The Southwestern Luzon Climatological Rainfall Maximum

The Southwestern Luzon region is included in the highest rainfall areas of the Southwest Monsoon. The climatological rainfall maximum is primarily a reflection of the subsynoptic rainfall disturbances that occur during episodes of strong southwest monsoon flow, the so-called monsoon surges. These rainfall episodes, which could last for many days to a few weeks, are sometimes called in the vernacular as “siam-siam” (Tagalog) and “nep-nep” (Ilocano and Pangasinan). The episodes frequently cause floods in Metro Manila, central Luzon, as well as in Pangasinan, Bataan and Zambales.

The rainfall pattern, which is almost circular in shape, forms near the end of May at the beginning of the rainy season. It gradually intensifies and remains practically stationary throughout the entire season. The maximum intensity is attained in August. Subsequently, it weakens rapidly and completely disappears in October at the beginning of the northeast monsoon season.

The maximum rainfall described above is a newly discovered feature of the climatology of the Philippines-China Sea region. The variation in the strength and in the location of the feature is an important factor which controls the rainfall in Southwestern Luzon and the neighboring regions.

This paper is an observational and diagnostic study of the rainfall and wind climatology of the Southern Luzon Region due to the annual Southwest monsoon.

[Presented at the Samahang Pisika ng Pilipinas (SPP) Congress, 2000.]