Persistent convection over Tutuila, American Samoa late on 30 April Samoa Standard Time
GOES-18 imagery/products, shown below, depict an area of enhanced instability and moisture centered on American Samoa, and convection develops and persists over/around the main island of American Samoa, Tutuila. The convection appears to form along an east-west line that drifts over the island between 0400 and 0800 UTC.
Total Precipitable Water fields, below, suggest the convection was at the leading edge of a somewhat more moist airmass: TPWs increased from 1.8″ (reddish in the enhancement) to 2.0+” (lilac/purple in the enhancement) as the convection developed on the leading edge of the enhanced moisture.
CSPP Geosphere visible/night microphysics RGB imagery, below, (click here for a faster animation, also from 0010 – 1210 UTC on 1 May 2025) shows the progression of the convection from low-level clouds to strong showers. The low-level outflow boundaries show up nicely as light blue/white/pink arcs that become progressively more dark red as the convection develops.
What other products are useful in diagnosing the atmosphere? The SSEC/CIMSS Tropical Weather page includes wind diagnostics over the South Pacific, and they can be retrieved from the data archive at that page. The vorticity analysis from 21 UTC/30 April 2025 to 120 UTC/1 May 2025, below, shows an increase in low-level vorticity over Samoa, reflecting the convective development that is occurring there. The center of the vorticity slowly drifts north as well during the animation. The vorticity fields have a structure that is in agreement with the observed convection; however, upper-level divergence and low-level convergence fields (not shown) do not.
MIMIC Total Precipitable Water fields (source), below, also show the subtle increase in TPW that accompanied the convection over Samoa.
Scatterometry data for this event was sparse. The toggle below — images that were taken from the manati website (https://manati.star.nesdis.noaa.gov/) — shows OSCAT imagery at 1128 UTC and MetopC imagery at 2013 UTC (both on 30 April 2025). There is speed convergence to the southeast of American Samoa.
LightningCast probabilities (available from an American Samoa Sector at this website) shows increasing probabilities from 0300 UTC to 0600 UTC, with GLM lightning observations starting at 0440 UTC.
LightningCast probabilities can also be calculated with CSPP Geo software, and the output for a longer period is shown below.
