Heavy rainfall events preceding the arrival of tropical cyclones

During recent active Atlantic hurricane seasons, observational evidence has suggested that heavy rainstorms sometimes can form unexpectedly well in advance of landfalling and near-coastal tracking tropical cyclones (TCs). A total of 47 of these predecessor rain events (PREs) occurring downstream of 21 TCs were identified between 1998 and 2006, representing approximately one third of all landfalling TCs during this time period. The more extreme PREs can cause significant flooding with little prior warning and can have even more serious impacts if they fall into the 25% of cases in which the TC itself later produces additional heavy rainfall across the same region. The relative frequency of these high-impact weather events therefore adds considerably to the forecast challenge prior and subsequent to TC landfall. The purpose of this presentation is to synthesize a statistical study of relevant PRE characteristics with a physical understanding of the linkages between the synoptic and mesoscale forcing mechanisms responsible for PRE formation.

A comprehensive data catalog of TCs and their associated PREs occurring between 1998 and 2006 was constructed using radar imagery from NCDC, NHC best-track data, the NPVU online QPE archive, and NWS precipitation data gleaned from online sources. NCEP/NARR gridded datasets provided insight into the key synoptic-scale features responsible for the PREs. Required criteria for inclusion of an event in the PRE dataset were evidence that the TC circulation advected moist tropical air toward the region of PRE formation, and that maximum rainfall directly attributed to the PRE exceeded a rate of 100 mm in 24 h.

Statistical analysis showed that a “typical” PRE is located ~1000 km ahead of the parent tropical system, occurs 1-2 days prior to the arrival of the TC at the latitude of the PRE, and lasts for ~12 hr, though some significant variability was noted. Detailed examination of environmental flow patterns revealed that a middle- and upper-level jet-entrance-region confluence zone well downstream of an approaching TC was a favored synoptic location for PRE development. The orientations of midlatitude troughs and ridges poleward of the TC modulated whether and where a PRE formed, but there was a strong tendency for the TCs in the dataset to spawn PREs left of their eventual tracks. A climatological and case-study approach will be used to demonstrate these points.