Hurricane Joaquin, while a category 4, unleashed torrential rain over the Bahamas, uprooting trees, disrupting power and ripping rooftops. Joaquin lingered in the tropics, shedding vast amounts of moisture that became part of a complex mix of weather systems in the eastern U.S., creating soggy conditions even before Joaquin shifted northeastward from the tropics offshore of the U.S., giving a one-two punch for the eastern states. Punch one was life-threatening inland flooding, and punch two was storm surge along coastal areas.
Soaked region just fresh off a ‘King Tide’ prepared for more water from multiple sources
The weekend before Joaquin started to move northeastward in the Atlantic, the U.S. East Coast had a ‘king tide’ weekend from the pull of a rare supermoon lunar eclipse. Such higher-than-normal high tides, combined with sea level rise, can create local coastal flooding. At the same time weather patterns brought a soggy end to the month between September 23 and 30, with high rainfall totals for Woolwine, VA (>16 inches), Beaufort, NC (>13 inches), and Destin, FL (>12 inches), and broke daily rainfall records for the last day of the month in Portland, ME, Boston, MA, and Providence, RI. Moisture from several parts of the tropics, including Joaquin, contributed to these extreme rainfall totals.
A deep jet stream dipping down to Florida plus tropical moisture sources combined to dump historic rain of a gargantuan scale for the Carolinas (Figure 1). This creates an increased risk for landslide or debris flows in the inland parts of the Carolinas.
Figure 1 Precipitation gauges in South Carolina with huge 24 and 48 hour rainfall totals October 3-4, 2015. Data source USGS.
Charleston, SC was caught between the extreme precipitation and the coastal storm surge, which made preparation to protect the cherished historic districts challenging. Not only were the huge precipitation rates over a 24-hour period on October 4 enough to deal with, the runoff may not flow as quickly to the lowest endpoint—the sea. This is because the tide gauge at Charleston, SC had two feet higher than the projected tide. The onshore winds are piling up water and creating storm surges that are likely to continue over several tidal cycles. The tide gauge at Charleston, South Carolina was over 2 feet above predicted tide level during high tide for October 4, 2015 (Figure 2).
Figure 2 Charleston South Carolina tide station with water levels above predictions on October 4, 2015. Source: NOAA
The situation is not quite over with these complex weather patterns, and it warrants paying close attention to evolving forecasts, planning accordingly, and avoiding flood-prone or landslide-prone areas.