It’s possible that the most damaging tropical system to strike the United States in 2019 won’t have a name.
Although Hurricane Dorian brought widespread devastation to the Bahamas, it did relatively modest damage to the southeastern United States. According to some insurer’s estimates, Dorian caused between $500 million and $1.5 billion in insured losses due to property damage and business-interruption claims.
This week an unnamed low pressure system—it only has a 30 percent chance to develop into a tropical depression or storm before moving inland into the upper Texas coast, the National Hurricane Center says—will bring a surge of tropical moisture into the state of Texas. Although it is not possible to say where the heaviest rainfall will occur, the Houston metro area falls into the highest risk category.
Forecast models have been all over the place, but it’s possible that widespread areas will see five to 10 inches of rain, with isolated totals of 15 inches or more as the system moves slowly north. This is manageable for most of Houston over two or three days, but the problem with extremely moist tropical air such as this is its capability to generate intense hourly rainfall rates.
One measure of atmospheric moisture is known as “precipitable water,” and this system will likely bring values of 2.4 to 2.6 inches into the Texas coast. This means that, in a column of air from the planet’s surface to outer space, if one were to squeeze out all of the moisture, it would measure more than 2 inches high. Such levels are nearly 200 percent of normal levels for September in Texas.
This high amount of atmospheric moisture will lead to high hourly rainfall rates. Typically, the roadways of Houston are designed to flood during heavy rainfall, carrying water away from yards, into bayous, and out toward the Gulf of Mexico. However, their capacity is about 1 to 2 inches of rain per hour. During tropical events such as this, rainfall rates of three, four, or even five inches per hour are possible.
This flooding threat comes only a little more than two years after Hurricane Harvey ravaged the upper Texas coast, including Houston, with widespread areas receiving 40 inches or more of rainfall. This tropical system will not approach Harvey in terms of scope or rainfall intensity, but it nonetheless has a flood-weary region on edge.
In 2001, a tropical storm barely worthy of the name (Allison)—so weak were its winds—pushed into the Texas coast and dropped 15 to 25 inches over much of the Houston area. It caused $5 billion in damages, and it remains the costliest tropical storm on record for the United States. Since then, considerable development has occurred in the region. Some has been responsible, and other subdivisions have been thrown up in or near floodplains, so the region is more vulnerable than ever to intense rainfall events.