FORECAST DISCUSSION

SUNDAY 17thMAY 2026 PMSHIFT

TONIGHT & TOMORROW:

Satellite images this evening show convective clouds and thunderstorms present over the islands. This is due to the current position of the South Pacific Convergence Zone (SPCZ) which generated heavy downpours and a few thunderstorms in some areas, during the morning. ASCAT swaths indicate 20 knots southeast winds east and west of the forecast zone with isolated convections embedded along the SPCZ instability line. Although observations of cloud and rain intensity has been reduced compared to the early hours of today, models are still forecasting instability and moisture to concentrate over Samoa through tomorrow mainly in the afternoon. An upper westerly wind flow is anticipated to swerve slightly Northwest which will remove airmass from atmospheric column favorable for cloud development. These severe weather conditions are predicted to cease tomorrow night as a ridge of high-pressure builds northward of the island chain.

TUESDAY THROUGH SATURDAY:

The abovementioned surface ridge will reintroduce stable weather across the islands for the mid-week as the SPCZ tracks further South. Such is due to the development of a weak circulation forecasted over Fiji group expected to move southeastward through the Mid-latitudes. The progression of this synoptic system will cause steering winds to vary more Westerlies until the end of this week. Strong upper westerlies will turn more southwest by Wednesday allowing dry polar airmass at the uppers preventing deep and long-lived convections to persist during this period. Thus, expect mainly fine weather conditions apart from brief showers late in the week.

MARINE OUTLOOK:

Wave and swell observations remain within the range 2.5 – 3 metres across Samoa’s Southern Seas. The eastward movement of the high-pressure system station over New Zealand, maintain the abovementioned wave activities as well as Southeast winds of 25 – 35 km/h across the Southern Marine Waters. These severe weather conditions are expected to persist through tomorrow night.

ADVISORIES/WARNINGS:

VALID: MARINE ADVISORY – SOUTHERN MARINE WATERS

VALID: HEAVY RAIN ADVISORY – ALL OF SAMOA

Weather Duty Team:

Gafa, Kalolo, Huch & Baker.