Severe Weather Awareness Week: flooding in Alabama can be serious
Areal flooding, river flooding and flash flooding all occur here
MONTGOMERY, Ala. (WSFA) - It may not come across as a type of severe weather, but flooding does fall into that category. Flooding is actually the 2nd-leading cause of weather-related fatalities in the U.S. based on data for the last 30 years.
So it’s vital that during Severe Weather Awareness Week in Alabama we discuss flooding, the different types of flooding and forecasting flooding. Like our discussions about tornadoes, damaging thunderstorms, hurricanes, and winter storms, this is an attempt to increase awareness and preparedness about a potentially dangerous weather phenomenon.
Just two feet of moving water can pick up and sweep a car, truck, van, or SUV away. What makes that fact particularly dangerous is it’s often difficult to see just how deep or strong floodwater is. What may look like an inch or two of standing water could be much worse. That is especially true at night; flooding becomes more dangerous when it’s dark outside.
TYPES OF FLOODING ALERTS...
There are a few types of flood-related alerts that you may see issued by the National Weather Service here in Alabama. You have a Flood Advisory, a Flood Watch, a Flood Warning, a Flash Flood Watch, and a Flash Flood Warning.
Those alerts are issued based on the type of flood threat we are set to experience. There is areal flooding, river flooding and flash flooding. Areal flooding is usually a result of prolonged and persistent moderate to heavy rainfall that causes gradual ponding or buildup of water in low-lying, flood prone areas. This threat would be covered by a Flood Watch/Flood Warning.
River flooding is when a river or stream overflows its banks as a result of recent heavy rainfall or prolonged rainfall over the particular river’s drainage basin. This causes areas that are normally dry to become inundated, such as low-lying areas near the river’s bank. River flooding is covered by a Flood Watch/Flood Warning as well with a focus on rivers.
Flash flooding is typically the most dangerous and severe type of flooding. Flash flooding usually occurs within 6 hours of heavy or intense rainfall. Quick rises often happen, resulting in a dangerous situation that can be a significant threat to life and/or property. This is covered by a Flash Flood Watch/Flash Flood Warning.
There is also a Flood Advisory that can be issued. This is issued more frequently than the other alerts and is issued when flooding that is not considered a significant threat to life or property is expected or occurring.
PREDICTING THE EXCESSIVE RAINFALL/FLASH FLOODING THREAT...
Ahead of any threat for heavy rain and flooding you may see us show outlooks issued by the Weather Prediction Center that highlight areas at risk. These excessive rainfall outlooks are very helpful because they allow us to show who has the greatest risk of experiencing very heavy rain and flooding.
There are 4 categories that can be used: we call them “low,” “medium,” “high,” and “very high.” If you go to the official Weather Prediction Center site you will notice the categories are labeled “marginal,” “slight,” “moderate,” and “high.” Our naming system is just to provide an even simpler way of describing your threat for flooding.
If your town or city is located in one of the threat colors, you have a chance of seeing heavy to extremely rainfall during the period in which the map is valid. For the green “low” risk, there is at least a 5% chance of seeing excessive rainfall leading to flash flooding within 25 miles of your location. In the yellow “medium” risk, you have at least a 15% chance of that occurring. For the red “high” risk, your chance is at least 40%. And for the pink “very high” risk, your chance is at least 70%.
It’s not often that we see high and very high risks issued, but those are days that you should treat very seriously. Any of the threat categories should be treated seriously, but when you get into those top two categories you’re looking at a day or night with a significant to extreme risk of flooding/flash flooding.
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