We have another flooding event upon us at NWS Huntsville, giving 9 straight days now with measurable precipitation. Moderate flooding is occurring or forecast along some of the Tennessee River tributaries such as the Big Nance and Paint Rock Rivers and minor flooding along the Tennessee River. Over 5 inches of precipitation have been measured at the Huntsville International Airport during this time, while amounts around 6 to as much as 10 inches of rainfall have been reported or estimated by regional radars in some areas. This type of rain can often lead to flooding here in the Tennessee Valley, but even heavier precipitation has fallen in the past with barely any flooding. Such was the case with the passage of the remnants of Tropical Storm Lee across the area in early September 2011. One of the important factors to consider when assessing the potential for flash flooding or areal flooding is antecedent soil moisture conditions. Historically, this has been difficult to do, and forecasters often have to make estimates based largely on in-situ observations from very few sites, or from anecdotal evidence and subjective experience. This is the reason the SPoRT LIS has become a popular tool with forecasters, particularly in the past year, when trying to assess flooding potential. As early as the 4 am discussion on January 8th, preceding the prolonged heavy rain event, forecasters noted that “the NASA LIS indicates that soil moisture [values] are already fairly high…”.
The SPoRT LIS images below show the 0-10 cm relative soil moisture (Image 1) and the 0-200 cm relative soil moisture (Image 2) on the morning of the 7th (which would have been the latest imagery available to forecasters at the time).
Notice that relative soil moisture values (wilting vs saturation point) in the 0-10 cm layer exceeded 60 percent across much of the area. In the deeper 0-200 cm layer, values exceeded 65 percent across a large portion of the area. Using these data so far (and this is still subject analysis at this point), forecasters have noticed that when values exceed about 60% and the area receives a “standard” 1-3 inch synoptic rainfall event, flooding issues often arise. SPoRT LIS soil moisture values were referenced in the Area Forecast Discussion Product not only on the morning of the 8th, but also on the morning of the 9th and the afternoon of the 12th. These data are quickly becoming a valuable situational awareness tool for forecasters here at the Huntsville NWS office. Not only are the soil moisture values being used to assess flood potential, but also for drought monitoring and local modeling efforts. For future efforts, we hope to work with the SPoRT team to conduct more case studies, and determine thresholds based on proper objective analysis.