Area Forecast Discussion for Hawaii

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Issued: Oct 01, 2016 3:24 AM HST

Synopsis
High pressure north of the state will drive fresh trade winds through the weekend. Showers are expected to increase in coverage during the weekend, with locally heavy downpours becoming more likely early next week, as a moist airmass associated with the remnants of Ulika moves over the state.

Discussion
Showers embedded in fresh trade wind flow have increased across windward and mauka areas during the night, with some of the showers spilling over into leeward areas of the smaller islands. Rainfall amounts during the past 12 hours have been mostly light, although an isolated heavy shower over Honaunau produced over 3 inches of rain during a couple of hours late Friday evening. Surface analysis depicts high pressure far north-northeast of the state, with fresh trade winds over the islands. The 12z Hilo sounding shows PW values increasing to 1.83 inches with the inversion rising to over 12 thousand feet, while the 12Z Lihue sounding continues to depict typical trade wind conditions with PW of 1.15 inches and the inversion near 7000 feet. MIMIC-TPW satellite imagery depicts an area of increased moisture with estimated PW values up to 1.8 inches moving into the Big Island and Maui, with a drier airmass retreating west of Oahu. The plume of enhanced moisture extends several hundred miles further eastward, to the remnants of former Tropical Storm Ulika about 550 miles east of Hilo. Aloft, water vapor imagery shows a weak trough extending eastward from the islands to over the remnants of Ulika, with a ridge centered north of the state.

During the weekend, moisture will continue to increase over the islands from east to west, while the upper trough initially over and east of the state will evolve into a pair of upper lows, one centered a few hundred miles northwest of Kauai and the other centered several hundred miles east of the Big Island. Surface high pressure will shift slowly southward far to the northeast of the state, maintaining fresh trade winds over the islands. An increasingly wet trade wind pattern is the likely result, with frequent showers over windward/mauka areas and scattered showers spilling over into many leeward areas. Model consensus indicates that the atmosphere will become only slightly unstable during the weekend, so most of the showers should remain light to moderate although a few heavier downpours will be possible. The frequency and persistence of the showers could still result in some considerable rainfall totals during the weekend across windward areas.

Moisture and instability is forecast to increase further across the state early next week, as a very moist airmass with PW values up to 2 inches move overhead, and the closer upper trough drifts eastward with the trough axis moving over the islands. Localized heavy downpours appear increasingly likely during this time, and isolated thunderstorms cannot be ruled out. Low level winds are forecast to veer slightly toward the east-southeast, as a passing front over the North Pacific weakens the Northeast Pacific surface high. This flow pattern will cause the smaller islands to become blocked by the Big Island, with afternoon seabreezes likely on Tuesday and Wednesday. These could act as a focus for locally heavy downpours. The heavy rains will likely be somewhat spotty and not occurring all the time, but there is sufficient confidence at this point to add locally heavy rainfall wording to the forecast for Monday through Wednesday. While later shifts may need to refine the timing of this event, a flash flood watch could become necessary for the early to middle part of next week if current forecast trends continue.

Trade winds should gradually return late next week, with moisture and instability slowly decreasing as the upper trough moves away and a drier airmass filters back into the state.

Aviation
Moisture moving westward on the trades will continue to push occasional MVFR conditions across windward locations. AIRMET SIERRA, for mountain obscuration above 2500 feet, is now in effect for north and east sections of Maui and the Big Island. Leeward terminals and terminals on the western islands should remain VFR through the rest of tonight and Saturday. Thanks to locally strong trade winds, AIRMET TANGO remains in effect statewide for mechanical turbulence below 8000 feet immediately over and south through west of the mountains.

Marine
Fresh to strong trade winds associated with high pressure north of the region will continue through the weekend. The highest winds and seas are forecast across the typically windier locations between Molokai and the Big Island each day. The small craft advisory will remain in place for these windier zones and has been extended through Sunday. Near advisory-level seas will be expected across the Alenuihaha Channel and the waters south of the Big Island through this time. Winds and seas will trend down next week as high pressure north of the region weakens.

Rough surf will persist along east facing shores into Monday in response to slightly enhanced trades between former tropical cyclone Ulika and high pressure to the north. Surf along east facing shores will begin to trend down by mid-week as the winds ease and shift toward the east-southeast. Elsewhere, small surf is expected through the upcoming week. Surf along north facing shores will likely build next weekend due to a gale that is projected to develop over the northwest Pacific by Wednesday.

Watches/Warnings/Advisories
Small Craft Advisory until 6 PM HST Sunday for Maalaea Bay, Pailolo Channel, Alenuihaha Channel, Big Island Leeward Waters, Big Island Southeast Waters.


DISCUSSION...Jacobson

AVIATION...Powell

MARINE...Gibbs