SYNOPTIC DISCUSSION
TUESDAY 16th DECEMBER 2025 PM SHIFT

TONIGHT, INTO TOMORROW:

The South Pacific Convergence Zone (SPCZ) remains to the south of Samoa in a fairly active state, resulting in easing weather conditions as observed in most areas today. Embedded along the SPCZ is a low-pressure system located southeast of Samoa while Tropical Depression (TD03F) is centred east of Vanuatu. Sounding ascents from Pago Pago over the past 12 hours (12z and 00z) indicate a moist vertical atmospheric profile with precipitable water values reaching approximately 60mm, and good instability as reflected by CAPE values exceeding 1500J/kg. MIMIC Total Precipitable Water (TPW) imagery also reveals abundant equatorial moisture advection over the region as the deepening northwesterly winds persist. Convectively conducive environment remains south of Samoa in terms of relative vorticity at 850-700mb, weak deep-layer wind shear and good upper divergence. However, this weather system is forecast to progress over the group by tomorrow, bringing scattered showers with brief periods of heavy downpours and thunderstorms, which maintain the risk of flooding in rivers and low-lying vulnerable areas. But for tonight, expect isolated showers with possible brief moderate to heavy falls in some areas and a potential for a few thunderstorms.

THURSDAY THROUGH MONDAY:

Global weather models suggest that the SPCZ will fluctuate over Samoa throughout this outlook period, maintaining atmospheric instability and persistent moisture advection across the region. The weather system is expected to bring rainfall conditions through at least Thursday with the potential for heavy downpours and gusty winds, before convective activities move northward in a disorganized formation. The low-pressure system currently positioned southeast of Samoa may migrate east-southeastward, while another low-pressure system may develop to the south by Friday. The lack of significant mid to upper-level dynamical forcing over the forecast area during this outlook period, suggest that convective development will be largely diurnally driven and self-sustaining.

MARINE OUTLOOK:

The latest marine observations from King Poloa and the Aunu’u wave buoys as well as altimeter data continue to indicate that wave heights remain below 2 metres. However, long-period southeasterly swells may enter the forecast area by Friday with the potential to exceed the advisory threshold of 2.5 metres across all marine waters of Samoa.

ADVISORIES/WARNINGS:
VALID: HEAVY RAIN ADVISORY

VALID: FLOOD ADVISORY

Weather Duty Team:
F. Kalapani, A. Niko, A. Sofeni & S. Tavana.