Use of SPoRT LIS Products to Evaluate Flooding Potential during a December 2014 Event in Central NC

NASA SPoRT has developed a real-time application of the NASA Land Information System (LIS) that runs over much of the central and eastern United States. The LIS produces several products, including a suite of soil moisture products that can be used to help assess drought and flooding potential. There are four LIS soil moisture products that are being assessed by WFO Raleigh forecasters in AWIPS-2. The products are also available online at http://weather.msfc.nasa.gov/sport/case_studies/lis_SEUS.html for the Southeast and http://weather.msfc.nasa.gov/sport/case_studies/lis_NC.html for North Carolina.

After a dry start to the month, multiple rainfall events occurred across central NC during the middle and latter portion of December 2014. During the afternoon of 22 December, forecasters were analyzing rainfall from the previous 24 hours which ranged from a tenth of an inch in the Northwest Piedmont to an inch or more across the eastern Piedmont and Coastal Plain regions. This event was the 3rd fairly significant rainfall event (>0.5 inches) over the central NC since 10 December. Forecasters became concerned as another storm system would impact the Carolinas during the next few days and was expected to produce between 1.25 to 2.0 inches of rain from 22 to 25 December (see Fig. 1). This additional rain had the potential to produce some flooding on main stem rivers, especially across the Coastal Plain.

Fig. 1. WPC 72-hour QPF forecast valid 12 UTC 22 December through 12 UTC 25 December, 2014.

Fig. 1. WPC 72-hour QPF forecast valid 12 UTC 22 December through 12 UTC 25 December, 2014.

A SPoRT-LIS field that forecasters have found useful is the one-week change in total column relative soil moisture (RSM, 0-2 m). The RSM is the ratio of the current volumetric soil moisture between the wilting and saturation points for a given soil type, with values scaling between 0% (wilting) and 100% (saturation). The one-week change product valid at 15 UTC on 22 December, just prior to the rain event, is shown in Fig. 2 with the NWS CWAs outlined in yellow. Note that multiple significant rainfall events occurred across central NC during the previous two weeks. Not surprisingly, this product indicated that much of central NC had experienced a relative soil moisture increase from the previous week.

Fig. 2. The SPoRT LIS one-week change in total column relative soil moisture valid at 15 UTC on 22 December 2014 with the WFO CWAs outlined in yellow.

Fig. 2. The SPoRT LIS one-week change in total column relative soil moisture valid at 15 UTC on 22 December 2014 with the WFO CWAs outlined in yellow.

Another SPoRT-LIS field that forecasters found useful is the SPoRT LIS 0-200 cm Relative Soil Moisture (%) analysis product. The LIS 0-200 cm Relative Soil Moisture (RSOIM) analysis from 15 UTC on 22 December is shown in Fig. 3. The RSOM values in the area outlined by the red box across the northern and central portions of the Coastal Plain are highlighted in the deeper and darker green shading and generally exceed 55% and in many locations exceed 60%. Subjective analysis of the RSOIM product by previously by WFO Huntsville AL during several synoptic rainfall events suggests that when the 0-200 cm RSOIM values exceed 55%, the risk of flooding on larger rivers increases substantially.

Fig. 3. The SPoRT LIS 0-200 cm relative soil moisture (%) analysis valid at 15 UTC on 22 December 2014 with the WFO CWAs outlined in yellow.

Fig. 3. The SPoRT LIS 0-200 cm relative soil moisture (%) analysis valid at 15 UTC on 22 December 2014 with the WFO CWAs outlined in yellow.

Significant rain did fall across central NC during the days leading up to Christmas. An analysis of precipitation across central NC from 23 to 25 December shown in Fig. 4 indicates a large area of 2.0 to 2.5 inches of rain fell across the Coastal Plain of NC with an average of around 2.0 to 2.25 inches across the Tar and Neuse River basins. These same locations were noted in Fig. 2 with RSOIM values that exceeded 55%.

Fig. 4. An analysis of precipitation across central NC from 23 to 25 December indicates a large area of 2.0 to 2.5 inches of rain across the Coastal Plain of NC with lesser amounts in the 1.0 to 2.0 range across the western and northern Piedmont of NC.

Fig. 4. An analysis of precipitation across central NC from 23 to 25 December indicates a large area of 2.0 to 2.5 inches of rain across the Coastal Plain of NC with lesser amounts in the 1.0 to 2.0 range across the western and northern Piedmont of NC.

