WeatherWeather Science


Flash flooding: How warnings are issued and what you need to do

Posted at 7:13 PM, Jul 22, 2019

Summer severe weather requires a well developed coordination between the National Weather Service (NWS), media, and emergency managers. The NWS in Pueblo has specific criteria when issuing warnings. Our job in the First Alert 5 weather center is to relay those warnings to you immediately. A severe thunderstorm warning is very straight forward across the country. They are issued when storms may produce 58 mph winds and hail of 1 inch in diameter.

But, flash flood warnings are more complicated and depend on many factors. Greg Heavener, the Warning Coordination Meteorologist at NWS Pueblo says, "flash flood warnings across the region can vary based on topography, population, urban versus [rural areas], but even off of burn scars as well. We have three highly active burn scars across our area; Spring Creek, Junkins and Hayden pass. They will typically have lower thresholds of rainfall for any given hour."

Criteria for flash flood warnings

More specifically, urban areas with concrete and pavement, like Colorado Springs and Pueblo, have a threshold rain rate of 1- 2 inches per hour. Burn scars only require a rainfall rate of 0.5 inches, or less on newer scars, for a warning to be issued. This is also terrain dependent. An area like Manitou Springs, with sloped topography that funnels water, will have a lower threshold than the Plains, where the terrain is relatively flat. This criteria is different for all areas of the country. Another aspect that will impact southern Colorado this week, is the soil moisture. Recent rains can lower the threshold needed for a warning, due to the soil being saturated already. Saturated soil has a harder time taking in more water.

What to do when a warning is issued

Heavener reinforces the phrase, "turn around, don't drown". If you come across a flooded roadway, you should find an alternate route. A foot of flowing water can float a car and carry it away. Plus, a flooded road may not reveal the full depth of water or if the road has been washed away. If a warning is issued in your area, it is best to stay home and avoid roads. If you are outdoors, find higher ground.

How to receive warnings

Smartphones are automatically equipped with emergency alerts. On IPhone, go to Settings - Notifications. Scroll to the bottom to find Government Alerts. You can toggle on emergency alerts. On Android, open your messaging app and scroll to the bottom. Tap menu - settings - emergency alerts.

These options will only give you tornado and flash flood warnings. If you would like to receive severe thunderstorm warnings, and much more, you can download the First Alert 5 weather app for free in the app store.