The SPoRT LIS soil moisture data have continued to be useful at NWS Huntsville, as a tool for assessing drought and flooding potential. Recently, forecasters found utility in assessing the threat for flood potential leading up to a heavy rain event forecast for the area. A “typical” 2-3 inch rainfall event, even in the cold season, can lead to some instances of localized flooding, especially if embedded convection exists. Nevertheless, just stratiform rain events of this magnitude can cause gradual rises of streams and minor flooding if antecedent soil moisture values are relatively high. So, in order to get a more quantitative understanding of soil moisture amounts, forecasters have come to rely on the SPoRT LIS data. On Nov 25th, as moderate to heavy rain was approaching the area, one of the Huntsville forecasters consulted the LIS data to determine soil moisture values and the flooding threat. In her forecast discussion, she noted, “THE AXIS OF HEAVIEST PRECIP IS CURRENTLY FORECAST TO EXTEND FROM AROUND TUSCALOOSA UP ACROSS THE SPINE OF THE APPALACHIANS…WITH AMOUNTS ACROSS OUR CWA RANGING FROM 1.5-3+ INCHES. CULLMAN/MARSHALL/DEKALB COUNTIES WILL BE MOST LIKELY TO SEE THESE 3+ INCH TOTALS. WITH 3 AND 6 HR FLASH FLOOD GUIDANCE OVER 2 INCHES ACROSS THE CWA…AND DRY SOILS INDICATED IN THE NASA LIS DATA…THIS SUGGESTS WE WILL BE ABLE TO HANDLE MUCH OF THIS RAINFALL SINCE IT WILL BE OCCURRING OVER THE ENTIRE DAY.” The following grapics show the shallow and deep layer relative soil moisture on the morning of Nov 25th, before much of the heavy rain began to affect the area.
Notice that values in much of north central Alabama, in the area of expected heaviest rainfall, were around 30 to 45%. Although still somewhat anecdotal and subjective, local use has shown that values under 50% during a typical 1-3 inch stratiform rain event will not lead to flooding, or only very isolated instances of minor flooding. Forecast rainfall amounts were on target, as about 1.5 to 3 inches resulted across the area. No flooding was reported. The next graphic shows the deep layer soil moisture as of this morning. Soil moisture values even after the heavy rainfall of 2-3 inches in portions of north central Alabama only climbed to about 40 to 50%.