We got through most of last night without any issues in Houston. Storms lined up well southwest of Houston near Matagorda Bay after midnight and have arrived across most of the metro area early this morning.

Radar as of 5:40 AM shows moderate to heavy rainfall across most of the area (with the exception of Freeport and north of The Woodlands). But everyone should see at least some rain through the morning.

As the day progresses, this rain will taper off or become more scattered in nature generally north and west of US-59/I-69. But heavy rain should continue into this afternoon generally south and east of there. A Flash Flood Watch remains hoisted for most of the area. We also have most areas south and east of US-59 in a “high risk” for flash flooding from the Weather Prediction Center today.

The WPC doesn’t hand out high risks easily, so that speaks to this system’s potential. A quick reminder from us: Please use caution when out and about today. It would be good to limit travel when you can on days like this, but if you are commuting or have to be heading out somewhere, be very careful and never drive through flooded roadways.

Total rainfall through this evening is expected to be on the order of 1 to 3 inches north and west of US-59. Some folks, particularly up in far northern regions (Conroe, Navasota, Cleveland to suggest a few) may see a bit less. The trouble spots are expected south and east of US-59. We expect 3 to 6 inches on average south and east of there, but some places will see higher amounts on the order of 6 to 10 inches or even a bit more.

There are still some uncertainties with how this will exactly play out, but in general this will work for today as a forecast. This outcome would likely cause a good deal of street flooding in parts of southeast Harris, Galveston, Brazoria, and Chambers Counties. Bayou or creek flooding is also possible, especially along Clear Creek or perhaps some of the bayous in southeast Harris County.

The bottom line today: Flash flooding, particularly street flooding is likely in areas south and east of downtown Houston at times today, especially through early afternoon. Some more serious street or even creek and bayou flooding is also possible in a few places, depending on exactly how much rain falls and how quickly it does. Rain should taper off a bit, or at least become more infrequent toward mid-afternoon.

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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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