Eric Berger

Today, the National Hurricane Center upgraded the low-pressure system in the Gulf of Mexico into a tropical depression. (Update: at 12:45pm CT the National Hurricane Center upgraded the system to Tropical Storm Imelda). As a result of this, a Tropical Storm Warning has been issued for the coast of Texas from Sargent to Port Bolivar. The storm’s forecast track, through Thursday morning, is shown below. The system should run rougly parallel to Interstate 45 once inland.

The formation of a depression increases the possibility—slightly—of this system bringing tropical storm-force winds into the Houston metro area later today and on Wednesday. However, the overall probabilities are low:

By far the greatest threat from this system remains heavy rainfall from now through Thursday. As noted this morning, the depression is bringing an exceptional amount of tropical moisture into the state of Texas. What we don’t know yet is where the heaviest rains will fall on Wednesday and Thursday. We think a few areas—maybe some isolated locations in Galveston or Houston or Beaumont or Lake Charles—could see peak amounts of 15 to 20 inches. But most of our region probably will receive considerably less than that, on the order of 5 to 10 inches, which hopefully will be mostly manageable.

Bottom line, there’s a lot of potential here for heavy rainfall, and we’re going to have to really track this closely as it brings flooding into Southeast Texas on Wednesday and Thursday. We’ll have a comprehensive update around 4pm this afternoon outlining what we know, and what we don’t.

And so it begins. Tuesday marks the first of three days during which the Houston area will face the threat of heavy rainfall as moisture levels surge due to a tropical system moving toward us. As the system has become a little better organized overnight, it’s forecast track has also shifted subtly such that its very loosely defined “center” should now pass just to the west of Houston this week as it slowly lifts north. Here is a rough approximation of a 5-day track from the GFS model ensembles.

This path likely sets up the heaviest rainfall between Matagorda Bay and the Beaumont area, with Houston very much in the midst of the bullseye. Over the next three days most or all of Houston falls under the moderate chance of “excessive rainfall” according to maps published by the Weather Prediction Center—a strong indicator of where the best experts think the heaviest rain is likely to occur.

So what does this mean for Houston? It means that Tuesday will be somewhat wet, and Wednesday and Thursday have the potential to be very wet. In terms of travel and normal business, I do not think rainfall on Tuesday will be too disruptive, especially for central and northern parts of the Metro area. But there are strong signals in the models that Wednesday afternoon, night, and Thursday could be more problematic.

Where? The models have drifted a little bit eastward with this heaviest rainfall during the overnight hours—if this trend continues the heaviest rains could come down just east of Houston. But it is far too hard to parse such details at this point, and frankly there is a lot of moisture coming into Texas so everyone is going to get rain. I think widespread totals of 5 to 10 inches are likely for most people over the next three days. Some areas probably will see 10 to 15 inches of rain. This is manageable for most if it is spread over two to three days. But 15 inches in five hours would not be. Unfortunately, as we know, tropical rain often comes in clumps. The system has a shot at becoming a tropical depression or storm before landfall tonight—30 percent per the National Hurricane Center—but that doesn’t really matter. It is going to be a rainmaker regardless.

For now we are holding this flood event at “Stage 2,” because there is still the potential for the heaviest rainfall to remain offshore, or come in east of Houston, but we remain very close to escalating it to Stage 3 (as in, we think there probably is about a 50 percent chance of a Stage 2 event, and 40 percent chance of a Stage 3 event, and a 10 percent chance of Stage 4). There is just a lot of potential rainfall here to be tapped, and we’re quite concerned about what could happen Wednesday night and Thursday over the region.

If there’s any good news, it’s that this event seems likely to come to an end by Friday morning or so, by which time we can begin to dry out. Chances are also decent that we may finally see the season’s first front by the end of the month, although we’re not close to being able to be definitive about that.

Some of our best forecast models continue to indicate that the low-pressure system approaching the Texas coast is going to deliver on its promise of being a real soaker for parts of the state. We are also increasingly concerned about the flood potential from this system, but we cannot pinpoint where the greatest threat lies—although Houston is very much in the running for the highest rain totals this week.

