SYNOPTIC DISCUSSION

TUESDAY 22nd JULY 2025 PM SHIFT

TONIGHT:

Settled weather conditions have been observed across Samoa over the past 12 hours. The ASCAT swaths and streamline analysis collection indicate a weak trough of low pressure over the group, which is responsible for stratiform clouds and few congestus, as seen in satellite data. The ascent from Pago Pago this afternoon [00z] depicts a stable atmosphere, dry profile through the uppers with a temperature inversion that jointly inhibits and limits convective storm development and growth. The total precipitable water amounts to 36.52mm, which is also reflected in MIMIC TPW data. A high-pressure ridge lies over Samoa at the mid levels whereby adds to the unconducive environment. In addition, the westerly aloft continues to persist with a strong wind shear environment. Therefore, mostly fine weather tonight, then becoming cloudy at times with some isolated showers possible later tonight through the morning as the weak trough lingers within the vicinity.

WEDNESDAY THROUGH MONDAY:

The embedded trough and convergence zone of the SPCZ are forecast to move over Samoa from the south on Thursday. The weather system is likely to linger over the group through Saturday, bringing periods of light to moderate rainfall activities in most areas, brief downpours in few areas. The convergence zone may establish over Samoa by Saturday with associated southeasterly winds to increase at 15 – 20 knots, particularly off the southern areas. This surge of moderate to fresh southeasterly winds may move across the islands late Sunday into Tuesday, so there's a chance of windy conditions by the start of the new week. A broad area of high-pressure is forecast to station north of Samoa at the mid levels, resulting in westerly steering winds to predominate through most of the outlook period. In addition, the westerly aloft will likely persist through at least Monday of the new week, before an upper trough progresses from the west of the group, resulting in northwesterly veering of winds aloft.

MARINE OUTLOOK:

The latest marine observations from the Aunu’u and King-Poloa wave buoys, as well as altimetry data, indicate wave heights fluctuating below the advisory threshold of 2.5 meters. Global wave models suggest this wave trend to continue through at least Friday, then a potential increase in wave heights exceeding 2.5 meters is expected from Saturday through the new week as southerly swells enter the forecast zone. The increasing southeasterly winds of the SPCZ by the start of the new week may contribute to elevating the wave heights across the islands’ marine waters.

ADVISORIES/WARNINGS:

NONE

Weather On-Duty Team:

F. Kalapani, A. Sofeni, F. U. Namulau’ulu & N. Tutasi.