As Tropical Depression Bill swirls its way through northern Texas this afternoon, the storm looks more impressive over land than it ever did over the ocean. The storm is still producing very heavy rainfall, and flash flooding is likely as it continues sloshing towards Oklahoma and eventually the Midwest.

Tropical Depression Bill is still maintaining its strength even though it’s hundreds of miles inland now. This morning’s advisory from the Weather Prediction Center* shows that Bill has 35 MPH winds as it churns near Dallas, Texas. The agency expects it to maintain tropical depression status through Friday morning, and there’s a slight chance—not great, but not exactly zero, either—that the storm could briefly restrengthen into a tropical storm over the next couple of hours.

The phenomenon at hand has been the talk of science and weather blogs for the past couple of days now. Dr. Marshall Shepherd—you may know him from WXGeeks on The Weather Channel—was part of a team that researched the “brown ocean effect” that could have an effect on Tropical Depression Bill’s organization and maintenance of strength so far inland. In a moisture-rich environment with saturated soils, storms can maintain their strength or even strengthen over land, as the soggy soil mimics the favorable environment found over the ocean. It wouldn’t be the first time this has happened—the most famous case was Tropical Storm Erin in 2007, which gained strength and even developed an eye as it moved over the Oklahoma City metro area. (The animated radar loop of Erin is shown above.)

This region of the country has dominated weather headlines over the past couple of months due to the extreme rainfall and destructive flooding that’s resulted from a conga line of supercells parking themselves over the region day after day. The waterlogged soil and tropical moisture in place over Texas seems to be keeping Bill alive and well, as much as a tropical system could be a few hundred miles inland, anyway.

Here’s the radar from 3:10 PM CDT today, which doesn’t show an image as impressive as Erin eight years before it, but it’s not too shabby, all things considered. Doppler radar still indicates a closed, tight center of circulation almost smack dab over Fort Worth, Texas, with a large rain shield entering southern Oklahoma with rainbands of various intensity radiating from the general circulation. Radar measurements show that winds just above the surface are between 40-50 MPH just a few hundred feet above the surface, which would put Bill at tropical storm strength if they managed to translate to the surface.

Why does this matter? Rain! And lots of it.

Bill produced a widespread area of six or more inches of rain around Victoria when it made landfall, and this swath of hefty totals continues north-northwest towards Dallas, eventually filling in through Oklahoma and around the heat ridge in the southeast. Bill’s continued strength/structure matters because it will allow heavy rain to continue unabated along its path until the storm finally begins to fall apart and all that’s left is residual moisture from the system.

Thankfully, the worst of the rainfall stayed offshore. A nasty train of supercells associated with Tropical Storm Bill set up shop just off the coast from Corpus Christi yesterday, lasting for more than twelve hours as they dumped more than twenty (two-zero!) inches of rain over the open waters. If not just for the flooding disaster they averted, the fact that these storms occurred over open water is a saving grace to countless people today. These supercells likely produced numerous tornadic waterspouts over the course of the event, which could have been a devastating tornado outbreak in the Corpus Christi area if it had occurred on land a few dozen miles to the west.

The storm—or what’s left of it in a day or two—will continue moving up and around the ridge of high pressure that’s locked over the southeastern United States, which is making millions of people sweat in record high temperatures. Even if the rain shield as we see it now dissipates, the deep, tropical moisture Bill contributed to the atmosphere will continue riding around the ridge, allowing any thunderstorms that develop to tap into that sweet, sweet fuel and produce extremely heavy rainfall.

As for timing, the heaviest rain should be in the Tulsa/Joplin/Fayetteville area by tomorrow evening, near southern Illinois by Friday evening, and in the Ohio Valley by Saturday evening. Flash flood watches are in effect for just about everyone expecting more than an inch or two of rain, as heavy rain over a short period of time will inundated already-saturated soils and storm sewers, leading to rapidly rising waters. Pay attention to warnings and weather radar as you go about your day, and make sure you have a plan in place in case your home, business, or roads along your travels are flooded.

Do not drive through a flooded roadway. You may think you can make it, but you won’t. Even if you could, assume you won’t. It takes just a couple of inches of water to sweep you off of your feet, and a little more than that to sweep away even a large vehicle. Not only do you risk your life by driving through high waters, but you risk the lives of those who have to rescue you or recover your dead body. It’s not worth it.

*The National Hurricane Center handles advisories on a tropical system until it makes landfall. After landfall, forecasting responsibility shift to the Weather Prediction Center, which issues advisories on the system until it dissipates.

[Satellite: NASA | Maps: author]

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