Harvey Phase ll: Chaotic river flood patterns

The warnings from meteorologists last week were alarming and dire.

Tweets from meteorologists across the country got relatively little attention when they started flying last Wednesday. That's when models began to show Harvey would rapidly intensify and stall over Texas.

It turns out many of the apocalyptic model rainfall forecasts from last week were largely accurate.

The National Weather Service used new language last weekend to describe this unprecedented flood event.

Tonight the developments continue to unfold from of one of the biggest rainstorms in U.S. history.

Phase ll

Now Phase ll of Harvey centers on a different problem. How does the hydrology of the Houston area handle the unprecedented yet uneven rainfall and runoff patterns?

We're starting to find out. Flood control managers are faced with some tough and untested choices about how much water to release from area reservoirs.

Real world experiment

Houston is no stranger to floods. But the area has never seen this much rainfall over a large area surrounding the city. Flood control systems have never been tested at this level.

Rainfall vs. runoff-driven flood patterns

It may sound like semantics if the water is rising around your home. But the flood patterns around Houston over the next 48 hours may be different from what we saw over the weekend.

Here's why.

• The unprecedented rainfall totals between 20" and 40" were not evenly distributed around Houston.

• Heavy rain fell in different local watersheds. (Spatial variation)

• Heavy rain fell at different times. (Temporal variation)

• The result will be chaotic local flood patterns as flows peak through different river systems at different times.

• Rainfall rates may be lower than last weekend, but many rivers will still rise.

• Flooding will be more runoff and less rainfall dependent over time.

• Locations that escaped major flooding last weekend may see peak flood levels in the next 48 hours.

So one thing to watch over the next 48 hours is how river basins around Houston react to unprecedented flow rates moving through the region.

Flood control systems are stressed to the max and failures can occur with little warning.

The rain may be easing off over the next 48 hours. But all that water will still be running hard to get to the Gulf of Mexico. And many of those pathways run through the City of Houston.