Eric Berger

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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It now appears less likely that a tropical blob in the southern Gulf of Mexico will develop into a tropical depression, but for our purposes that doesn’t really matter—a large amount of moisture is headed up the Texas coast and this will bring heavy rainfall to parts of the area beginning as early as tonight through Thursday morning. High tides of 2 to 3 feet above normal are a secondary concern for coastal areas, but the biggest threat from this system is high hourly rainfall rates.

As with most tropical events in the region, this one is difficult to forecast. With such a moist atmosphere, there is the potential for a lot of rain, but with this system the heaviest rains could remain offshore. In issuing a flash flood watch for Houston, the National Weather Service is predicting widespread totals of 2 to 4 inches, with isolated areas seeing 6 inches or more (most likely somewhere near Galveston Bay). This seems about right. Due to the potential for high hourly rainfall rates, which can quickly flood streets, we are predicting this will be a Stage 2 flood event on the Space City Weather flood scale for Houston. This essentially means that while we anticipate the potential for significant street flooding—turn around, don’t drown—we do not think there will be much structure flooding.

In terms of timing, there are some regional models that show heavy storms moving into Houston as early as 9 or 10pm tonight, but others delay the heavier rain until after sunrise on Wednesday. Basically, I think the simplest, most accurate forecast is to say the potential for heavy rainfall will begin after sunset on Tuesday night, and last through Thursday morning.

If there’s good news, it’s that the upper-level pattern supports pulling this very moist air mass away from the area on Thursday afternoon or evening, and replacing it with drier air—so this wet pattern is unlikely to linger. We’ll stay on top of this through the entire event.

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Houston’s hot weather—the region got its first real taste of the mid-90s this weekend—will continue for one more day before moisture begins streaming into the area from the south. From late Tuesday through Thursday, a tropical disturbance moving up the coast will bring a healthy chance of rain to the region. While some heavy rain is likely, we don’t have any major flood concerns at this time. The National Hurricane Center gives the system a 60 percent chance of becoming a tropical depression or storm, and the ceiling on intensity is almost certainly a low-end tropical storm. More on this below.

Monday

Conditions today will be a lot like Sunday, with partly to mostly sunny skies, and high temperatures in the low to mid-90s. Sunscreen, water, and repeat. Lows Monday night will only fall into the upper-70s.

Tuesday

Skies will turn partly cloudy as moisture begins to move into the area from the south. Rain chances should remain fairly low—20 to 30 percent —for most of the day before the tropical tropical disturbance begins pulling north toward the region. Timing is a bit tricky, but the heavier rain should hold off through the daytime hours on Tuesday. Highs will be around 90 degrees during the daytime.

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