textproduct: Honolulu

SYNOPSIS

Moderate trades will persist through today, leading to some showers along windward and mauka areas. An upper-level trough just west of the state will move produce high clouds and trigger a few heavy showers and thunderstorms across interior Big Island and around Kauai. Trades will weaken and veer southeasterly by the latter half of the weekend as low pressure deepens northwest of Kauai. Another, more potent upper-level trough will bring renewed chances for isolated heavy showers early next week. By Tuesday, light winds and increased afternoon shower activity is anticipated across island interiors.

DISCUSSION

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

An upper level trough and surface trough to the west can be seen on satellite imagery and radar this morning bringing in some mid and upper level clouds over the state. Some light to moderate showers can be seen from these clouds. The mid and upper level clouds should linger through much of the morning and possibly the afternoon. 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 trough to the west lingers.

VFR conditions are expected to generally prevail across most airports. 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. 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 FL230 and FL320 due to the potential for some moderate mid level turbulence. This AIRMET will likely be needed through the morning.

MARINE

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 Maalaea Bay- Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big Island Southeast Waters.