National Weather Service Pago Pago Facebook posts on 11 December describe an atmosphere primed for heavy rain and lightning, the latter Facebook also shown below. Band 13 animation, above, shows persistent deep convection over the Samoan Islands. The convection develops and drifts southward.

MIMIC Total Precipitable Water over the south Pacific shows moist air over Samoa being drawn poleward in an atmospheric river. (Cyclone Jasper (see this also) along the northeast coast of Australia is also apparent).

ASCAT Scatterometry data from Metop-B, below (source), shows strong surface convergence over the Samoan Islands.

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Lightning events on 12 December are shown below from this link. In this case, GOES-18 Flash Extent Density field are overlain on GeoColor imagery. Abundant lightning is occurring north and south of the Samoan Islands. An Airmass RGB animation from the same website (here), shows the persistent southward motion of moist tropical air over the Samoan Islands.

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