SYNOPTIC DISCUSSION
SATURDAY 02ndAUGUST 2025 PM SHIFT
TONIGHT:
A line of low-level cloud patches has been present over Samoa for the past 12 hours, as a result of a weak trough of low pressure located in the vicinity of the group. This weather system triggered thick low clouds and thus showers, particularly over the highlands and southern areas, in conjunction with surface heating. The ascent from Pago Pago this afternoon [00z] depicts good moisture at the lower levels, where the precipitable water amounts to 43.80mm. However, a very dry and subsiding airmass aloft lingers over Samoa, as high-pressure systems dominate the mid-to-upper level atmosphere, resulting in weak instability. These are stabilizing factors that suppress further vertical cloud development. While the surface has enough moisture for showers, the lack of upward movement of air at higher levels means there's not enough instability for thunderstorms or continuous rainfall. In addition, CIMSS data also indicates very weak relative vorticity at 850mb and an area of upper convergence over the forecast zone – both of which are unfavorable for deep convection. For tonight, expect cloudy periods with few showers in some areas of the group.
SUNDAY THROUGH FRIDAY:
Global weather models suggest minimal rainfall activity throughout this outlook period as high-pressure systems are likely to dominate Samoa’s weather. This weather pattern is expected to direct south to southeasterly surface winds through Monday, shifting more easterly from Tuesday into Wednesday. A short-wave high-pressure ridge may move over Samoa from the west, resulting in surface winds to veer northeasterly to northerly from Thursday. This equatorial wind flow may advect sufficient moisture to trigger some isolated showers on Thursday. In addition, a ridge of high-pressure is also expected to dominate the mid-levels throughout this outlook period, maintaining a dry and subsident airmass aloft. At the upper levels, southwesterly to westerly upper winds are forecast to persist which may keep a convective suppressive environment over Samoa.
MARINE OUTLOOK:
High swells are expected to affect the coastal and marine waters of Samoa from Monday, with swells originating from the south. Waves and swell heights may exceed 2.5 metres and are forecast to persist through at least Friday, creating potentially dangerous conditions for those engaging in marine activities. Global weather models suggest these waves and swell heights to exceed 3 metres by late Tuesday. Therefore, the marine advisory is now in force for all marine waters of Samoa.
ADVISORIES/WARNINGS:
VALID: MARINE ADVISORY
Weather On-Duty Team:
F. Kalapani, A. Niko, P. Huch & N. Tutasi.