Eric Berger

Imelda shifting north, risk shifting east of Houston

As of midday Wednesday, we’re starting to feel pretty good about the chance of Houston avoiding significant flooding from the remnants of Tropical Storm Imelda. We’re not entirely in the clear, to be sure, but the threat is shifting east away from the metro area. First of all, let’s take a big picture look at the radar at Noon.

Radar image at noon CT. (kktv.com/Space City Weather)

As you can see, Imelda’s remains are producing a very broad circulation, with several distinct banding features from along the Texas coast to well into Louisiana and northeast Texas. Houston residents will also take note that these strongest bands are generally moving away from the metro area.

Based upon the latest modeling trends, we expect this trend to continue today. While the Houston metro area may see an additional 1 to 4 inches of rainfall over the rest of Wednesday and through Thursday (especially to the east of Interstate 45), the threat of sustained, heavy, and flooding rainfall is diminishing. We can see this based upon the HREF model output, which is an ensemble of several models that attempt to predict smaller-scale convection during rainfall events. Of all the models we’ve looked at so far during this event, the HREF has arguably done the best. Here is its output for now through Thursday.

HREF ensemble forecast for rain accumulation from midday Wednesday through Thursday evening. (NOAA)

The bottom line is that right now we expect the Houston region, including hard hit areas in Brazoria and Galveston counties, to generally see improving conditions from here on forward. Intermittent storms may well pop up, but right now, we anticipate that most areas can safely return to business-as-usual activities. For areas east of Houston through Beaumont, however, we definitely have concerns about ongoing heavy rainfall, and will continue to monitor that situation.

Imelda hammering the upper Texas coast, Houston mostly spared for now

Let’s begin this morning’s post with a quick look at 24-hour rain totals for the broader southeast Texas region. Imelda moved inland near Freeport on Tuesday, meandered up toward Sugar Land, and is now near, or just west of The Woodlands. Since that time the storm has brought a devastating amount of rainfall to coastal Brazoria County (nearly 20 inches in some locations), and upwards of 8 inches or more for places to the right of the storm’s track near Alvin, Friendswood, Pearland, and Clear Lake.

24-hour rain totals from Tuesday morning through Wednesday morning. (TexMesoNet)

So what comes next? Imelda’s heaviest rains are continuing well to the south and east of the storm’s center, with much of the most intense rainfall just offshore. (This is good, because an area near Sargent recorded a 5-inch hourly rainfall rate last night).

This intense coastal band is, at present, capturing the deeper flow of Gulf moisture before it can move inland. Long story short, the band highlighted in the radar map below is effectively a shield this morning preventing heavy rainfall over the central Houston area.

Houston radar at 8am CT Wednesday. (kktv.com/Space City Weather)

Beyond this morning, however, we’re kind of flying blind as to where storms will redevelop later this afternoon and evening. Sometime later today the feeder band shown above should have broken down, and along with some daytime heating we can probably expect storms to re-form over inland areas. But whether that’s over Clear Lake, downtown Houston, or points east is pretty hard to determine. Our short-range modeling does a pretty poor job during these kinds of situations.  If you want my best guess—and this really is a guess—we’ll see some kind of mass of storms form over the central Houston area this afternoon or early evening, and then push into Liberty County later tonight. These could be relatively moderate storms, or quite intense.

For now, at least, Imelda looks to be manageable for all of the region except for the immediate coast, as well as flooding along Clear Creek to the southeast of Houston. More later.

Imelda dissipating over Houston, but heavy rains likely to persist

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.

24-hour rainfall totals for Tuesday. (HCOEM)

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.

Imelda’s position as of 12:10am CT Wednesday. (kktv.com/Space City Weather)

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.

TS Imelda comes ashore, flooding likely as heavy rains begin

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.

(Space City Weather)

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).

Tropical Storm Imelda banding on Tuesday afternoon. (kktv.com)

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.