The significant rain combined with the wet antecedent conditions did result in flooding at several forecast points across central NC with a few locations in the Coastal Plain reaching moderate flooding. The observed hydrograph and multiple forecast traces for Smithfield NC (SMFN7) on the Neuse River is shown in Fig. 5. The observed stage is noted by the nearly continuous red dots surrounded by blue circles that exceed the orange horizontal line (flood stage) and the red horizontal line (moderate flood stage). The river exceeded flood stage at Smithfield during the afternoon of 24 December and reached moderate flooding less than 24 hours later.

Fig. 5. The observed hydrograph and multiple forecast traces for Smithfield NC (SMFN7) on the Neuse River from 12 UTC on 23 December through 12 UTC on 27 December.  The observed stage is noted by the nearly continuous red dots surrounded by blue circles that exceed the orange horizontal line (flood stage) and the red horizontal line (moderate flood stage) while the forecast traces are noted by the narrower lines with dots every 6 hours.

Fig. 5. The observed hydrograph and multiple forecast traces for Smithfield NC (SMFN7) on the Neuse River from 12 UTC on 23 December through 12 UTC on 27 December. The observed stage is noted by the nearly continuous red dots surrounded by blue circles that exceed the orange horizontal line (flood stage) and the red horizontal line (moderate flood stage) while the forecast traces are noted by the narrower lines with dots every 6 hours.

Several days prior to flooding, the hydrologic situation was discussed in the NWS Raleigh Area Forecast Discussion (AFD) issued at 230 PM EST on Monday, 22 December (text shown below). In the AFD, the antecedent conditions were discussed with multiple SPoRT LIS products referenced. While the rainfall during the following few days exceeded the initial forecast and the anticipated impacts, the availability of the SPoRT LIS products lead to increased awareness of the flooding potential which proved especially helpful as the observed rainfall amounts increased and river levels rose.

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE RALEIGH NC

.HYDROLOGY…
AS OF 230 PM MONDAY…

ANTECEDENT RAINFALL OVER THE PAST 24 HOURS RANGED FROM A MINIMA OF ABOUT A TENTH OF AN INCH IN THE NORTHWEST PIEDMONT (THE UPPER YADKIN/PEE DEE AND UPPER HAW RIVER BASINS) TO A STRIPE OF 1 INCH PLUS ACROSS THE SOUTHEAST (THE CENTRAL NEUSE AND CENTRAL CAPE FEAR BASINS). WILL SEE ONLY MINOR RISES ON THE MAINSTEM RIVERS IN RESPONSE…BUT THIS IS THE 3RD FAIRLY SIGNIFICANT RAINFALL EVENT (>0.5 INCHES) OVER THE AREA SINCE 12/10. RELATIVE SOIL MOISTURE PERCENTAGES IN THE 0-200 CM COLUMN HAVE BEEN INCREASING…WITH HIGHER PERCENTILES NEARER THE SURFACE…SO QUICKER RUNOFF IS EXPECTED FROM OUR UPCOMING RAIN EVENT.

CURRENT QUANTITATIVE PRECIP FORECASTS FROM THE GEFS AND NAEFS ENSEMBLES ARE IN LOCKSTEP AGREEMENT AT PRESENT…WITH HEAVIEST RAIN (~1.3-1.5 INCHES) FROM TOMORROW NIGHT THROUGH CHRISTMAS MORNING. RAINFALL AMOUNTS IN THIS RANGE COULD POTENTIALLY CAUSE SOME MINOR FLOODING ON THE NEUSE RIVER AND TAR RIVER LATE ON CHRISTMAS DAY…BUT IT WOULD BE LOW IMPACT WITH THOSE RIVERS BARELY REACHING MINOR FLOOD STAGE. THE UPSHOT…RIVER FLOODING WILL ONLY BE AN ISSUE IF RAINFALL FORECASTS BEGIN TRENDING HIGHER…INTO THE 2 INCH PLUS RANGE.
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One thought on “Use of SPoRT LIS Products to Evaluate Flooding Potential during a December 2014 Event in Central NC

  1. This is a great application of the data and we appreciate the informative and detailed post! Based on this, it looks as though resulting streamflow (at least along the Neuse River) may be even more sensitive to antecedent soil moisture conditions than those here in the TN Valley. Of course, this is just one event, and it’d be good to go back and do an investigation of soil moisture and precipitation prior to previous flooding events to determine what thresholds might be more relevant to flooding along this river. Based on the moderate flooding outcome, lower threshold values for both rainfall and 0-200 cm RSM might be more applicable. Anyway, great case…and thanks for sharing!

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