FORECAST DISCUSSION

WEDNESDAY 20thMAY 2026 PM SHIFT

TONIGHT:

Fair weather conditions persisted across Samoa over the past 12 hours as a ridge of high-pressure predominates the region. Satellite data indicates mostly cloud-free conditions over the group, with isolated convections observed to the southeast. The afternoon radiosonde ascent from Pago (20/00z) depicts conditional instability with a precipitable water value reaching 52.38mm, thus suggesting a conducive environment for convective development. MIMIC Total Precipitable Water (TPW) imagery indicates adequate moisture across the region as deepening westerly winds aloft continues to aid equatorial moisture advection. Relative vorticity fields at multiple atmospheric levels reveal a negative vorticity environment over the forecast area, indicative of anticyclonic curvature and lack of significant lifting to trigger convection. In addition, strong deep-layer wind shear persists, inhibiting vertical development of convective storms. These atmospheric conditions prompts the cancellation of the heavy rain advisory for all land areas of Samoa.

OUTLOOK: THURSDAY THROUGH TUESDAY

Global weather models suggest some showery conditions on Thursday morning as a weak trough from the low-pressure system centred southeast of Tonga, extends over the group. From Saturday through Sunday, southeasterly winds are likely to dominate the surface as a ridge of high-pressure builds from the south. By Monday, easterly trade winds are forecast to predominate across most of the region. At mid-levels, a high-pressure area may linger to the north of Samoa throughout this outlook period, directing westerly steering flow across the group. Strong westerly winds are forecast to persist at the uppers throughout this outlook, and with the absence of a significant upper-level forcing, cloud development will be largely diurnally driven. Therefore, mostly fine weather conditions are expected across Samoa during this outlook period, apart from localised convective cells.

MARINE OUTLOOK:

The latest marine observations from Aunu’u and King-Poloa wave buoys indicate that wave heights are fluctuating below the advisory threshold of 2.5 metres. Global wave models suggest that wave activities will remain below 2.5 metres through at least Saturday, before a southerly swell enters the southern marine waters of Samoa with a low probability of exceeding the advisory threshold.

ADVISORIES AND WARNINGS:

CANCELLED: HEAVY RAIN – ALL OF SAMOA

Weather Duty Team:

F. Kalapani, A. Sofeni, U. Namulau’ulu & N. Tutasi.