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Island-effect thunderstorms affecting Guam

By Scott Bachmeier •

Himawari-8 Visible (0.64 µm, left) and Infrared Window (10.4 µm, right) [click to play animated GIF | MP4]

JMA Himawari-8 Visible (0.64 µm) and Infrared Window (10.4 µm) images (above) showed the development of “island-effect” thunderstorms over Guam on 05 October 2022 — as mentioned in the NWS Guam Area Forecast Discussion, these storms produced 2-3 inches of rainfall over parts of western/central Guam (prompting the issuance of a Flood Advisory: Text | Tweet).

This type of island-effect convection occurred because the typical easterly trade wind flow within the  boundary layer was disrupted when a trough of low pressure became situated across the region, which led to a regime of light surface winds across the Mariana Islands (below) — and daytime heating of the islands contributed to “land breeze” convergence which aided convective development.

Himawari-8 Visible (0.64 µm) image at 0000 UTC on 05 October, with GFS model 1000 hPa wind barbs plotted in yellow [click to enlarge]

Hourly images of the MIMIC Total Precipitable Water product, viewed using RealEarth (below), indicated that abundant moisture was in place over Guam and the Mariana Islands during this period — for example, 57.75 mm (2.27 inches) in the vicinity of Guam at 0000 UTC (the Guam rawinsonde TPW value at that time was 54.48 mm or 2.15 inches).

Hourly MIMIC Total Precipitable Water product [click to play animated GIF]

A toggle between Suomi-NPP VIIRS True Color RGB and Infrared Window (11.45 µm) images valid at 0332 UTC (below) showed a thunderstorm that was developing over the western edge of the island.

Suomi-NPP VIIRS True Color RGB and Infrared Window (11.45 µm) images, valid at 0332 UTC [click to enlarge]

Another example of island-effect convection over Guam on the following day is discussed here.

Categories: Heavy rain / flooding, Himawari-8, RealEarth, Red-Green-Blue (RGB) images, Suomi NPP, VIIRS