High Risk of Flash Flooding in Texas as Remnants of Patricia Arrive (FORECAST)

Very dangerous flash flooding continues to develop over parts of Texas, and the situation will only worsen this weekend as moisture associated with Hurricane Patricia adds to an already water-logged storm system.

For the second time this year, flooding is striking drought-parched areas of Texas. A series of heavy rainfalls in May dumped more than 20 inches of rain on parts of the state, virtually wiping out a long-term drought and causing destructive flash floods. Tropical Storm Bill brought another round of widespread heavy rain in June. After that, the state snapped back into drought, setting the stage for wildfires earlier this month.

(MORE: Drought Returns Again | May Floods Leave Death, Destruction)

Some of the same areas hard hit in May and June have already been hit hard in the last couple of days. On Friday, the National Weather Service issued a flash flood emergency for portions of Navarro County, and another such dire bulletin was issued Friday night.

In that county, the Corsicana Airport recorded 17.50 inches of rain in the 22-hour period ending at 4:53 a.m. CDT Saturday. Numerous roads in the city were left impassable from high water Friday, and by Friday night high-water rescues were reported on at least two stretches of Interstate 45 through the county.

Rainfall totals of 5 to 10 inches were common across the Dallas-Fort Worth Metroplex and areas just to the south and east Thursday and Friday.

On Saturday morning, torrential rains pushed south into the Austin area, dumping up to 7 inches of rain in 3 hours and flooding numerous low water crossings in the area.

Here are the details on the heavy rain threat and how the tropics may be involved.

Heavy Rain, Flood Threat in the Southern Plains

Persistent southeasterly winds are pumping moisture from the Gulf of Mexico into the southern Plains, where it will interact with an area of low pressure in the upper atmosphere. This will contribute to heavy rains across much of Texas through Saturday, expanding east into portions of the Lower Mississippi Valley and Gulf Coast by Sunday and Monday.

Flash flood watches have been issued by the National Weather Service for much of Texas, including the Dallas-Fort Worth metroplex. The watches were recently expanded east into portions of Louisiana.

Areas of rain and thunderstorms will spread across central to eastern sections of Texas, southeastern Oklahoma and into parts of northwest Louisiana and central Arkansas through Saturday. A separate weather disturbance moving through the southern tier of the U.S. will prolong the heavy rain threat into Sunday across those states.

In addition, moisture and energy from Hurricane Patricia in the eastern Pacific will factor into this setup this weekend. Computer forecast models are showing that the mid-level remnants of Patricia will get pulled northeastward toward the Texas Gulf Coast by a southward dip in the jet stream over the western and central United States.

(MORE: Category 5 Hurricane Patricia Bears Down on Mexico)

In all of the above mentioned areas, flash flooding will occur where heavy rain persists the longest.

The greatest risk Saturday and Saturday night is over central and southeast Texas, including Austin, San Antonio and Houston. Widespread rainfall totals of 5 to 10 inches are expected in this region along with locally even higher totals. Despite the recent drought, rainfall rates of 2 to 4 inches per hour will be more than enough to cause rapid runoff and flash flooding.

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In addition, persistent onshore winds will bring some minor coastal flooding, high surf and dangerous rip currents through at least this weekend to the Texas and Louisiana coasts.

Tides are expected to run at least 1 to 2 feet above normal for several tidal cycles over the next few days for much of the Texas coast, eventually eastward to as far east as the New Orleans area by Sunday and Monday.

Coastal flood advisories have been issued for portions of the Texas and Louisiana coasts, where widespread minor to moderate coastal flooding is expected.

(FORECASTS: S. Padre Island | Corpus Christi | Houston | Lake Charles, Louisiana)

While the rain is causing dangerous flooding, it will at least alleviate the drought that set in over the southern Plains and Lower Mississippi Valley from mid-summer on.

(MORE: Expert Analysis | Hurricane Central)

Timing the Flood Threat

Saturday:

• Main Threat Areas: central Texas to start the day, then spreading east across Texas, into southeastern Oklahoma, Arkansas and possibly western Louisiana.
• Cities: Austin | Dallas-Fort Worth | Houston | Little Rock

Sunday:

• Main Threat Areas: eastern Texas, Louisiana and southern Arkansas.
• Cities: Houston | Lake Charles | Shreveport

Monday:

• Main Threat Area: Louisiana, although a broad area of scattered light rain is expected from eastern Texas to as far east as parts of Georgia. 
• Cities: Lake Charles | New Orleans | Shreveport

Improvement Later Next Week

Low pressure and the swath of greatest moisture are forecast to move north through the Mississippi Valley early next week.

This will be in response to an upper level trough picking up the energy and merging the system with another surface low, diving east through the north-central states.

The result will be a decrease in intensity of rain Monday into Tuesday across the Texas and Louisiana vicinity, with drying conditions for the middle of the week.

At the same time, coastal flooding concerns will ease after Monday as low pressure lifts north and away from the Deep South.

PHOTOS: May 2015 Southern Plains Flooding