Relatively Wet Soils and Future North Carolina Rains…

A cold front will sag slowly southward across the mid-South and Mid-Atlantic during the upcoming weekend, and will likely stall over the Carolina Piedmont region as a couple of waves of low pressure move along the front. The broad scale lift of the moisture-laden airmass ahead of and over the shallow front will be capable of producing moderate to heavy rainfall in the Carolinas through early next week. Recently, the Raleigh NWS forecast area experienced heavy rainfall (see recent blog post from the Raleigh NWS office) which lead to the moistening of soils and some instances of flooding. If the heavy rains in the latest forecast by the Weather Prediction Center (WPC) materialize, then current soil moisture levels suggest the risk for flooding will be increased. Take a look at the SPoRT LIS 0-200 cm Relative Soil Moisture (%) analysis below from 12Z this morning (Image 1).

Figure 1.  SPoRT LIS 0-200 cm Relative Soil Moisture (%, RSOIM), valid 12Z 11 Sep 2014.  An area with RSOIM values around 50-60% is circled in black centered on the eastern part of the Raleigh forecast area.

ImageĀ 1. SPoRT LIS 0-200 cm Relative Soil Moisture (%, RSOIM), valid 12Z 11 Sep 2014. An area with RSOIM values around 50-60% is circled in black centered on the eastern part of the Raleigh forecast area.

The area circled in black indicates portions of the Raleigh NWS forecast area where 0-200 cm Relative Soil Moisture (RSOIM) values exceed 55%. Here in the Huntsville area, subjective analysis of several synoptic rainfall events suggests that when 0-200 cm RSOIM values exceed this threshold, the risk of flooding of basins and rivers in our area is increased substantially. Next, let’s take a look at the latest 5-day precipitation graphic produced by the Weather Prediction Center (WPC, Image 2).

Image 2.  WPC (HPC in label) 5-Day Total Precipitation ending 12Z 16 Sep 2014.

Image 2. WPC (HPC in label) 5-Day Total Precipitation ending 12Z 16 Sep 2014. The region with relatively moisture 0-200 cm soils is circled in black.

In this latest update from the WPC, precipitation amounts totaling around two to four inches are forecast for portions of the Raleigh NWS forecast area. Of course, these totals are likely to be adjusted over the days ahead. Additionally, some of the precipitation during this period is likely to be convective in nature, which will make the resulting precipitation in the region more heterogeneous, and it may fall in several distinct episodes, reducing the overall average rainfall rate. Nevertheless, since soil moisture levels have now exceeded a seemingly critical threshold, this particular area may bear watching for potential flooding if the rains materialize.

The Raleigh WFO together with the Huntsville and Houston WFOs are participating currently in a more formal assessment of several SPoRT LIS variables. Although this and other soil moisture variables have demonstrated utility for assessing both drought and flood risk in the Huntsville forecast area over the last few years, this is the first formal assessment to evaluate the utility in different environments simultaneously.

One thought on “Relatively Wet Soils and Future North Carolina Rains…

  1. Nice post, Kris! Thanks for giving the Raleigh office a heads up on this possible event this weekend.

    It will be interesting to see how this event plays out over the weekend. It will be a good test of your 60% threshold. When we do some further analysis of this event (and others over Huntsville), we’ll need to take into account the different soil types. It may be more appropriate to develop objective methods for determining areal flood potential binned by soil type because different soil types will retain water at different rates.

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