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

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.

"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.

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.

In the Midwest, quarter-sized hail and an isolated tornado cannot be ruled out, while in the South, flood alerts are in effect for areas that were devastated by flash floods last week.

Straighter, smoother streams flow more violently than those that meander due to a reduction in friction. Areas with steeper terrain can see water accelerate downhill at a much faster rate causing a more destructive flash flood than flatter, open areas.