Everyone knows about windspeed differential in doppler velocity mode. It replaced the hook echo 25 years ago as the strongest indicator of a rapidly rotating mesocyclone. It is the first indication of spin turning from horizontal to vertical (i.e. pointing down at the ground). As we know, mesocyclones don't always touch down. My understanding is that this is the trigger for the NWS to issue tornado warnings.
The flooding equivalent of this does not require human intervention or cell service or landlines.
By my count on the topo map there are ~100 upstream tributaries above Hunt on the South and Central branches of the Guadalupe. Tributaries is really the wrong word, they are giant rain gutters carved from granite.
Let's speculate that there was a gauge measuring 1) flow in cubic feet per second and 2) the speed of the rise in water level. If that data stays logarithmic for all gauges you have X amount of time before the river rises to Y level at speed Z downstream. We don't need AI to predict the results. It's basic fluid dynamics and hydrology. In a logarithmic situation there is "more" time and you might not need sirens to stop death in the ancient river channel (which is half a mile wide). Anything in that channel, as we witnessed, is toast. It would be common sense not to build structures or sleep in them anywhere in that half-mile-wide channel, but that's another topic.
I've kept this clean, and used capitalization and punctuation, which as everyone knows, is not my forum style. But I have to break with civility for a moment.
A quick aside to address the KerrCo "people" who resisted sirens. You dumb sonsofbitches, you live in fucking TEXAS, where we have TORNADOES ALL THE FUCKING TIME, and you don't want storm sirens? "Sirens wouldn't have helped in time" is a LOSER EXCUSE (TM-wheels). KerrCo "leaders" LOST a lot of innocent tourists and LOST 2-dozen+ young girls. Kerr County and Kerrville leaders and their base are LOSERS.
"Sirens wouldn't have helped in time" grinds my gears.
The Prestige: if more than, say, 4 or 5 of those gauges show data going linear, i.e. exponential, the gauge should fire off a "distress signal", a "radio beacon", it could even use off-the-shelf ADS-B commercial aviation transponder hardware. No cells. No landlines. No humans. The beacon goes off, the sirens turn on. Why? Because God has sounded the alarm.