Before getting into the forecast, let me just say that Harvey remains an unpredictable system in terms of intensity before landfall, and especially its track after landfall. This creates considerable uncertainty in the forecast. What you’ll find below is our best attempt, but conditions will change, and so will the forecast. As always, we’ll remain on top of it as best we can—because like you, we’re making critical decisions about travel, our families, and our property.

Harvey has improved its satellite appearance markedly this morning, and a reconnaissance aircraft found the storm’s central pressure had dropped considerably. The system now has 65-mph winds, and as it passes over warm water there is little to slow down intensification of the system prior to landfall. Because the storm has gotten its act together, and still has about 36 hours over water, the National Hurricane Center has amped up its wind speed forecast considerably. Harvey is now anticipated to come ashore with 115-mph winds, just over the threshold of Category-3, or major hurricane status. Here’s the 10am CT track update from the hurricane center:

Winds

Now that Harvey appears to be rapidly strengthening, we need to carefully consider the wind impacts of the storm. Winds are always highest on the right side of a hurricane, where the counter-clockwise rotation drives wind and storm surge inland for maximum effect. Therefore the location of the storm’s landfall is critical. Right now the hurricane center forecast predicts a landfall between Port Mansfield and just north of Matagorda. Such a landfall location translates to the following probabilities for sustained, tropical storm-force winds:

For the greater Houston area, this translates to about a 70 percent chance of TS-force winds, 39 mph or greater. This probability has increased due to the strengthening of Harvey, and it would increase further if Harvey takes a more northerly track toward Matagorda. The current probability of hurricane-force winds in Houston and Galveston is presently less than 20 percent, so I’m hopeful we won’t see anything too extreme.

As for timing, we can expect these winds to arrive late Friday night or Saturday morning, but certainly both days will probably be gusty for the region. After Harvey moves inland on Saturday morning, the winds should slacken some over Houston.

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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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