Here at the Huntsville National Weather Service Forecast Office (NWSFO), we’ve been using various SPoRT LIS parameters, namely absolute and relative soil moisture and skin and soil temperatures, for situational awareness purposes for some time now. Specifically, I’ve been using soil moisture data to help with drought monitoring application and subsequent feedback to the U.S. Drought Monitor for a few years. But, we’ll leave that particular use for another post. Today, I wanted to post about the relevance of these data on the far other end of the hydrologic spectrum, that is, flooding. Let’s take the most recent example here in Alabama and adjacent areas of Mississippi and Florida. I’m going to start with a graphic of precipitation over the region during the 24 hour period ending at 12UTC 7 April 2014.
As shown in the graphic above, some locations on the south side of the Birmingham metro received around five to six inches of rain in about a 12 hour period. This lead to significant flash flooding across parts of the metro where some water rescues were even necessary. Rainfall amounts here in northern Alabama ranged from around one inch in the far northwestern part of the state to about 3.5-4 inches in parts of north central and northeastern Alabama.
Now, let’s investigate the antecedent soil moisture conditions before heavy rainfall moved across the region. Let’s begin by taking a look at the shallow layer (0-10 cm) relative soil moisture produced by the SPoRT LIS.
Relative soil moisture values generally south of Birmingham, and in particular in the “black belt” region of Alabama, a region characterized by rich, black topsoil, but underlying less permeable chalks and clay soils, were around 75% to 90%. So, even before widespread heavy rains of three or more inches moved across the region, soil moisture values were relatively high. Note here in northern Alabama however, that values were lower, at around 50%. The combination of lower antecedent soil moisture conditions and similar to lower rainfall led to less incidence of flash flooding in northern Alabama. In fact, only two flash flood warnings were issued for the event and were for the same location: encompassing parts of DeKalb, Jackson and Marshall Counties in northeastern Alabama. Incidentally, soil moisture values were higher in the Big Wills Valley in DeKalb County where most of the flash flooding occurred. Although, other factors should be considered there such as the narrow and steep-walled drainage basin characteristics.
Now, let’s take a look at the deep layer (0-200 cm) antecedent relative soil moisture conditions across the region (below).
In this image, the solid darker green indicates relative soil moisture values around 60% or higher, while blue shades indicate values in the 70s-90s%. Notice that values were this high or higher across much of Mississippi and central and southern portions of Mississippi. Next is a map of flooding locations as of this morning (April 11th…unfortunately, I don’t have one from earlier).
Streams and rivers with orange and red boxes indicate locations of minor and moderate flooding, respectively. Notice that many of these are located where rains were heaviest, but also where antecedent soil moisture values were highest. The main take away item here is that while rainfall values of this magnitude can occur in this part of the country, importantly, soil moisture values must be included to make a more complete assessment of the threat for flooding. We certainly have had higher rainfall amounts in the region with much less flooding. Such was the case with the passage of Tropical Storm Lee in September 2011, especially here in northern Alabama.
Here at the Huntsville NWSFO it has taken some time, but we have noticed that when deep layer (0-200 cm) relative soil moisture values exceed about 60%, we are at a greater risk for longer term flooding on local stream and river basins when we receive a “typical” synoptic rainfall event totaling around 2-3 or more inches. Values in northern Alabama before this event were generally under 60%, with the exception of the Big Wills Valley in DeKalb County and in some small portions of the Paint Rock Valley in Jackson County. Incidentally, in addition to the flooding along the Big Wills Creek in Fort Payne, minor flooding occurred along the Paint Rock River in Jackson County here in northern Alabama. This was one of the locations with some indication of relatively wet antecedent soils (relative soil moisture values ~60%). Thus, once again, we received further confirmation in these rough thresholds. These two locations contain streams that are particularly prone to flooding, but it has been difficult to gauge what rainfall amounts are necessary. This is because an important component of that assessment was lacking until the advent of LIS soil moisture data into our operations beginning over a year ago.
Thanks to the SPoRT team, we now have these data back in AWIPS (II) and can overlay other important data, such as QPF from the Weather Prediction Center. Although these flooding threat analyses are mostly qualitative and subjective at this point, the plan is to undertake a more objective study of soil moisture and rainfall thresholds in our more problematic drainage basins to have a better understanding of threats for flooding in the future.