textproduct: Honolulu

SYNOPSIS

Moderate trades will persist through today, leading to some showers along windward and mauka areas. An upper-level trough moving overhead will produce high clouds and may trigger a few heavy showers and thunderstorms around Kauai and Big Island. Trades will weaken and veer out of the east-southeast across the western half of the island chain tonight 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 afternoon and Monday. Light winds favor chances for mainly afternoon showers Tuesday and Wednesday, followed by rebuilding trade winds Thursday and Friday.

SHORT TERM UPDATE

Issued at 847 AM HST Sat Apr 18 2026

A somewhat unstable moderate trade wind flow remains in place this morning. A strong 1032 mb surface high centered far north of the state along 40N latitude is driving the moderate trades. The local pressure gradient and resulting trade winds are being slightly weakened by a surface trough parked roughly 300 miles northwest of Kauai. A weakening negatively tilted upper-level trough approaching from the west has triggered clusters of thunderstorms within the trough and along a shallow band of moisture less than 100 miles south of Big Island. Southwest winds aloft just ahead of the upper trough are spreading thick high clouds from the nearby thunderstorms over much of the state. This may act to suppress thunderstorm activity over the islands this afternoon, since weak ridging below 700 mb is producing a low-level inversion around 4000 ft. If this holds, modest rainfall will remain confined to mainly windward slopes today. However, in the event that the high clouds thin and the inversion erodes as the upper-level trough moves overhead, we have still maintained some chances of afternoon showers over leeward and interior areas, with thunderstorms possible near Kauai.

PREV DISCUSSION

Issued at 326 AM HST Sat Apr 18 2026

Moderate trades will persist for one additional day as a slow- moving surface high pressure system north of the Hawaiian Islands tracks eastward. Simultaneously, a weakening upper-level trough positioned just west of the state continues to generate high-level cloud coverage and diminish overall atmospheric stability. Increasing diurnal heating throughout the morning and afternoon is expected to heighten instability, potentially triggering another round of thunderstorms across the interior of the Big Island.

Satellite imagery indicates persistent thunderstorm activity northwest of Kauai, associated with a developing low pressure system. Although this system remains relatively weak and is situated several hundred miles to the west, it may cause trades to veer southeasterly and weaken. This shift could result in isolated, potentially heavy showers over leeward sections of all islands. However, as the upper-level trough moves further away tonight, the threat of localized heavy rainfall will diminish, shifting shower activity predominately toward windward and mauka regions.

A more potent upper-level trough is forecast to approach the state during the latter half of the weekend, introducing increased instability from tomorrow night into Monday. Global models suggest that the majority of moisture and shower activity will remain concentrated along a quasi-stationary surface trough linked to the low pressure system northwest of Kauai. Elsewhere, rainfall will be focused over island interiors, with continued potential for thunderstorms on the Big Island.

Forecasters are monitoring the possibility of localized heavy showers anchoring along the Koolau Mountains on Oahu Sunday night. While current moisture levels do not appear significant, veering flow with height often facilitates heavy rainfall in this specific terrain; therefore, close monitoring of this trend is warranted.

By Tuesday, the upper-level trough is expected to drift east of the islands, followed by the rapid arrival of weak ridging. This transition should lead to a general decrease of shower activity statewide. However, model guidance indicates that the surface trough previously northwest of Kauai will migrate over the western portion of the state. This movement will likely promote light and variable winds along with the increased possibility of afternoon rainfall over island interiors.

AVIATION

Issued at 847 AM HST Sat Apr 18 2026

A jet streak at the base of an upper level trough to the west will continue to supply mid and upper level clouds over the state with embedded showers through much of the morning and possibly into the afternoon. In addition, a line of eastward-moving thunderstorms can be seen about 200 miles northwest of Kauai, but most model guidance has this line weakening and passing just north of the area today.

Otherwise, a mix of moderate to locally breezy easterly trade winds with some sea and land breezes will be in place through the weekend as the surface trough to the west lingers and VFR conditions are expected to generally prevail across most airports. Brief showers will move into windward and mountain areas on the trades, and afternoon showers and thunderstorms will remain possible over interior Big Island. Clouds and showers may also develop over the interior portions of the smaller islands during the day as sea breezes develop. Any afternoon showers could become briefly intense due to the upper level trough providing some instability. These showers would then decrease after sunset as local land breezes occur.

AIRMET Tango is in place between 100 and FL350 due to the potential for some moderate mid to upper level turbulence. This AIRMET will likely be needed through the morning. Light icing will also be possible between 140 and FL250 within the deep layered clouds.

MARINE

Issued at 326 AM HST Sat Apr 18 2026

A high pressure system far north of the state will slowly drift eastward this weekend. Easterly trade winds continue today 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 western Hawaii region from Sunday into early next week.

A small pulse of medium period swell energy from the northwest should produce a slight bump in north facing surf heights today. 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 Monday night, peaking from Tuesday to 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 this evening for Alenuihaha Channel-Big Island Leeward Waters-Big Island Southeast Waters- Maalaea Bay-Pailolo Channel.