In late September, a slow moving upper-level low pressure system was forecast to drop into the Southeast and migrate across the eastern Gulf States. In addition, Hurricane Joaquin was over the Bahamas and was, for a time, forecast to lift north along the Atlantic seaboard with heavy rain likely impacting the eastern Carolinas. The QPF forecast evolved from focusing near Joaquin’s forecast track near the coast to further inland as it became clearer that the upper-level low and not the hurricane would be the driving force behind the precipitation. At one point, the forecast for most of North Carolina, including almost all of WFO Raleigh’s (RAH) area, was for rainfall to exceed 6 inches (right).
Even through central NC had been slowly edging back into more severe drought conditions and streamflows were quite low, a forecast of 6 inches of rain was alarming, and a flood watch was issued for all of RAH’s forecast area. In addition, contingency forecasts from the Meteorological Model Ensemble Forecast System (MMEFS) predicted as much as an 80 percent chance of moderate or greater flooding for many of RAH’s river flood forecast points.
NASA SPoRT has developed a real-time configuration of the NASA Land Information System (LIS) that runs over much of the central and eastern United States at 3-km grid spacing. The 0-200 cm relative soil moisture product from the LIS leading into the event (right) depicted very dry soil conditions with a majority of the area with only 20-25 percent saturation. This was a consideration when river flood warnings were issued, as while the total amount of forecast rainfall was impressive, the event was going to be spread mainly over a 48 hour (10/1-3) time window, which would allow the very dry subsoils to effectively buffer the infiltrating rainfall.
Total rainfall from the event generally ranged from 3 to 5 inches (right), with both the amount and coverage of the precipitation occurring just under critical thresholds to avoid significant flooding issues. A number of flood warnings and advisories were issued, with the impact and damage relatively limited to travel inconvenience. There were 5 river forecast points which exceeded minor flood stage, with none reaching moderate flood stage.
The rainfall’s long duration allowed the bulk of the rain to infiltrate deeply rather than run off, which provided major benefits for late season agriculture, both for harvest and soil preparation for winter crop planting. The 0-200 cm relative soil moisture recharge was the most dramatic we have seen here at RAH to date (below), with post-storm percentages rebounding to 35 to 55 percent of capacity.
In addition, public water supply reservoirs rose to target pools and the U.S. Drought Monitor removed all drought conditions from the Tar Heel state for the first time since the beginning of summer.