Very heavy rainfall continues north of the Houston metro area, focus may shift south by early Friday

In brief: Houston has largely been spared by heavy rainfall during the last two days, but the story has been different to the north of our region, especially along and north of Highway 105. This could change beginning late Thursday night, when the city of Houston will see the potential for heavy rainfall. Next week still looks rather hot.

It has been a wet and stormy night for areas north of Houston, including locations such as College Station, Huntsville, and Livingston. Since Wednesday evening, accumulations for areas north of Lake Conroe and around Lake Livingston have reached 7 to 10 inches, leading to flash flooding. This is certain to lead to additional, significant downstream flooding along the San Jacinto and Trinity rivers during the coming days.

Rain totals from 6 pm Wednesday to 6 am Thursday show a sharp increase in accumulations north of Highway 105. (Harris County Flood Control District)

So far Harris and surrounding counties have been spared by the latest round of showers. Although the atmospheric conditions supporting heavy rain remain most favorable for locations north of the Houston metro area in the coming days, some of this activity will push southward. At this time, the most likely period for heavy rainfall across the Houston metro area will now come early on Friday.

To that end we are maintaining a Stage 1 flood alert for the Houston area today and Friday. For the northern locations described above, the regions around the lakes Conroe and Livingston, we are definitely approaching Stage 3 conditions on our flood scale. But I want to be clear that, for now, we don’t expect that kind of flash flooding conditions in the Houston metro area. It really has been a sharp gradient in rainfall. For example, during the last two days The Woodlands has recorded less than one-quarter of an inch of rain. Less than 30 miles to the north, New Waverly has received nearly 8 inches.

Thursday

I expect the band of heavy rainfall to the north of the Houston metro area to persist through the morning hours before there is some weakening. It will likely move slightly south, covering much of Montgomery County, but it should be diminishing in intensity through the morning hours. Later this afternoon I expect to see some scattered showers and thunderstorms pop up across Houston, but at this time they don’t look too terribly organized. For the most part, in the city of Houston and points south, we should see partly to mostly cloudy skies with muggy air and highs in the mid-80s. There likely will be a bit of a break from showers this evening for the area.

Thursday night into Friday morning

While there is the usual uncertainty, it does appear as though another round of showers and thunderstorms will develop later on Thursday night, likely after midnight. At this time there is a greater likelihood of rainfall across most of the Houston area, including locations south of Interstate 10. Roughly speaking, there is the potential for an additional 1 to 4 inches of rainfall accumulations, with the risk for higher totals running from the wee hours of Friday morning into the middle of the day.

There could be some higher bullseyes. I don’t feel overly confident in the details of this forecast, so expect an update on this website later this afternoon as we get better data. The bottom line is that it could be a wet night for Houston, but I’m not guaranteeing it. Because of this potential for additional rain tonight into Friday, we’re holding on to the Stage 1 flood alert for the entire Houston area.

NOAA rain accumulation forecast from noon Thursday to noon Saturday. (Weather Bell)

Friday

We probably will see the heavy rainfall threat ending later on Friday, so expect partially clearing skies to go along with highs in the low- to mid-80s during the afternoon hours.

Saturday and Sunday

The weekend, for the most part, should bring partly sunny skies and modestly warm weather to Houston. Look for highs in the mid-80s. While rain chances overall in Houston are quite low, they are not zero. Unfortunately, chances appear to be a bit higher for locations north of the city, including the hard hit areas north of Highway 105 described above. To be clear we’re not talking about inches and inches more of rain, but there will be the potential for additional showers and thunderstorms this weekend.

Next week

We’re still looking at the warmest weather we’ve seen so far in 2024 next week. Expect highs in the low-90s by mid-week, with plenty of humidity to make it feel rather warm out. It’s not full on summer, but it will definitely feel hot since it’s been half a year since we’ve felt heat like this. There is a chance, but only a chance, that we may see a final, weak-ish front trail into the area some time next weekend to bring us some drier air. A man can hope …

We’re issuing a Stage 1 flood alert for tonight and tomorrow across Houston, with the greatest risk north

In brief: Although considerable uncertainty remains, the potential definitely exists for heavy rainfall across the Houston metro area beginning this evening, and persisting through Thursday. The greatest risk for strong storms is north of Interstate 10, but heavy rain could still develop further south.

We’re continuing to watch for a potential flash flooding event in the Houston metro area later this evening, overnight, and Thursday. This will be driven by a fairly potent atmospheric disturbance that will drive one or more rounds of showers during the next 24 hours. The heaviest rain is likely to fall between midnight tonight and noon on Thursday, so definitely plan to check conditions before venturing out tomorrow morning.

The area in red is most at risk for heavy rainfall and flooding. (NOAA)

Some of these storms could be severe, with the threat of damaging winds. But primarily, the main concern is heavy rainfall. This is particularly acute for areas north of Houston, some of which already received 5 to 10 inches of rainfall earlier this week. Soils are wet and rivers and creeks are already high. Major reservoirs are full. So these showers will not be particularly welcome.

