All eyes are on North and South Carolina as Major Hurricane Florence approaches the region over the next two days. One important component to the official forecast is for the storm to slow down as it approaches the coast, due to the collapse of major atmospheric steering currents. As a result, the NCEP Weather Prediction Center is predicting extreme rainfall amounts, especially for southeastern coastal North Carolina where 15-20”+ of rainfall is anticipated over the next 7 days (Fig. 1).An examination of the antecedent soil moisture is helpful to qualitatively assess the ability of the ground to absorb some of the moisture from the incoming rainfall. Figure 2 shows a collage of shallow to deep soil moisture percentiles from 12 September within the four layers of the Noah land surface model, as being run in real time within NASA SPoRT’s configuration of the Land Information System (i.e., “SPoRT-LIS”). The percentiles are derived from a 1981-2013 database of SPoRT-LIS daily soil moisture values in order to compare the present-day soil moisture to historical values on any given day of the year. In Fig. 2, we see that recent soil moisture values are historically quite dry over central/northern South Carolina and into far southern North Carolina, with values under the 10th percentile (and even 2nd percentile, yellow/red shades) in some areas. Meanwhile, as one traverses inland and northward, the soils steadily moisten to anomalously wet conditions (green/blue shades), especially over interior North/South Carolina to the Appalachian Mountains. The dry soil moisture anomalies near the coast suggest that the soils will initially be able to absorb incoming rainfall fairly effectively. However, as prolonged heavy rainfall continues with the expected slow movement of Hurricane Florence, the soils should quickly become saturated, thereby leading to enhanced runoff and flooding potential over time. So while having dry soils will be of some help early in the event, a prolonged exceptional rainfall up to 20”+ will lead to substantial flooding regardless of the initial soil moisture distribution.
The blog author documented a similar scenario (also over South Carolina), where substantial moisture from Hurricane Joaquin in Autumn 2015 led to 20”+ rainfall totals, largely occurring over dry soils in an area of moderate to severe drought, thereby completely eliminating the drought classification in South Carolina and producing substantial flooding. A similar scenario was also seen associated with Hurricane Harvey in southeastern Texas last year, where very dry soils were prevalent prior to Harvey’s landfall north and west of Houston Metro. However, given the very prolonged exceptional rainfall event, incredible soil moistening and flooding occurred anyway in much of southeastern Texas.