SYNOPTIC DISCUSSION
TUESDAY 20th JANUARY 2026 AM SHIFT
TODAY:
Active convections remain further west of Samoa, affecting the regions of Solomon Islands, Vanuatu and New Caledonia. Meanwhile, isolated thunderstorms persist around the island chain due to a trough of low-pressure lying within the vicinity of Samoa. The morning ascent from NSTU Pago Pago (12z) indicates an unstable atmosphere as reflected by a CAPE of 1247J/kg and a moist vertical profile with precipitable water reaching 61.87mm. MIMIC Total Precipitable Water (TPW) imagery reveals abundant moisture advection over the region, continuing to aid convective storm growth and development. The relative vorticity fields at 850-700mb show positive vorticity signals over the forecast area, indicating cyclonic curvature of the trough that deepens through the mid-levels. In addition, an upper trough positioned just south of the group induces some instability that contributes to sustaining short-lived thunderstorms in the short term. A weak to moderate deep-layer wind shear environment persists over the forecast area, suggesting a lack of disruption in the vertical ascent of air parcels. As a result, expect isolated showers with heavy falls in some areas, gusty periods and a few thunderstorms today.
A Tropical Disturbance (TD05F) was analyzed and named by RSMC Fiji to the northwest of New Caledonia at 191800UTC. Organization has improved in the past 24 hours. Cyclonic circulation extends up to 500hpa. System lies to the south of upper level ridge with good upper divergence. GLOBAL MODELS HAVE PICKED UP THE SYSTEM AND MOVE IT SOUTHEAST WITH LITTLE INTENSIFICATION. THE POTENTIAL FOR THIS SYSTEM TO DEVELOP INTO A TROPICAL CYCLONE IN THE NEXT 24 HOURS IS LOW TO MODERATE.
WEDNESDAY THROUGH SUNDAY:
Global weather models suggest that moist and unstable atmospheric conditions will persist over Samoa throughout this outlook period. On Wednesday, a surface high-pressure system may develop south of Fiji, interacting with a ridge further north of Samoa, establishing a convergence zone with an embedded trough (essentially the SPCZ) near the group. This displacement of the SPCZ is forecast to direct northwest to westerly winds from midweek onward, with a gradual intensification of convective activities through the weekend; however, mainly concentrate to the north of Samoa. An upper trough just south of Samoa is forecast to progress northward by Wednesday, allowing the southwesterly winds aloft to predominate.
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:
Kalapani, Kalolo, Uili & Baker.