Up to a foot of rain is possible in areas of Gulf Coastal Texas as repeated rounds of heavy rain target the Lone Star State. The heaviest is expected through Thursday morning in a strip stretching from Corpus Christi northeast toward Houston.
Preparations are underway now for a potentially significant flood event south of Interstate 10 and east of Route 281 in Texas. A tropical disturbance meandering in the western Gulf will sit and spin through the midweek period, sweeping incessant bands of heavy rain and thunderstorms onshore. While wind and coastal surge will not be an issue, a high-end rain event appears to be underway.
While there is uncertainty as to where the greatest totals will be tallied, a widespread 5 to 8 inches is likely with a few spots in between approaching a foot.
Despite the horrific scenes fresh in our minds from Hurricane Harvey, flooding of that proportion is not anticipated with this system. That said, some areas will probably see flooding, particularly in urban areas where concrete and pavement make the ground impermeable. Small streams and creeks may overflow their banks in a few instances, and inundation of low-lying areas will probably occur. The extent of the flooding may be limited as the rain occurs intermittently over a series of days rather than all at once and/or nonstop.
Showers and thunderstorms will continue to blossom and spread northward into the Houston area through Monday afternoon. After sunset, the axis of heaviest rainfall will shift farther southeast, with the tropical firehose setting its sights on Corpus Christi. As that first band begins to fizzle into Tuesday morning, a second vigorous moisture conveyor belt looks to set up over Houston/Galveston, wobbling back and forth a bit until the rain eventually tapers down Wednesday night and Thursday.
Two main features will drive the development of heavy rain in eastern Texas. The first is a rather unorganized blob of convection previously associated with a tropical wave of energy at the surface. This blob is loaded with moisture, itching to fall out of the clouds in the form of drenching downpours.
The energy at the surface will get an assist from low pressure in the upper levels hovering near the Gulf Coast of Texas. Both the surface and the upper level lows will slowly push into eastern Texas by Tuesday, aligning in such a way as to send wave after wave of heavy rain toward the Houston/Galveston area.
Exactly where the axis of heavy rain will set up is a point of contention. Uncertainty will probably remain high for the duration of the event as model forecasts have been a moving target so far.
One piece of good news in all of this is, believe it or not, Texas needs the water.
The U.S. Drought Monitor lists communities near Rockport and Refugio as being in “extreme” drought. Surprisingly, this is the same exact area that was razed last August by the eyewall of landfalling Category 4 Harvey. Houston’s William B. Hobby Airport has received 16.51 inches of precipitation since January. That is about 6 inches below normal. Corpus Christi is down a whopping 7 inches. In fact, Corpus Christi has picked up only 4.74 inches since Jan. 1, less than 40 percent of what should have accumulated as we head into the heart of summer.
Because of this, the ground is dry. That means the soil can afford to be a bit more absorbent, cutting down on the overall flood risk. In other words, Texas’s tolerance for heavy rainfall is a bit higher. That said, a few downpours may drop more than 2 inches per hour, so training of storm cells could easily overwhelm storm drains and lead to major issues in a few spots.
Houston has never recorded a three-day period with a foot or more of rain – aside from during Harvey last August. Records at the airport date back as far as 1969. Moreover, six of the top 10 heaviest one-day totals have been since 2000, highlighting the trend for flooding rains to become increasingly common. Adding one such stretch to the record books is certainly possible this week.
As we look toward the future, it is likely these events will become part of the new normal. As the temperature of Earth increases, the atmosphere’s ability to contain and unload moisture does, as well. Climate scientists have found that for every degrees Celsius the climate warms, the air can hold 7 percent more water.
Since 1970, Houston has warmed more than 3 degrees Fahrenheit. In that time frame, Houston’s average annual rainfall has climbed about 4 inches. While that eighth-of-an-inch per year increase may not sound like a lot, it is the equivalent of seeing an entire month “extra” of precipitation nowadays.
While it is impossible to attribute a single event to climate change, it is correct to say events of this nature will be more common in the future, and events that do occur are likely to be more extreme.
Capital Weather Gang’s Greg Porter contributed to this post.