Matt Lanza

Good morning. We’ve got a full rundown on Tropical Depression 14 here today, and we’ll touch on Tropical Depression 13 as well. First, let’s get you through Sunday’s forecast.

Today through Sunday

The next three days have some similarities, and they should be mainly sunny with slight shower chances. The maximum coverage of showers may be today and particularly focused south and west of Houston. Saturday should see a bit less coverage but maybe more of a chance in other parts of the area. As the broader circulation from TD 14 nears on Sunday, we could see offshore winds develop, or at least more of a northeasterly component to them, which would probably shut down rain risks, especially south and east of Houston. Basically, you shouldn’t be expecting rain this weekend, but you shouldn’t be surprised if you see some either, especially Friday and Saturday.

Look for highs in the middle or upper-90s and lows generally in the 70s through Monday morning.

Monday and beyond (or the TD 14 discussion)

Alright, on to the main event. There is good news and bad news today on the Tropical Depression 14 front. The good news is that the storm is not any better organized this morning. The bad news is that because the storm is not any better organized this morning, we remain with more questions than answers.

Let’s start with a satellite loop of the system this morning.

Most folks will look at that loop and basically see a “swirl,” which is what you expect, even if thunderstorm coverage is a bit sparse. Well, the “swirl” is actually in the mid-levels of the atmosphere, not at the surface. At the surface, which matters greatly for the future of this thing, the National Hurricane Center (NHC) says that satellite passes and earlier reconnaissance aircraft data can’t really hone in on one defined center. Broadly speaking, this means TD 14 is disorganized this morning. That’s a good thing, of course, but it’s also a tricky one because without a defined starting point, you degrade the forecast of your ending point, thus increasing uncertainty in the forecast after a day or two.

Let’s look at the official NHC outlook as of 4 AM CT.

The NHC track remains pointed right at Houston, and the wrinkle overnight was that it explicitly shows a hurricane in the Gulf as it approaches. How do you interpret the map above? While the forecast shows the track pointed at Houston, by day 5, the reality is that the center could be anywhere from near Matagorda Bay to perhaps near New Orleans. We always say “look at the cone, not a point on a map.” You really need to take that advice with this system, because there is an inordinate amount of uncertainty still today.

We know that TD 14 will emerge in the Gulf on Sunday. It will probably be a tropical storm at that point, but given the current situation off Honduras, that’s not a guarantee. It may be weaker. Over the Gulf, it should find some opportunity to strengthen a bit given very warm water temperatures (as noted yesterday afternoon).

But there are three significant hurdles that will impact how TD 14 may or may not strengthen and track.

1.) Wind shear: We know wind shear is expected to be low as it emerges in the Gulf. But per the NHC, it’s expected to increase in the northern Gulf. According to the SHIPS model, by late Monday or Tuesday, shear is around 15 to 25 knots. That’s not massive, but it’s enough to give a tropical system some problems. Alternatively, there is some risk that the wind shear will lessen or focus more inland. In that case, it may actually help TD 14 to strengthen a bit as it approaches land. This remains a difficult item to pin down, but the going theory is that TD 14 will have to fight some wind shear as it approaches Texas or Louisiana.

2.) Dry air: If you look at the GFS model on Monday evening at 7 PM, you’ll see TD 14 probably as a moderate tropical storm in the Gulf. If you look at relative humidity on the GFS model, you’ll see the storm is lopsided with the best moisture north and east of the center and a pretty significant amount of dry air south and west.

We have had some lower humidity than usual lately here, right? You’ve felt that. It’s still plenty hot, but dewpoints (at IAH) have been as low as 59 degrees in recent days! That’s really, really comfortable for Houston in summer. And rare too. That’s the first time that has occurred in August since 2015, and of the 7,180 August hours in the last 10 years, only 20 (0.25%) have had air that dry. Well, dry air is going to linger a bit longer, and it would seem that it will impact TD 14 in some capacity. Could TD 14 fight off dry air? Sure, but it better get its act together quickly if it can become robust enough a storm to accomplish that feat.

3.) Tropical Depression 13: One of the most common questions I’ve seen from people is some variation of “Could TD 14 merge with TD 13 as it comes into the Gulf and create a Category 6 megacane, which would be soooo 2020!?”

