought conditions could promote flash flooding as rain runs off the ground. If thunderstorms come in hot and heavy with torrential rain, flash flooding would be possible early in the multiday event.
At some point, as rain continues, the ground will be able to absorb more water and head off the risk of flooding for a while. However, rain is expected over the course of three or four days, with the heaviest downpours expected later in the event. At that point, the soil in the Houston metro area could be saturated to the point that it cannot absorb more, and the flash-flooding risk would increase.
Forecast models are predicting six or seven inches of rain in the Houston area through Tuesday morning. That amount will possibly not be widespread, but it’s possible that isolated strong thunderstorms could dump even more rain than the upper threshold of the forecast.
The National Weather Service predicts a general four to five inches of rainfall through early next week.
The source of all this moisture will be the Gulf of Mexico, where low pressure will strengthen over the weekend. Even though the National Hurricane Center does not expect the system to develop into a tropical storm, it will still generate significant rain.
The amount of moisture in the air will be near-record for June, according to Matt Lanza at spacecityweather.com. “In other words,” Lanza wrote Wednesday morning, “about as much moisture as the atmosphere can hold this time of year.”
Lanza says that because the ground is starting dry, and the bayous and creeks are low, the rain will be absorbed initially. “But,” he said, “if we do see repeated rounds of storms Saturday, Sunday, and Monday, we may start to see the ground become saturated and flash flooding will become more of an issue in some spots.”
The Houston region — and much of Texas — has been very dry. Rainfall in Houston itself is running more than four inches below average for April, May and June. A moderate drought has taken hold of parts of South Texas, though the forecast rain should mitigate those conditions.
This heavy rain event threatens Houston less than a year after Hurricane Harvey unloaded up to several feet of rain on the region, the most extreme rain event in the city’s history. Lanza said this will not turn into a repeat. “I know people here are on edge … but this is not Harvey,” he said. “Events like the one we’ll likely see here Sunday through Tuesday, I would spitball, happen once every year or two.”