As anticipated, Hurricane Florence resulting in monumental rainfall totals, particularly across southern and eastern North Carolina. This past week’s rainfall totals are depicted in Figure 1, derived from the NOAA/National Weather Service Advanced Hydrologic Prediction Service (AHPS). Widespread totals exceeded 10” across most of southern/eastern North Carolina and far eastern South Carolina, with maximum rainfall of more than 20” along and within a few counties of the Atlantic Coast.
The extreme rainfall dramatically impacted the soil moisture, which underwent a substantial transformation from very dry to near-saturation across south-eastern North Carolina. Figure 2 shows soil moisture retrievals before and after Hurricane Florence from NASA’s Soil Moisture Active Passive (SMAP) mission, which estimates near-surface soil moisture (~top 5 cm) in near-real-time derived from passive microwave satellite observations. A 6-day animation of hourly SPoRT-LIS simulated 0-100 cm relative soil moisture images overlaid with Multi-Radar Multi-Sensor rainfall contours (Fig. 3) nicely shows how the predominantly very dry soils across North and South Carolina prior to Florence were quickly moistened to near saturation over just a few days. [Ongoing research at SPoRT seeks to further improve the experimental soil moisture estimates by assimilating SMAP retrievals into the SPoRT-LIS framework.]
Similar to that shown in a companion blog article, Figure 4 shows the evolution of shallow (0-10 cm) to total column/deep (0-200 cm) soil moisture percentiles relative to the SPoRT-LIS 1981-2013 climatological database. Anomalously dry soil moisture is depicted by orange/red colors, while anomalously wet soil moisture is given by green/blue colors. Prior to Hurricane Florence, much of South Carolina and southern parts of North Carolina were experiencing unusually dry soil moisture for this time of year. Despite the capacity for the soils to receive moisture, the historic rainfall was enough to overcome soil moisture deficits, quickly leading to near-saturated soil conditions in all model depths, and ultimately substantial flooding. An interesting feature to note after the storm impact is the very high soil moisture percentiles greater than 98th percentile across South Carolina in the shallow layers. Meanwhile, the deeper soils experienced excessive soil moisture percentiles above the 98th percentile predominantly over North Carolina where the heaviest rainfall occurred and where the pre-storm dry anomalies were not as large as in South Carolina.
Figure 4. Daily animations of SPoRT-LIS soil moisture percentiles relative to 1981-2013 climatology, valid from 10 to 17 September over model depths at (top image) 0-10 cm, (2nd image) 0-40 cm, (3rd image) 0-100 cm, and (bottom image) total model column 0-200 cm. [Click on each individual image for full resolution]
Finally, the dramatic transformation in soil moisture is nicely highlighted by examining the present-day, county-averaged values relative to the 1981-2013 climatological distributions, as shown in Figure 5 at four select counties in North Carolina. Robeson and Cumberland counties (first and third images in Figure 5) experienced the driest soils prior to Hurricane Florence (westernmost counties in Fig. 1), whereas New Hanover and Craven counties (second and fourth images in Figure 5) were more moist prior to Florence’s rainfall. Each of the four sampled counties ultimately experienced record daily soil moisture values by 17 September (99.9th percentiles). However, the moist antecedent soils in Craven county led to record soil moisture values by 15 September, whereas the other counties that began with drier soils achieved record values by 16 or 17 September. In summary, despite predominantly dry soils prior to Hurricane Florence across much of the Carolinas, the tremendous 10-30”+ rainfall totals led to a quick saturation of the soils and massive, widespread flooding.
Figure 5. Daily animations of SPoRT-LIS total column, county-averaged soil moisture histograms, valid from 10-17 September 2018 for (top image) Robeson county, NC [city of Lumberton], (2nd image) New Hanover county, NC [city of Wilmington], (3rd image) Cumberland county, NC [city of Fayetteville], and (bottom image) Craven county, NC [city of New Bern]. Gray bars represent frequency distribution of 1981-2013 climatological 0-200 cm relative soil moisture values, vertical colored lines are reference percentiles, and black dashed line is present-day, county-averaged soil moisture value. [Click on each image for full resolution]