Parts of the Texas Gulf Coast are experiencing their heaviest rainfall event since Hurricane Harvey in 2017, with a dangerous deluge dumping rainfall at rates approaching 4 to 6 inches per hour in spots. The rains are from slow-moving thunderstorms associated with tropical depression Imelda, which made landfall Tuesday as a weak tropical storm.
Wednesday marks day two of a three-day drenching that has the Houston and Galveston metro areas squarely in its sights, the latest in several seemingly routine hundred-year rain events to plague the region in recent years. Flash-flood watches blanket the map, and the Weather Prediction Center in Maryland has declared the region to be at “high risk” for excessive rainfall.
Through daybreak Wednesday, Houston had dodged most of the heaviest rain bands, while Galveston — the coastal city 45 miles to the southeast — has been clobbered by repetitive rounds of squalls all morning - with rainfall totals topping 8 inches.
Radar-estimated rain totals of 13 to 15 inches had been observed along the Matagorda coast near Freeport, where Imelda — the super-soaked depression spurring the inundations — made landfall Tuesday afternoon.
By now, the center of Imelda’s remnant circulation has moved into the northwest Houston suburbs, but the system is tapping into moisture from the Gulf of Mexico and directing it over land in the form of towering showers and thunderstorms.
With little storm motion expected within the next 24 hours, Houston could start to see some of the persistent, heavy rain bands on Wednesday. A widespread 2 to 5 inches, with localized 7-inch amounts, is possible within the city and northwest, though that could change, particularly in southeastern parts of the city.
Radar on Wednesday morning showed the northern edge of the heavy precipitation shield backing into Houston. Moderate to heavy rainfall is likely in the city of 2.3 million by noon.
From just east of Matagorda Bay, 60 miles southwest of Galveston, up through Port Arthur, 90 miles east of Houston, a band of 10 mile-high thunderstorms continues to drop rainfall at ridiculous rates, briefly spiking to nearly half a foot per hour. A gauge within that band in Bay City recorded 3.88 inches of rain in one hour.
Doppler radar estimates a total of nearly two feet over the San Bernard National Wildlife Refuge. Within the band, a general 10 to 18 inches, with isolated 2-foot amounts, can’t be ruled out. The most likely spot for this would be near the Brazoria/Matagorda County line.
A 20-inch total was reported near Sargent, Tex., Wednesday morning.
This one particularly heavy band of thunderstorms exists along a sharp gradient of instability along the immediate coastline. That focuses the storms, allowing them to ride along like rail cars on a train track, which meteorologists refer to as “training.”
At the same time, extremely efficient moisture transport in the atmosphere is, in essence, pumping in a conveyor belt of fuel. It’s like someone handing you a saturated sponge, you wringing it into a bucket, and then being handed another sponge.
And another. And another.
Eventually that bucket’s going to overflow.
Data suggests this band will largely remain the focus of the heaviest rainfall from this system through Wednesday afternoon, drifting slightly east throughout the course of the day. That may shift the focus away from the Lake Jackson area to the Palacios corridor in Texas this evening, while heightening the flood risk on the east side of the band near the Texas/Louisiana region.
If this scenario comes to pass, the Beaumont and Port Arthur region may become a hot spot for flooding Wednesday afternoon into early evening.
It then looks as though any holes in the radar now over Houston may start to fill in overnight into Thursday morning. Although Houston may not end up with the highest rainfall totals, dangerous flooding is possible if any rain bands target the vulnerable, concrete-blanketed city.
Some models indicate the possibility of this occurring. The flood risk for Houston will increase Wednesday afternoon and remain into early Thursday morning.
By Thursday, the remnants of Imelda’s circulation are likely to spin to the northeast. This could focus the heaviest rain in Port Arthur, Beaumont and Lumberton, with virtually all computer models showing the axis of heaviest rain sitting there with next to no movement Thursday.
On Friday, the system will pull away from the coast. Removed from its fuel source and conveyor belt of moisture, it will start to dissipate — but not before drenching places like Tyler, in East Texas; Shreveport, in northwest Louisiana; and Texarkana, at the Texas-Arkansas state line. The bulk of the heavy rain will largely have wound down by the time the weekend rolls around, with only remnant showers and isolated downpours over extreme eastern Oklahoma and Arkansas.