Eric Berger

For much of the Houston area, the first day of widespread rainfall from the tropical storm formerly known as Imelda has been mostly a non-issue. Large swaths of the metro area north of Interstate 10 have received less than one-half an inch of rainfall. Count yourselves lucky. Parts of southern Brazoria County, as well as the Alvin, Clear Lake, Pearland, and Friendswood areas have received 6 to 8 inches of rainfall over the last 24 hours. Soils south of Houston are sodden, and some stretches of Clear Creek are nearing their bankfulls.

As of shortly after midnight, Imelda’s “center” appears to have moved almost directly over Sugar Land as it slowly wobbles northward across the Houston metro area. This “drunken sailor” motion will probably continue for the next day or so as the system slowly lifts northward. Looking at the radar at this time, there are several clusters of fairly heavy showers and thunderstorms, particularly right along the coast, moving from Brazoria County toward Galveston County.

But fortunately we have yet to see the really intense rainfall rates of 2.5 to 5 inches per hour that can quickly back up bayous, roads, and flood yards. For the rest of the night, some of the high resolution models are continuing to show rain showers really blowing up between midnight and mid-morning Wednesday, but so far we’re not seeing that really verify on radar. However, the scencario remains plausible given the highly efficient moisture transfer by Imelda from the Gulf inland, along with plenty of atmospheric instability. The biggest area for concern is probably south and east of Interstate 69 for the rest of the night.

As for Wednesday and Thursday, both days will have the potential for heavy rainfall. Hard to say which has the higher potential, but for now I’d probably go with Wednesday as the system should really begin to pull away from Houston on Thursday.

Tropical Storm Imelda formed Tuesday afternoon just before making landfall near Freeport, Texas. The storm will bring some moderate wind gusts into the upper Texas coast over the next day or two, but by far our bigger concern is heavy rainfall and inland flooding as the storm pulls copious moisture inland. To that end, we are escalating our alert to Stage Three on our flood scale. This means we are likely to see significant flash flooding. This post will explain why.

Timing

Probably the most notable shift in the modeling guidance has been an acceleration of when we expect heavier rainfall to begin. It now appears likely that heavier rains will move into Houston this evening, with banding features consistent with a tropical storm. We now also have increasing confidence that the worst of this will be over by Thursday afternoon or evening—so Houston appears to be locked into this mess for the next 48 hours or so. (It’s nice to have a fairly clear end point).

Tuesday night rains

A good portion of Harris County, and coastal counties, will likely see between 3 and 6 inches of rainfall this evening, during the overnight hours Tuesday, and through Wednesday morning. There may also be a narrower band that sees as much as 12 to 15 inches tonight, although it is not clear whether this will occur along the coast, offshore, or somewhere east of Interstate 45. Where this heavier rainfall occurs we probably will see significant street flooding due to extremely high rainfall rates, and potentially water getting into homes. During any flash flooding please remain in your location and do not venture out.

Wednesday afternoon through Thursday

As Imelda slowly lifts north, we expect the more intense band of rainfall to lift with it. So our biggest concerns for heavy rainfall later on Wednesday and Thursday will probably migrate to the Interstate 10 corridor and further inland. But frankly, we’ll have to reassess this after we get through Tuesday night.

As for overall event totals through Thursday, it seems a fair bet that most of the Houston metro area will pick up 5 to 10 inches of rainfall, with isolated totals of 20 inches or greater. Most likely these higher totals will occur somewhere east of Interstate 45, but whether that’s over Galveston Bay; East Harris County; or Liberty and Montgomery Counties, is difficult to say. More later. Hang in there, y’all.

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.