SYNOPTIC DISCUSSION

FRIDAY 12th SEPTEMBER 2025 AM SHIFT

TODAY:

A convergence zone has moved just north of Samoa, allowing moderate southeasterly winds to prevail and resulting in improving weather conditions across the group. Satellite imageries reveal suppressed cloud development over the region, suggesting the dominance of dry airmass and subsidence aloft. Relative vorticity field at 850mb shows weak negative anomalies just north of the group, associated with the convergence zone. Meanwhile, positive anomalies exist at 700-500mb, indicating anticyclonic curvature, which is promoting large-scale descending air and further suppressing convective development. In addition, the southwesterly winds dominate the uppers, bringing polar airmass that refines dry air advection over the forecast domain and further restricting vertical growth of clouds. As a result, the heavy rain advisory has been cancelled; but for today, expect partly cloudy skies with a chance of brief moderate showers in the afternoon over highland areas.

SATURDAY THROUGH WEDNESDAY:

Global weather models suggest that the trade wind regime will dominate throughout this outlook period, resulting in minimal rainfall activity. Showers will be primarily diurnally driven, with a tendency to favor the interior highland areas. A high-pressure ridge is forecast to dominate the mid-levels during this outlook period, maintaining subsidence aloft and a relatively dry profile over the forecast domain. The southwesterly to southerly winds will persist at the uppers through Sunday, further reinforcing dry airmass advection associated with polar outflow. This synoptic configuration is likely to suppress deep convection by limiting vertical development, thereby restricting the growth of significant convective storms during this outlook period.

MARINEOUTLOOK:

The latest marine observations indicate that waves and swell heights are gradually falling below the advisory threshold of 2.5 metres collectively. Global wave models suggest that these wave activities will continue to decrease below 2 metres by tomorrow as the moderate southeasterly winds ease at 10-15 knots. As a result, the marine advisory is now cancelled for all marine waters of Samoa.

ADVISORIES/WARNINGS:
CANCELLED: HEAVY RAIN ADVISORY

CANCELLED: MARINE ADVISORY

Weather Duty Team:
F. Kalapani, L. I. Tuatipi, S. Tavana & N. Tutasi.