By Jim Stefkovich, Meteorologist, Alabama Emergency Management Agency
CLANTON – Sunday, 4:00 pm December 4, 2022
Several waves of heavy rainfall will affect areas north of I-20 from Monday through early Thursday. Total rainfall amounts from Monday through Tuesday morning will be 1-3 inches, and some areas could receive 3+. There is likely more rain in the forecast Tuesday afternoon through Thursday, which could produce an inch or more rainfall. Flooding is possible, especially since this area is already saturated from the strong to severe storms a few days ago.
The National Weather Service (NWS) may issue a Flood Watch by Monday.
I also want to quickly go over the official NWS river forecasts found on their homepages. You should be very careful in situations when heavy, widespread rainfall is expected from days 2-7 in the future.
The example I use is the forecast for the next five days which was produced this afternoon, December 4th, for the Paint Rock River near Woodville (between Huntsville and Scottsboro) in north Alabama.
As you can see, through the morning of December 9th, the official NWS forecast is for the river to remain near 3 feet, even dropping slightly Monday afternoon – Thursday.
Here’s the problem:
The NWS only puts in 24 hours or 48 hours (depending on the location) of rainfall amounts to determine how creeks/rivers will be affected across all of the state, then “assumes” no precipitation beyond 24-48 hours to produce their official 5-7 day forecasts.
So, in the example above, the forecast is assuming no rainfall from Monday afternoon into Tuesday, even though that’s when the bulk of it will occur.
A lot of times, this doesn’t cause any major problems to the river forecasts, especially if conditions have been dry. However, almost every winter/spring, when heavy rain has fallen, and systems are expected to produce more heavy rainfall 2-7 days in the future, highly inaccurate river stage forecasts can and do occur.
As shown in the next graphic, when putting in rainfall amounts for all of next week, the “most likely” river stage will be between 7-16 feet beginning on Tuesday, NOT 3 feet. That’s a huge difference.
The bottom line is if you depend on river stage forecasts for making critical decisions, please use these forecasts with extreme caution. The NWS is well aware of this issue and is working on a solution, but this is unlikely to occur through at least this coming spring.