SYNOPTIC DISCUSSION
FRIDAY 05th DECEMBER 2025 PM SHIFT

TONIGHT:

A weak trough of low-pressure lies just northwest of Samoa, allowing the southeast-to-easterly winds, light and variable at times, to dominate the group. Satellite data indicate suppressed convection over the region with some congestus development over the interior highlands during the afternoon. The ascent from Pago Pago this afternoon (00z) indicates a stable and dry atmosphere with a temperature inversion at lower levels, inhibiting further vertical growth of convective storms. The precipitable water value reaches 32.23mm, which is well reflected from MIMIC Total Precipitable Water (TPW) data. The lack of relative vorticity signals at 850mb to 700mb over the forecast zone signifies an unfavourable environment for convective development. In addition, the southwesterly aloft continues to advect a dry and descending airmass that induces subsidence aloft over the region. Therefore, expect partly cloudy skies with some possible showers across Samoa tonight.

SATURDAY THROUGH THURSDAY:

Global weather models suggest that a broad area of low-pressure currently to the northwest of Samoa will move westward as a high-pressure ridge gradually builds over the group by Sunday. This ridge is forecast to relax southward by Monday, allowing the northeasterly winds to dominate through at least Thursday. Such weather pattern is forecast to induce equatorial moisture advection, which will likely bring rainfall conditions from Tuesday. The deepening northeasterly winds are also forecast to prevail at the mid-levels, refining that moisture advection over the forecast region. The absence of a significant upper-level trough throughout this outlook period suggests that cloud development will be largely diurnally driven. However, ensemble models hint at the potential for heavy downpours in the new week.

MARINE OUTLOOK:

The latest marine observations indicate that wave and swell heights are fluctuating below the advisory threshold of 2.5 meters. However, the highest tide of the month is expected this evening, which will likely bring very rough seas and strong currents across all marine waters of Samoa. In addition, global wave models suggest that north to northeast swells will affect Samoa’s marine waters by mid-week of the new week, but at a low probability of occurrence.

ADVISORIES/WARNINGS:
NONE

Weather Duty Team:
F. Kalapani, S. Kalolo, A. Uili & N. Tutasi.