Thursday afternoon: Hurricane Harvey rapidly strengthens, nearing Texas

We are now about 36 hours from the landfall of Hurricane Harvey, which is taking advantage of 87-degree waters in the Gulf of Mexico to intensify today, and may continue to do so right up until landfall late Friday night or Saturday morning. The rapid intensification we’ve seen today (as of 1pm Harvey had sustained winds of 85 mph) is the reason why meteorologists are so watchful of tropical lows in the Gulf during August and September—in the absence of wind shear, the bath water out there is highly favorable for storms to strengthen.

Harvey has transformed from a tropical depression into an impressive hurricane in less than 24 hours. (NOAA)

Today’s strengthening, and increasingly bullish model forecasts for intensity, have caused the National Hurricane Center to increase its predicted wind speed for Harvey’s landfall to 125 mph, just shy of Category 4 hurricane intensity. Although intensity forecasts are imprecise, it seems safe to conclude that a powerful hurricane will be coming to the Texas coast soon.

With a stronger storm, we need to be more concerned about the landfall location. Confidence continues to increase in a landfall between Corpus Christi and Port O’Connor. This obviously will have big implications in terms of wind and storm surge for Corpus Christi (where there is a voluntary evacuation), and points north to Freeport. We will have more about the impacts to Texas in an update later this afternoon or early evening.

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Harvey nearly a hurricane: Updating the major threats to Texas

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:

Official forecast track for Harvey at 10am CT.

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:

(National Hurricane Center)

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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Likely a hurricane at landfall, Harvey approaches a wary Texas

Good morning. Tropical Storm Harvey has shown signs of better organization overnight, and now appears likely to become a Category 1 hurricane before landfall. However, given the warmth of the Gulf of Mexico and lowering wind shear levels, a stronger storm seems possible. As discussed Wednesday, models are increasingly focused on a landfall between Brownsville and Corpus Christi, and the National Hurricane Center has issued hurricane warnings for locations from Port Mansfield to Matagorda, Texas.

Tropical Storm Harvey forecast track. (National Hurricane Center)

This much of the forecast seems clear enough. Over the next two days, before Harvey makes landfall late on Friday or early Saturday, the storm will bring heavy rainfall, storm surge, and possible hurricane conditions to portions of the lower Texas coast. Through Saturday, some areas from South Padre Island through to Matagorda Bay, may receive a dozen inches of rain, or more. Barring a major change in forecast, locations further up the Texas coast, including Houston, will see seas rise 3 to 5 feet above normal levels, gusty winds, and perhaps 2 to 4 inches of rain through Saturday.

The key question for the upper Texas coast, including the Houston region, is what comes next.

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As Harvey approaches, here’s how to prepare for inland flooding

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.

Rainfall totals from TS Allison (NOAA)

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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