So, this might beg the question…”what is elevated?” Well, it’s all relative of course. But, a preliminary analysis of previous events suggests that when 0-10 cm relative soil moisture values reach 60-65% or higher, the flash flooding and/or areal flooding threat increases markedly with a “typical” 1 to 3 inch rainfall event. Now, as mentioned, this is still all preliminary and determing the potential for flooding can be complicated and involve several variables (e.g. soil moisture, rainfall rates, land use, etc), but forecasters like to be able to put a value on soil moisture saturation. Often, this is done in a very qualitative and rather subjective way. If it “seems” to have rained a lot recently, a forecaster may perceive soil moisture values to be elevated. But, it’s always nice to have a number to put to the actual degree of saturation. The NASA LIS data provide just that. With that said, it’s still important to keep in mind that these numbers represent model output. Fortunately though, we are blessed here in northern AL with a number of USDA Scan sites (Figure 2) that monitor soil moisture values at various depths with which to do grount-truth comparisons.
All of this acts to increase forecaster awareness of soil moisture values across the area, and raise confidence in a given flooding scenario. So, with this information in mind on the 20th, forecasters elevated the threat for flash flooding and even mentioned this threat while referencing NASA LIS soil moisture values in the afternoon Area Forecast Discussion (AFD), as noted in the Hydrology section below…
“.HYDROLOGY… HEAVY RAINFALL FROM RECENT EVENTS HAVE LED TO ABOVE AVERAGE SOIL MOISTURE CONDITIONS…RATING IN THE 60-80% SATURATED ACROSS THE AREA. WITH 1-3 INCHES OF RAINFALL EXPECTED OVERNIGHT…FLASH FLOODING IS A LARGE CONCERN. IN ADDITION TO THE FLASH FLOODING THREAT… AFTER THIS RUNOFF DRAINS INTO AREA CREEKS AND RIVERS…RIVER FLOODING WILL LINGER INTO EARLY NEXT WEEK. A FEW SITES ACROSS THE AREA HAVE JUST RECENTLY CRESTED WITHIN THE PAST 24 HOURS FROM HEAVY RAIN 2 DAYS AGO…AND THESE SITES WILL LIKELY SPIKE BACK ABOVE FLOOD STAGE QUICKLY.”
So, what happened? Well, as all forecasting events can prove to be challenging, this one was no less so. Expected rainfall amounts did not quite materialize with this first system, as shown in Figure 3 and Figure 4.
As the figures above show (and, sorry about the fuzziness of the images), precipitation amounts totaled around 1 inch for this first event, which was towards the lower end of what was expected. However, this was followed up by a heavier rain event late on Sunday into Monday morning (Figure 5).
Notice the swaths of heavy rainfall (~1-3 inches) that did occur with this event. The NASA LIS 0-10 cm relative soil values by the morning of the 23rd (0900Z) show the increase in soil moisture values after most of the heavy rain had fallen (Figure 6).
So, with the generally elevated soil moisture values and this 2nd round of 1-3 inch rainfall…did we get flooding? Yes, quite a bit actually. You can click here to see the many flooding reports received at our office late on the 22nd into the 23rd.