Generally, I think areas north of Interstate 10 can expect 2 to 4 inches of rainfall through Thursday night, but what concerns me is the potential for higher bullseyes of 6 or more inches. These accumulations will depend upon the speed at which these lines of storms move through, and that’s just not clear right now. For areas south of Interstate 10 the risk is not zero, but it is lower. I’d expect between 0.5 and 2 inches of rain for locations closer to the coast.

This is a difficult forecast because we’re getting mixed signals. On one hand, there is the potential for a pretty heavy rainfall event, especially north of Houston. Yet there are some scenarios in which things don’t fully pan out. So we’re going with a Stage 1 flood alert to raise awareness about the potential for flooding tonight and on Thursday. Things could get worse than that up north, however.

We’ll be back with an update early on Thursday morning.

More showers and thunderstorms are likely later today and tonight, with additional flooding possible

In brief: The overall forecast looks more ominous, with atmospheric conditions favoring another round of strong showers and thunderstorms later today and tonight in the Houston region. We are instituting a Stage 1 flood alert for the entire area, and may possibly upgrade that to Stage 2 for areas north of the city later today.

Wednesday

Conditions this morning will be fairly calm, with only a smattering of light showers across the region. Highs today will reach the mid-80s, with mostly cloudy skies. Winds from the southeast will be a little higher, at about 10 mph, with gusts to 20 mph.

However, our confidence is increasing in another bout of fairly strong showers and thunderstorms later today or tonight. These storms will likely fire up late this morning, or during the early afternoon to the west of Houston and then move through this evening or during the overnight hours. I want to stress there is a fair amount of uncertainty in the timing and location of these storms, which will bring the potential of damaging winds and heavy rain. Given this uncertainty, we will likely post an update this afternoon to the website.

Outlook for excessive rainfall today and tonight. (NOAA)

At this time the greatest threat of heavy rainfall lies to the north of Interstate 10. However, some of the latest modeling and data point to a more widespread event that could bring heavy rainfall across the Houston metro area down to the coast. Regardless, I’m concerned about the potential for additional flooding along the East Fork of the San Jacinto River, which has already reached a major flood stage due to heavy rainfall earlier this week north of Houston.

In terms of rainfall accumulations, our model guidance is not at all consistent. Much of the area is likely to see an additional 1 to 3 inches of rainfall, but given the increasingly favorable setup for heavy rain I could see some locations picking up as much as 5 inches or more of rainfall tonight into Thursday morning. For now we are issuing a Stage 1 flood alert, which means nuisance street flooding mostly, for the entire Houston area. We may modify this later today.

Thursday

Rain chances will start to diminish during the morning hours on Thursday, but we’re likely to see some additional scattered showers throughout the day. Skies will again be mostly cloudy, with highs in the mid-80s. Lows Thursday night will be in the 70s—so fairly muggy and warm.

Friday

This will be another mostly cloudy day, with highs in the mid-80s and a chance of scattered showers and possibly a few thunderstorms. Rain chances should end at some point on Friday.

Saturday and Sunday

The weekend looks partly sunny and a bit warmer, with highs in the mid- to upper-80s depending on the extent of sunshine. Winds will be from the south-southeast, gusting up to 20 or 25 mph so there should be a breeze all weekend. With low to non-existent rain chances, the region will get a chance to dry out.

Some areas of Houston are likely to hit 90 degrees by Tuesday. (Weather Bell)

Next week

No changes in the forecast for next week as high pressure settles in. We can expect warmer weather, with highs likely reaching 90 degrees by Tuesday or Wednesday. The rest of the week looks warm and muggy with low rain chances. I’m hopeful that a weak front may arrive in time to cool us down slightly by next weekend, but that is something I certainly would not bet on.

The SCW Q&A: First, the bad news; obscured by clouds; shuttered; humidity is relative; predicting it.

Welcome to the “just in the nick of time” April edition of the SCW Q&A. Each month Eric and Matt tackle the most interesting questions you’ve thrown at them. You can leave more questions in the comments here, or hit the Feedback link in the blog’s sidebar. We’ll also scan our social media channels (Facebook, Instagram, Threads, X/Twitter, Mastodon, Bluesky) for queries. And we’ll try to get the Q&A posted sooner in May! 

– Dwight

Q. Could this be is our last front until Sept/Oct? April is coming to a close soon, and I can’t recall many pronounced cooldowns in May.

A. Let’s get the bad news out of the way first. We are very near the end of the road when it comes to cool fronts in Houston. However, it is not unheard of to see cooler nights in May. That is not to say cold. If I squint into the distance, I can see a decent chance of a front around the May 11-13 period, so a little less than two weeks from now. That’s a long way out to have some confidence, but I’m modestly hopeful we may have one final fling with drier air before sultry summer settles upon us for months on end.

–Eric

Q.  I had read before the event yesterday that clouds sometimes clear during an eclipse due to the cooldown. Is that what happened yesterday in Houston when we had a thinning of clouds enough to see the crescent?

