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1-minute imagery to monitor flash flooding potential across American Samoa

By Scott Bachmeier •

1-minute GOES-18 Infrared (10.3 µm) images with an overlay of GLM Flash Points and the Total Precipitable Water derived product, from 2201 UTC on 04 May to 1300 UTC on 05 May [click to play MP4 animation]

During a period when American Samoa had been under a Flash 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, satellite imagery can be a critical tool for monitoring the development of deep convection). 1-minute GOES-18 Infrared imagery with overlays of GLM Flash Points and the Total Precipitable Water (TPW) derived product (above) showed clusters of deep convection with intermittent lightning activity that moved across the main island of Tutuila (located just south of where the imagery is centered) as well as the smaller Manu’a Islands (~65 miles to the east) during a 15-hour period on 04-05 May 2026. At the Pago Pago METAR site on Tutuila, most of their calendar day 24-hour precipitation for 04 May (2.21 inches) occurred during the period shown in the GOES-18 animation above — and a Flash Flood Warning was issued at 0154 UTC on 05 May.

GOES-18 Infrared (10.3 µm) image at 0250 UTC on 05 May, with a cursor sample of the coldest cloud-top infrared brightness temperature near the island of Tutuila [click to enlarge]

The coldest cloud-top infrared brightness temperature exhibited by storms in the vicinity of Tutuila was -78.14ºC (above) — which was at an altitude near the Most Unstable (MU) air parcel’s Equilibrium Level (EL), according to rawinsonde data from Pago Pago (below). That sounding also depicted an atmosphere which was very moist and unstable, with parameters that were favorable for the further development of deep convection. In addition, rain shower and thunderstorm activity was enhanced by the presence of a surface trough / stationary front in the vicinity of the Samoan Islands (05 May surface analyses: 0000 UTC | 0600 UTC | 1230 UTC).

Plot of rawinsonde data from Pago Pago, American Samoa at 0000 UTC on 05 May [click to enlarge]

It is noteworthy that according to the climatology of TPW for all 0000 UTC soundings at Pago Pago (below), the TPW value of 2.81 inches sampled at 0000 UTC on 05 May 2026 was significantly higher than the previous maximum value on record for that particular time/date (2.63 inches).

Plot of Total Precipitable Water climatology for all 0000 UTC soundings at Pago Pago, American Samoa [click to enlarge]

The MIMIC Total Precipitable Water product (below) showed the band of high moisture that was moving southeast across the Samoan Islands during this flash flooding event.

Hourly MIMIC Total Precipitable Water images, from 0000 UTC on 03 May to 1500 UTC on 05 July [click to play animated GIF]

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