The 22-26 December 2013 Caribbean Rainstorm

Between 22 and 26 December 2013, a prominent upper-level cutoff cyclone formed over the northwestern Caribbean and promoted subsequent convection that produced prolonged heavy rain over the Lesser Antilles. Estimated storm total rainfall that exceeded 200 mm in some locales during this period led to widespread flooding, numerous mudslides that threatened infrastructure, and delayed access of vital recovery resources to residents. Although impactful heavy rain events in the Caribbean are typically associated with tropical cyclones, noteworthy heavy rainfall events can occur during the Caribbean “cool” and dry season. Locations in the Lesser Antilles received rainfall amounts greater than 90% of the December (1971-2017) average rainfall as well as over 44% of the rainfall normally observed during the entire cool season (D, J, F; 1971-2017) during the December 2013 event. December 2013 was the third wettest December on record (1953-2017) at St. Thomas, United States Virgin Islands. This poster will present an analysis of the structure and evolution of the synoptic-scale flow pattern that enabled the aforementioned extreme rainfall event of December 2013 to occur.

The antecedent synoptic-scale conditions to the December 2013 Caribbean rainstorm featured upper tropospheric anticyclonic wave breaking (AWB) over the western subtropical Atlantic Ocean and potential vorticity streamer (PVS) to the south over the Caribbean Sea. Once formed, this PVS elongated and thinned in response to AWB-related larger scale deformation processes. Satisfaction of the necessary condition for barotopic instability at the southwestern end of the PVS in response to this elongating and thining culminated in the formation of a cutoff cyclone via the potential vorticity (PV) fracture process. Diabatic processes also contributed to the PV fracture process through interaction of the PVS with deep tropical moisture on its equatorward flank in a region of synoptic-scale ascent ahead of the newly formed cutoff cyclone. Deep convection that developed along the equatorward flank of the PVS ahead of the aforementioned cutoff cyclone assisted the PV fracture process and contributed to heavy rainfall over the Caribbean islands. A quantitative analysis of the role of AWB on the development of this PVS and resultant cutoff cyclone will be conducted by taking advantage of the high horizontal, vertical, and temporal (hourly) resolution of the new ERA5 global gridded reanalysis dataset.