SYNOPTIC DISCUSSION

MONDAY 19th JANUARY 2026 AMSHIFT

TODAY:

The active flank of the South Pacific Convergence Zone (SPCZ) remains further west of Samoa, affecting the region of Solomon Islands, whereas occasional thunderstorms are seen around the group. Meanwhile, a trough of low-pressure system with associated rain and thunderstorms to the east, is moving towards Samoa and is expected to bring deteriorating weather today. The ascents from NSTU Pago Pago (12z) indicate a moist and unstable atmosphere over the past 12 hours. The latest sounding shows a CAPE of 1133J/kg and precipitable water of 62.39mm, generally indicating conducive environment for convective storm developments. MIMIC Total Precipitable Water (TPW) imagery well reflects this sufficient moisture advection over the region. For today, expect this weather system to bring scattered showers with brief heavy falls, gusty periods and thunderstorms across the group.

TUESDAY THROUGH SUNDAY:

Global weather models suggest that moist and unstable atmospheric conditions will persist over Samoa throughout this outlook period. A trough of low-pressure is expected to affect the region through at least Tuesday, maintaining an elevated risk of heavy downpours across the group. From Wednesday, a surface high-pressure system may develop south of Fiji by Wednesday, interacting with a ridge further north and establish a convergence zone with an embedded trough (essentially the SPCZ) over Samoa. This displacement of the SPCZ is forecast to direct northwest to westerly winds from midweek onward, with a gradual intensification of convective activities through the weekend; however, mainly concentrate to the north of Samoa. The absence of a significant upper-level dynamical forcing over the forecast area suggests a lack of enhancement in convective storm growth and development during this period.

MARINE OUTLOOK:

The latest marine observations from Aunu’u and King Poloa wave buoys, as well as altimeter data, indicate that wave and swell heights remain well below 2 metres. Global wave models suggest that this wave trend is likely to persist through at least Friday. A northerly swell event is forecast to arrive later in the week and may coupled with the fluctuations of the SPCZ axis within our forecast area.

ADVISORIES/WARNINGS:

VALID: HEAVY RAIN ADVISORY

Weather Duty Team:

Kalapani, Kalolo, Huch & Namulau’ulu.