What we know

The low-pressure system will approach the Texas coast on Tuesday, and likely move inland on Tuesday night or early Wednesday. There is a small chance it will strengthen into a depression or tropical storm near the coast, or just inland, but rainfall remains the primary threat.

Once inland, the system is likely to wobble for a couple of days, as high pressure to its north prevents a turn in that direction.

What we don’t know

At some point the storm will begin to slowly pull north, and this likely will bring the stronger core of rains toward Houston. This period of heavy rainfall for Houston may begin later Wednesday, or early Thursday, but it’s hard to say for sure. Heavy rainfall is possible any time beginning Tuesday afternoon through Friday morning.

We don’t know where the heaviest rains will fall, and we’re probably not going to know much about exactly how much rain any specific location will receive until maybe Tuesday, or more likely later Wednesday—and even that’s not certain. That’s the reality of these events. We can offer ranges, but we’re not going to offer false certainty.

The forecast

If you’ve seen any individual computer model maps for precipitation in the coming days, please take each one with a grain of salt. Such maps are good for giving a sense of potential rainfall, but generally they’re pretty poor at determining precise amounts and locations. This map, of European ensembles, offers a rough guess at what we might expect.

Overall, our forecast for rainfall between now and Friday for the greater Houston area are:

• Widespread total of 5-10 inches, on average, meaning that some some areas will see less, but some will see more.
• Bullseye totals of 15-20″ cannot be ruled out in a few isolated areas.

We still have a lot of questions about this system, most notably whether the heaviest rainfall will occur over an area such as Matagorda Bay, or Houston, or Beaumont, or—best case scenario—just offshore. But we do have serious concerns for Texas for the next several days. We will be watching this closely. Matt and I have discussed escalating this to a Stage 3 flood on our scale (we’re currently at Stage 2) but we’re not quite there yet.

We’ll have a comprehensive update in the morning. Also, as a reminder, if you would like complete updates during storms such as this delivered immediately after publication to your inbox, subscribe in the form on this page.

The big weather story for our week will be a slow-moving tropical system pushing into Texas beginning late Monday, and bringing the potential for heavy rainfall from Tuesday through Thursday. It still is not entirely clear where the heaviest rain will come, but there definitely is the potential for some 10-inch bullseyes, or greater. The region most likely to see this rainfall is somewhere between Matagorda Bay and Beaumont. We are more concerned about this event than we were on Sunday, and are forecasting it to be a Stage 2 flood event.

As of Monday morning, here are rainfall predictions from the National Weather Service and NOAA Weather Prediction Center for this system, and we see no issue with these forecasts after looking at the current models:

• Widespread rainfall of 3 to 6 inches through Friday
• Some areas are going to see amounts of 6 to 9 inches, with isolated amounts in excess of 12 inches possible.
• Rainfall rates of 2 to 3 inches per hour are going to be common during the event, which will quickly back up streets and push water into yards.

One problem with forecasting the locations facing the greatest rainfall threat is that this tropical system probably won’t move too much until later this week, as high pressure to the region’s north should allow it to meander for a couple of days.

To be clear, this is most definitely not a Hurricane Harvey type situation—it will not produce anything like Harvey’s totals, nor over such a large area. There also remains a reasonable chance—perhaps 25 percent—that the forecast busts and most of Houston sees 1 to 2 inches of rain this week. However at this time we definitely see the potential for flooding and are classifying this as a Stage 2 flood event on this site’s flood scale. For now, we don’t envision significant home flooding, but these are definitely “turn around don’t drown” conditions, during which isolated underpasses and high-water crossings may become life threatening.

The general pattern we can probably expect is for scattered showers on Monday and likely most of Tuesday. The period of greatest rainfall threat will probably come from Tuesday night through Thursday—with some of the latest models pinpointing Thursday as the day of biggest concern. Days will be mostly cloudy otherwise, with highs of around 90 degrees. Partly sunny skies should return by the weekend.

For those who live along the coast, we also have some high-water concerns as the low pressure system approaches Texas. According to the National Weather Service, through Tuesday morning, low lying areas along the immediate coast may experience minor coastal flooding during high tide times. Additionally, strong rip currents along Gulf facing beaches, and minor flooding along flood prone roads such as Highway 87 near High Island and Blue Water Highway may be possible.

We will update accordingly throughout the week.