textproduct: Honolulu

SYNOPSIS

A high pressure system far north of the Hawaiian Islands will slowly drift east over the next two days, however a broad ridge north of the region will remain in place keeping easterly trade winds in the forecast through the end of next week. A low level trough moving across the region today and an upper level trough approaching the islands on Monday will deepen into an upper low by Tuesday, leading to increasing cloud and shower trends across the region lasting into the end of next week. Shower activity during this time period will favor windward mountain areas in the overnight to early morning hours with brief periods of showers drifting into leeward areas.

DISCUSSION

Satellite imagery this morning shows a low level trough drifting westward through the islands on the easterly trade winds today enhancing clouds and showers across the state. The highest coverage for clouds and showers will develop along the eastern mountain slopes of each island as trade winds lift the cloud bands over mountain ridges enhancing rainfall activity. An upper level trough farther to the northwest of the islands continues to track towards the state. Expect wind speeds to pick up this afternoon over the eastern islands and then later tonight over the western islands as the surface trough continues to drift westward and higher pressure builds in.

By Monday and Tuesday an upper level trough will approach the Hawaiian Islands from the northwest direction. Models are in good agreement that this upper trough will deepen into a closed low on Tuesday near Kauai. Expect temperature inversion heights to rise from 6,000 to 8,000 feet elevation, deepening cloud heights, producing a wet trade wind weather pattern across the state.

This upper low will slowly drift eastward across the region on Tuesday with a troughing pattern lingering just northeast of the state into Friday. The upper low's upstream position will lift the clouds to higher heights just east of the island chain, producing continued wet trade wind weather favoring windward island mountain slopes, especially in the overnight to early morning hours. The long range rainfall forecast grids were adjusted higher to account for these increasingly unsettled weather trends.

AVIATION

Moderate to locally breezy trades through early next week. Low cigs and SHRA may occur over windward sites, bringing brief MVFR conditions, however broadly VFR conditions are expected through the period.

No AIRMETs in effect. Low-level turb is possible should trade winds strengthen, however conditions are not expected to warrant a turb AIRMET at this time.

MARINE

The easterly trades have eased slightly compared to the past couple of days as a weak surface trough moves through the region from east to west. Guidance indicates trades will strengthen back to fresh to locally strong levels tonight into early this week once this feature shifts west of the area. This will likely support a return of Small Craft Advisory conditions across the typically windier waters surrounding Maui County and the Big Island. Strong high pressure rebuilding far north of the state by midweek should maintain strong trades, with advisory-level winds potentially expanding to additional coastal waters statewide.

Surf along north facing shores will peak today near the late- April seasonal average, then gradually ease through the first half of the week. Offshore and nearshore buoy observations support this trend, showing a decent medium-period pulse moving through early this morning. An upward trend is expected during the second half of the week as swell generated by a broad low lifting north into the Bering Sea near the Date Line reaches the islands. Looking farther ahead, confidence remains lower due to model differences, but some guidance continues to suggest a more potent storm could develop over the far northwest Pacific Monday into Tuesday. If this scenario materializes, a longer-period northwest swell could arrive by next weekend, with surf heights potentially approaching advisory levels along exposed north and west facing shores.

Surf along south facing shores will trend up, though likely remain somewhat inconsistent, as a southwest swell arrives and peaks over the next 24 hours. This will be followed by a gradual downward trend by Monday. Another small south-southwest pulse may arrive next weekend from activity within our swell window east of New Zealand.

Surf along east facing shores will remain relatively small and choppy due to a combination of locally wind-driven seas and a small northeast groundswell moving through today.

HFO WATCHES/WARNINGS/ADVISORIES

None.