Eric Berger

Over the last couple of hours, the radar has shown the heaviest rainfall congealing offshore, and this seems to be in response to falling pressures as the tropical disturbance moves north across the Gulf of Mexico. The net effect of this is that the offshore storms are robbing inland storms of moisture, and thus sapping their ability to produce really high rainfall rates like we saw earlier this morning near Wharton. This is good news for the rest of today, although we’re still going to see some scattered storms, and areas immediately along the coast, such as Galveston, may see intermittent heavy rainfall this afternoon.

Most of the region has seen manageable rains today, and rainfall rates below 2 inches per hour. But we definitely saw the bullseyes we feared develop in Wharton, Fort Bend, and Matagorda counties this morning. One gauge north of Palacios recorded 14+ inches of rain, and a pretty large region saw 6+ inches since midnight. These totals are shown below.

The forecast for Wednesday evening through Friday remains tricky. Our best guess is that storms begin to redevelop onshore tonight, but the favored area is probably just east of Houston through the Beaumont area, and into southwestern Louisiana. In Houston, on Thursday, we will probably see less coverage than Wednesday—perhaps quite a bit less. We still have some concerns about later Thursday night and Friday, and we’ll address those in posts to come. My sense, however, is that the worst of this system, in terms of organized activity, is now slowly moving away from the metro area.

A large mass of storms continues to work through the Houston metro area this morning, but the rains have been manageable for the most part, with accumulations of 1-2 inches. However, we do have some significant concerns about areas to the southwest of Houston.

A particularly strong band of showers has established itself in Wharton and Fort Bend counties this morning. One location on the Colorado River, south of Wharton, briefly recorded rainfall rates in excess of 4 inches per hour, and has measured nearly 10 inches of rainfall since shortly before sunrise. A flash flood warning is in effect for this area through 9:45 a.m., and we anticipate these storms will slowly move east, toward Brazoria, southern Harris, and Galveston counties this morning. So if you live in the southern half of the metro area, pay close attention to the radar for the next several hours.

Most of the high-resolution modeling suggests this very moist air mass will shift mostly offshore by this afternoon, perhaps by around 2pm to 4pm. This should then lead to a reprieve in widespread storms at least through this evening.

The forecast for Thursday and Friday is not particularly clear, but we do not expect quite the organized storms that we are seeing across much of the Houston metro area this morning to persist. Nevertheless, heavy rain will remain a threat heading into the weekend. We will try to firm up this forecast in a new post later today.

Storms that fired up on Tuesday afternoon have died down this evening, in part due to the loss of daytime heating. Unfortunately, this is just a lull, as the tropical disturbance that will make for wet conditions through Thursday morning remains in the Gulf, roughly centered near the border between Texas and Mexico. This pre-sunset satellite image captures the size of the tropical system, and amount of moisture it can potentially bring into Texas and Louisiana as it moves north-northeast. Fortunately, it appears the heaviest rains will probably remain offshore, but we’re still definitely in watch-and-wait mode.

Our best tools to forecast rain totals over the next 24 to 36 hours, high-resolution models, have not done a tremendously good job so far today, and they’re not initializing well. This means that even 1 or 2 hours into an individual model’s run, it already is pretty far off what is actually happening on radar. So, to some extent, we’re flying blind in terms of forecasting.

Still, with high atmospheric moisture levels (precipitable water values will rise above 2.2 inches over the next day or so, which is a prerequisite for high hourly rainfall rates) and low pressure at the surface we’re all but guaranteed heavy rainfall for at least part the metro area. After the present lull, it appears as though rain showers will build back up sometime after midnight offshore, and begin to migrate inland again. It’s quite possible that Wednesday morning’s commute will be a soggy one, and with the potential for flooded streets please check road conditions before heading out. We’ll have an update before sunrise.

NOAA rainfall total estimates for now through Friday evening. (Pivotal Weather)In terms of storm totals, as mentioned, the forecast models are all over the place, but areas around Galveston Bay, including southeast Harris County, probably are at the most risk for heavy rainfall that will quickly flood streets. We’re still looking at likely rainfall totals of 2 to 4 inches for most areas, with higher localized amounts of 6 inches or greater. The region’s bayous are in pretty good shape and should be able to handle most of that. It still appears as though the heaviest rains will clear out of the area by or before noon on Thursday.

I want to preface this post by saying that this is a low-confidence forecast, overall. What we can say with some confidence is that heavy rains are moving toward Houston, and in some cases already moving inland between Freeport and Galveston. This will set the stage for a wet period in Houston from Tuesday evening through Thursday morning. A flash flood watch is in effect, and we are predicting a Stage 2 flood threat for the area. What we are not so confident about are the details—such as where will the heaviest rainfall occur, and will the big bullseyes of rainfall be 6 inches, 10 inches, or more? Let’s take it day by day.

Tuesday evening through Wednesday afternoon

We anticipate that this initial round of showers moving into the coast this evening probably will reach about to Interstate 10 before winding down, probably by or before midnight. After this, models are indicating that a more organized band of storm activity will move into the coast between midnight and sunrise Wednesday. This could occur anywhere between Matagorda Bay and Galveston Island, although at this point I’d probably lean toward the southwest, and Matagorda/Brazoria county area. These storms may very well be efficient rain producers, with rainfall rates of 2-, 3- or more inches per hour, which will quickly back up streets.

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