Eric Berger

Good morning. Tropical Storm Nicholas has a better defined center of circulation this morning, and it has reformed north of where forecasters believed it to be earlier. This, combined with an expectation that Nicholas will now move a little bit more quickly once inland, have led to some changes in timing and rainfall amounts in the forecast. As a reminder, we’re continuing to predict Stage 3 flood conditions for some parts of the greater Houston area through Wednesday. Some, but not all, of our region will see prolonged heavy rainfall beginning later tonight.

Storm status

Tropical Storm Nicholas has strengthened to have maximum sustained winds of 60 mph this morning, as of 4am CT, and is moving to the north-northwest. The storm will likely make landfall this evening between Corpus Christi and Matagorda Bay.

This will bring tropical storm force winds to the Matagorda Bay region on Monday afternoon, and these could spread up the coast to Galveston Island on Monday evening. Tropical storm force wind gusts will spread inland into portions of Harris, Fort Bend, and Colorado Counties late Monday or early Tuesday before beginning to weaken during the afternoon hours. These winds should not be great enough to produce widespread power outages in the Houston metro area, but some local problems are definitely possible.

Storm surge values of 3 to 5 feet are possible from Port O’Connor to Sargent, and 2 to 4 feet from Sargent to Galveston to High Island. Some significant inundation is possible around Matagorda Bay.

For nearly all of us in the greater Houston metro area the biggest concern, by far, will be rainfall.

How wet will it get, and when?

The Houston region should see scattered to numerous showers on Monday, but the heavier rainfall that is most concerning to us should fall between a 36-hour period from Monday evening into Tuesday night, or Wednesday morning. So this is the time when you should plan for the most significant impacts.

The flood threat is very real, but it is slightly mitigated by the fact that Nicholas is now expected to continue moving to the northeast after it reaches Houston, and seems less likely to slow down. This should, hopefully, bring down rain totals somewhat. To that end, the map below represents an average expectation of the rainfall your location will get from Nicholas. For much of our region, these should be mostly manageable totals.

However, it is important to note that we are most concerned about locally higher amounts, which are likely during a tropical system that produces training thunderstorms with extremely high rainfall rates. Therefore, some locations may see as much as 10 to 20 inches of rainfall from Monday evening through Wednesday morning. While these bullseyes are more likely near the coast, they are definitely possible as far inland as Harris, Montgomery, or Waller counties.

Please note that we have issued a Stage 3 flood alert, on the Space City Weather scale, in expectation of these bullseyes.

Conclusions

Nicholas will soon be here. While conditions across the region should be OK for much of today, they may deteriorate fairly quickly this evening or after midnight. In our next post we will try to provide a firmer time for the onset of heavier rain in the metro area—whether that’s 8 pm, midnight, or later. Our next post will come no later than 10:30 am CT.

Good afternoon. The purpose of this post is to emphasize that while we have a pretty good handle on the forecast for Tropical Storm Nicholas over the next 24 to 36 hours, after that many questions remain. And for this reason, we want readers to understand that while Houston may see heavy rain over the next three days, other parts of the state are very much at risk as well.

As of 1pm CT, Nicholas still has 40 mph winds, and it is moving north-northwest across the southern Gulf of Mexico. Winds from the storm may reach South Texas by Monday morning, and the upper Texas coast by around Monday night. It’s possible that Nicholas becomes a Category 1 hurricane, and there will be some effects from this wind and surge. However the greatest threat remains rainfall, so that’s what we’ll now discuss.

I know you’re tired of reading about “uncertainty,” but you’re going to have to bear with us for another 12 to 18 hours on that score. Why? Because Nicholas only now is beginning to develop a reasonably well defined center of circulation, and we’re starting to get better data from reconnaissance airplanes flying into the storm. This improved information will be fed into models. So the computer model outputs after 6pm today will be better, and then models released after midnight tonight should be pretty good. The bottom line: When you wake up on Monday, we should be able to give you a more precise forecast.

In the meantime, it’s important to stress that the greatest threat for heavy rainfall and flooding is not coming on Sunday night Monday, but rather Tuesday and Wednesday as the center of Nicholas moves into the Texas coast. After that, the rains will be largely dependent on the track of the storm—which again we should have a better handle on by tomorrow morning.

Put more explicitly, it is quite possible that some location in Texas inside a triangle from Austin to Corpus Christi to Beaumont sees 20 or more inches of rainfall on Tuesday or Wednesday. (Most areas, of course, will see considerably less). I still probably would favor the Houston, Galveston, or Beaumont regions for the heaviest rainfall, but we just cannot say for sure. So what should you do in the face of this uncertainty? Well, if it helps, you can blame us. But what I would advise is keeping an eye on the forecast, especially tomorrow, when we should have a better handle on things. And then be prepared for the potential of flooding on Tuesday or Wednesday.

Matt will have another update on the situation this evening, before 9pm.

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Ok, now on to the forecast. With this evening’s post we are going to give a general sense of the timing for what to expect, and when. We have some answers about the near-term forecast through Monday, but the bigger and perhaps more important question concerns what comes in the middle of the week.

