Forecast Discussion
Monday, 18 May 2026, PM Shift

Tonight and Tomorrow:

The South Pacific Convergence Zone (SPCZ) remains fluctuating over the islands, producing sustained cloud cover and episodes of heavy rainfall. The system's periphery, characterised by intense downpours, thunderstorms, and strong winds, is currently situated just offshore of the southernmost islands. Total precipitable water (TPW) continues to accumulate to the south, aligning with the SPCZ's current position, while persistent mid- to upper-level westerly winds are displacing the system and its moisture southward. These meteorological conditions are forecast to gradually subside by late tomorrow, resulting in improved weather across Sāmoa as the system exits the area of responsibility. The Heavy Rain Advisory remains in effect for all land areas of the island group.

Extended Outlook: Wednesday through Monday
Global weather models indicate the SPCZ will remain over Samoa through early Tuesday, with periods of rainfall and thunderstorm activity. A ridge of high pressure is forecast to develop north of Samoa by Tuesday, likely displacing the SPCZ southward and away from the islands. In addition, a low-pressure system is expected to form southeast of Fiji and move southeastward during the week, further shifting rainfall activity south. Consequently, weather conditions are anticipated to improve by Wednesday as the high-pressure ridge becomes dominant. Upper-level westerly winds may transition to a more southwesterly direction by Wednesday, enabling a dry polar airmass to prevail and suppressing deep, persistent convection. Predominantly fine weather is expected across Samoa from mid-week onward.

Marine Outlook:
Recent marine observations indicate that wave heights exceed 2.5 metres across the southern marine waters of Samoa. This is due to an intense high-pressure system over New Zealand, generating elevated combined waves and swells, along with southeasterly winds of 25 to 35 km/h over the southern seas. These conditions are forecast to persist through tonight, with wave heights potentially surpassing 3 metres.

Current Advisories and Warnings
VALID: HEAVY RAIN – ALL OF SAMOA
VALID: MARINE ADVISORY – SOUTHERN MARINE WATERS

Weather Duty Team:
MJS, Lao Ioane Tuatipi, Punipuao Huch & Ne’emia Tutasi