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This is sort of on the more realistic side of worldbuilding, but maybe somewhat exaggerated for the sake of a setting. What's the fastest atmospheric pressure drop in weather records? This question is kind of more asking how quickly a flash flood could happen, especially maybe a decade in a future. Of course, heavy rainfall contributes to that, but a storm surge caused by wind and pressure is what I'm interested in.

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    $\begingroup$ Define flood. My street can flood in 30 minutes of hard rain. $\endgroup$ – paparazzo Jan 10 '17 at 23:23
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    $\begingroup$ In a flash. Can you define the geography and weather conditions you're talking about? $\endgroup$ – Samuel Jan 10 '17 at 23:30
  • $\begingroup$ A surge because of a strong storm or a limited strength hurricane, and for an area that's at sea level. $\endgroup$ – Tom Jan 10 '17 at 23:38
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    $\begingroup$ You ask about atmospheric pressure, but flash floods often occur far outside the area where the rain actually fell. youtube.com/watch?v=ORZQUlk8vxg $\endgroup$ – Cort Ammon - Reinstate Monica Jan 11 '17 at 0:21
  • $\begingroup$ Practical example: Northern California last Sunday. Get a heavy warm rain on top of a good mountain snowpack, and it doesn't take long at all. $\endgroup$ – jamesqf Jan 11 '17 at 4:12
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Global warming ==> big hurricane with storm surge ==> landfall into a funnel-shaped bay ==> huge wave/flood up the bay!

FYI, the sharpest barometric pressure drops are (I'm pretty sure) inside tornados -- but those are too short-lived and move too erratically to cause the sort of flooding you're seeking.

Instead, consider the phenomena knowns as Tidal Bores:
https://en.wikipedia.org/wiki/Tidal_bore which tells us that the largest one on earth is:

Qiantang River, China, which has the world's largest bore, up to 9 meters (30 ft) high, traveling at up to 40 kilometers (25 mi) per hour

So this gives you both a ballpark flood height and a speed.
It's apparently popular for extreme surfing (video; I couldn't make this up): https://www.youtube.com/watch?v=XvO5Ckd7mlU

Now, imagine the storm surge from a big (possibly global warming enchanced) hurricane, making landfall into a bay/river prone to tidal bores. Head for high ground.

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My creek flooded in 10 minutes. One moment I was standing on the bank, the next I was watching my dumpster float away, after that, I was carrying my baby brother and my precious belongings up the hill because we could not get out of the valley. So flash floods can happen fast.

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  • $\begingroup$ Thank you for the answers guys. Just a part of an action scene I was imagining. I might end up using a flood not caused by weather, but I'm not sure, so I appreciate the answers to this somewhat vague question. $\endgroup$ – Tom Jan 11 '17 at 17:45

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