FORECAST DISCUSSION
MONDAY 13thAPRIL 2026 – P.M SHIFT
TONIGHT:
A moist easterly wind flow continues to prevail across Samoa, while a trough of low-pressure lies west of the group with associated deep convection. A weak upper trough positioned just west of the group aided convective development and growth of this surface system, with northwesterly winds aloft steering upper clouds over Samoa. Such synoptic weather pattern triggered a few thunderstorms just west of Savai’i while bringing cloudy periods across most of the land areas. The afternoon ascent from Pago (13/00z) indicates weak atmospheric instability alongside a fairly dry vertical profile, with precipitable water reaching 50.55mm. A strong temperature inversion is also present at lower level, which may significantly inhibit cloud development in the nowcast period. The aforementioned weak upper trough may continue to move over the group later tonight, and possibly aid convective development if conditions destabilise. But for tonight, expect partly cloudy skies with some showers, brief moderate falls possible in some areas due to low-level clouds streaming along the wind flow.
TUESDAY THROUGH SUNDAY:
The gentle to moderate easterly trade winds are forecast to dominate the surface through at least Thursday, with an embedded weak trough of low-pressure that may linger over the group on Wednesday and Thursday. A high-pressure system currently centred over southern Australia is expected to move across the Maritime, pushing the South Pacific Convergence Zone (SPCZ) near south of Samoa during the weekend, likely increasing showery conditions. Therefore, a broad area of low pressure may reside just south of the group, prompting the development of embedded low-pressure systems. Light and variable steering flow is forecast over the forecast area through Wednesday, suggesting the lack of vertical disruption in convective development. Northwesterly to westerly steering flow may predominate from Thursday, possibly enhancing moisture advection over the region. The abovementioned weak upper trough is forecast to exit through the north of Samoa by Tuesday, allowing strong southerly winds aloft to dominate.
MARINE OUTLOOK:
The latest marine observations from Aunu’u and King Poloa wave buoys indicate that wave heights occasionally exceeded 2.5 metres over the past 12 hours. These wave activities may continue to exceed 2.5 metres mainly over the southern marine waters from through at least Tuesday. During this outlook period, there is a low probability of significant wave heights entering the northern marine waters. Although this may be the case, activities may not pose serious wave-related hazards to the group’s marine waters.
ADVISORIES/WARNINGS:
NONE
Weather Duty Team:
F. Kalapani, A. Sofeni, C. Baker & S. Maualaivao.