With Tropical Depression Harvey moving toward the Texas coast, our largest concern for the region is not the wind or the potential storm surge. Instead, Harvey’s greatest impact will come from inland flooding. Current forecasts predict anywhere from 10 inches to more than 20 inches of rain during this event for parts of Texas, which may well include Houston. As the storm approaches, you should be aware of what inland flooding is, and how to prepare for it.

What is inland flooding?

When a tropical system makes landfall, it begins to deteriorate and collapse without the energy from the warm ocean to feed it. The moisture within the storm precipitates out, often in torrential rainstorms that can last for days.

Additionally, if the system moves slowly or stalls, heavy tropical downpours can lead to flood conditions hundreds of miles inland, far from the point of landfall. Tropical systems hold millions of TONS of water, and all that water falls as rain, eventually. This leads to major flooding, something Houston is all too familiar with. With inland flooding from tropical storms, it can mean—in a few extreme cases—getting 30 inches of rain, or half a year’s worth, in three or four days.

Keep in mind, wind speed of a tropical system is not an indicator of how much flooding may occur. Tropical Storm Allison never became a hurricane, and was barely even a tropical storm by wind speeds, but because it stalled over our region Houston experienced days of these heavy tropical rainstorms.

The figure above also shows the difficulty in predicting what areas will flood the most. In Harris County alone, rainfall totals ranged from less than 5 inches to nearly 40 inches during Allison. Any small change in the storm’s path or intensity is the difference between flooding or light precipitation. And to be clear, we are not saying Harvey will be another Allison, only that there is potential for severe inland flooding from Harvey.

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As of 2:30pm CT, Harvey remains a depression in the southwestern Gulf of Mexico. We remain most concerned about the potential of this tropical system to drop a prodigious amount of rain from Friday through Tuesday or Wednesday of next week, primarily due to the fact that it will likely stall, or nearly so, once it moves into Texas this weekend. Let’s jump into the forecast.

Harvey track and intensity

The depression remains fairly poorly organized this afternoon, and that’s a good thing, because once it begins to develop a tightly organized circulation, there will be little to inhibit from strengthening further. The official forecast calls for a 75-mph hurricane to make landfall along the Texas coast Friday afternoon. It’s worth noting that the models (including the European forecast system) have shifted a bit to the left, meaning that a landfall between Brownsville and Corpus Christi now seems most probable. That may be good for the Houston region in terms of storm surge, but as we’ll see, it may matter little in terms of rainfall.

Rainfall, and timing

Please note that the following is speculative, but we’re attempting to affix a time frame to this storm and its effects because we’ve had a lot of travel questions. The following forecast assumes a Friday landfall along the south Texas coast, and therefore the forecast is subject to change. You have been warned.

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Just a quick post to note the official redevelopment of Harvey as a tropical depression in the southern Gulf of Mexico. The forecast track mirrors what we’ve been discussing for a couple of days now—the movement of a tropical storm into the Texas coast, after which its slows down, and drenches the eastern part of the state.

The official forecast predicts Harvey will reach hurricane strength—barely a Category 1 storm with 75mph—before it reaches the Texas coast. Tropical Storm watches are in effect for Galveston and the Galveston Bay area. Hurricane watches are in effect for Brazoria County, and areas southwest of Houston. But the primary concern with this system remains widespread rainfall and inland flooding. The hurricane center’s rain forecast for Harvey reflects this:

Our forecast from this morning largely holds up, as it is based on similar model data that made up the National Hurricane Center’s outlook. We will post a more comprehensive update this afternoon, around 2:30pm. In the meantime, here is what Matt and I feel you should bear in mind about the next several days.

• The rainfall hype is real and serious, and although it’s impossible to specifically forecast it at this time, you need to be aware of the threat.
• The forecast is going change, and in systems like this trying to pin down who gets maximum rainfall is extraordinarily difficult. Patience required.
• The rains will be spread out from Friday through Tuesday, but for Houston we continue to expect the heavier rain during the latter half of that period.