SYNOPTIC DISCUSSION

SUNDAY 29th  MARCH 2026 AM SHIFT

TODAY:

An active trough embedded within the South Pacific Convergence Zone (SPCZ) remains over Samoa, bringing brief heavy downpours and thunderstorms as observed in some areas of the group. The morning ascent from Pago (28/12z) indicates moist and unstable atmospheric conditions with precipitable water reaching 62.99mm. MIMIC Total Precipitable Water (TPW) imagery also reflects abundant moisture advection over the region as the deepening moist northwesterly flow persists. In addition, an upper trough lies over the group, inducing moderate upper divergence with a weak deep-layer wind shear environment. However, vorticity fields at 850mb and 700mb reveal that strong vorticity signals are positioned just east and west of the forecast area, suggesting a relatively weak vorticity environment over the group. These environmental conditions suggest a high risk of heavy downpours and flooding across Samoa today.

MONDAY THROUGH SATURDAY:

Unstable weather conditions will remain over the area of forecast as the SPCZ axis fluctuates along our island chain for the start of the new week. A weak low-level circulation is still predicted to the west of the Savaii early Tuesday, which will exceed the steering level. This surface to mid-level development and synoptic setting is expected to drive a moderate to fresh Northerly flow over the group through mid-week, hence, maintaining moisture over the area. Intensified wind speeds are forecasted at the gradient level with short duration as it rapidly exits the region due to the extension of a ridge from the north through the latter days of this outlook period. Trade wind regime will be introduced on Thursday as the SPCZ axis migrates further southwest, allowing the aforementioned ridge to take control of stable weather later in the week.

MARINE OUTLOOK:

Generally, the observed marine data show that wave and swell heights (waves: surface water movement; swells: longer-period waves from farther away) across Samoa’s waters are still below the 2.5 metre threshold, so there is no threat. Wave models suggest this will continue for the forecast period.

ADVISORIES/WARNINGS:

VALID: HEAVY RAIN WARNING

VALID: FLOOD WARNING

Weather Duty Team:

F. Kalapani, S. Kalolo, S. Maualaivao & F. Namulau’ulu.