SYNOPTIC DISCUSSION

THURSDAY 18thSEPTEMBER 2025 PM SHIFT

TONIGHT INTO TOMORROW:

The transitioning conditions of the weather pattern persist across the group tonight due to the continuing prevalence of the strong southeasterly wind flow over all the land areas of Samoa tonight, which maintains the in-force of the Wind Warning for the whole of the country. The intensified high pressure system to the Northeast of New Zealand which caused the strengthening of the aforementioned wind flow. The northern part of the forecast zone is likely to experience partly cloudy with some showers, while the southern part of the islands is expected to be cloudy with isolated showers and the possibility of brief moderate falls at times. In addition, the anticipation of strong and gusty winds at times dominates the area of responsibility. Moreover, the Pago Pago sounding profile of this afternoon [00Z] illustrates the dry airmass advection in the upper – level atmosphere and also signals the decreasing moisture of 40.31 mm, which mainly focuses at the 900 – 800 mb and 550 – 500 mb level. This correlates with the TPW animation loop signal broad vapors lying to the north and south of the area of concern tonight. Also, the CIMMS observations declare the suppressing of the vertical storm's development at the islands’ surface due to the non – existence of the upper divergence and the availability of a strong wind shear environment. Furthermore, according to global weather models, which forecast the gradually decreasing of strong southeasterly wind strengths, that will dominate the area of the forecast tomorrow. This is due to the movement of the intensified high pressure system to the southeast of the group further eastward. Thus, it is likely to downgrade the enforced wind warning to an advisory level. Therefore, for tomorrow, the island’s surface is expected to be affected by partly cloudy with some showers, and expect strong and gusty winds to become less intense as the evening progresses.

SATURDAY THROUGH WEDNESDAY:

Global weather models indicate that the light southeasterly trade winds will prevail at least until Monday of the upcoming week, bringing a cool and dry air mass to the area. The lack of upper-level forcing in the forecast region during this period suggests that showers will mainly occur during the day, particularly in the interior highlands. As a result, only a few showers are expected. Additionally, a convergence zone is predicted to form northeast of Samoa by the start of the new week and move over the islands by Tuesday. This weather system may lead to scattered showers with the possibility of moderate rainfall across the region until the end of this forecast period.

MARINE OUTLOOK:

The intensified high-pressure system to the northeast of New Zealand was driving strong southeasterly winds of 55'– 60 km/h over the group’s marine waters, causing the elevation of waves and swells. Recent observations indicate waves and swell heights remaining above 3.5 metres across the country’s marine waters. These severe weather conditions experienced over land and sea may continue to affect the group through tomorrow. Thus – warrants the continuing validation of the Marine Warning for all marine waters of Samoa tonight. 

ADVISORIES/WARNINGS:

VALID: WIND WARNING

VALID: MARINE WARNING

Weather Duty Team:
Gospel. N, Albert. N, Andrew. S & Samuelu. T