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1-minute GOES-18 imagery to monitor the potential of flash flooding in American Samoa

By Scott Bachmeier •

1-minute GOES-18 Infrared images with an overlay of the Total Precipitable Water derived product (in cloud-free areas) and GLM Flash Points (white), along with plots of METAR surface reports (cyan), from 0401-1400 UTC on 09 October [click to play MP4 animation]

Due to a lack of radar coverage over American Samoa, WSO Pago Pago requested a 1-minute Mesoscale Domain Sector over the islands to monitor convective development and the potential for flash flooding. GOES-18 (GOES-West) Clean Infrared Window (10.3 µm) images (above) showed rain showers that developed in the general vicinity of the American Samoa island of Tutuila (where Pago Pago International Airport NSTU is located) on 09 October 2025. GLM Flash Points indicated that intermittent lightning occurred very near the island of Tutuila — although no thunderstorms or lightning were explicitly reported during that particular 10-hour period at NSTU. The northwest-to-southeast orientated band of deep convection was aligned along a trough of low pressure that was located across the islands.

A Flash Flood Warning was issued at 0335 UTC (25 minutes prior to the start of the animation shown above), which was valid until 0700 UTC — 0.60″ of rainfall was recorded during the 6-hour period ending at 0600 UTC, but there no reports of flooding on Tutuila that were received by WSO Pago Pago.

GOES-18 Total Precipitable Water derived product values in the vicinity of American Samoa were generally in the 2.0-2.4 inch range — and a toggle between Pago Pago rawinsonde data at 0000 UTC and 1200 UTC (below) indicated that Precipitable Water (PW) increased from 2.19 inches to 2.40 inches during that 12-hour period. In addition, after 1200 UTC the coldest cloud-top infrared brightness temperatures associated with some of the rain showers and thunderstorms across the region were around -80ºC (darker shades of purple embedded within brighter white regions) — which represented a small overshoot of the Most Unstable (MU) air parcel’s Equilibrium Level (EL).

Plots of rawinsonde data from Pago Pago (NSTU) at 0000 UTC and 1200 UTC on 09 October [click to enlarge]

Categories: GLM, GOES-18, Heavy rain / flooding, Lightning