Moderate trade winds will hold through Saturday, bringing modest showers to windward slopes. An upper level trough will move overhead, producing some high clouds and triggering a few heavy showers and thunderstorms interior Big Island and around Kauai. Trades will weaken and veer out of the southeast across western half of the island chain late Saturday and Sunday as low pressure deepens several hundred miles northwest of Kauai. Another passing upper level trough will bring renewed chances for isolated heavy showers Sunday and Monday. Light winds favor chances for mainly afternoon showers Tuesday into Thursday.
A slightly unstable trade wind flow is in place this afternoon. Surface high pressure passing well north of the area has boosted trades to moderate strength this afternoon, something we have not seen for some time. Little organized moisture within the trade wind flow has led to scant rainfall across most areas today. An upper level trough approaching from the west is producing increased high clouds and a decrease in stability aloft. Combined with afternoon heating, this instability was sufficient to trigger a couple of thunderstorms across the high interior terrain of the Big Island, though deep convection elsewhere has remained confined along a weak surface trough 150 to 200 miles northwest of Kauai.
Thunderstorms will diminish by sundown on the Big Island, but a few could flare up west of, and possibly over, Kauai overnight as the upper level trough inches closer to the islands. Elsewhere, continued moderate trades will focus modest rainfall across windward slopes.
Only subtle changes are expected on Saturday. The upper level trough will weaken as it swings over the islands, likely maintaining enough instability to trigger another round of showers and thunderstorms over the Big Island interior. Developing surface low pressure centered several hundred miles northwest of area should keep the threat of widespread heavy showers and thunderstorms northwest of Kauai. However, the developing low may cause the trades to weak and shift south of due east by afternoon, which could allow for isolated showers, some potentially heavy, to form over leeward terrain of all islands. The upper level trough should move off to the northeast overnight, diminishing the threat for localized heavy showers and focusing modest rainfall over windward areas.
On Sunday and Monday, trade winds will weaken everywhere and will shift southeasterly across the western half of the island chain, while another upper level trough brings instability, mainly Sunday night into Monday morning. For now, it appears that the deepest moisture and highest chances for widespread rainfall will remain along a surface trough northwest of Kauai, and while there will be some potential for heavy rainfall around Kauai, a Flood Watch does not seem warranted. Elsewhere, expect additional rounds of afternoon interior showers and possibly a thunderstorm on Big Island, and another item to monitor will be a chance for localized anchored heavy showers along the Koolau Mountains of Oahu Sunday night. Even though moisture does not appear to be significant on Oahu, the expected veering flow with height has a tendency to produce heavy rain events on the Koolau.
Chances for heavy rainfall look to diminish Tuesday into Thursday. The upper level trough should move off to the east late Monday, allowing a weak ridge aloft to settle over the state. The GFS and ECMWF are hinting that the surface trough that had been lingering northwest of Kauai will be pushed over the western end of the island chain. This would favor light and variable winds with higher rainfall chances over island interiors during the late morning and afternoon hours.
Moderate to locally breezy easterly trade winds filled in earlier today and may linger into tomorrow. However, model guidance differs on the predominant direction of the wind field for Saturday. Most of the short term guidance indicates a slight veering of the winds as a trough approaches from the northwest, which would lend to a more sea breeze driven pattern, while the remaining models have easterly trades persisting. With these model discrepancies, confidence on the wind forecast for Saturday is a bit lower than normal.
Otherwise, VFR conditions are expected to generally prevail. Brief light showers will move into windward and mountain areas on the trades, and afternoon showers and thunderstorms will remain possible over interior Big Island again on Saturday. If the winds veer slightly, clouds and showers may also develop over the interior portions of the smaller islands during the day on Saturday as sea breezes develop.
No AIRMETs are in effect.
A weak trough northwest of the state will lift north and away from the area through the weekend. High pressure then builds northeast of the area, which will allow trade winds to gradually strengthen tonight. Moderate to fresh trade winds are expected across area waters, with locally strong trades over the windier waters around Maui County and the Big Island. A Small Craft Advisory (SCA) is in effect for the windier waters through Saturday. The trough to the north develops into a weak low by Sunday, with a trailing trough remaining to the west of the main Hawaiian Islands. This will cause trades to weaken slightly and veer to the east-southeast. Gentle to moderate east-southeast winds are expected Sunday into early next week, though winds could be southerly near the trough axis in the northwest waters for Monday and Tuesday.
North shores are nearly flat with a few spots seeing tiny surf this afternoon. A small pulse of energy from the north should produce more widespread (tiny) surf on Saturday. A storm force low near Attu Island, Alaska will bring the next larger pulse for north-facing shores. This will be a small to moderate, medium-period northwest swell arriving Monday night and filling in by mid-week. A gale force low advancing south along the west coast of the US next week also produces a moderate, medium period northeast swell that arrives Tuesday and peaks around the middle of next week.
A small to moderate, medium-period SSE swell will begin to subside tonight and fade through the weekend. Surf is expected to return to background levels along south facing shores by early next week. Surf along east-facing shores remains below the seasonal average as trades remain light. The aforementioned northeast swell also boosts surf along east-facing shores mid-next week. Small background energy from the west will be possible this weekend into next week from Typhoon Sinlaku, but confidence remains low.
Small Craft Advisory until 6 PM HST Saturday for Maalaea Bay- Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big Island Southeast Waters.
