As expected, Tropical Depression Harvey became a tropical storm overnight, and forecasters believe it will reach hurricane strength before striking Texas late Friday or early Saturday. As Harvey trudges through the Gulf of Mexico, forecasters have been dancing around the "A" word — Allison. As in Tropical Storm Allison, the 2001 storm that dumped more than three feet of rain on Houston and caused some of the worst flooding in the city's history.
Harvey remained more than 400 miles off the coast of Texas as of Thursday morning, and its path is far from certain. The most aggressive forecasts, which call for more than 20 inches of rain to fall in isolated areas, fall short of Allison's 38-inch output. But Harvey bears more than a few similarities to Allison.
“I think Allison was a rather remarkable storm, so I would be very hesitant to say this will be another Allison,” said meteorologist Eric Berger, who runs Houston weather blog Space City Weather. “But worst case scenario, it is a possibility.”
Allison, the first named storm of the 2001 Atlantic hurricane season, made landfall at Freeport before moving up the coast. After moving north through Houston, Allison was infamously stopped by a high pressure system. The storm reversed course, traveling south back to the city. Over six days, 38.6 inches of rain fell on Houston. Most of the city's bayous overran their banks, flooding 73,000 homes and leaving many residents without power for days. Twenty-two people in the Houston area died in the storm.
Harvey is projected to make landfall at Matagorda before following a similar path as Allison up the coast. It is slow moving and has a similar wind speed to Allison. Crucially, a high pressure system is sitting over Texas and threatens to stop Harvey, a low pressure system, in its tracks. The National Weather Service predicts Harvey will slow down as it makes landfall and then drag along the coast, dumping rain while drawing more fuel from the very warm gulf waters.
“You've got a situation in place where all the ingredients are there,” Berger explained. “While a storm might otherwise be pulled northeast after 12 or 13 hours of heavy rainfall, in this case, you could get two to three days of rain.”
In that scenario, Houston could see substantial flooding. The National Weather Service is urging Texas residents near the coast to prepare for 10 to 15 inches of rain across the region, with isolated areas receiving as many as 20 inches, as well as a potential storm surge of 4 to 6 feet. Forecasters believe Harvey is the most dangerous storm Houston has faced this year.
To put Harvey's potential in perspective, northwest Houston received between 6 and 8 inches of rain earlier this month when the Buffalo, Greens and White Oak bayous overran their banks. During the devastating Tax Day Floods in 2016, areas of Harris County received around eight inches of rain, with isolated areas receiving up to 16 inches.
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The forecast for Harvey on Thursday morning was more aggressive that Wednesday'a predictions. Meteorologists at the National Hurricane Center believe Harvey will likely become a hurricane before making landfall, and Governor Greg Abbott has issued a disaster declaration for 30 coastal counties. The Houston area remains is now under a Tropical Storm Warning while coastal counties to the south are under a Hurricane Warning. A Storm Surge Warning extends from Port Mansfield to San Luis Pass, while a Storm Surge Watch is in effect south of Port Mansfield to the Rio Grande and north of San Luis Pass to High Island.
In Houston, Mayor Sylvester Turner said the city is placing barricades near flood-prone streets and underpasses so they may be closed quickly when flooding arrives.
We will have continuous coverage of Harvey as it approaches Texas.