SYNOPTIC DISCUSSION
FRIDAY 13th MARCH 2026 AM SHIFT

TODAY:

The South Pacific Convergence Zone (SPCZ) lies further west of Samoa, with associated deep thunderstorm activity affecting Fiji and parts of Vanuatu and Solomon Islands. Meanwhile, mostly cloud-free skies persist over Samoa as the easterly trade winds dominate the region. The latest ascent from Pago (12/12z) indicates modest CAPE with a relatively dry vertical profile where precipitable water reaches 48.64 mm. Deepening moderate to fresh easterlies from the surface to mid-levels continue to advect abundant moisture surge westward and away from the group, which is well depicted in MIMIC Total Precipitable Water (TPW) imagery. Relative vorticity fields at 850mb and 700mb indicate weak vorticity signals, suggesting limited large-scale lifting over the forecast area. These environmental conditions occur within a moderate deep-layer wind shear environment. In addition, strong southeasterly flow in the upper-levels is advecting a relatively dry polar airmass over the region, which may significantly suppress convective storm growth and development. Despite these suppressing factors, localized surface heating and orographic effects may still trigger a few afternoon thunderstorms with brief showers over inland highland areas. Otherwise, mostly fine weather conditions are expected across the group today.

SATURDAY THROUGH THURSDAY:

Global weather models suggest minimal precipitation during this outlook period as easterly trade winds are expected to predominate across the region. A weak short-wave trough may move across Samoa on Saturday, extending through the mid-levels of the atmosphere. However, the main cyclonic curvature associated with this trough is expected to remain south of the group, where the majority of the accumulated rainfall is likely to occur. Strong southeasterly wind flow is forecast to dominate the upper levels through at least Sunday, associated with an upper trough stretching north of the group. This setup suggests that the upstream of the upper trough will lie over the forecast area on Saturday, which may induce subsidence aloft, suppressing convective storm growth and development across Samoa. Overall, fair weather conditions are expected during this period, with the potential for localized afternoon thunderstorms. Some isolated showers may occur as the weak trough passes through on Saturday, but widespread precipitation is unlikely.

MARINE OUTLOOK:

The latest marine observations from the Aunu’u and King Poloa wave buoys, as well as altimeter data, indicate wave and swell heights fluctuating around 2 meters, suggesting no imminent threat across Samoa’s marine waters. Global wave models suggest that wave activities will persist below the advisory threshold of 2.5 metres throughout this outlook period.

ADVISORIES/WARNINGS:
NONE

Weather Duty Team:
F. Kalapani, L. Tuatipi, A. Uili & F. Namulau’ulu.