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1-minute GOES-18 images to monitor heavy rainfall potential across American Samoa

By Scott Bachmeier •

1-minute GOES-18 Clean Infrared Window (10.3 µm) images with an overlay of GLM Flash Points, from 1901 UTC on 24 March to 0400 UTC on 25 March [click to play MP4 animation]

Due to a lack of radar coverage over American Samoa, WSO Pago Pago requested 1-minute Mesoscale Domain Sector coverage over the islands during a period of heavy rainfall risk. GOES-18 (GOES-West) Clean Infrared Window (10.3 µm) images (above) displayed showers and thunderstorms that moved across the American Samoa island of Tutuila (where Pago Pago International Airport NSTU is located) on 24 March 2025 — which produced periods of moderate to heavy rainfall (leading to flash flooding and landslides, prompting the issuance of a Flash Flood Warning; there were local storm reports of up to 4.5″ of rainfall in 3 hours). The coldest cloud-top infrared brightness temperature associated with these thunderstorms was -86ºC (brighter shades of white embedded within dark black regions).

A listing of Pago Pago surface observations is shown below — highlighting the time period displayed by the 1-minute GOES-18 imagery (during which NSTU received 3.72″ of rainfall in 3 hours). Local time in American Samoa is 11 hours behind UTC.

Surface observations from Pago Pago, American Samoa (NSTU) — the red box highlights the time period covered by the 1-minute GOES-18 imagery [click to enlarge]

Around the time of onset of the thunderstorms that moved across the island of Tutuila, nearby satellite-derived Total Precipitable Water values were as high as 2.39 in (below). This thunderstorm development was focused along a west-to-east oriented surface trough (Fiji Meteorological Service surface analyses: 2100 UTC | 0000 UTC | 0300 UTC).

GOES-18 Clean Infrared Window (10.3 µm) image with an overlay of the Total Precipitable Water derived product and GLM Flash Points at 2206 UTC on 24 March [click to enlarge]

The heavy rains that occurred were within a large region of atmospheric moisture as shown in the MIMIC TPW animation below of Total Precipitable Water. MIIMIC TPW values started decreasing around 0900 UTC on 25 March 2025.

MIMIC estimates of Total Precipitable Water 0000 UTC 23 March 2025 – 2000 UTC 25 March 2025 (Click to enlarge)

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