In brief: In this afternoon’s update we discuss the region’s diminishing heavy rainfall threat even though a Tropical Storm is likely for form off the Texas coast on Wednesday. We also go over what you can expect from that.

The Woodlands had a day

Rather than closer to the coast, the most persistent band of storms lined up along northern Harris and southern Montgomery counties on Tuesday. Some gauges along Spring Creek, including areas near Tomball and The Woodlands, picked up as much as 6 inches of rain in short order.

When we speak about highly variable rainfall totals with a few bullseyes, this is exactly what we mean. Much of Harris County received 0.5 inch or less on Thursday, but for a relatively small part of our region there were very heavy and persistent rains leading to significant street flooding. This is the textbook definition of a Stage 2 flood event in Houston. It’s pretty brutal if you’re sitting right under it, but for most of the rest of the metro area, things are just fine. Anyway, as of 4:30 pm CT the radar is largely quiet across the region.

Speaking of that Stage 2 flood alert, we are going to hold on to that overnight out of an abundance of caution. We’ll revisit things on Wednesday morning.

Tuesday night

Although some sporadic showers are possible during the overnight hours, the radar should remain fairly quiet into early Wednesday morning. So go about your business as usual.

Wednesday

Some time on Wednesday a low pressure system may well become a tropical storm as it traverses across the Western Gulf, near the Texas coast. Its center may briefly touch land from time to time, but overall the system should continue to track to the northeast. By Wednesday evening it should be inland for good, somewhere near Beaumont or Lake Charles.

Houston’s coastal areas (including Galveston Island) may see tropical storm force wind gusts on Wednesday morning, and these will probably peak around sunrise. In terms of storm surge, expect water levels to be 1 to 3 feet above normal tide levels. So this could cause some minor flooding for low-lying coastal areas.

So what about rains? Yes, we’re going to see the continued possibility of showers and thunderstorms, and the threat of heavy rainfall looms for tomorrow. But my expectations for Wednesday is that most of Houston probably sees 0 to 1 inch of rain, with some bullseyes of 3 inches possible for a few isolated areas, most likely near the coast. Needless to say this should be pretty manageable.

The rest of the week

We’ll have more detail tomorrow, but we are now reasonably confident that Thursday and Friday will be mostly sunny days with highs in the low 90s and fairly low rain chances (if not zero). I’m not particularly confident about Saturday’s forecast as there’s a chance of some storms back-dooring in from the northeast (think 1 inch of rain, or less). Sunday looks mostly sunny and rain free.

In brief: We expect widespread showers and thunderstorms today, particularly south of I-10, where flash flooding is possible. Our current thinking is that a tropical disturbance expected to track across the Western Gulf on Wednesday will remain far enough offshore to spare Houston from significant flooding, but we’re going to watch things closely.

Behold, the tropics

As of this morning a low pressure system located over South Texas is starting to move toward the Gulf. On Wednesday it should move to the northeast, just off the Texas coast, toward Louisiana. At this time it may have a brief period during which it can spin up into a tropical depression, or possibly a named storm (it would be called Arthur). The National Hurricane predicts there is a 60 percent chance this occurs. Although certainly there would be a lot of noise around a tropical storm forming in the Gulf so close to Texas, we’re here to tell you the impacts from this system are going to be similar regardless of its development in the coming days.

And there’s some good news in the forecast. Remember how we’ve been talking about the boom or bust potential later this week, on Wednesday and Thursday, due to heavy rains from this tropical system? As of Tuesday morning we are leaning toward bust. Put another way, after today and this evening, our rain chances will probably trend down somewhat. And we think the likelihood of very heavy rainfall (i.e. bullseyes of 10 inches or above on Wednesday and Thursday) in the Houston metro area is low. It’s not zero yet, but things are trending in the right direction. There will be some winds and tides to contend with, but nothing too significant. More on that below.

Tuesday

In terms of heavy rainfall, conditions will be most favorable today across the Houston region with rich tropical moisture spreading into the area. There is a thick band of heavy rainfall offshore the Texas coast this morning, spreading all the way from Corpus Christi to Port Arthur. The key word there is offshore. Some of this activity is spreading inland this morning, and we expect rains to move into the Houston area today. Due to the tropical nature of this airmass, rainfall rates could be quite high, quickly leading to street flooding (and hence, our Stage 2 flood alert).

