SYNOPTIC DISCUSSION
MONDAY 17TH NOVEMBER 2025 PM SHIFT

TONIGHT:

The South Pacific Convergence Zone (SPCZ) lies within the vicinity of Samoa, keeping unsettled weather conditions over the region. During the day, most of the active convections remain just north of the group, with strong northwesterly winds aloft steering upper clouds and moisture over the forecast area. These conditions result in gloomy weather with periods of heavy downpours and gusty winds across parts of Samoa throughout the day. The afternoon ascent from Pago Pago (00z) depicts an unstable atmosphere as reflected by a CAPE value of 2206J/kg and moisture content of 56.13mm. MIMIC Total Precipitable Water (TPW) data also reveal abundant moisture advection over the region. In addition, an upper low-pressure system lies over Fiiji with its downstream flank extending over Samoa, providing upper divergence that enhances convective storm growth and development in the short term. Therefore, anticipate occasional rain with heavy falls, strong gusty periods and thunderstorms for tonight.

TUESDAY THROUGH SUNDAY:

Global weather models suggest that the SPCZ will fluctuate near Samoa throughout this week in a relatively active state, before gradually intensifying and becoming more established over the group during the coming weekend. This variability in the weather system is expected to maintain atmospheric instability and abundant moisture over the region, potentially triggering periods of heavy falls and thunderstorms across the group, with easing periods around mid-week. The upper-level low currently positioned over Fiji is forecast to move eastward and gradually dissipate into a trough, residing just east of Samoa by Friday. However, associated upper-level diffluence is likely to continue providing significant dynamical forcing, enhancing convective storm development across the region.

MARINE OUTLOOK:
The latest observations from Aunu’u wave buoy indicate that wave and swell heights remain fluctuating above the advisory threshold of 2.5 metres. This is driven by an intense high-pressure system centred further southeast of Samoa, generating high southeasterly swells,. These swells are further elevated by wind waves produced by strong gusts associated with convective activity around the islands. These hazardous wave conditions are expected to persist across Samoa’s marine waters throughout this outlook period.

ADVISORIES/WARNINGS:
VALID: HEAVY RAIN WARNING

VALID: FLOOD WARNING

VALID: WIND ADVISORY

VALID: MARINE ADVISORY

Weather Duty Team:
F. Kalapani, A. Niko, N. Tutasi & C. Baker.