FORECAST DISCUSSION

WEDNESDAY 13thMAY 2026 PMSHIFT

TONIGHT:

The South Pacific Convergence Zone (SPCZ) remains to the south of Samoa under suppressed atmospheric conditions. Meanwhile, fair weather conditions continue to persist across the group with some localised cloud development over the interior highlands during the afternoon. The afternoon sounding from Pago (13/00z) indicates a dry and stable atmosphere where dry slots exist throughout most of the vertical profile with a noticeable temperature inversion near 800mb. MIMIC Total Precipitable Water (TPW) imagery reveals a dry airmass advection over the region, which is reflected in the sounding trace where the value is 34.85mm. CIMSS data shows a weak positive vorticity environment at the lower level with significant negative vorticity at the uppers, jointly suggesting a weak cyclonic curvature to trigger significant convection. As strong westerly winds aloft continue to prevail alongside an unfavourable wind shear environment, cloud development remains suppressed over the region.

THURSDAY THROUGH TUESDAY:

Global weather models suggest that the easterly winds will dominate the surface through at least Saturday with typical localised convection in the afternoon over highlands. A light to gentle and variable steering flow may persist through at least Monday, becoming northwesterly in the remaining days of this outlook. In addition, an intense high-pressure system sitting over the Tasman Sea is expected to move eastward, pushing the SPCZ to reside just south of Samoa by Sunday and inducing rainfall activities. This synoptic weather pattern may also direct strong southeasterly winds across the south of the group from Sunday evening. The strong southwesterly to westerly winds aloft is forecast to persist at the uppers during this outlook period, suggesting a lack of significant upper-level forcing for convective enhancement.

MARINE OUTLOOK:

The latest marine observations from Aunu’u and King Poloa wave buoys, as well as altimeter data, indicate that wave heights continue to fluctuate well below 2 metres. However, a southeasterly swell event is anticipated to enter our southern seas by the end of the week as an intense high-pressure system stationed over the Tasman Sea advances eastward. JMA predictions are hinting at a 30% chance of elevated swells and waves of more than 3 metres during this period and may prompt the issuance of a marine advisory if wave models persist in the next 24-72 hours.

Furthermore, tide table predictions are hinting the peak tide of May on Sunday at 06:10am, which will likely elevate swells for mostly our Southern Seas.

ADVISORIES/WARNINGS:

NONE

Weather Duty Team:

Kalapani, Niko, Baker & Maualaivao.