Here is the forecast track of TD 13, by the way:

The answer to that one is complicated. For the sake of time and simplicity, I’ll be brief here. There is a meteorological process known as the Fujiwhara effect. You can read about it here. In a nutshell, if two tropical systems get within about 800 miles of each other, one can impact the circulation of the other. The diagram below shows this in the case of a much stronger storm. In this case replace Bopha with “TD 14” and Saomai with “TD 13:”

In an idealized Fujiwhara situation (unlikely), what would likely happen is that TD 13 would move into the Gulf from south of Florida, and as it comes north, TD 14 would probably slow down offshore of Texas or Louisiana, allowing 13 to go inland (likely to our east) and 14 to meander for a day or so offshore before going inland too. Now, to be clear, this is not the base case; it’s not what we expect. However, there could very well be some element of interaction that occurs between TD 14 and TD 13 because of their expected proximity to one another. That would depend on strength, orientation, etc. And since we don’t really know those things right now, it’s difficult to say how that interaction will (or will not) occur. We have also never seen that before in the Gulf. Like, ever. We have had simultaneous systems in the Gulf, but they’ve never been close enough to each other to risk that sort of interaction. In that respect, this is basically unprecedented and there will be unknowns.

So to summarize: It is likely that TD 14 will enter the Gulf Sunday and strengthen, possibly to a hurricane, tracking broadly toward Texas or Louisiana. The Houston area is very much in the cone, and we’d advise folks to continue to prepare as if a hurricane were coming. There are some hurdles for TD 14 and numerous things we do not yet know, however, and impacts could realistically range from hardly anything to a full-blown hurricane in our area. Eric and I will keep you posted through the weekend.

Quick note on rainfall: While we’re trying to figure out the tropical side of this thing, we are obviously sensitive to people’s concerns about rain, flooding, and a Harvey stall type scenario. Right now we do not expect a “stall” situation. No reliable model is hinting at that, and there’s nothing in the data right now that would argue that it’s a serious concern. The storm may slow down a little as it moves through, but slowing down is not equivalent to stalling and the differences are significant. We expect heavy rain, particularly if the storm comes in more to the southwest of Houston, and flooding is always possible in tropical systems. But we are not especially worried about any one possible outcome at this time. We will update you on the rainfall aspect more this weekend as things become clearer.

Special note: Today, at 1 p.m. CT, Eric and I will be conducting a Facebook Live to provide up-to-date forecast information, and answer your questions about what to expect from this tropical system. This event is sponsored by Reliant.

Good afternoon. Tropical Depression 14 formed late this morning from the disturbance previously dubbed Invest 97L. This is the primary system we will watch over the next few days as it moves toward the Gulf. Here’s a brief rundown of what we know and, more importantly what we don’t know.

Now

Tropical Depression 14 is located east of Honduras and south of Cuba over the western Caribbean Sea.

It’s basically what you would expect from a tropical depression: sort of messy, but with numerous storms both east and west of the center, so it’s not terribly asymmetrical.

The latest forecast from the National Hurricane Center has the system moving into the Gulf and remaining a tropical storm through early next week.

It’s important to note that while a “center line” points this right at Houston, the cone is the cone for a reason. The storm may track well to our east or well to our southwest. Our impacts here in Houston would vary considerably based on that, so this map should not be considered a license to panic or to write it off entirely.

Over the next two days, TD 14 will enter an environment that’s pretty conducive for strengthening. Shear will continue to relax. As it moves toward the Yucatan Peninsula of Mexico, look for this to intensify into a tropical storm and possibly a hurricane. Water temperatures in the western Caribbean are more than supportive of strengthening.

Once it gets to the Yucatan and beyond, the questions begin to mount quickly.

Here is what we know:

We know that TD 14 should move across the northeastern part of the Yucatan Sunday morning. We know it will likely be at least a tropical storm. We know it will be pulled north by a trough over the South and nudged west by the Bermuda high as it expands westward across Florida. We know that if impacts occur in Houston, they would likely begin Monday night or Tuesday.

Here is what we don’t know:

We do not know where TD 14 will ultimately end up and how strong it will be when it gets there. I know this makes you throw your hands up in the air in frustration, but those are the realities.

We do not know how much wind shear TD 14 will encounter over the Gulf. As the trough over the South weakens and pulls out, there will be some wind shear left behind. If that ends up over the Gulf, that will help mitigate TD 14’s ultimate strength. If that wind shear goes north as the storm comes north, then we need to watch for TD 14 to intensify more significantly.

We also don’t even fully know how much land interaction TD 14 will have near Honduras and Mexico. The latest advisory suggests TD 14 is farther south than originally thought, so that is something that could ultimately impact the system.

We don’t know how much rainfall we will see, as it will depend on how organized TD 14 becomes and exactly where it goes.

We don’t know how TD 13 may impact what happens with TD 14, so there’s a good deal of uncertainty there.

More ambiguous things we can say:

A stronger storm would likely go to our east, whereas a weaker one would come closer to us. But weaker does not necessarily mean “weak,” so keep that in mind.

We do not expect a stall/Harvey scenario at this time or even an Imelda one but we are not entirely sure how things will play out. Flooding is possible in any tropical system, so we cannot rule any sort of flooding out right now.

We should be able to narrow the cone a bit in the morning, and we should know a lot more later tomorrow and Saturday morning. So stay tuned.

