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A “multi-day heavy rain” event could spark flash flooding across the Rio Grande Valley and the Ranchland counties in deep South Texas starting Thursday, according to the National Weather Service in Brownsville (NWS).

Meteorologists say chances of excessive rainfall have increased since initial forecasts on Monday. As of Tuesday morning, the Valley is under a slight to moderate risk of excessive rainfall. And while severe weather is not expected, stronger thunderstorms could produce small hail, the NWS said in an email advisory on Tuesday, September 29.

The weather threat is expected to come as the remnants of Hurricane Polo and tropical moisture in the Pacific Ocean and Gulf of Mexico “interact with… (a) slow-moving cold front” that will push southward into Texas later this week. The colliding weather systems are expected to produce so-called “training” showers and thunderstorms — waves of rainfall that dissipate and reform over the same areas repeatedly. That could spark flash flooding.

“Most vulnerable are urban areas, roads, small streams and washes. Isolated significant flash floods possible. Floodwaters may cause road closures,” the NWS said.

All four Valley counties, the Northern Ranchland counties, and the southernmost portions of the Texas coast are projected to see between 2 and 4 inches of rain over 48 hours Thursday into Friday, with heavier rain expected closer to the border, according to NWS forecast models.

Areas west of U.S. 281 — also called Interstate 69-C — are at moderate risk for excessive rainfall late Thursday into Friday morning, with a 10% chance those areas could see as much as 4 to 6 inches of rain by the time the precipitation tapers off on Saturday, the NWS said.