textproduct: Honolulu

SYNOPSIS

A high pressure system far north of the Hawaiian Islands will slowly drift east, however a broad ridge north of the region will remain in place keeping easterly trade winds in the forecast through into the weekend. An upper level trough approaching the islands today will deepen into an upper low near Kauai by Tuesday, leading to increasing cloud and shower trends across the region through the week. Shower activity will favor windward mountain areas in the overnight to early morning hours with brief periods of showers drifting into leeward areas.

DISCUSSION

The satellite picture this morning shows clouds surrounding a low level trough that continues to drift westward with the trough axis currently just west of Kauai. High level cirrus clouds, associated with a sub tropical jet stream south of the islands, will continue to stream over the Big Island today. An upper level trough appears on water vapor imagery approaching the island chain from the northwest direction

An upper level trough approaching the Hawaiian Islands from the northwest direction will enhance shower activity across the region through Tuesday. Models are in good agreement that this upper trough will deepen into a weak closed low near Kauai by Tuesday. A weakness in the 500 mb height fields are a good indicator of a weak upper low that the global models are unable to resolve due to resolution issues. Expect temperature inversion heights to rise from 6,000 to 8,000 feet elevation during this time period, deepening cloud heights, producing a wet trade wind weather pattern across the state.

This developing 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, just northeast of the islands, will lift the clouds to higher heights, producing continued wet trade wind weather conditions. These showers will favor windward island mountain slopes, especially during the diurnal rainfall maximum overnight to early morning hours. Periods of passing showers will continue throughout the week and into the weekend due to the persistent unstable trough setting up just northeast of the islands.

AVIATION

Moderate trades will continue through the period. Intermittent MVFR is possible across windward sites due to low cigs and showers, but VFR conditions are broadly anticipated. Shower activity may ramp up starting around sunset as an upper level system moves toward the islands.

No AIRMETs are in effect, however a mountain obscuration AIRMET may be needed if low clouds become more widespread due to oncoming shower activity.

MARINE

Moderate to fresh easterly trades will continue through Tuesday, then strengthen by midweek as a weak trough shifts to the west and the ridge builds north of the state. This will likely correspond to Small Craft Advisory winds across the typically windier waters surrounding Maui County and the Big Island beginning by Wednesday.

Surf along exposed north- and west-facing shores will gradually ease into Tuesday as a northwest swell lowers. An upward trend is expected during the second half of the week as swell generated by a broad low that has lifted north into the Bering Sea near the Date Line reaches the islands. Looking further ahead, confidence remains lower due to model differences, but some guidance continues to suggest a more potent storm evolving over the far northwest Pacific over the next couple of days. If this scenario materializes, a longer-period northwest swell could arrive by the weekend, with surf heights potentially approaching advisory-levels along exposed north and west facing shores.

Surf along south-facing shores will remain small through much of the week, with mainly background south to southwest swell expected. Another south-southwest pulse may arrive by this weekend from recent activity within our swell window east of New Zealand.

Surf along east-facing shores will remain relatively small and choppy due through midweek, with a slight rise possible later in the week as trades strengthen.

HFO WATCHES/WARNINGS/ADVISORIES

None.