textproduct: Honolulu

SYNOPSIS

An upper level trough passing through the region this morning will maintain some higher level clouds over the islands with scattered showers and the chance for a thunderstorm or two, mainly over the western islands. Lighter winds are expected to spread across the region Tuesday and Wednesday, leading to some afternoon clouds and showers. A ridge builds back in north of the state by Thursday, allowing moderate easterly trade winds with passing windward showers to return lasting through the upcoming weekend.

SHORT TERM UPDATE

The forecast has been updated this morning to adjust the PoPs/Wx/Sky based on the latest model guidance. The shortwave trough swinging through the region continues to spark some thunderstorm activity, but that activity has remained in the offshore waters to the north of the islands. The chance for thunderstorms does remain this morning over/near Kauai and Oahu.

PREV DISCUSSION

Issued at 329 AM HST Mon Apr 20 2026

Looking into the infrared satellite imagery this morning we see a Mesoscale Convective System with moderate to heavy showers and thunderstorms collapsing as scattered thunderstorms riding in on an outflow boundary fades as it approaches the western coastal waters of Kauai. A few of these thunderstorms will likely reach these western coastal waters early this morning, however the entire system will likely collapse into a more moderate rain band by the time it reaches the islands of Kauai and Niihau around sunrise. Elsewhere the passing upper level trough shown on satellite water vapor imagery will increase shower trends today across the state, especially along southeastern slopes of Haleakala on Maui and the southeast slopes of Mauna Loa on the Big Island this afternoon. Rainfall and thunderstorm coverage in the next 24 hours of the statewide forecast was adjusted upward to account for these smaller scale weather impacts.

An upper level trough will swiftly pass from west to east across the state today in southeasterly wind flow briefly enhancing shower activity as it passes over the island chain. An upper level ridge will build in behind the trough as the trough moves away from the islands from tonight into Tuesday. Expect drying trends as the upper level ridge builds stabilizing subsidence, downward moving warming and drying air, strengthening and lowering the subsidence inversion heights across the Hawaii region.

Stronger subsidence will build the surface ridge north of the islands, allowing trade winds to return to the region from Wednesday night on into the upcoming weekend. Subsidence inversion heights will range from 5,000 to 7,000 feet during this time period, high enough for brief passing showers favoring the typical windward and mountain areas in the overnight to early morning hours into Sunday.

AVIATION

Light easterlies persist windward areas along with land- sea breezes over leeward and interior regions. Winds will begin to veer more ESE late this afternoon into the evening hours. Light showers possible windward and mauka. Heavier showers likely develop over island interiors during peak heating this afternoon. As the upper-level disturbance and associated surface trough approach the Hawaiian Islands from the west, expect cloud and shower activity to tick upward. VFR prevails, but MVFR within heavier showers.

No AIRMETs are currently in effect.

MARINE

Issued at 329 AM HST Mon Apr 20 2026

A trough of low pressure passing northwest of the state will act to weaken and veer winds east-southeasterly through mid-week across the local waters. Light wind flow should allow for afternoon sea breeze development near the shores. Isolated thunderstorms are expected to continue over the northwestern waters in the vicinity of the trough through tonight. By Thursday, high pressure ridging builds back in north of the state with fresh northeasterly trade winds returning to the region into next weekend.

Swell and surf will remain small along all shores through late today.

A small, medium-period northwest swell arrives tonight, bringing an uptick in surf along north and west facing shores for Tuesday before peaking on Wednesday. A moderate, medium- period north- northeast swell arrives on Tuesday and peaks also on Wednesday, bringing moderate surf to north and northeast facing shores, before slowly declining through the end of the week.

Another small, long-period west-northwest swell will enter the waters Wednesday night and provide a slight bump up for surf along west and northwest facing shores. Surf will continue to be small along south facing shores through the period. East facing shores will see an increase in surf with the arrival of the aforementioned north-northeasterly swell despite below-seasonal average trades.

HFO WATCHES/WARNINGS/ADVISORIES

None.