SYNOPTIC DISCUSSION
WEDNESDAY 09th APRIL 2025 PM SHIFT.
TODAY:
The moist easterly wind flow prevails over Samoa, while a low-pressure trough with an embedded convergence zone lies north. Therefore, major active convections lie across the north of the group with cloud tops being steered over the region by the strong west-northwesterly aloft, providing cloudy periods as observed. The vertical ascent from Pago Pago this afternoon [00Z] depicts a good instability as the CAPE = 2796J/kg, with a fairly moist profile where the precipitable water is 57.38mm, which is also well reflected from the MIMIC TPW data where the moisture lies within 50-60mm. In addition, convective conducive environment lies more north in terms of relative vorticity, upper divergence and deep-layer wind shear. The absence of a synoptic lifting over Samoa will not trigger thunderstorm activities within the vicinity, however, the deepening easterlies may direct single cells developing over waters toward Samoa later tonight. Therefore, expect cloudy periods with isolated showers, brief heavy falls and a few thunderstorms.
THURSDAY THROUGH TUESDAY:
Global weather models suggest the deepening easterlies to persist through at least Friday then gradually veer northeasterly by Saturday and Sunday as a trough with an embedded convergence zone to the north progresses over Samoa. The winds are forecast to strengthen at 15-20 knots for Friday and Saturday but mostly in the southern areas due to a strengthening high-pressure system centred further south. In addition, the easterly trades will then dominate the weather for the new week with improving weather conditions. The westerly aloft is forecast to persist through at least Thursday then veer southerly from Friday and expected to strengthen by Monday, establishing that strong upstream which will likely suppress convective activities. Therefore, isolated showers with periods of heavy falls are expected across Samoa through Sunday where most of the convections may focus north and west of the group.
MARINE OUTLOOK:
The latest marine observations from King-Poloa and Aunu’u wave buoys indicate wave and swell heights fluctuating within the range of 1.5 – 2 meters, which is well below the advisory threshold of 2.5 meters. However, the global wave models suggest the southerly swells to enter the southern marine waters of Samoa, likely by Friday and may bring waves exceeding 2.5 metres.
ADVISORIES/WARNINGS:
VALID: HEAVY RAIN ADVISORY
Weather On-Duty Team:
F. Kalapani & S. Tavana.