Heavy Precipitation Associated with Southern Appalachian Cold-Air Damming and Carolina Coastal Frontogenesis in Advance of Weak Landfalling Tropical Storm Marco (1990)

An analysis is presented of Tropical Storm Marco (1990), a storm that dropped copious amounts of rain over the southeast United States. Marco was noteworthy because of its role in the formation and evolution of two distinct episodes of cold-air damming and coastal frontogenesis over Georgia and the Carolinas. These mesoscale features led to greater than 300 mm of precipitation in 2 days over the near-coastal southeast United States; much of the rain occurred while Marco was over 400 km away. This case is further complicated by two other nearby tropical cyclones, which affected Marco’s track and the overall rainfall distribution. Synoptic and mesoscale analyses of the development of the coastal front and cold-air damming episodes show that the location of Marco helped to orient low-level winds toward the Appalachians. As rain developed inland, a pocket of relatively cool air, the “cool pool,” formed near the mountain slopes and was partially blocked by the higher terrain. Low-level analyses show that the coastal front on the oceanward edge of the cool pool became a focusing mechanism for ascent and precipitation, as moist, tropical air advected inland by Marco was forced upward at the density gradient. The results indicate that a weak tropical cyclone can directly effectuate intense precipitation distant from the storm center, both by causing moist tropical flow toward land and by inducing mesoscale features that focus the precipitation and lead to heavy rainfall and flooding.