A bad situation has turned worse. Earlier tonight we discussed two major concerns for the rest of the night—that a primary rain band moving north-northeast across Houston would stall, and that a second rain band moving toward west Houston would strengthen. Both subsequently happened, creating a super-mega-rainball of doom that that stretched across Houston from Tomball and The Woodlands down to Texas City. And as of 12:45am CT on Sunday, it shows no signs of abating.

Among the extreme rainfall data we have already seen tonight is that 9.92 inches of rain fell within three hours over portions of Southeast Harris County, and 6.60 inches fell within a single hour along Clear Creek in Friendswood. According to the Harris County Flood Control District, both of these totals are 500-year flood events.

Another data point that is concerning in the extreme: The Houston-Galveston office of National Weather Service has issued three Flash Flood Emergencies tonight for the Houston region. It had only ever issued one before in the five years or so that the forecast product has existed.

At some point, this must end. But it does not appear likely to do so anytime soon. It is not clear to me whether the homes flooded by Harvey in the greater Houston area will tally in the thousands, tens of thousands, or hundreds of thousands, but it will be many. This community will have to come together after another devastating flood that seems likely to approach the same magnitude as that of our previous reference event, Tropical Storm Allison. That is two historic floods in less than two decades. Hopefully, we will learn from this. Certainly, we will rebuild.

Our general thinking at Space City Weather has been that tonight, and Sunday morning, would be the worst of the flooding Harvey will throw at us. We had hoped to get through the next 12 to 24 hours with minor wounds, but that will not be the case. Now, future rainfall from Harvey over subsequent days, although unlikely to approach the magnitude of tonight’s terror, will only worsen an already deep wound.

Key messages

• This is an extremely dangerous flooding situation for the Houston metro area
• Stay in your location
• The heavy rains from Harvey are likely to continue into Sunday morning, worsening an already borderline catastrophic situation

Posted by Eric at 12:45am CT on Sunday

A major rain band has slammed Houston this evening beginning on the west and southwest side, progressing through central Houston, and now it is approaching a line from The Woodlands to Baytown to League City. It has produced tropical-storm-esque rainfall rates of 4 inches per hour (and higher), which has backed up bayous. The National Weather Service issued a rare flash flood emergency, indicative of the dire state of affairs.

As a result of this, Houston on the cusp of a major, widespread flood event of that could affect thousands of homes. Only the fact that this main band has progressed slowly to the north-northeast has saved the region from greater flooding. So far, this has mostly been a street flooding issue. (Mostly). And while some bayous are teetering on the edge or overtopping their banks, for the most part the region’s bayou systems are holding. So the critical question is, what comes next? Is this it? Or will one or two more bands fire up before Sunday morning and push through Houston.

So, as of 10:30pm CT, here is what we are worrying about.

1. Will further rain bands from Harvey exacerbate a bad situation? Another rain band pushing into Houston from the west within the next several hours, of similar intensity to the first band, (Band 1 in the graphic below) would likely push bayous out of their banks, and turn a major street flooding problem into a major structure flooding problem.

2. Will Band 1 slow down as it moves toward a line from, say, Humble to Seabrook, or along the Gulf Freeway? There are some meteorological reasons to think it might, and this could potentially become a devastating event for the east side of Houston. A nearly stationary line of showers producing 4 inches of rain per hour does not take long to flood structures.

3. What comes next? Tropical Storm Harvey may be weakening, but its rainmaking potential isn’t going anywhere. The latest forecast track from the National Hurricane Center suggests this. We might hope that some of its ability to produce such massive, powerful rainbands diminishes over the next couple of days. But still, it’s going to be there in some form or fashion. Alas, we’ll have to worry about this one tomorrow.

We will continue updating tonight as warranted. Or as long as the beer here at Space City Weather HQ holds out.

Posted by Eric at 10:45pm CT on Saturday

Earlier, Matt called attention to the potential for very heavy rainfall tonight from Tropical Storm Harvey, which is drifting east-northeast between San Antonio and Houston. Unfortunately this is exactly what is happening in the Houston metro area. Now is the time to get off the roads, get to your residence, and wait out a potentially long night of flooding.

Before jumping to the forecast, we’ve had some questions about why Harvey can maintain its strength and rainfall generating potential a full 24 hours after making landfall along the Texas coast. The following satellite photo, taken by NOAA’s GOES-16 satellite at sunset tonight, tells the story. We have a vigorous low-pressure center inland, but it remains near enough to the Gulf of Mexico to pull copious amounts of moisture into the state.

Rainfall rates

Beginning at about 7:00 pm a large feeder band began moving into the Houston area, and it has brought an impressive amount of rainfall in a short period of time. Some areas near Waller, northwest of Houston, and Stafford, southwest of Houston, have seen rainfall rates of 4 inches per hour during the last hour. That is about twice the carrying capacity of most bayous in the Houston area, and we have started to see flooding along some nearby bayous. We are already seeing deep street flooding in parts of western central Harris County, near Interstate 10 and the Sam Houston Tollway.

Here’s a look at the rainfall band’s position at around 9pm tonight. You will note the main feeder band extends over the Gulf of Mexico, from which it is pulling moisture into Houston. Additionally, a second rain band near El Campo and Bay City appears to be strengthening, and will move through Houston later this evening.

High resolution models (which are run frequently) continue to show high rainfall totals tonight and into Sunday morning, with widespread accumulations of 3 to 6 inches, and some locations receiving in excess of 10 inches. On top of the rainfall, we continue to see indications of tornadoes on the radar. As Matt suggested, now would be a good time to make sure you have your mobile device emergency alerts notification set to “On” tonight.

We remain concerned about the potential for flooding in the Houston region tonight, and will do our best to keep you apprised of the situation.

Posted by Eric at 9:10pm CT on Saturday