Rain accumulations today should be highest along and to the south of Interstate 10. In these locations 2 to 4 inches of rain will be possible, with some isolated areas of 6 inches possible. Further north totals should be less. Due to the rain-cooled air mass temperatures today for most locations should be in the 80s, with lower 80s likely near the coast. Rain chances should slacken somewhat overnight, but there will still be pockets of activity.

Wednesday

The aforementioned tropical system should pass closest to Houston on Wednesday. At this time we think it will track southeast of our region, remaining off the coast. In this case it should keep the heaviest rains offshore. The major risk at this time is if the system tracks more to the north, and moves inland into the Upper Texas coast. This would mean more rain over the Houston metro area. To be clear, we don’t think this will happen, but we are watching for this possibility and will of course keep you informed.

Anyway, our best-guess forecast at this time for Wednesday is for about a 60 percent chance of rain showers, with better chances and higher accumulations closer to the coast. For now rain totals look fairly manageable, with 1 to 2 inches, but again this would change if the tropical system took a more northerly route.

We are also likely to see some coastal wind gusts of about 30 mph, starting early Wednesday morning. I’m not sure how far inland these will extend, but it’s possible they could reach areas such as Pearland or League City. (Again this is dependent on the tropical system’s track). Water levels will also be 1 to 3 feet above normal tide levels, so there could be some minor coastal flooding. High temperatures on Wednesday should be in the upper 80s.

Thursday and Friday

At this time we expect rain chances to step back on Thursday and Friday, likely back into the 40 percent range. These likely would be more of your typical summertime showers and thunderstorms rather than anything too serious in terms of flooding potential. Highs will likely reach the lower 90s with more sunshine.

Saturday, Sunday, and beyond

The weekend looks to be partly to mostly sunny, with high temperatures in the lower 90s. There probably is about a 40 percent chance of rain on Saturday (hard to have too much confidence one way or other here), with lower chances on Sunday. Most of next week will probably see highs in the lower 90s, with the potential for scattered showers but fairly low chances overall. Basically it should be a return to normal programming for June in Houston.

In brief: In this afternoon’s post we recap our rains so far and look ahead to the rest of the week. We also answer the question on everyone’s mind: Is this gonna be another Hurricane Harvey?

The situation so far

Our rains today have been well within the bounds of expectations, with most locations picking up 1 to 2 inches of rainfall. Some high water marks (pun intended) were located just northwest of central Houston, where a few spots near Inwood Forest picked up 4 inches. It has made for some messy driving, but overall there have been no major issues beyond inconveniences. Our radar is mostly quiet as of later Monday afternoon, and I’m hopeful it will remain so through the evening hours and into early Tuesday.

What happens on Tuesday?

My baseline expectations are that conditions will be similar on Tuesday. We’ll probably see increasing showers and thunderstorms during the morning hours and then build to a crescendo during the middle of the day, before waning during the afternoon or evening hours. I think most areas will see an additional 1 to 2 inches with some higher bullseyes. Our Stage 2 flood alert covers this period well. That’s if things behave according to expectations and, in this atmospheric environment, we’re just going to have to wait an see.

That blob in the Gulf is now orange

Yeah, we saw that too. The National Hurricane Center is now forecasting a 50 percent chance that a low pressure system will become a tropical depression or storm later this week after it moves back over the Gulf late Tuesday night or Wednesday morning. I would not focus too much on that because the impacts from this system (heavy rainfall, mostly) are going to be similar regardless of development. And it’s not like this is going to suddenly blow up into a hurricane.

Is this going to be another Hurricane Harvey?

No.

Umm, could you expand on that a little more?

Sure. Hurricane Harvey brought five days of very heavy rainfall to Houston. What we experienced today was not at all Harvey-like, and at this point I don’t expect that Tuesday will be as well. We’re seeing 2 inches of rain on the daily, not 10 inches.

With that said we do have some concerns about Wednesday and potentially Thursday. Some of our modeling is showing 20-inch bullseyes (in terms of accumulated rainfall through Thursday) and we cannot ignore this possibility. To be clear not all of the modeling data shows this. That’s because there is not great confidence yet in the evolution of the aforementioned tropical system as to where it will go, and specifically where its hardest rains will fall. The difference between the GFS model rainfall output at 11 am today (left) and 5 pm (right) highlights this inconsistency.

