Generation of unusually large runup events

Large runup events can be a danger to the public and can cause significant changes to coastal morphology. On January 16, 2016, several unusually large runup events were captured on video and noted in injury reports along a large stretch of the west coast of the United States, ranging from Northern California to the Washington coast. We present an examination of the near- and off-shore conditions before and during these events. Although these events resemble small tsunamis or meteo-tsunamis, no earthquake or significant atmospheric disturbances were observed during this time. Coincidental observations of long and rapidly increasing peak wave periods, both on- and off-shelf, suggest that long and large swells generated by distant storms were linked to these events. We also find that swell heights, to some degree correlate, with water level fluctuations at the coast. Similar patterns of alignment of rapidly developing swells offshore and large water level oscillations at the coast and correlation of swell height and water level at the coast, were seen in two additional instances of documented large runup events. Herein, we discuss potential generation mechanisms.