Eric Berger

Good morning. Before jumping in to today’s post, I wanted to apologize for site outage problems during the overnight hours. This is very unfortunate timing and we will correct the problem going forward.

On Wednesday night, as anticipated, much of the Houston region passed a relatively quiet night. However, the same cannot unfortunately be said for regions to the east and northeast of Harris County. An extreme rainfall event has unfolded over Jefferson and Orange counties. This morning, the National Weather Service took the rare step of issuing a “Flash Flood Emergency” for the region, warning residents that, “This is an extremely dangerous and life-threatening situation. Do not attempt to travel unless you are fleeing an area subject to flooding or under an evacuation order.”

This is really a terrible situation for Beaumont, Winnie, and surrounding areas. There are major issues on Interstate 10 through the area. Some areas have seen 10 to 15 inches of rainfall during the overnight hours, and a thick band of rainfall due to Imelda continues to saturate the area. It is showing no signs of movement for the next couple of hours.

With respect to Houston, we have some concerns later today. A related band of storms near Conroe and Cleveland will likely eventually sag southward today. Accordingly, parts of Montgomery County will likely see 2 to 8 inches of rainfall today, and some northern and central parts of Harris County may see 2 to 6 inches of additional rainfall. These areas of the Houston region have, until now, been somewhat unaffected by Imelda’s heaviest rains so they can probably handle most of this rain. That doesn’t mean there won’t be any street flooding, however.

We probably will see some additional storm development later today, but most of the activity should wind down this evening as the sun sets. By Friday morning we should return to more normal, business-as-usual pop-up thunderstorms. So we’ve all got to get through one more day of Imeda-misery. We will update accordingly.

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.

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.

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.

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.