Area Forecast Discussion

SYNOPSIS

A light land and sea breeze pattern will produce afternoon clouds and spotty showers over land and partial clearing at night. Trade winds will redevelop late Wednesday and strengthen Thursday and Friday, focusing rainfall over windward and mauka areas.

SHORT TERM UPDATE

Emergent sea breezes will at least produce interior clouds and a few showers, except for the Big Island where greater coverage and intensity of showers is likely. Isolated heavy rainfall is not out of the question given the resident moist airmass characterized by dewpoints in the 70s and PWAT values around 1.5". However, like yesterday, high cirrus threatens to weaken sea breeze convergence just enough to limit interior shower potential. Inherited chc/sct PoPs look reasonable. Short term forecast unchanged.

PREV DISCUSSION

Issued at 334 AM HST Tue Apr 21 2026 Light winds prevail as somewhat stable conditions develop. A surface low sitting about 500 miles north of Oahu is producing a very weak pressure gradient and resulting background winds over the Hawaiian Islands, leading to light land breezes. While there is no organized moisture around the state, dew points remain elevated in the upper 60s to 70 F in places. This modestly high moisture has kept overnight temperatures a few degrees warmer than normal and has allowed for isolated showers overnight, though nothing more than a few hundredths of an inch at most sites. Mid- to upper-level ridging is building overhead from the west as an upper-level trough moves east of the islands. This is reflected in an inversion near 5000 ft on the Hilo sounding, while lingering deeper moisture near Kauai kept the inversion suppressed on the Lihue sounding.

A humid land and sea breeze regime will prevail over the next 24 hours or so. Isolated showers near the coasts will dissipate during the early morning, followed by interior clouds and spotty showers in the afternoon. While stability is slowly returning, a briefly heavy shower is still possible in the afternoon and evening.

Trades will slowly redevelop on Wednesday as the low to the north drifts east and weakens. The combination of the building trades and gradually increase stability will lessen chances for afternoon showers.

Trade winds will strengthen Thursday and Friday and will persist into early next week as surface high pressure becomes established north of the islands. The moderate to locally breezy trades and more stable conditions will push dew points back into the lower 60s, providing relief from the recent humidity. Showers will become focused over windward slopes.

AVIATION

Issued at 334 AM HST Tue Apr 21 2026 Light and variable winds continue across all terminals this morning, triggering land-sea breezes which are expected to persist through midweek. Onshore sea breezes will bring clouds and possible light showers over island interiors this afternoon as peak heating occurs, which may lead to periods of MVFR conditions. Otherwise, VFR conditions prevail.

No AIRMETs in effect.

MARINE

Issued at 334 AM HST Tue Apr 21 2026 A weak low north of the area extends a trough south toward the islands, which will keep light to locally moderate east-southeast flow in place across the local waters through early Wednesday. Light wind flow should allow for afternoon sea breeze development near the shores. During the latter half of the week, surface high pressure building north of the area will bring a return of moderate to locally strong northeasterly trade winds.

A small, medium-period, northwest swell build today, providing a small bump to surf along north and west-facing shores. At least one other northwest swell of similar size and period arrives late today which will support elevated surf along west-facing shores through mid-week.

A moderate, medium-period north-northeast swell also arrives today and peaks on Wednesday, bringing moderate surf to north-facing shores, before slowly declining through the end of the week. East- facing shores will see an increase in surf with wrap from the north-northeasterly swell, despite below seasonal average trade winds. Strengthening trades late this week will support closer to seasonal average surf for east facing shores into the weekend.

Surf will continue to be small along south facing shores through the period.

HFO WATCHES / WARNINGS / ADVISORIES

None.