As we’ve been telling you for essentially weeks now, the tropics are becoming very active. Some of this is just basic math—as we approach late August and September, the tropics usually come alive. But there is more at play this year, with things happening in the background that should allow for basically an “outbreak” of sorts of tropical systems across the Atlantic basin in the days ahead.

Tropical outlook in a sentence

There will be at least two legitimate opportunities for tropical systems to develop and potentially impact some part of the Gulf of Mexico over the next 10 days, and an additional system or two may develop farther out in the Atlantic as well.

Invest 97L: First in line

The system the models are most bullish on is Invest 98L, which we’ll discuss below. But the one we are most concerned with more immediately is Invest 97L, because this is closer to us, and it is the type of system that can sneak up on us and cause some problems.

Invest 97L is currently over the eastern Caribbean Sea. It’s not terribly well organized, and you can see a satellite loop from Tuesday afternoon showing mostly just a “blob” of storms at this point.

Over the next few days, 97L will plod along in the Caribbean, interacting at times with a disturbance over South America. So development is only expected to be slow or gradual. Eventually, it will end up in the western Caribbean, somewhere near the coast of Honduras or the Yucatan. I can hear 75% of our audience now saying, “Wait, that’s how Harvey happened, right?” And the answer is, “Sort of, yes.” It’s the elephant in the room here, obviously, so let’s discuss and compare real quick.

By the time the disturbance gets into the Western Caribbean this weekend, the upper air pattern will be favorable for it to gradually lift north. Here’s a map from the GFS ensemble valid on Saturday afternoon. We’re looking at the 500 mb level, or about 20,000 feet up, basically the large-scale steering pattern for this time.

On Saturday afternoon, 97L should be off the Yucatan or Belize or just north of Honduras. It will be steered to the west by high pressure near Bermuda, while at the same time, a trough of low pressure in the upper atmosphere over the Lower Mississippi Valley is going to try and “pull” Invest 97L to the north. So at this point, the system should be moving to the northwest.

So in that respect, a middling system that gets pulled north and west across the Yucatan and into the Gulf of Mexico, Invest 97L is similar to Harvey. But I’m going to intentionally stop with the similarities there because the potential of what this does is much different from Harvey.

There is a chance that the trough remains strong enough to continue in its effort to “capture” Invest 97L, allowing it to come north into the Gulf and toward Louisiana or perhaps the Upper Texas Coast. That scenario can’t be ruled out. There is also a chance that Invest 97L gets “missed” by the trough and ends up stationary without much to steer it over the Gulf of Mexico. While that’s not a huge rainmaker on land, that becomes problematic because there would be a chance Invest 97L could strengthen while sitting offshore. Another possibility is that the Bermuda high continues to expand so much so that it just pushes 97L into Mexico as a weak rainmaker. Harvey was a once in a lifetime storm in terms of rainfall, so we don’t want folks to worry about a “Harvey” type storm right now. There are indeed some similarities in the initial overall big picture, yes. But that’s where it ends right now. What we should be doing (and what Eric and myself will be doing) is making sure we are prepared to take action late this weekend or early next week just in case it becomes clear that 97L will become a problem for us. We will keep you posted.

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Back on Friday, I wrote in our weekend forecast discussion that “if the chance of a shower is about 10 percent Friday and Saturday, maybe it inches up to 15 or 20 percent Sunday.” We pride ourselves on honesty here at Space City Weather, and that will end up likely proving inaccurate. The reality is that compared to Friday, our Monday front is a bit faster and has a burst of upper level support in the atmosphere this afternoon which should allow storms to blossom today.

Radar is quiet as of this writing, but as an upper level disturbance interacts with the ample heat and humidity over our area, it may be just the trigger needed for strong thunderstorms to develop north of Huntsville and Conroe by about 3 or 4 PM. We expect a complex of storms to develop up that way and drop into places like Conroe and The Woodlands between 7 and 9 PM or so.

After that, things do become a little more uncertain. Most modeling does bring those storms into the I-10 corridor and possibly points south after 8 to 9 PM or so. We’ll see if the storms can survive the trip, but the bottom line is a likely dry afternoon for Houston and points south, with storm risks increasing later in the evening.

This does not look like a high-end severe weather outbreak, but storms could certainly become strong to severe as they drop through the region. We are in a “marginal” risk (1/5) of severe weather. Hail is a possibility, but as is often the case in these sorts of setups, strong winds may be the more common problem. With the storms moving at a pretty healthy clip, look for gusty winds ahead of the rain this evening, especially north of I-10.

Storms should dissipate or move offshore overnight, leaving us mostly quiet to start Monday. We can’t rule out more showers and storms tomorrow. The cold front should arrive later tomorrow, ushering in slightly less humid air for midweek, which will be most noticed at night via slightly cooler nighttime lows. Eric will have more on that in the morning and an update on the tropics, which he has already touched on today.