The weather system previously known as Tropical Storm Harvey has begun to reorganize over the Gulf of Mexico and is expected to slam into Texas’s coast Friday, unloading dangerous amounts of rain.
The National Hurricane Center said in its 11 a.m. update Wednesday that Harvey reattained tropical depression status and is expecting it to strengthen into a tropical storm before the day ends. The storm is over very warm water (86 to 88 degrees) in the Gulf of Mexico, which is a very favorable ingredient for intensification.
The Hurricane Center predicts Harvey will reach the Texas coast Friday afternoon as a strong tropical storm, but it could almost just as easily intensify into a hurricane.
A hurricane watch covers a large section of the coast of Texas, including Corpus Christi. Tropical storm watches cover areas just to the south and north, including the Houston and Galveston areas.
On Wednesday morning, the storm was about 400 miles southeast of the Texas-Mexico border, moving to the northwest at 9 mph.
By far, the hazard of greatest concern with this system is its rainfall. While the center of the storm is expected to reach the coast Friday afternoon, heavy rain is likely to begin in the morning. Because of weak atmospheric steering currents, computer models indicate Harvey will stall over the Texas-Louisiana area through most of the weekend, at least, dispensing potentially incredible amounts of rain.
“Harvey is expected to produce total rain accumulations of 10 to 15 inches with isolated maximum amounts of 20 inches over the middle and upper Texas coast and southwest Louisiana through next Tuesday,” the Hurricane Center said.
Some computer model forecasts suggest the storm could linger over Texas through early next week, producing astronomical rainfall amounts between 30 and 50 inches in areas. Experience suggests this kind of forecast may be somewhat overdone and all models do not predict such an extreme scenario, but all available information points to a very serious, if not historic, rainfall event.
— Ryan Maue (@RyanMaue) August 23, 2017
Even if more conservative rainfall projections play out, they would result in areas of life-threatening flooding from Corpus Christi through Houston and as far east as New Orleans and Mobile, Ala.
In parts of this region, the soil is especially moist because of higher-than-normal rainfall in August, which increases the flood risk.
— Jonathan Erdman (@wxjerdman) August 23, 2017
The most recent series of model runs generally bring the storm’s intensity up to a borderline Category 1 hurricane (on the Saffir-Simpson scale of 1 to 5), while the consensus is a strong tropical storm.
Even if it does not become a hurricane, the danger posed by such extreme rainfall must be taken seriously. Or, if it becomes a hurricane and is rated only a Category 1 or 2, it’s important to remember the phrase “there’s more to the story than the category” with storms like this.
Many people tend to focus too much on the “category” label or where the storm will make landfall, but those factors are not relevant when considering the risks posed by heavy rain and flooding — which can occur in weaker systems and far away from the storm center.
History shows that tropical depressions, tropical storms and low-end hurricanes are capable of causing widespread devastation. Recall Agnes in 1972, Allison in 2001, and Irene in 2011, for example.
Because of Harvey’s potential, and the short amount of time before it makes landfall, NOAA, NASA and the Air Force are flying aircraft into and around the system Wednesday to get a better handle on its intensity, structure and environment. Data collected will go to forecasters and be fed into computer models.
While the hazard of greatest concern is rainfall, the storm may produce a dangerous storm surge, or rise in ocean water near the coast when it comes ashore — which would flood normally dry land. The Hurricane Center has issued a storm surge watch along a large section of the Texas coast, including Houston and Galveston, for the potential rise in water of 4 to 6 feet above the ground, if the storm arrives at high tide.
— NHC_Surge (@NHC_Surge) August 23, 2017
Finally, strong tropical-storm-force winds are likely when the storm makes landfall, and hurricane-force winds are possible — raising the specter of downed trees, building damage and power outages.
Beyond the weekend into next week, computer models forecast the remnants of Harvey to track across the Southeast and possibly toward the Mid-Atlantic, expanding the area that may be affected by its heavy rainfall. But storm specifics, such as its timing, track and rainfall amounts, are uncertain right now.