SYNOPTIC DISCUSSION
FRIDAY 12thDECEMBER 2025 PM SHIFT

TONIGHT:

An easterly wind flow continues to prevail across Samoa, while the South Pacific Convergence Zone (SPCZ) remains south of the group, stretching over the Solomon Islands. The afternoon ascent from Pago Pago (00z) generally indicates a dry atmospheric profile with precipitable water reaching 43.65mm, and a sufficient CAPE of 1529J/kg. MIMIC Total Precipitable Water (TPW) data shows a dry surge over the forecast area, with moisture advection across the north and south of Samoa. In addition, the strong southwesterly winds aloft steer upper-level clouds and moisture over the group, resulting in a fairly moist upper profile in the sounding. Over the past 12 hours, subsidence aloft persist over the region as a high-pressure system dominates, significantly suppressing convective storm development and growth. Relative vorticity fields at 850-700mb signify none-to-weak vorticity signal over the region, suggesting unconducive environment in the short-term. Therefore, for tonight, expect partly cloudy skies with a few showers possible in some areas as patches of low clouds stream along the easterly wind flow.

According to Fiji RSMC, Tropical Disturbance TD03F center [1003hpa] was analyzed near 10.2s 163.6e at 120600utc. TD03F slow moving with position fair based on himawari-9 IR imagery and peripheral surface observations. The system lies under weak upper divergence, low to moderate shear with good poleward outflow. SST around 30 degrees Celsius. The system is expected to be steered to the southeast by the near equatorial ridge. Global models have picked up the system and move it southeastwards with some intensification. The potential for this system to develop into a tropical cyclone is low and moderate after 36 hrs.

SATURDAY THROUGH THURSDAY:

Global weather models suggest that a trough of low pressure to the North may move over Samoa by late Saturday, bringing showery conditions with a potential of moderate to heavy falls through Sunday. This weather system, is forecast to gradually merge with the SPCZ by Monday and lingers over the group throughout the new week. Consequently, deteriorating weather may intensify through the week, likely to induce heavy downpours in all land areas of Samoa. Light and variable winds are forecast to persist at the mid – levels, providing a favorable environment with respect to wind shear and steering. An upper diffluence is likely to emerge to the west of Samoa, from Sunday which may assist in the development and growth of convective storms 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 heights continue to fluctuate below the advisory threshold of 2.5 metres. Global wave models suggest that wave and swell heights will continue to persist below the advisory threshold through at least Tuesday of the new week, before increasing wave conditions enter the forecast area.

ADVISORIES/WARNINGS:
NONE

Weather Duty Team:
F. Kalapani, S. Kalolo, A. Sofeni & F. U. Namulau’ulu.