Arthur is no longer a Tropical Storm, but its remnants will cause major problems for the central Gulf Coast region today. The National Oceanic and Atmospheric Administration has issued a rare “high risk” for excessive rainfall for today into early Friday. Here’s what you should know.

"High Risks are not issued lightly and indicate the potential for both widespread and life-threatening flash flooding," wrote the NOAA’s Weather Prediction Center early Thursday morning." Some parts of coastal Alabama, Louisiana, and Mississippi could receive 1 to 2 feet of rainfall over a very short period of time as the remnants of the first named storm of the Atlantic basin season creeps across the Deep South. Meteorology lecturer and geographer Jordan McLeod spent the past several years of his career at the University of South Alabama, which is in the bulls-eye.

“This is probably a conservative estimate in some locations along the Gulf Coast because this is designed to be a smoothed product,” McLeod wrote on his Facebook page. “It’s amazing how weak tropical systems (or their remnants) can produce such extraordinary rainfall totals, but we as meteorologists have been preaching that for years now," he went on to say. He’s right. I published a 2007 research study with my colleagues at the University of Georgia showing how the weaker hurricanes and tropical storms are the most robust rainfall producers rather than the stronger hurricanes.

A “high risk” is rarely issued, but if you see it, that means there is a 70% or greater chance of excessive rainfall and life-threatening flooding. High risk days are only issued by the Weather Prediction Center on 4% of days from 2010 to 2022. However, they accounted for a third of all flood-related fatalities and 80% of all flood-related damages. There is your “so what?” right there.

When I look at how much moisture is in the column of air, I understand why there is such a high risk. Meteorologists launch weather balloons to learn about the structure of the atmosphere from the surface to the tropopause. If you look at the plot of data, it’s called a sounding, from the weather balloon launch at Slidell, Louisiana, the two bold black lines (temperature and dewpoint temperature) are very close or touching each other through much of the entire troposphere. That’s a signal precipitable water content is very high. In other words, there is a lot of water vapor that can be converted to rain

If you are in affected regions, please take this seriously and do not walk or drive through flooded roadways.