SYNOPTIC DISCUSSION
TUESDAY 20th JANUARY 2026 PMSHIFT
TONIGHT INTO TOMORROW:
As of now, the trough of low pressure system lies closely to the southwest of Samoa, while maintaining unstable weather conditions in the past 12 hours. The observed sounding profile, delineated an abundant amount of moisture content extending through the mid level, with moderate instability for surface lifting. On that note, the pre-dominate wind flow slowly veer northeast-to-northwest with weak low-to-mid level steering winds. Furthermore, present (850-500) mb negative vorticity signatures that coincides with the current system’s location, a clear indication of synoptic lifting. Notably, a weak upper trough of low pressure lingers across the area of concern generate a weak upper divergence aloft plus a weak wind shear. This promotes vertical cloud formation along with diurnal variation and topographic effect, generate short-lived afternoon showers with a few thunderstorms but it dissipate at night. But, it generate 7.2 mm of rainfall from Saoluafatastation. Late night, a patches of cumulus and towering cumulus drifted over the northern areas and developed overnight single cells of both island, provided brief heavy falls with gustiness during showers. Thus, also accumulate a 5.6 mm rainfall from the same station – Saoluafata.
For tomorrow, the model consensus indicate increased in precipitation amount where the trough of low pressure remains to near the southwest of Samoa, with a surface high pressure interact with another separate high pressure system extending from the east towards the north of Samoa. This would established a weak convergence zone with a low-to-moderate probability of severity. Moreover, expect weak and variable winds in the mid atmosphere, this may promote vertical cloud growth. Anticipate the upper trough of low pressure to relocate to the north of the area of forecast, and introducing southeasterly aloft.
From the FMS, the Tropical Disturbance (TD05F) was analyzed near 22.5S 162.2E at 200600UTC. Organization has slightly improved in the past 24 hours. Cyclonic circulation extends up to the 500 hpa. System lies to the South of upper level ridge with good upper divergence.
Global models have picked up the system move it southeast with little intensification. The potential for this system to develop into a Tropical Cyclone in the next 24 hours is very low. And it pose no threat for Samoa.
THURSDAY THROUGH MONDAY:
The model consensus, forecasted that the coupled systems – trough of low pressure with an associated convergence zone (essentially the SPCZ) may linger over the area of concerned on Thursday, while slowly moving northward as a low level circulation is likely to formed south of the Southern Cooks on Friday. The system is forecasted to slowly descend over Samoa on Saturday through the later part of the outlook period. At this period, there’s a likelihood of increased of wind strength may reach or even 25 knots. In addition, a upper diffluence that induces upper divergence aloft with weak wind shear over the area, enhancing the surface system. So, if the severe weather conditions persist then the given advisories may considered upgrade with an included wind advisory. But for now, we’ll continue to monitor and update accordingly.
MARINE OUTLOOK:
The latest marine observations from Aunu’u and King Poloa wave buoys, as well as altimeter data, indicate that wave and swell heights remain well below 2 metres. Global wave models suggest that this wave trend is likely to persist through at least Friday. From Saturday, increased wave activities may enter the forecast area and are likely to exceed the advisory threshold of 2.5 metres.
ADVISORIES/WARNINGS:
VALID: HEAVY RAIN ADVISORY
VALID: FLOOD ADVISORY
Weather Duty Team:
A. Liu, A. Sofeni, P. Huch, & N. Tūtasi.