textproduct: Honolulu

SYNOPSIS

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.

SHORT TERM UPDATE

Issued at 818 PM HST Fri Apr 17 2026

Latest satellite and radar imagery depicts a light band of moisture associated with an upper-level trough moving northeastward across Kauai and extending into Oahu this evening. Increased precipitation probabilities over Oahu a bit to show this feature, extending the line of isolated to scattered showers more eastward to cover both islands. Otherwise, the remainder of the forecast remains on track showing moderate trade winds holding throughout Saturday, bringing some decent shower activity to windward slopes before weakening and veering for the remainder of the weekend period.

PREV DISCUSSION

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.

AVIATION

Issued at 818 PM HST Fri Apr 17 2026

A mix of moderate to locally breezy easterly trade winds with some land and sea breezes will be in place through the weekend. Most of the short term guidance indicates a slight veering of the winds as a trough approaches from the northwest tonight through tomorrow morning, 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 and Saturday night is on the lower end.

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. Clouds and showers may also develop over the interior portions of the smaller islands during the day on Saturday as sea breezes develop.

AIRMET Tango is in place between FL230 and FL320 due to the potential for some moderate mid level turbulence. This AIRMET will likely be needed through the night and potentially tomorrow morning.

MARINE

Issued at 818 PM HST Fri Apr 17 2026 A high pressure system far north of the state will slowly drift eastward this week. Easterly trade winds continue through Saturday with Small Craft Advisory conditions in effect for the windier waters and channels near the Big Island and Maui. A series of low pressure systems developing northwest of the islands will weaken the ridge over the state, causing weak southeasterly winds to develop over the Hawaii region lasting into early next week.

A small pulse of swell energy from the north should produce a slight bump in north facing surf heights on Saturday. A small to moderate, medium period northwest swell will move into the region by Monday night and continue through Wednesday. A moderate, medium period northeast swell will arrive by Tuesday, peaking early Wednesday, and then slowly declining through Thursday.

A series of overlapping small south swells will move into the region into next week. Surf heights along east facing shores remains on the smaller side due to weaker trade winds.

HFO WATCHES/WARNINGS/ADVISORIES

Small Craft Advisory until 6 PM HST Saturday for Alenuihaha Channel-Big Island Leeward Waters-Big Island Southeast Waters- Maalaea Bay-Pailolo Channel.