Invest 94L

As of Saturday afternoon, the Gulf storm, or Invest 94L, remains poorly organized (see for yourself). It is difficult to assign a center to the low pressure system, and this lack of definition is one reason why forecast models are struggling a bit. However, the overall picture seems fairly clear. As the system moves northwest across the Bay of Campeche, it is likely to become better organized on Sunday, and should become a depression by Sunday night or Monday. By that time it should be approaching the northern Mexico or southern Texas coast. After this things the forecast becomes murky. By Monday night or so, the system may either move inland near the Rio Grande River, or turn northerly and track up the Texas coast. If it remains offshore, Invest 94L has a chance to become a reasonably strong tropical storm on Tuesday.

Sunday

Skies should remain mostly clear overnight, but clouds will begin to build across the upper Texas coastal region on Sunday. However, for much of the day any showers and thunderstorms we see should be scattered in nature.

Monday and Monday night

By Sunday night or Monday morning we should start to see more organized storms moving in from the southeast, off the Gulf of Mexico. It’s important to note these rains will not be directly related to the core of Invest 94L, which will remain far to the south. However, as a slug of moist tropical air moves inland we should see widespread accumulations of 1 to 3 inches for inland areas, and 3 to 5 inches of rain for coastal counties. We can’t rule out some bullseyes of 7+ inches. It seems possible that this initial round of storms may wind down on Monday evening or over night.

Tuesday and Wednesday

The real question is what comes after this point, and that will be determined by what the center of Invest 94L does. If the storm’s core moves up the Texas coast (the “Coast hugger” scenario in this morning’s post), our region would probably see manageable rain totals on Tuesday and Wednesday, with the heavier rains offshore or falling over southwestern Louisiana. However if the low moves into Texas and then wanders upward through state—perhaps along the Highway 59 corridor or further inland as in the “Tour of Texas” scenario—we could see a second round of even heavier showers on Tuesday night and Wednesday. For now, it is impossible to say which of these situations will play out, but ultimately this is probably the most important thing to watch in terms of flooding for the greater Houston region. If we’re going to get rocked, it probably would come then.

Our confidence is reasonably high that the rainfall event, whatever comes our way, will begin to wind down on Wednesday afternoon or evening.

Matt will have a full update in the morning, and we’ll have all-hands-on-deck coverage with myself, Matt, and Maria tomorrow.

Good morning. It’s going to be a beautiful Saturday in Houston, in early September. We totally understand that people are focused on 9/11 remembrances, family gatherings, or other activities today. But there is a potentially serious weather situation developing in the Gulf of Mexico that you need to keep one eye on this weekend. I say potentially because there are some scenarios in which Houston sees only moderate rainfall, and there are others in which we get drenched.

Here’s what we know for sure: It is highly likely that a tropical depression or storm will form over the southwestern Gulf of Mexico within a day or two. This extremely moist system will bring heavy rains, winds, and high seas to the Texas coast beginning late on Sunday. But it’s the details that matter, and we can’t pin those down yet. Based upon the latest modeling and the atmospheric setup, I want to present three plausible scenarios for the next week. As we will discuss, the effects range from mild to extreme, and that’s one reason why it is so difficult to provide an accurate forecast at this time.

Three scenarios

The map below shows the European model’s ensemble forecast for the evolution of the tropical system, which has been designated Invest 94L. It shows tracks from Friday night through Tuesday evening, and I’ve boxed what I consider three of the most likely possibilities. Let’s discuss each of them.

Scenario A: Coast hugger

In this case the storm moves close to the Texas coast on Monday, but perhaps doesn’t come fully ashore. It then moves fairly rapidly to the north and then northeast, bringing its center close to the Houston metro area on Tuesday or Wednesday. In this case we would see higher tides—although probably not too great of a storm surge—and wind gusts of tropical storm force in the metro area. However, the heaviest rainfall would likely fall off the Texas coast and potentially in southwestern Louisiana. In this scenario Houston might see 2 to 4 inches of rainfall, with the 20-inch bullseye remaining offshore.

Scenario B: Tour of Texas

In terms of rainfall, this scenario is more ominous. A likely weak tropical storm would move into South Texas on Monday or so, and slow down for a couple of days. The center would remain close enough to the coast that the storm would be able to tap into Gulf of Mexico moisture. In this case, Houston might expect 5 to 15 inches of rain, with a 20-inch bullseye of rainfall coming somewhere in the area from Corpus Christi to Beaumont. But it’s impossible to say where. The potential for heavy rainfall would linger through Wednesday. Winds and seas would be much less of a factor.

Scenario C: Riding the Rio

It still is possible that the low moves into northern Mexico and basically tracks along the Rio Grande River, dying out after a few days of being inland. In this case the Texas coast would see some moderate rainfall for a few days, but totals for most areas would probably be 2-4 inches, or even less. This scenario seems a little less likely at this time, but it would certainly have the most benign effects for Houston.

Conclusions

So what actually happens? I wish I could tell you. But we’re talking about a tropical wave that has yet to form a semblance of a circulation, so the forecast models are going to struggle with its track. After that, it’s not entirely clear how much the wind shear currently hampering its organization will weaken over the next couple of days as the system moves northwest or north toward Texas. And these are just the beginning of our questions.

The bottom line is that Saturday should see fine weather in Houston. Sunday should start out sunny as well before rain and thunderstorm chances increase during the afternoon and evening hours. After that heavy rainfall is possible through Wednesday, and we just really can’t say how much. But this is a forecast you should be monitoring closely. We’ll have an update later this afternoon or evening.