Unexpectedly heavy rainfall due to mesoscale features Induced by a landfalling and non-transitioning tropical storm

Landfalling and near-land tropical cyclones (TCs) provide a challenging forecast problem, especially regarding major threats like storm surge, high winds, and heavy rainfall. Although the dangers from strong winds generally weaken rapidly with increasing distance from the core of the circulation, the precipitation structure associated with a landfalling TC does not fit any specific model, and is far more likely to have maxima far removed from the main circulation. Mesoscale phenomena such as coastal fronts, orographic enhancement, and cold air damming can have a marked influence on the areal precipitation coverage, even while the storm is greater than 250 km away. Normally considered as cold-season phenomena, coastal fronts and cold-air damming can be induced by a nearby tropical cyclone in the right position with respect to the coastline.

Atlantic Tropical Storm Marco (1990) was chosen because two distinct incidences of coastal frontogenesis, cold air damming, and orographic enhancement significantly alter the final precipitation distribution. Marco was responsible for greater than 500 mm of precipitation in parts of Georgia and South Carolina, although the storm never reached hurricane intensity. Marco's track and its associated wind field likely led to the heavy precipitation and flooding recorded during this period. Synoptic upper air analyses from NCEP/NCAR's North American Regional Reanalysis (NARR), NHC Best Track data, and hourly high-resolution surface data from the USAF's DATSAV3, NCEP's ADP, and ICOADS will be used to explain how the coast's physical features combine with the TC to induce the mesoscale features and thus modify the total precipitation distribution.