Before the remnants of Tropical Storm Lee impacted the Tennessee Valley, portions of the region had gone 3 full weeks without measurable rainfall. (For parts of Texas, this seems laughable, but this was one of the longest dry stretches of 2011 for the southeast.) The U.S. Drought Monitor placed parts of northern Alabama into D1 (moderate) drought, but larger portions of central and eastern Alabama have been in D3 (extreme) drought for several weeks.
From the NASA Land Information System (LIS) perspective, this is what the dry spell looked like across Alabama. This is the relative integrated soil moisture from the surface to 2 meters deep, as of 0900 UTC 4 September:
Portions of the Huntsville county warning area (CWA) have as little as 15-20% relative soil moisture, with most of the region averaging 25-30%. Portions of east central Alabama are below 5% RSM, further validating the extreme drought classification.
Then came Tropical Storm Lee. Multiple computer models predicted that almost a foot of rain would fall across the Huntsville county warning area, raising concerns about significant flooding. But a funny thing happened: most of the rain fell over a 24-hour period instead of a shorter time span, lessening the flash flood threat; and the very dry ground soaked up nearly all of the rainfall. Even the anticipated river flooding mostly failed to materialize.
Here is the NASA LIS relative integrated soil moisture from 0900 UTC on 7 September (this morning), telling a very different tale:
Portions of the HUN CWA are now up to 60 to 65 percent RSM, while the driest soils in east-central Alabama are up to 10 to 15 percent. It is still remarkable to see such little flooding from such high rainfall totals, but the availability of the NASA LIS, combined with the low rainfall rates, help explain the low-impact event. You can even watch the relative integrated soil moisture change as the remnants of Lee move across the area on 5 September; the black contours denote precipitation.
For more information about T.S. Lee’s impact on the Tennessee Valley, including rainfall totals from the event, visit NWS Huntsville’s T.S. Lee recap page.