Strong Storms Form Near the Samoan Islands
During the day of 27 March 2026, a band of strong strorms propagated from north of the Samoan islands southward to the island themselves. These storm were notable for deep convection and strong environmental instability and were the cause of flood warnings across American Samoa.
A good first look at the environment supporting these storm comes from the NUCAPS vertical profile retrievals. Here’s a profile from the NOAA-20 satellite from just south of the area where convection formed. This is from 13:23 UTC, which corresponds to 2:23 AM in American Samoa Standard Time. Even though it’s the middle of the night, 4822 J/kg of surface-based CAPE is present. The downdraft CAPE is also significant at 895 J/kg, and the sounding is moist (although not atypically high for this region) at 1.87 inches of precipitable water. Together, these values indicate a strong possibility of deep convection and significant rainfall.
The CIMSS MIMIC-TWP2 brings some larger-scale context to the high amounts of water vapor. This loop spans from 0400 UTC on the 27th to 0500 UTC on the 28th. Looking at the Samoan Islands (southwest of the intersection of 10 S and 170 W), it’s clear that they lie in the heart of a strong plume of atmospheric moisture.
The Band 13 (10.3 micron) imagery from GOES-18 also helps show the intensity of the convection, with numerous overshooting tops seen throughout this field of view. These are seen as the grey regions embedded in the darker black areas in the enhanced color scale on the Band 13 imagrey.
It may be easier to recognize the areas of intense convection using the Day Convection RGB. Here, the most vigorous updrafts are visible as bright yellows. Note that the colors seem to fade to a more pastel tone as the loop continues. This is because this particular loop runs between 0100 to 0450 UTC on the 28th. That’s 2:00 to 5:50 PM in local time, and sunset is at 6:28 PM. This product depends on several different shortwave channels (0.64, 1.6, and 3.9 microns) and thus the fading daylight at the end of this loop makes this product increasingly unreliable.
An interesting phenomenon can be seen in the visible imagery during the morning. Lots of outflow boundaries are seen propagating southward of the main convection located to the north of the Samoan islands; note the thin bands of clouds that form in the middle of the image at the end of the loop. Here is where we are seeing the impact of the strong downdraft CAPE (DCAPE) from the NUCAPS sounding at the top of this post. DCAPE represents the tendency of a parcel that is perturbed downwards to keep going down. Areas with high DCAPE are prone to strong evaporatively-cooled downdrafts which force strong winds when they collide with the surface and propagate outward. These downbursts fizzled out before they reached land which makes it hard to determine just how strong they are given the lack of surface observations in the ocean.
