Month: June 2024

In brief: We’re continuing to track the potential for heavy rainfall in the greater Houston area due to a tropical system in the southern Gulf of Mexico. Whether or not it becomes Tropical Storm Alberto, the effects will be the same for Houston. We now think the period of Tuesday afternoon through the middle of Wednesday will have the highest chance for heavy rainfall.

On Monday afternoon the National Hurricane Center said it would begin issuing forecasts for a “potential tropical cyclone” in the southern Gulf of Mexico. This is the tropical system that we’ve been talking about for several days now. This action indicates that hurricane center forecasters believe the system is likely to become a tropical depression or storm (it would be named Alberto) in the next day, but it has yet to do so.

In truth, this changes virtually nothing about the forecast for elevated easterly winds, higher seas, and heavy rainfall that Houston will experience over the next couple of days. The purpose of this post is to attempt to set some expectations in terms of timing of the worst of the rainfall. Tropical weather is inherently unpredictable, but now that we’re closing in on Tuesday and Wednesday, our confidence is a little bit higher. Emphasis on a little.

Monday evening and overnight

As expected we’ve seen scattered showers and a few thunderstorms today, and generally I expect this pattern to continue this evening and overnight. These storms should pose no threats for flooding risks beyond putting a damper on outdoor activities.

Tuesday

Generally, I expect Tuesday morning will not bring anything too crazy in terms of sustained heavy rain. However, by around noon or shortly after that it appears a deeper slug of tropical moisture will move inland into the upper Texas coast. This is when we expect the development of more widespread showers, especially along and south of Interstate 10. The heaviest rain is likely to fall closer to the coast, because that is the source of the moisture. It is too early to say whether heavy rainfall on Tuesday afternoon and evening will impact commute home, but it’s a distinct possibility.

Tuesday night and Wednesday

Our modeling continues to suggest that the heaviest rainfall will arrive later on Tuesday, and persist through Tuesday night through much of the day on Wednesday. By Wednesday evening or night, it does appear that the the worst of the rains will be exiting to our west.

I’m hesitant to speak about accumulations with too much certainty. There continues to be a lot of noise in the models about what to expect. There are some scenarios in which the bulk of the heavy rainfall goes south of Houston (and Galveston) on Tuesday and Wednesday. In this case the region (even coastal regions) might pick up only a handful of inches of rainfall—totally manageable.

However, I would rate as slightly more likely the possibility that the southern half of the Houston region picks up 4 to 8 inches of rainfall through Wednesday, with higher isolated amounts. This, obviously, would create some flooding issues and is the reason why we’ve instituted (and maintained) a Stage 2 flood alert for the entire region through Wednesday.

The thing with tropical rain is that it comes in bunches. And with this system, we have the very real possibility that at least some portions of our area will see sustained tropical rainfall during the next 48 hours. So please be prepared for that possibility.

Our next update will be posted early on Tuesday morning.

In brief: Our pattern is changing this week with arrival of tropical moisture. This will lead to heavy rainfall over the Houston area, especially on Tuesday and Wednesday. However, our concerns for Houston are lower today with much of our model guidances now projecting the most extreme rainfall further south, near the Coastal Bend and areas such as Corpus Christi.

Tropical moisture arriving

A surge of tropical moisture will move into Texas this week. However, based upon trends in the forecast over the last 24 hours, the threat of the heaviest rainfall has shifted south of the greater Houston region, including Galveston Island. This does not mean it is not going to rain in the Houston area this week. However, it does mean that our risk of seeing extreme rainfall totals and widespread flooding is lower.

For the time being we are going to maintain a Stage 2 flood alert for the Houston area through Wednesday given that the potential for high rainfall rates remains. Localized areas, especially near the coast, are still likely to see flooding. However, a Stage 2 event for Houston can now be considered a worst-case risk scenario.

Monday

A large area of low pressure in the far southwestern Gulf of Mexico is starting to push a surge of moisture toward Texas this morning, and we’ve already seen some streamer showers. Atmospheric moisture levels will rise today, and as a result this afternoon I anticipate seeing fairly widespread showers and thunderstorms. A few areas could quickly pick up a couple of inches of rain, but totals for most locations should be less. Rain chances will slacken a bit tonight. With partly to mostly cloudy skies, highs today will be in the upper 80s for most locations.

High seas, high winds

Beginning today we’ll also see increasing winds from the east due to the low pressure system. These will peak on Tuesday and Wednesday. While they won’t be too significant for much of the area—we are talking gusts of up to 25 to 30 mph—they could be higher along the coast, including Galveston Island. A coastal flood watch is also in effect for this week, with tides likely to run a couple of feet higher than normal, and a high risk of rip currents. Low-lying coastal areas may flood.

Tuesday and Wednesday

These days will see the highest threat for heavy rainfall, as tropical moisture levels peak in the atmosphere. As noted above, a majority of our modeling now indicates that the greatest potential for heavy rainfall—that is, the areas where we could see 12 inches or more of rainfall—are now south of Houston, in locations such as Matagorda Bay and Corpus Christi.

That does not mean that Houston is out of the woods, so to speak. The environment is still favorable for heavy rainfall this week, it’s just a bit less favorable. In terms of expectations, much of the area along and north of Interstate 10 is likely to see between 1 and 5 inches of rain this week, with areas south of the freeway and closer to the coast likely to see 3 to 6 inches. The risk, for coastal areas, is that localized areas could see higher amounts. That remains possible given the state of the atmosphere. And high rainfall rates can quickly backup streets.

Highs on Tuesday and the Juneteenth holiday are likely to be in the low- to mid-80s due to cloudy skies and the likelihood of rainfall.

