What’s a ‘flash flood warning’ anyway?

Your NOAA weather radio and your smartphone are working overtime lately cranking out flash flood warnings.

The onslaught of flash flood warnings this week got me thinking. Precisely what does a "flash flood warning" mean? And why do they seem to be happening more often lately?

National Weather Service: Over 100 products

Believe it or not, The National Weather Service has about 122 different types of watches, warnings and advisories they send out for various weather phenomenon.

Even for meteorologists, the list of NWS products can be overwhelming. This "periodic table" poking fun at the number of NWS watches, warnings and advisories was a hit at this month's National Weather Association conference.

So it's not surprising that there is confusion about certain types of NWS warnings.

What is a 'flash flood warning?'

A flash flood warning is what it suggests. Flooding that can happen in a flash, meaning rapidly. The National Weather Service issues flash flood warnings when flash flooding is imminent or occurring.

My MPR News weather colleague meteorologist Bill Endersen adds this excellent contribution.

Another key aspect of flash floods is that they occur where you would not normally expect significant water accumulation such as intersections, homes and campgrounds. Flooding, in contrast, occurs when bodies of water rise and flood out of their basins or banks.

Also of note, flash flood warnings are generally short duration events issued for several hours. Flood warnings may be issued for days at a time, and are usually for specific river systems.

Here's the NWS description of a flash flood warning.

Flash Flood Warning: Take Action! A Flash Flood Warning is issued when a flash flood is imminent or occurring. If you are in a flood prone area move immediately to high ground. A flash flood is a sudden violent flood that can take from minutes to hours to develop. It is even possible to experience a flash flood in areas not immediately receiving rain.

Flash flood forecasting and warnings

How does the NWS decide when to issue flash flood warnings?

The NWS uses county-level flash flood guidance to keep track of how much rain in certain period it will take to cause flash flooding. That number of inches depends on many factors, most notably soil moisture at the time of the event.

About Flash Flood Guidance:

National Weather Service (NWS) River Forecast Centers routinely issue Flash Flood Guidance throughout the day for every county in their area. The river forecast centers determine 1- 3- and 6-hour flash flood guidance values for all counties, and 12- and 24-hour values for parts of the eastern United States. The NWS Weather Forecast Offices use this guidance when issuing flash flood watches and warnings to the public.

Flash Flood Guidance estimates the average number of inches of rainfall for given durations required to produce flash flooding in the indicated county. These estimates are based on current soil moisture conditions. Note, in urban areas, less rainfall is required to produce flash flooding.

The last sentence raises a good point about flash flooding in urban areas. More pavement produces faster runoff that can overwhelm storm sewers. Flash flooding in urban areas can be more frequent and severe.

Here's a good resource on flood safety from NOAA.

Climate change role

It is increasingly clear that climate change is paying a role in increasing the increasing frequency and intensity of heavy rain events in Minnesota and across the U.S..

As predicted, a warmer atmosphere holds more water vapor capable of producing heavier downpours.

For every 1-degree Celsius of warming, the air above us can hold about 4 to 5 percent more water vapor. That in turn can increase rainfall by 10 to 15 percent in extreme rainfall events, according to climate experts like Kevin Trenberth with the National Center for Atmospheric Research.

These more extreme rainfall events are leading to a new breed of flash flood that quickly overwhelms drainage infrastructure built for a previous climate. Storm sewers, streets, basement drainage systems, and even sewage treatment systems are more vulnerable to being overrun by flash flooding as our climate warms.

Like it or not, flash floods are becoming an increasingly frequent and impactful part of Minnesota's climate system.