With the potential for heavy rain and flooding over the coming days I wanted to review a couple of items. The National Weather Service produces a product called flash flood guidance. FFG allows a forecaster or anyone really to see how much rain it would take to cause flooding. There are various parameters such as 1 hour rain, 3 hour rain, and 6 hour rain. Back in May and June most of Texas had FFG values down towards half an inch to one inch meaning that’s how much rain it would take to cause flooding. Its a drastically different picture today with very dry soils across the eastern two-thirds of Texas. It would take over five inches of rain falling in a 6 hour period to cause widespread flash flooding across the eastern half of Texas. That’s not to say urban areas wouldn’t have problems heavy rain.
Once we start seeing soils saturate from rain the flash flood guidance values will become lower. Think of the ground like a sponge. It can absorb plenty of water for a while before becoming totally saturated. Once the sponge is saturated any additional water simply flows right out of it. Soil is similar in that once its saturated water just runs off and you start experiencing flash flooding. The good news is we really do need the rain and it looks like much of Texas is going to get it over the next few days.
Parts of Texas will likely get too much rain. The latest Weather Prediction Center rain forecast for the upcoming event has widespread 3 to 5 inch totals across a majority of Texas with ligher amounts in East Texas. 5 to 7 inch totals are shown across North Texas, parts of Central and South Texas. Various weather models say that localized amounts over 9-10 inches will be possible in those areas. So you see while the ground can take a lot of water – it cannot absorb all of what is being depicted. As it becomes more clear where the heaviest rains will fall I expect Flash Flood watches to be issued. Now is the time to start preparing for the threat of flooding across North Texas, Central Texas, and even parts of South Texas towards the coast. Hopefully we can get through this event without widespread flooding but it looks like we’ll have to deal with localized flooding at least.