textproduct: Honolulu

SYNOPSIS

A moist air mass will linger over the islands through the weekend, with showers favoring windward and mauka areas tonight as moderate easterly trades redevelop. Winds will weaken and veer east-southeasterly Saturday, then become light and southerly on Sunday, allowing land and sea breezes to dominate and drive afternoon showers over interior and leeward areas. Forecast confidence decreases early next week as one or more weak fronts approach the state, though limited upper-level support should keep rainfall impacts modest.

DISCUSSION

A moist and somewhat unstable air mass remains in place across the state this afternoon. Light to moderate east-northeasterly winds are present over the western islands, while light east- southeasterly winds continue over the eastern end of the island chain. Earlier today, weak low-level convergence along a fading surface boundary over the central islands provided the focus for locally heavy rainfall and isolated thunderstorms. That boundary will continue to weaken tonight, allowing moderate easterly trade winds to become more uniform across the area as high pressure slides eastward north of the state. As a result, clouds and showers will focus over windward and mauka areas overnight.

Looking ahead to the weekend, that same high pressure system will continue shifting eastward in advance of the next front approaching from the northwest. This will cause winds to gradually veer east-southeasterly on Saturday, then weaken further and turn southwesterly across the western half of the state by Sunday. With this lighter background flow, land and sea breezes will become more dominant each day. Meanwhile, the upper-level trough that has been supplying instability will slowly pull away to the east, allowing atmospheric stability to increase from west to east as mid-level ridging builds in. That said, the air mass will remain quite moist and some instability will linger, especially on Saturday. Model guidance keeps precipitable water values well above seasonal averages through the weekend, so even with improving stability, isolated to scattered showers are still expected, especially across interior and leeward areas each afternoon where sea breeze convergence sets up. While an isolated thunderstorm cannot be ruled out, the probability remains too low to include in the forecast at this time.

Forecast confidence decreases heading into next week as models continue to struggle with the timing of an early-week front, and even more so with a possible follow-up front later in the week. With mid-level ridging forecast to persist over the state, these fronts are expected to arrive with limited upper-level support. That should keep shower activity fairly modest and allow the cooler air masses behind them to only briefly influence the islands before sliding east. For the first front early next week, the 12Z ECMWF is faster than the latest GFS, bringing the boundary through early Monday, while the GFS delays passage until later Monday into Tuesday. Behind the front, winds are expected to quickly veer easterly, then southeast on Tuesday, before weakening and turning southerly by Wednesday. This would once again favor a land and sea breeze regime midweek, ahead of another potential front toward Thursday or Friday.

AVIATION

Scattered showers, some with heavy rain, have mostly ended. Isolated showers continue over portions of Maui County as of mid- afternoon, with most other locations remaining dry. Winds will remain light (5-15 mph) and mostly out of the northeast, but will begin to veer easterly tonight, and southeasterly Saturday. VFR conditions are expected in most areas, but showers with isolated MVFR are forecast for Saturday.

No AIRMETs in effect.

MARINE

High pressure currently north of the state will drift eastward over the weekend as another front approaches the state from the northwest. Moderate trades will veer to a more southeasterly direction on Saturday then become light southerlies Saturday night into Sunday. Winds should become light enough for land and sea breezes to develop during this time. The tail end of the cold front will likely move through the state late Sunday into Monday with strong north to northeast winds following behind the front. There is still some uncertainty regarding the timing and the strength of the winds behind the front and the forecast should be fine tuned in the coming days.

The current medium period north-northwest (330-340 degree) swell will continue to decline tonight into Saturday. Surf heights have fallen below advisory thresholds and the High Surf Advisory has been cancelled. A series of small overlapping northwest swells will maintain below average surf over the weekend through early next week. The first pulse should arrive late Saturday and peak on Sunday, followed by another pulse late Monday. A slightly larger north-northwest (330 degree) swell is expected Tuesday into Wednesday. In the long range, there is potential for a large long-period northwest towards the end of next week, but the latest ECWAVE guidance seems to be trending lower, while the WW3 guidance continues to stay persistent with a warning level swell by the end of next week. Current forecast continues to reflect the WW3 solution, but stay tuned for updates over the next few days.

Surf along east-facing shores will remain small into next week due to the lack of persistent trade winds locally and upstream. South-facing shores will remain nearly flat through the weekend. A tiny long-period south-southwest swell is possible Monday into Tuesday.

HFO WATCHES/WARNINGS/ADVISORIES

None.