Relationships between tropical systems and rainfall in the Baja California Peninsula, Mexico

The influence of tropical weather systems in the rainfall patterns over the Baja California peninsula is examined. The impact of these systems, over the southern portion of the peninsula, is analyzed and the study period is limited to the summers of 1970 through 2006. To determine the spatial coverage and intensity of rainfall, daily data from a ground-based network of rain gauges is applied.

The best-track dataset, from the United States National Hurricane Center, is used to examine the tropical cyclones that approached the peninsula at various ranges from less than 400 km through 1,200 km. Another portion of these cyclones made landfall and this occurred late in August or during the entire month of September. Some landfall events occurred along the Pacific coast and are related to moderate amounts (400-500 mm) of total rainfall over the rain gauge network. In a unique case, the slow motion of Hurricane Juliette (2001) brought accumulations that exceeded 800 mm over periods of 3-4 days. Strong tropical cyclones tend to make landfall over the southeastern peninsula and, among this group, Hurricane John (2006) remained over land for more than 40 hours. Maximum rain rates (270-450 mm d-1) from John set new records at several stations with observations since 1970. It is interesting to note that most of the previous maxima (< 425 mm d-1) were associated with the approach or landfall of 6 tropical cyclones between 1976 and 2003.

Time series of rain, at selected stations along the southern peninsula, are constructed and they are used to determine several characteristics of the rainfall episodes during the development of tropical systems. The analysis of these time series also helps to identify other (secondary) sources of heavy rainfall, which are related to the development of Mesoscale Convective Systems (MCSs) that a) initiate over the Sierra Madre Occidental, cross the Gulf of California and reach the peninsula or b) MCSs that form over the peninsular mountains and have slow westward motion away from these mountains. These MCSs are part of the North American Monsoon System that is established during the warm season over northwestern Mexico and the southwestern United States.