Month: September 2020

4 pm CT Tuesday update: We’re starting to get a better handle on the forecast for the rest of today, tonight, and Wednesday when it comes to the potential for heavy rainfall. The good news is that we’re reasonably confident that this mess will begin to clear out of here by Wednesday morning, if not sooner for some areas. The bad news is that we’ve got to get through later today and tonight.

Beta synopsis

Beta is now a depression, with sustained winds of 30 mph. In its 4 pm CT advisory on the system, the National Hurricane Center notes that its speed to the northeast has increased a little bit today, from about 2 mph to 5 mph. This speed should increase further over the next 24 hours. The storm’s “center” will pass across the Houston area on Wednesday, probably before noon. Its passage should be imperceptible.

Tuesday evening and Wednesday morning

As anticipated, the radar cleared out over much—although not all—of the Houston metro area earlier this afternoon providing a reprieve for southern regions after more than a full day of rain. Aside from Clear Creek on the southern side of Houston, most bayous are within banks at this time. The Clear Creek watershed, by contrast received some of the heaviest rains and some home flooding in Pearland and Friendswood has been reported today. Based upon current levels, it looks like flooding along Clear Creek can be classified as a 10-year flood, although that will have to wait for a post-storm analysis.

As the remains of Beta move northeast, we anticipate the storm’s circulation will draw additional moisture in from the Gulf of Mexico, and form new rain bands. These will likely produce training over the region (at 4pm CT we’re seeing this from downtown Houston up Highway 290). Most model guidance suggests 2 to 6 inches of rain over these areas, but the National Weather Service has indicated a risk of some areas seeing 10 inch bullseyes.

At this time it appears the heaviest rains tonight will probably miss hard-hit areas in Fort Bend, Brazoria, and Galveston counties—they may go further north. However, it’s going to be close. For example, and take this with a grain of salt, here is a 12-hour rainfall forecast from the HRRR model showing accumulations from 4pm Tuesday through 4am Wednesday.

Beta is a messy storm and unpredictable, so this is just one forecast and far from gospel. Still, this model (and others) indicate the heaviest rains during the next 12 hours will probably stay north, and current radar trends support the idea that high rainfall rates will be most prominent across north, northeastern Harris County and into Liberty County.

Were such rain totals to come further south into Harris County they would pose a big risk to areas like Meyerland and Westbury, which have recorded in excess of 10 inches of rain over the last 24 hours. However, we have seen some good falls in Brays Bayou today which are continuing. So a few inches, which is our current expectation, are probably manageable, but 10 inches would be pretty bad.

As Beta continues to slog to the northeast, most model guidance shows the precipitation exiting the western half of the metro area by around midnight, or the wee hours of Wednesday morning; and exiting Harris County by around sunrise or shortly thereafter. By the middle of Wednesday morning the focus of heavier rainfall will probably shift toward the Beaumont area. So it looks like we have about 12 to 18 more hours to battle with Beta. Is anyone else sick of this storm?

Our next update will come by or before 8 pm CT Tuesday.

 

11:15am CT Tuesday: For much of the last 24 hours a rain band of at-times broken storms has tracked upward along the Interstate 45 corridor and then more or less curved left, following the South Sam Houston Tollway. This has dropped about a foot of rain on Pearland and Missouri City, and nearly as much in other communities adjacent to the Gulf Freeway and on the south side of Houston. Some bayous are at bankfull, and others have overtopped their banks. Roads are a mess.

Over the last couple of hours this band, and the strongest inflow of moisture from the Gulf of Mexico has shifted somewhat east, coming across Galveston Bay and into Baytown. These areas have been relatively untouched by Beta’s heaviest rains over the last two days, and can generally handle the rainfall.

As a result of this shift, the hardest hit communities south and southwest of Houston will see more sporadic, less intense showers as we head into the late morning and, presumably, early afternoon hours. The question is what happens then? Our high resolution forecast models have not performed great, but right now there is a general consensus that this trend will hold. While the greater Houston area will remain at risk for heavy rainfall for the rest of today, by this evening or Wednesday morning the bulk of the activity should shift east of Interstate 45.

So cautiously, I would say the worst is probably over for areas southwest of Houston, including Sugar Land. That may become increasingly true for Pearland and hard-hit communities along the Gulf Freeway as well later today and tonight. Whether that means those areas will pick up another 1 or 3 inches of rain is not clear. This is all subject to change, of course, but at least it does seem consistent with radar trends and the overall setup of the atmosphere.

Our next post will come no later than 4pm CT Monday.

8:10 am CT Tuesday: Effective immediately, we are elevating our Flood Scale alert to Stage 3 for parts of the Houston metro area along, and south of Interstate 10. This is where the heaviest rain should again fall today and perhaps tonight as Tropical Storm Beta continues to pump moisture into the region. For areas further inland, we are maintaining a Stage 1 flood alert.

Why did we elevate the risk? For one, Beta produced more rains on Monday and Monday night than we generally anticipated, up to 10 to 12 inches in some locations. While this is not a catastrophic storm yet, it has produced widespread street flooding in central and southern parts of the Houston area. Many roads, including much of Highway 288, are impassable at this time. Residents are strongly encouraged to stay home today. Finally, many bayous in hard hit areas are at or near bankfull, meaning that we could see some home flooding today if heavy rains continue. The best web site to track the water level of your nearest bayou or creek is Harris County’s Flood Warning System.

One of Beta’s strongest bands has again established itself along roughly the Interstate 45 corridor this morning, and this is dragging showers capable of producing 1 to 1.5 inches of rainfall an hour into the city, affecting some already hard-hit areas. This band appears to be lifting slowly north, and behind it there is an area largely free of rain. It’s possible this area of more stable air may move into Houston, providing a bit of a reprieve later this morning. But I’m making no promises on that.

Right now, I would generally expect 2 to 4 inches of additional rainfall across the Houston metro area today, but we cannot rule out higher bullseyes.

Our next update will come no later than 11 am CT.