A. This is a great question. The answer is yes. During the eclipse in Houston, we had an issue with various sorts of clouds. We had higher level cirrus clouds, mid-level clouds, and low-level clouds. Most of the low-level clouds were cumulus clouds. What usually causes cumulus clouds to form? Heating. You heat the ground during the daytime, the heated air parcels rise, cool, condense, and you get clouds. Well, as you begin to block out the sun during an eclipse, you begin to reduce that heating mechanism. And in time, a lot of those cumulus clouds (or what we often call “fair weather clouds”) will dissipate. The animation below is from the 2017 eclipse, and you can see how the cumulus cloud field over Missouri and Illinois really dissipated as the eclipse unfolded.

A satellite loop of the 2017 total eclipse over Missouri and Illinois. (University of Wisconsin CIMSS)

This is likely what “saved” much of Texas this year. There were still plenty of clouds higher up, which are less impacted by daytime heating and more driven by physical storm systems. But by virtually eliminating a layer of lower cumulus clouds, you had one less obstruction to view the sun, which periodically broke through the higher clouds.

–Matt

Q. How far inland would you recommend having hurricane shutters to board up if a storm comes inland? I find myself trying to calculate what would be the strongest type of hurricane that we could feel in order to decide if I want to invest in shutters, but can’t find any info. Would you guys know?

A. We’re not storm damage experts, but whether you should have hurricane shutters depends upon several factors. First of all, the most important factor is whether you’re close to the coast, as winds do die down significantly as a storm moves inland and encounters friction with trees, homes, high rises, and other structures.. The second factor is the proximity of stuff, like trees and unattended deck chairs and you name it. The most common reason windows break is due to flying debris. So if there’s lots of things around that you can imagine flying into your windows, that’s a risk factor.

Maximum sustained winds from Hurricane Ike in 2008. (NOAA)

I’ll be honest with you. I’ve lived through flood storms in Houston. I’ve lived through surge storms. But I’ve not really experienced a wind storm. Hurricane Ike was the closest thing, but it barely brought Category 1 winds to parts of Houston. This was bad enough, knocking power out to the city for up to two weeks. At the time I lived in Clear Lake and we had to get a new roof. I guess what I’m saying is that it’s not just your windows, it’s also your roof. And a prolonged period without power.

I guess what I would advise is that, if you live south of Interstate 10, and can afford it, hurricane shutters are not a bad idea. You will probably never need them, but if it proves peace of mind, that is not a bad investment in an uncertain world. Make sure they’re easy to put up on short notice.

–Eric

Q. On your weather app I notice that the humidity (sorry I mean HOUmidity) can be low in the evening and then go way up in the morning, coming back down again as the day goes on. Why?

A. Relative humidity (RH) is just that, relative. Dewpoint temperature is the temperature to which you’d need to cool the air to saturate it. In other words, the higher the dewpoint, the damper the air is. Dewpoint can fluctuate, but it’s usually more stable than temperature. So, let’s say you have a dewpoint of 72°. The morning low is 75° in this example. That means your morning relative humidity at 75/72 would be 90 percent. Obviously, you warm up during the day, so with a high of 88 degrees but a steady dewpoint still sitting at 72, your RH at 88/72 has dropped to 59 percent. As the air cools around sunset bringing the temperature relatively closer to the dewpoint, the humidity rises again. It limits the usefulness of RH. This is why we flout other metrics like dewpoint to drive home how humid it actually feels.

–Matt

Q. After your earlier answer, I’m confused by the description of what “percent chance of rain” represents. I was taught in my meteorology classes that probability of precipitation in weather forecasts means the probabilistic chance that it will rain at the point specified in a given time period. For example, if I enter my zip code to get a National Weather Service forecast for my area today, a 40 percent chance of rain means each point within my zip code has a 40 percent chance of seeing rain, not that 40 percent of the land area within my zip has a chance of seeing rain. Insights? Or is this just splitting hairs?

A. It’s more or less splitting hairs. But this is a fun topic so why don’t we dive a little deeper. Let’s go directly to the National Weather Service, where we find this delightful introduction to its “probability of precipitation” explainer:

The probability of precipitation forecast is one of the most least understood elements of the
weather forecast.

I just love the “most least” phrasing. A perfectly confusing introduction to a confusing topic. Anyway, fundamentally you are correct that it is a point forecast rather than an areal forecast. But it is a point forecast based on an areal forecast. If that makes sense. Which it probably doesn’t. And that’s ok.

There are two factors that go into making a “probability of precipitation” forecast. First there is the forecaster’s certainty that precipitation will form or move into the area, and this is multiplied by the areal coverage of the precipitation that is expected. Here are two examples for how one might derive a 40 percent chance of rain:

(1) If the forecaster was 80 percent certain that rain would develop but only expected to cover 50% of the forecast area, then the forecast would read “a 40 percent chance of rain” for any given location.

(2) If the forecaster expected a widespread area of precipitation with 100 percent coverage to approach, but he/she was only 40 percent certain that it would reach the forecast area, this would, as well, result in a “40 percent chance of rain” at any given location in the forecast area.

Does that clear things up? Probably not.

But given the general inaccuracy of precipitation forecasts, I think it’s fine to think of a “40 percent chance of rain” as either a point forecast or, more generally, that about 40 percent of an area will receive rainfall during a given period of time. This is not an exact science, it’s just a forecaster making a most best guess of things.

– Eric