Area Forecast Discussion

Synopsis

There are three key features of note observed in latest satellite imageries that will drive the weather pattern across American Samoa over the next 7 days:

First is the high pressure ridge to the south and southeast that has driven the trade wind pattern over the past several days. This can be seen in latest ASCAT satellite wind estimates as a counterclockwise circulation to the south of the Cook Islands.

Second is a developing surface low pressure system just west of the aforementioned ridge. This surface low is developing beneath an upper-level trough in the subtropical jet stream and is clearly evident by a large area of heavy cloud cover and clockwise circulation observed via ASCAT between Fiji and New Zealand.

Third is a surface trough to the north, currently positioned near the Tokelau island chain. This trough has been held to the north of American Samoa over the past several days by the aforementioned high pressure ridge.

Short term [tonight through Wednesday night]

00Z RAOB upper-air observation from this afternoon revealed that a strong stable trade wind inversion is still in place above a shallow moist layer. This limits convective cloud growth to this shallow moist layer, below the stable temperature inversion. This will keep rainfall activity limited to scattered shallow, brief, and mostly light showers while the trade wind pattern remains.

The high pressure driving the trade winds has weakened slightly since its peak, but winds should continue to remain gusty through at least this evening. The high pressure system will then be gradually pushed out as the developing low pressure system moves eastward and overtakes it. This will lead to further decreases in wind speed tonight into Tuesday night. By Wednesday as the low passes to the south of American Samoa, it will completely cut off the islands from the easterly trade winds, bringing light and variable wind conditions.

As the weather pattern changes Wednesday, the trade wind inversion will rapidly erode. Associated with the upper-level system will be a cold front extending from the surface low into the tropical South Pacific. This cold front will approach American Samoa from the south by late Wednesday. Additionally, the surface trough to the north of the islands will begin to drift southward and closer to the American Samoa islands as the high pressure system is pushed out of the area, but is expected to weaken significantly as it does so. Erosion of the stable inversion layer, lift along the cold front, the surface trough approaching from the north, and upper-level forcing from the jet stream will all combine to increase rain chances by late Wednesday. Numerous to widespread showers are expected by this time frame. Heavy uncertainty remains regarding whether there will be any threat for flash flooding, with most recent model runs not very bullish for heavy rainfall.

Long term [Thursday through Monday]

Rain chances will remain elevated on Thursday as the active weather system remains in the area. Model data indicates that American Samoa will be on the cold side of the frontal boundary by Thursday morning. Therefore, temperatures are likely to be well below average on Thursday as southerly winds transport cool air from near New Zealand into the tropics.

By late week through the weekend, yet another high pressure system will take over to the southwest of the territory, turning winds back out of the east to southeast. Models indicate that this high pressure system will be considerably weaker than the one that is currently in the area, depicting trade winds at only a modest 5 to 15 mph.

Marine

Observations from PacIOOS Aunu'u buoy indicates wave heights remaining in the 9 to 12 foot range driven by breezy trade winds and southerly swells. A new, very long period, south swell reached the shorelines last night, increasing peak period times to over 20 seconds. As a result, the High Surf Warning remains in effect for today as these peak period times continue to favor surf near the 15 foot threshold. Sea and surf conditions will begin to gradually improve starting tonight and continuing through midweek as winds and swell strength both diminish.

The low pressure system is forecast to develop into a formidable midlatitude cyclone as it passes south of American Samoa. While the system will pass well to the south and not cause any direct impacts on the territory, it will generate yet another south swell. This swell is forecast to reach coastal waters by late week into the weekend and build seas back to at least 7 to 9 feet.

PPG WATCHES / WARNINGS / ADVISORIES

High Surf Warning remains in effect. Small Craft Advisory remains in effect.