Anatomy of a Flash Flood Watch

A Flash Flood Watch is in effect for most of Alabama from midnight tonight through tomorrow evening, and a Flood Watch is up for the Tennessee Valley.

A serious flood threat could develop tonight and tomorrow for a large part of Alabama. The heavy rain from last Saturday amounted to around 2 to 5 inches in total for most of Alabama, rivers are running fairly high, and the ground is still very wet; those antecedent conditions could cause any additional heavy rain greater than 2 or 3 inches could cause “flashy” creeks and streams to overflow their banks quickly. Village Creek and the Cahaba River are both rather “flashy” because of how quickly they rise when heavy rain moves in.

Here is the hydrograph of Village Creek at Avenue W in Ensley. Notice how the creek “flashed” from around 2.4′ to over 7′ in about 9 hours on Saturday as the heaviest rain ended and all the water from the surrounding area ran down into the creek.

Village Creek Hydrograph

Village Creek runs from western Jefferson County’s hills to near the Birmingham Airport, and on Saturday, it was not impacted by the heaviest rain; if the band of 3-5″ had fallen over Birmingport, Mulga, and the area surrounding Bayview Lake, the “flash” would have been much more significant. It probably would have spiked up above the flood stage into the 10-13′ range. At 11.5′ (just 1.5′ above flood stage) Avenue W becomes impassable; at 12.5′ some of the residential areas in Ensley have to be evacuated.

So, this is why a flash flood watch is in effect for tonight and tomorrow. Soggy ground, higher-flowing streams and rivers, and locally-heavy amounts of rain exceeding 3 inches could cause some quick rises on smaller streams and rivers such as Village Creek, the Cahaba River, and a number of smaller, lesser-known creeks that tend to flood in heavy rain events. The two things that matter most are obviously where the heaviest rain falls and the antecedent conditions.

Computer guidance can only take us so far with this situation. The models do a good job with the patterns that create heavy rain, but they are often less-than-spectacular when it comes to picking out the localized spots where the heaviest of heavy rain comes down. For instance, here are two “runs” of the same model. One run started at 9 PM, and the second run started at midnight:

Model Differences

It’s clear to see that the model “knows” some heavy bands of rain are likely; however, it cannot tell exactly where they will line up.

What should we make of it? First of all, as I always say in posts like this, models are “guidance,” not gospel. They are to be used to find a message instead of precise forecasts of rain amounts and timing. The message here is that heavy rain is likely, and the biggest totals could be as much as 4 or 5 inches. If that four or five inches falls over a region of hilly terrain, flashy creeks and streams (and even smaller rivers) in the valleys below will rise quickly. For Village Creek, that would have to happen over the creek and just north of it. For the Cahaba, it would need to rain heavily in the Trussville and Leeds areas. There are a number of other small streams that could be affected the same way, but we do not have specific streamflow information to answer questions about each individual place.

Larger waterways are affected by flooding about 1 to 3 days after the rain has ended. All of the swollen tributaries dump out into rivers like the Tennessee, Coosa, Tallapoosa, Alabama, and the Tombigbee. If 5 inches of rain falls in the basins of any of these rivers, there is a good chance that they could get close to or exceed flood stage sometime on Thursday or Friday.

Interested in learning more about hydrographs, flooding, and streamflow? The Southeast River Forecast Center’s page is nice and has a lot of up-to-date information: SERFC. To see the antecedent conditions that could lead up to an flooding event, this page from the Advanced Hydrologic Prediction Service allows you to look back at this week’s rain, yesterday’s rain, or the rainfall so far this year: AHPS. Above all, flood safety is something to always be aware of! The slogan used by the National Weather Service is “Turn Around, Don’t Drown!” Follow this link for more on Flood Awareness.

-Jason
Follow me on Twitter: @simpson3340

Category: Alabama's Weather, Met 101/Weather History