This is why we said this morning that we are monitoring trends closely for the second half of the week, and we may feel the need to escalate to a Stage 3 flood alert. But we’re not there yet. And it’s quite possible that the heavy rain misses the Houston metro area on Wednesday and Thursday, leaving us with more pedestrian-type showers. So we’re working hard to set expectations here: Yes, it could be really quite gnarly, or maybe it won’t be bad at all.

The bottom line is that this is not like Harvey when we knew some really bad stuff was coming, that it would last for several days, and that it would cover a large area. This is very unlikely to be that. Anyway, that’s what we know right now. I’m hoping to have some more clarity about the Wednesday-Thursday forecast in tomorrow morning’s post, which is when we’ll be back with you. As always, thanks for your trust and readership.

In brief: Houston will face the threat of heavy rainfall every day this week, but at the same time we also have some uncertainties. It is worth paying close attention to the weather in the coming days. There is a fair bit of nuance in the forecast, so please read carefully.

Assessing Houston’s flood risk

We have dual flood threats this week. The first one, up today and Tuesday, concerns a weak front that will move into the region and provide an impetus for showers and thunderstorms, with heavy rainfall a distinct threat.

Then, later on Wednesday and possibly lasting into Friday, a slow-moving tropical disturbance will bring a surge of moisture into the region and create conditions where very high rainfall rates and serious flooding are possible. However we simply do not know where the greatest rainfall totals from this tropical disturbance will end up.

We are maintaining our Stage 2 flood alert for now across the greater Houston region, but depending on the tropical disturbance we may need to escalate things for the second half of this week. Rest assured Matt and I are continuing to watch this evolving situation closely, and will update as warranted.

Monday and Tuesday

Around sunrise this morning we are seeing a large cluster of storms near College Station and Navasota, and this is being driven by a front (which unfortunately is not going to bring any drier or cooler air with it, this being June and all). Over the course of today this activity will probably move southward, into the Houston region as the weak front stalls over the metro area.

We expect a fairly stormy pattern to persist over the region through Tuesday. That is not to say it will rain all the time, everywhere, but the atmosphere will be very fertile for showers and thunderstorms. We generally expect accumulations of 2 to 4 inches of rainfall during this time period, but you should be prepared for high rainfall rates and local bullseyes of 6 or more inches. This is the kind of rainfall that leads to flash flooding, and hence our call for a Stage 2 flood alert. You will want to be weather aware. Things may calm down a bit by Tuesday evening.

In terms of temperatures, the clouds and rain-cooled air should limit high temperatures to the low- to mid-80s. Conversely, with a very warm and humid air mass in place, overnight lows will not drop too far below 80 degrees. Although we cannot entirely rule out some severe thunderstorms, the primary threat is heavy rainfall.

Wednesday, Thursday, and Friday

This is the period where we have even greater uncertainty, and potentially greater risk. This is due to a low pressure system that is presently just inland over northeastern Mexico. This system should move to the northeast this week, and reemerge over the Gulf of Mexico. After this point it likely will drift toward the upper Texas and Louisiana coasts. It will be slow moving, and bringing a lot of moisture with it, which as long-time residents will know is a potent recipe for a rainmaker.

We are talking potential bullseyes of 20 inches, or more, of total rainfall through Friday. As of today we simply do not know where this low will go, precisely, or where its heaviest rains will fall. It is quite possible that the heaviest rainfall occurs over Galveston Bay, or to the east of Houston over Liberty County or even Beaumont-Port Arthur. But we also cannot rule out very serious impacts over Galveston Island or even within Houston itself.

So how should you be thinking about this? It is distinctly possible that parts of Houston will see serious flooding during the second half of this week, but also possible that the heaviest rains will go east and we will be left with a largely manageable situation during the second half of this week. Until there is more clarity in the outcome, we simply cannot say. What does seem clear is that some part of the upper Texas coast, or perhaps southwestern Louisiana, is going to get rocked with some very serious rainfall and flooding in the late Wednesday or Thursday time frame.

When we can say more with confidence, we will.

Saturday, Sunday, and beyond

Rain chances do appear likely to back down somewhat this weekend, although probably not go away entirely. We can probably expect high temperatures in the vicinity of 90 degrees with partly sunny skies. Next week will likely get us back into the lower 90s, with partly to mostly sunny skies. We shall see!