Analysis of Landfalling Rapidly Weakening Tropical Cyclones in the Philippines

Rapid weakening (RW) of tropical cyclones (TCs) is defined as the 90th percentile of all 24-h over-water weakening rates in the Western North Pacific (WNP) basin, corresponding to a decrease of at least 20 kt in the JMA dataset and 25 kt in the JTWC dataset. RW tends to occur along the 20 – 30° N latitude of the WNP, which makes the probability of RW TC landfall in the Philippines low. Over the study period from 1951–2020, a total of 468 and 563 WNP RW TCs were recorded, where only 17 and 19 of those made landfall in the country based on the JMA and JTWC datasets, respectively. Analysis of potential wind threats of landfalling RW TCs shows significantly lower hazards than non-RW TCs, except for those that make landfall on northern Luzon. RW occurs more frequently outside of the southwest monsoon or Habagat season. Simulations of two recently landfalling RW TCs – Typhoon (TY) Maysak (2015) and TY Yutu (2018) – using the Weather Research and Forecasting (WRF) model show the decrease in the equivalent potential temperature (𝜽e), a measure of the amount of heat and moisture in the atmosphere, in the TC inner core region can be used to diagnose RW. Constantly decreasing 𝜽e values below 400 𝑲 caused by cooler underlying sea surface temperature and/or dry air intrusion lead to TC RW. RW can also occur in low-shear environments. Environmental conditions that result in RW are typically observed from October–April of the following year, which explains the higher occurrence frequency of RW in the inactive TC season of the WNP. While the impacts of RW TCs are lower, over-forecasting a TC in one event can lead to a complacent populace for the next, as well as damage the reputation of forecasters, hence the importance of understanding RW.