Thursday

Rainfall remains possible on Thursday, even likely. But as moisture levels start to decrease, so too will the threat of heavy rainfall. Look for highs in the upper 80s with partly sunny skies.

Friday, Saturday, and Sunday

We should see the return of partly to mostly sunny skies for the weekend, with highs in the upper 80s to lower 90s. However, as high pressure is unlikely to be in total control, each day may well have a decent chance of afternoon shower and thunderstorm activity with daytime heating. We’ll see.

In brief: Unfortunately, we are celebrating Father’s Day by issuing a flood alert for the Houston area. There won’t be any issues today, or likely on Monday. But the period of Tuesday and Wednesday is likely to experience heavy rainfall that could lead to flooding. Some of this flooding, especially for areas south of Interstate 10, could be significant.

We are continuing to track the threat of surging tropical moisture this week, loosely associated with a tropical low over the Southern Gulf of Mexico. While there remain major details to parse, the threat of heavy rainfall in the greater Houston metro area is sufficient to put a Stage 2 flood alert into place for the entire region—including Harris and all surrounding counties.

A Stage 2 flood alert signifies the potential for flash flooding. Events falling into this category may cause significant, widespread street flooding across large swaths of the area, flooding numerous cars. Some limited home flooding is also possible, but this is not widespread during a Stage 2 event.

In terms of expectations for flooding this week, for areas along and north of Interstate 10, a Stage 2 flood is likely to be a worst case scenario. However, for areas south of Interstate 10, and especially coastal counties, we may have to escalate to a Stage 3 flood alert depending on how the forecast evolves over the next few days. We’ll be monitoring this closely.

What to expect this week

We have no concerns about weather conditions today. Skies will be partly to mostly sunny, with high temperatures in the low 90s, and only a few, isolated showers. Conditions will be similar tonight. Showers will become more widespread and numerous on Monday, but again rain totals should be manageable for most.

However, the period from Tuesday morning through Wednesday night will see a surge a tropical moisture, and bring the best rain chances into the region. With this kind of tropical moisture we can see high rainfall rates that quickly backup streets, and flood yards. For any plans you have on Tuesday, Tuesday night, and Wednesday, you’ll need to be weather aware. Rain will remain possible for Thursday onward, but the threat of heaviest rainfall should diminish after Wednesday.

In terms of accumulations, I think most areas north of Interstate 10 will see 2 to 6 inches of rainfall, with higher localized amounts. However for areas south of the freeway, and especially closer to the coast and near Galveston and Matagorda Bays, there is the potential for 6 to 12 inches of rain, with higher bullseyes. Some uncertainty remains. Our forecast modeling remains a bit split as to whether the heaviest rain will fall near Galveston, or further south closer to the Coastal Bend. However, the clear message is that we need to be prepared for the possibility of flooding in Houston. (If you’re wondering about impacts for the entire state of Texas, Matt has you covered on The Eyewall).

In addition to the rains, we’ll see the breeze picking up from the Gulf of Mexico on Tuesday, with onshore winds of 20 to 30 mph, and higher gusts. These winds will be strongest right along the coast. With these winds tides will push a couple of feet higher, potentially flooding low-lying coastal areas such as parts of Bolivar Peninsula.

I do want to be clear that there is a difference between tropical rains and the types of violent wind storms we saw earlier this spring. Straight-line winds and hail are unlikely with this kind of system. The primary threat for most of is going to be heavy rainfall and accompanying flooding.

We’ll have full coverage for you all week.

In brief: We’re continuing to follow the possibility of heavy rainfall next week in the greater Houston area, especially for coastal areas. We’re still far from having all of the details, but we know enough to take the threat seriously.

Weekend outlook

Happy Saturday, everyone. If sunshine and heat are your jam, you’re in luck this weekend. Today, especially, will bring brilliant blue skies and temperatures in the mid-90s. There may be a few clouds tomorrow, but I still expect mostly sunny skies on Sunday, with high temperatures in the low- to mid-90s. It’s possible we’ll see a few isolated showers and thunderstorms during the afternoon and evening hours on Sunday, but most of us will simply be sunny and warm.

And then it won’t be.

Next week

The overall pattern appears unsettled for most of next week, with a very healthy chance of showers each day, and the middle of the week especially concerning. A low pressure system over the southern Gulf of Mexico will help push a large plume of tropical moisture into Texas and Louisiana. This threat of heavy rainfall next week will materialize whether the low in the southern Gulf becomes a tropical depression or storm. In fact, it doesn’t really matter. The table is set, regardless.

Over the last 24 hours some of our guidance has been showing excessive amounts of rainfall along the Gulf coast, but whether these bullseyes occur over Matagorda Bay, Galveston Bay, Port Arthur, or Southern Louisiana is something we cannot say. In terms of timing, the greatest threat of heavy rainfall likely will come during a period from Tuesday evening through Thursday morning.

So what does this all mean? If you live inland of Interstate 10, the overall risk is lower. Most of these areas are likely to pick up 2 to 4 inches between Monday and Friday. However, areas along and south of Interstate 10 are likely to see 4 to 6 inches of rainfall next week. Our concern is that some localized areas may see 10 inches or more of rainfall next week given the tropical nature of this rainfall. The expected level of moisture in the atmosphere—a value known as precipitable water—is very high, and such levels are capable of producing high rainfall rates that can quickly flood streets. We don’t know for sure whether this kind of pattern will establish itself over the Houston area, but it is a distinct possibility.

I’m writing all of this on a Saturday morning not to scare you, but rather to prepare you for the possibility of heavy rainfall next week. We will continue to watch this closely, and update this weekend as warranted.