FORECAST DISCUSSION

Synopsis

High pressure to the north of the islands will continue to provide the islands with trade wind weather for the rest of the week. Moderate to strong trades will weaken during the second half of the week, becoming light and variable by the start of the holiday weekend. A surface trough is expected to settle over Kauai by Wednesday and linger there into the Memorial Day weekend.

Discussion

We are looking a breezy trade wind pattern for the next 24 hours before a weakening trend settles in for the second half of the week. Showers could also be on the uptick for especially the western part of the island chain within the next 48 hours. The breezy trades, generated by a strong ridge of high pressure located north of the islands, will be undercut by a surface trough during the second half of the week. This trough which is now a stalled front and shearline, extends from 1000 to 1200 miles NE of Oahu to 85 miles N of Kauai. The models agree on the trough drifting and settling across Kauai Wednesday night. The showery weather may come sooner, as early as Wednesday afternoon, from pockets of showers running ahead of the trough. Oahu will be at the fringe of the activity so there will be periods of passing showers on Thursday. The GFS solution which is the aggressor of the two, holds on to a showery pattern for Kauai lasting into the weekend. Light to moderate trades should be in place by Friday. The trades will be weak enough for the development of local sea breezes on the smaller islands. This hybrid convective trade wind pattern will favor the central islands stretching into the weekend. At the upper levels, it was first thought the upper trough will play a bigger role in the weather this weekend but the models have eased off from that assessment. The islands will be at the fringe of a broad upper low with 500 mb temps of around minus 7 C on Thursday. The air mass cools to between minus 8 and minus 10 C by Friday afternoon with the cooler temps over Kauai. This means the air mass over the islands destabilizes to where it will support locally heavy showers for particularly Kauai. From this point on, the EC and GFS differs on the upper air pattern. While the GFS pinches off a low southwest of Kauai, the EC pushes the trough through the island chain and is followed by warming aloft. At the surface, both models have the trough moving just west of Kauai by Saturday night while retaining a showery pattern for the Garden Isle. Marginal low level moisture will likely limit the afternoon convective showers to scattered at most across the central island over the weekend. The background trade wind flow shifts to the south and southeast on Sunday as this surface trough deepens north and west of Kauai. As for Memorial day, it remains inconclusive at this point of the wind field. But it looks like light variable winds with a chance of afternoon showers. Currently, a patch of trade wind clouds and showers are moving through the central islands. It should reach Kauai around day break. Expect these clouds and showers to diminish and become isolated to scattered by noon. With the breezy trades, some of these showers will easily spill over into the leeward sections of the smaller islands. More shower bearing clouds are noted on satellite upwind of the islands. We expect these showers to reach the islands around sunset this evening. A thin layer of broken to overcast cirrus clouds over the Big Island will clear out later this afternoon or tonight.

Aviation

High pressure well north of the state will keep a breezy trade wind flow in place through tonight. Clouds and showers will favor windward and mauka areas, but will spread leeward across the smaller islands from time to time due to the strength of the trades. VFR conditions are expected at most of the TAF sites, but MVFR cigs/vsbys will impact windward areas, including PHLI, PHMK and PHTO from time to time. AIRMET Sierra remains in effect for mountain obscuration across windward sections of Oahu Molokai Maui and the Big Island, with these conditions expected to spread to Kauai after 15Z. Conditions should then temporarily improve from east to west later this morning, with another round of enhanced clouds and showers likely spreading from east to west and requiring a mountain obscuration AIRMET for some windward sections of the state this afternoon into this evening. AIRMET Tango remains in effect for low level turbulence over and downwind of the terrain of all islands due to the breezy trade winds.

Marine

Breezy trade winds will continue across large areas of the coastal waters through tonight, as high pressure passes far north of the islands. Trade winds will begin to diminish on Wednesday, then decrease further toward the end of the week. The Small Craft Advisory is posted for much of the waters through tonight, and has been extended for the typically windy zones around Maui County and the Big Island through Wednesday night. Northwest swell appears to be peaking now at the nearshore buoys, and should gradually decline today. The resulting surf will remain below advisory levels on north and west facing shores. This swell is forecast to decline further while veering around to the NNW tonight and Wednesday. A series of small and short-period NNE swells are expected later this week into the weekend. The current small south swell will linger through Wednesday. A larger south swell is expected to arrive Friday and continue through most of the Memorial Day weekend. This swell may produce advisory level surf of 8 feet or greater along south facing shores. In addition, the combination of swell energy, high astronomical tides associated with the new moon, and ongoing higher than predicted water levels due to oceanographic factors, will likely result in minor coastal flooding due to wave run-up along south facing shores later this week and during the weekend, especially near the high tide times. East facing shores will see short-period swell and choppy surf during the next couple of days due to the breezy trade winds.

HFO Watches/Warnings/Advisories

Small Craft Advisory until 6 AM HST Thursday for Maalaea Bay, Pailolo Channel, Alenuihaha Channel, Big Island Leeward Waters, Big Island Southeast Waters. Small Craft Advisory until 6 AM HST Wednesday for Kauai Northwest Waters, Kauai Leeward Waters, Kauai Channel, Oahu Windward Waters, Oahu Leeward Waters, Kaiwi Channel.

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DISCUSSION...Lau AVIATION...Jelsema MARINE...Jacobson

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