FORECAST DISCUSSION

MONDAY 18thMAY 2026 AMSHIFT

TODAY:

Moist and unstable weather conditions persist over Samoa as the South Pacific Convergence Zone (SPCZ) remains within the vicinity. Satellite data this morning indicate that major convection lies just south of Samoa, with isolated cells around the group bringing cloudy conditions and brief heavy falls in some areas. The morning ascent from Pago (17/12z) reveals a moist and unstable atmosphere with a precipitable water value exceeding 60mm. MIMIC Total Precipitable Water (TPW) imagery clearly reflects this abundant moisture advection over the region. Upper-air data indicate that strong westerly winds aloft persist with a divergence area lingering just south of the group. Given the weak vorticity environment and strong deep-layer wind shear over the forecast area, breaks between rainfall activity are expected. For today, anticipate cloudy conditions with scattered showers, heavy at times with thunderstorms and gusty periods.

TUESDAY THROUGH SATURDAY:

Global weather models suggest that the abovementioned convergence zone (SPCZ) will linger over Samoa through early Tuesday, keeping periods of rainfall and thundery activity across the group. A ridge of high-pressure may develop over the north of Samoa by Tuesday and is likely to force the SPCZ southward and away from the group. In addition, a low-pressure system is forecast to develop near southeast of Fiji and track southeastward through the week, dragging rainfall activities further south. Therefore, improving weather conditions are expected by Wednesday as a ridge of high-pressure dominates the region. Strong upper westerlies may veer more southwesterly by Wednesday, allowing a dry polar airmass to dominate, preventing deep and long-lived convection during this period. Therefore, expect mostly fine weather conditions across Samoa from mid-week.

MARINE OUTLOOK:

The latest marine observations indicate that wave heights continue to fluctuate above 2.5 metres across the southern marine waters of Samoa. This is due to an intense high-pressure system over New Zealand, directing high combined waves and swells, with southeasterly winds of 25 – 35 km/h over the southern seas. These wave conditions are expected to persist through tonight, with wave heights potentially exceeding 3 metres.

ADVISORIES/WARNINGS:

VALID: HEAVY RAIN ADVISORY – ALL OF SAMOA

VALID: MARINE ADVISORY – SOUTHERN MARINE WATERS

Weather Duty Team:

Kalapani, Sofeni, Tavana & Namulau’ulu.