Another half a foot of rain is possible in flood-pounded areas from central Texas to Mississippi this week

By Jennifer Gray, CNN meteorologist

A quarter of the state of Texas has been experiencing exceptional drought — the most extreme level of drought — and suddenly, the skies have opened up above the Lone Star State, nearly wiping away their summer rainfall deficit in 24 hours.

More than nine inches of rain has fallen at Dallas-Fort Worth International Airport, making this the second-rainiest 24-hour period on record. Unfortunately, too much has fallen, too quickly.

More rain is in the forecast: “Additional Rainfall amounts of 2 to 4 inches are possible in the warned area. Flash flooding is ongoing or expected to begin shortly,” said the National Weather Service office in Dallas/Ft. Worth in its latest flash flood warning.

The situation in Texas is still unfolding. For the latest on what’s happening and more videos from the Dallas-Fort Worth metroplex, follow along here.

“Trained weather spotters reported major flash flooding ongoing across Dallas with numerous roads and cars submerged, including Interstate 30 at Interstate 45 near downtown Dallas,” according to the flash flood warning statement issued at 3:21 a.m. CT.

This past week’s drought monitor mentioned how the same region in Texas has been 8-12 inches short on rainfall for the past six months. Much of it has now been erased, in just a few hours.

Texas has gone from a flash drought to flash flooding in only 24 hours; a real weather whiplash.

Timing out the torrential rain and flash flooding

While the heaviest rain is expected to move out of the Dallas Metroplex today, the rain for Central Texas, and other areas across the south is just beginning.

“Additional rainfall of 3-6 inches with isolated pockets of 7+ inches are possible over the next 5 days from central Texas to central Mississippi,” the Weather Prediction Center wrote in its morning weather discussion.

The extreme rainfall will stretch from the Austin/San Antonio region all the way east to Central Mississippi.

The culprit is an upper-level trough (an unstable line of low pressure high in the atmosphere), which is moving very little, resulting in extreme rainfall hitting the same areas for days on end.

“Much of this rainfall will be beneficial and welcome due to the effects of an ongoing drought, but the potential still exists for instances of flash flooding in urban areas and places with poor drainage,” the Weather Prediction Center explained.

In some of the more intense downpours, we could see rainfall rates of 2-3 inches per hour. Some storms could train one behind the other, resulting in torrential downpours lasting for long periods of time without breaks.

The National Weather Service forecast office in Austin/San Antonio is forecasting up to 3 inches of rain, with the potential for 5 inches in some of the harder hit areas. In Jackson, Mississippi, they are forecasting even more.

“This torrential rainfall will occur over the course of a few days. Global models show some 4-9 inches for some areas with locally higher amounts over the course of 2-3 days,” noted the National Weather Service office in Jackson, Mississippi.

I spoke with Greg Carbin, chief operations officer at the Weather Prediction Center. He said the bigger flooding concern is for the areas where the soils are already saturated.

“Northwest Louisiana, Southwest Arkansas, extreme Northeast Texas, parts of East Texas — all of those areas over the last two weeks are a few inches above normal for precipitation. That would add to the vulnerability if you get excessive rainfall in those areas, which it looks like we will. This whole mass of precipitation is moving east into that area today,” Carbin emphasized.

For some of these areas there is up to 90% soil moisture, according to NASA imagery, “… so any additional rainfall will create more dangerous flooding conditions,” the Weather Prediction Center stressed.

Imagine having a sponge 90% full of water. It leaves very little sponge left to soak up any additional moisture before it starts to leak out or overflow. The same is true with the ground.

The rain will occur in rounds between now and the weekend. Hours of very heavy rain will be followed by a dry period, then start up again. The heaviest rain will occur between now and Wednesday, then it will be spottier through Sunday.

“The models then indicate activity may gradually decrease from Tuesday afternoon into Tuesday evening as the atmosphere could become overworked and needs to take some time to recharge (destabilize),” the National Weather Service office in Austin/San Antonio explained.

I guess if there is a bright spot in all of this, it’s the temperatures. Some areas will feel more than 20 degrees cooler than in the last few weeks. The rain and cloud cover are keeping the relentless summer heat at bay, during this week of weather whiplash.

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