SYNOPTIC DISCUSSION
WEDNESDAY 11th MARCH 2026 PM SHIFT

TODAY:

A trough of low-pressure to the west, combined with a prevailing moist northeasterly wind flow, continues to bring unstable weather conditions across Samoa. From this morning through early evening, periods of heavy downpours with strong gusty winds and a few thunderstorms have been observed across the group. The afternoon ascent from Pago (11/00z) indicates significant convective potential energy, with precipitable water reaching 51.51mm, highlighting a moist atmosphere. However, deepening moderate to fresh easterly winds are gradually pushing significant moisture surge eastward, as reflected in MIMIC Total Precipitable Water (TPW) imagery. This has resulted in a decreasing trend of available precipitable water over the region during the past 24 hours. Relative vorticity fields at 850-700mb reveal a weak vorticity environment over the forecast area, accompanied by strong deep-layer wind shear, suggesting disruption of vertical development of convective storms. In addition, the lack of an upper-level forcing over the group suggests that overall weather conditions may slightly ease through tonight; nevertheless, isolated showers with heavy falls in some areas and a few thunderstorms remain possible. Strong gusty winds at times are also expected in all land areas.

THURSDAY THROUGH TUESDAY:

Global weather models suggest that the easterly trade winds will dominate the forecast area through at least Friday. Model consensus indicates surface winds of 15-25 knots on Thursday, with 850mb winds occasionally exceeding 25 knots. Some of this stronger momentum may mix down to the surface, producing periods of strong gusty winds across the islands. In addition, a weak short-wave trough is forecast to move across the group on Saturday, bringing showery conditions with the potential for moderate falls. From Sunday through the end of this outlook, the easterly trade wind regime is expected to persist across the region. The lack of a significant upper-level forcing through at least Sunday suggests that cloud development will be largely diurnally driven over the forecast area. By the start of next week, a weak upper-level trough may move over the forecast area from the north. This feature may enhance atmospheric instability and support locally driven afternoon thunderstorms across the island group.

MARINE OUTLOOK:

The latest marine observations from Aunu’u, King Poloa wave buoys and altimeter data indicate that the wave and swell heights fluctuate below 2 meters, suggesting no imminent threat across Samoa’s marine waters. Global wave models suggest that wave activities will persist below the advisory threshold of 2.5 metres throughout this outlook period.

ADVISORIES/WARNINGS:
VALID: HEAVY RAIN ADVISORY

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