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1-minute GOES-18 images to monitor the potential of heavy rainfall and flooding across American Samoa

By Scott Bachmeier •

1-minute GOES-18 Infrared Window (10.3 µm) images with an overlay of 1-minute GLM Flash Points (white dots), with plots of METAR surface reports (cyan), from 0801-1800 UTC on 29 March [click to play MP4 animation]

During a period when American Samoa had been under a prolonged Flood Watch, the Weather Service Office at Pago Pago requested that a GOES-18 (GOES-West) Mesoscale Domain Sector be positioned over the region (due to their lack of radar). 1-minute Infrared imagery with GLM Flash Points (above) displayed deep convection with intermittent lightning activity that moved southward across the main island of Tutuila (where Pago Pago, METAR identifier NSTU, is located) and the smaller Manu’a Islands (~65 miles to the east) during a 10-hour period on 29 March 2026. The development of rain showers and thunderstorms was enhanced by the presence of a surface trough of low pressure that was draped across the Samoan islands (surface analyses: 0600 UTC | 1230 UTC | 1500 UTC | 1800 UTC).

Rawinsonde data from Pago Pago (below) revealed that the atmosphere was rather moist and unstable, with parameters that were favorable for the development of deep convection. The coldest cloud-top infrared brightness temperature exhibited by storms in the vicinity of NSTU was -78ºC — which represented a small overshoot of the Most Unstable (MU) air parcel’s Equilibrium Level (EL).

Plot of rawinsonde data from Pago Pago, American Samoa at 1200 UTC on 29 March [click to enlarge]

Although rainfall across American Samoa on 29 March was not exceptionally heavy (and no Flash Flood Warnings were issued), there was a period when NSTU received 1.11″ of rain within about 2 hours as a thunderstorm with moderate rain showers passed over Tutuila (below).

List of decoded surface reports from Pago Pago (NSTU), with a red box highlighting the period (in Local Time) that 1.11″ of rainfall occurred within about 2 hours on 29 March [click to enlarge]


List of METAR reports from Pago Pago (NSTU), with a red box highlighting the period when 1.11″ of rainfall occurred within about 2 hours on 29 March [click to enlarge]

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