Friday the 13th Hailstorm in the Province of Bulacan, Philippines (13 August 2021): A Case Study

Abstract

This case study presents a thorough investigation of the environmental setup that led to the hail-producing severe storm that impacted the municipality of Norzagaray and City of San Jose Del Monte, including other nearby areas, in the province of Bulacan on the afternoon of August 13, 2021. During this period, 2–5 cm and potentially as large as ~8 cm diameter hail was reported over these locations of Bulacan. For this purpose, the combination of HIMAWARI-8 AHI, PLDN and its flash counts, and meteorological indices; synoptic, thermodynamic, and kinematic indices, calculated from the ERA5 reanalysis are utilized to understand the nature of the hail event. In the morning, the pre-convective environment was comprised by a warm inversion layer that inhibited storm initiation, until the arrival of ample moisture and convective heating in the afternoon. By the afternoon, model sounding analysis revealed that the environment transitioned into uncapped profile with steep low-level lapse rate owing to warm, moist south-westerly wind flow from the Manila Bay in the lower troposphere and north-easterlies aloft crossing the SMMR induced by a weak low-pressure system located in the eastern Philippine Sea, with minimal turning on the wind profile. This promoted low-level convergence within the area of interest and build up of instability. The updraft associated with convectively unstable atmosphere, sufficient cloud-layer bulk shear, and storm nudging at its maturing phase countered entrainment-driven dilution and aided the growth of ice crystals by rapid collection of supercooled cloud liquid particles, which ultimately led to formation of hailstones.