Nearly one year after Hurricane Harvey bared down on the Texas Gulf coast as a category 4 hurricane in late-August, fears are growing among some residents in the Houston area as the most significant round of rain since then is set to pummel the region. This will not be another Harvey where up to 60 inches of rain fell over about a week-long time frame, but some locations will likely receive over a foot of rainfall, which will lead to flash flooding in some locations.
The main driver of this heavy rainfall event this week is a trough of low pressure moving onshore and into the Texas and eastern Mexican coasts. This area of spin in the atmosphere, paired with a deep plume of tropical moisture, will aid in the development of long-lasting, heavy rains. This tropical moisture may be record-breaking in some cases. In Corpus Christi, the daily record precipitable water values over the course of the month of June are almost all less than 2.5 inches. With this week’s event, values will surpass 3 inches in some areas. This basically means that there is an abundance of moisture in the atmosphere that this disturbance will use, which will therefore lead to heavy rainfall and flooding.
The impacts are already beginning this Monday as rounds of heavy rain move onshore and into coastal sections of Texas and Louisiana. For the remainder of the day, the rain will become more scattered to perhaps isolated across Louisiana, while the main impacts affect Texas. The NAM model paints a very concerning picture of the development of a training, feeder band of heavy rain setting up right over Houston and surrounding areas beginning during the mid-afternoon. The HRRR model also outputs a somewhat similar solution, but more of a broken up line of thunderstorms instead of a constant barrage of heavy rain into Monday night. Even if this situation doesn’t set up (which we hope doesn’t because it would lead to flash flooding in some areas), there will still be rounds of rain and thunderstorms that affect the area into the overnight hours.
Father south, heavy rain will also be commonplace, especially in the Corpus Christi area. Rounds of showers and thunderstorms will hang on near the coast this afternoon and tonight. These storms will also be slow-moving, so flooding will be a risk as well despite the terrible drought currently plaguing the region. The extreme southern portion of Texas will experience occasional precipitation into tonight, but it won’t be as active of a day as areas to the north.
Tuesday will likely be the most impactful day as the deepest and most widespread tropical moisture moves onshore from the Gulf of Mexico. Widespread precipitable water values of over 2 inches will be in place, which will definitely promote heavy rainfall. The morning will feature the worst impacts in our opinion, as an expansive, heavy rain affects southeastern portions of Texas. By this time, the rain will overspread farther inland, affecting cities like San Antonio and Austin. Now during the afternoon and evening hours, the shield of rain will begin to break up, but scattered showers and thunderstorms will continue to slam similar areas with heavy rainfall.
Now by Wednesday, this trough of low pressure, which is the ultimate driver of this heavy rain event, will stall over southern Texas. This now shifts the heaviest of rain and the greatest concern for flooding to the south. A new area of heavy rain may develop across this part of the state, impacting Austin, San Antonio, and surrounding areas. Even though we are referencing the southern portion of Texas, not all of this region will deal with the steady, intense rainfall. In fact, it will be more spotty down towards Brownsville. On Wednesday, northeastern Texas and the ArkLaTex will now begin to get in on the rainfall action with scattered showers throughout the day. The rain will move to the north and east overnight. while the heavy rain continues to sit over southern Texas. This may be shaping up to be a big flood event for this part of the state.
On Thursday, a cold front associated with a low pressure moving east from the Central Plains will attempt to clear the moisture out of Texas. It will first focus and enhance the moisture on Thursday across eastern Texas, but it will then disperse and decrease the higher areas of precipitable water. Therefore, more rain and thunderstorm will be likely across much of eastern Texas on Thursday. The flood threat will continue for yet another day.
It’s not until Friday when much of the state and surrounding areas dry out. Aside for a few spotty showers and storms, it will be dry statewide. That will foreshadow a dry weekend overall with the occasional, isolated storm around. The flood threat will come to an end beginning Friday, however.
In terms of rainfall, it will be plentiful. Some locations will receive over a foot of rain, while a widespread 1 to 3 inches impact the eastern half of Texas. Closer to the coast, rainfall in excess of half a foot will be commonplace, making this the most concerning area for flooding.