textproduct: WSO Pago Pago

Long term [Wednesday through Sunday]

A complex surface and upper-level weather pattern will develop across the Southwest Pacific during the long term forecast period. A mid to upper-level trough in the subtropical jet stream should develop over the Southwest Pacific by mid to late week. GFS ensemble, operational GFS, UKMET, and ICON model data depict a rare pattern for American Samoa, with an anomalously strong and high amplitude upper-level trough for the area. In this high-end scenario, a surface cold or stationary front would likely become positioned over or just south of the American Samoa islands. Surface convergence along the front combined with the upper-level troughing would provide strong low and upper-level forcing for enhanced rainfall activity. Additionally, strong mid and upper- level winds associated with the subtropical jet would lead to anomalously strong deep-layer vertical wind shear (increase in wind speed with height from the sfc to 6 km above sfc) for our area, likely exceeding 25 knots, especially on Sunday when the subtropical jet is forecast to be closest to the territory. Strong deep-layer wind shear favors increased convective organization, and would hence lead to higher potential for thunderstorms and persistent heavy rainfall in this scenario, especially on Sunday.

While there is a strong signal for the aforementioned high-end scenario that would favor increased potential for thunderstorms and flash flooding towards the end of the forecast period, confidence remains very low at this time. Some model data, including the ECMWF, indicate a much weaker and less-amplified upper-level trough. A weaker trough would keep any frontal surface convergence and focused area of heavy rainfall well south of the area. The exact positioning and strength of these surface and upper-level features will be critical. Considering this uncertainty, we will forecast numerous showers late week through the weekend at this time, with potential for upgrades to widespread showers and inclusion of thunderstorms in later forecasts as model data becomes more clear.

Marine

A combination of breezy easterly winds and south swells will continue to hold rough seas through midweek, making navigation difficult for small vessels. Seas should subside by late week as trade winds diminish across the area.

PPG WATCHES/WARNINGS/ADVISORIES

Small Craft Advisory