Predecessor rain events in tropical cyclones

During recent active Atlantic hurricane seasons, observational evidence has suggested that heavy mesoscale 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 pose an important forecast challenge because they can cause significant flooding with little prior warning, especially in those 25% of cases in which the TC itself later produces additional heavy rainfall across the same region. The purpose of this talk is to describe PRE characteristics, to document PRE structure and evolution by means of representative case studies and to illustrate 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 the National Climatic Data Center, the National Hurricane Center best-track data, the National Precipitation Verification Unit online Quantitative Precipitation Estimate archive, and the National Weather Service precipitation data gleaned from online sources. The National Centers for Environmental Prediction/North American Regional Reanalysis datasets were used to help identify synoptic and mesoscale forcing mechanisms responsible for 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.

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 h. significant variability is noted. A middle- and upper-level jet-entrance-region confluence zone well downstream of an approaching TC is a favored synoptic location for PRE development. The orientation of midlatitude troughs and ridges lying poleward of TCs modulated whether, where and when a PRE formed while mesoscale surface boundaries acted as a focus for PRE heavy rainfall. Case study results will be used to show the details of PRE behavior.