The Role of the Subtropical Jet Stream in Dry Season Severe Caribbean Rainstorms: Climatology and Composite Analyses

Abstract Although high-impact heavy rainfall events in the Caribbean are typically associated with tropical cyclones (TCs) during the wet season, significant heavy rainfall events can also occur during the Caribbean dry season, absent of TCs. Dry season severe Caribbean rainstorms (CRs) can produce rainfall totals that surpass climatological dry season monthly mean rainfall, ranging from 25 to 200 mm, by >3 sigma over a relatively short duration (2–16 days), resulting in infrastructure- and life-threatening flooding and mudslides. This paper introduces a 35-yr (1983–2017) dry season severe CR climatology derived from the PERSIANN-CDR precipitation dataset, revealing that dry season severe CRs occur, on average, eight times per year with some interannual variability and exhibit a seasonality that includes relative maxima in November/December and April/May as well as a relative minimum in February. Self-organizing map (SOM) analyses are applied to detect subtropical jet stream (STJS) flow patterns governing events in this climatology which include an upper-level trough reaching the Caribbean (Trough cases, 23% of all events), a frontal boundary extending into the Caribbean attendant to an extratropical cyclone (EC) at midlatitudes (EC Front cases, 54% of all events), or a potential vorticity stream (PVS) forming near the Caribbean (PVS cases, 7% of all events). In these events, extratropical forcing, occasionally combined with orographic forcing, can promote intense, organized, and widespread deep, moist convection that produces heavy rainfall. The remaining events do not have a discernable extratropical connection and are not considered to be influenced by the STJS (16% of events). Significance Statement High-impact heavy rainfall events in the Caribbean are typically associated with tropical cyclones during the wet season, but significant heavy rainfall events can also occur during the Caribbean dry season without tropical cyclones. Dry season Caribbean rainstorms (CRs) can result in infrastructure- and life-threatening flooding and mudslides from rainfall totals that substantially surpass normal dry season monthly rainfall in a short duration (2–16 days). A 35-yr dry season severe CR climatology is constructed, indicating that these events occur on average eight times per year. The subtropical jet stream is recognized as an important driver and influencer of thunderstorms with heavy rainfall in most (84% of all cases) dry season severe CRs. The results of this study could help Caribbean weather forecasters recognize the nature of dry season severe CRs and associated weather patterns to provide more information and advanced warning to local emergency management and the public.