Residents living south of the San Saba River in San Saba County have been ordered to evacuate due to river surging.

"Life-threatening flash flooding" is ongoing in Kerr and Gillespie Counties -- including the areas of Kerrville, Comfort, Ingram, Hunt, Mountain Home, Waltonia, Harper, Kerrville-Schreiner Park and Cypress Creek -- according to the National Weather Service.

The Weather Prediction Center first issued a Level 2 out of 4 flash flood advisory for Monday, but has now extended the threat into Tuesday. A flood watch is now in effect for Monday.

The National Weather Service warned that the Guadalupe River could surge to nearly 15 feet—5 feet above flood stage—by Sunday afternoon.

Flash floods in Texas have killed at least 107 people over the Fourth of July weekend, with more than 160 still missing.

Scholars and designers of early warning systems say that there are still huge gaps in our ability to predict flash floods and warn those at risk.

Here's what to know about the deadly flooding, the colossal weather system that drove it and ongoing efforts to identify victims.

Heads up if you're in the Hill Country or know someone who is. The threat for flash flooding in the areas devastated by last Friday's flooding have increased.