Soil Moisture Before and After T.S. Lee

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:

NASA LIS Relative Integrated Soil Moisture, 0900 UTC 04 September

NASA LIS Relative Integrated Soil Moisture, 0900 UTC 04 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:

NASA LIS Relative Integrated Soil Moisture, 0900 UTC 07 September

NASA LIS Relative Integrated Soil Moisture, 0900 UTC 07 September

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.

6 thoughts on “Soil Moisture Before and After T.S. Lee

  1. Nice post and very dramatic pictures. A few comments / questions. A difference image would be nice to show. Why look at 0-2m soil moisture? The 0-10cm moisture layer would show much more variability relevant to forecasting? Jon Case, what do you think?

  2. I will work on a Diff image of 0000 UTC 7 Sep minus 0000 UTC 4 Sep and provide a follow-up blog post. That should be a compelling image!

    Regarding the total column soil moisture versus 0-10 cm: As Brian pointed out at the SPoRT virtual workshop, the 0-10 cm layer responds almost immediately to rainfall events, thereby saturating quickly. This layer does have much more variability and directly interacts with the PBL. However, from a drought-monitoring standpoint, the total column relative soil moisture reveals how dry or moist conditions have been over the longer term (weeks to months), since the deeper soil layers typically take much longer to respond to rainfall (or prolonged drying) events.

  3. Did you create the difference image yet Jonathan? I would like to see that.

    The deeper integrated soil moisture would be more useful for long term drought monitoring purposes, especially in areas where groundwater is being used as a primary water source or when water tables have fallen markedly and are being closely monitored. However, in times of more recent drought the upper-level soil moisture may actually be more useful as more immediate impacts to agriculture will take precedence.

    I think that difference images of these products could have very beneficial and practical applications for drought monitoring and for weekly response to the U.S. Drought Monitor! I’m looking forward to working with Jon on this!

  4. Pingback: Recent Flooding in the Huntsville Forecast Area and Use of the SPoRT LIS « The Wide World of SPoRT

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