Area Forecast Discussion for Hawaii

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Issued: Apr 27, 2015 4:00 PM HST

Synopsis
A stalled front over the eastern half of the state will dissipate tonight, allowing strong northeast trade winds north of it to spread across the Big Island. Persistent showers remain anchored over windward sections of the islands, and will be greatest in coverage across Maui county. Breezy trade winds and active showers will continue through much of the work week, with the strongest winds expected Wednesday.

Discussion
Water vapor imagery shows an upper level trough moving away east of the state, with a high building in just to the north. At the surface, a weakening front is nearly stationary between Maui and the Big Island, with high pressure building far north-northwest of the state. 00Z raobs show a weak and elevated inversion in Hilo, with a strengthening and lowering one near 7kft in Lihue. Precipitable waters are 1.27 and 0.89 respectively. Early afternoon mimic total precipitable water imagery shows the highest moisture focused near Maui county/big island, with drier air spreading across the state from the north.

The old front continues to weaken, and the cloud band associated with it is starting to break up. Northeasterly winds near the Big Island will increase further tonight as the front dissipates, and they start to feel the stronger gradient that is in place across the rest of the state. Moisture associated with this front will keep conditions cloudy and showery across much of Maui county, with the greatest rainfall across windward sections. As the front dissipates, some of the moisture will spread westward and enhance the windward showers across Oahu and Kauai overnight into Tuesday as well.

Strong winds will be a concern for the next few days. We stayed below advisory levels for the most part today, with peak gusts in the 30°S/LOW 40s mph. Even with building upper level heights, a lingering mid-level trough near 700mb will keep the lower inversion from becoming very well defined, and it does not look like the type of setup for downslope enhancement. The high builds north of the state on Tuesday, but remnants of the front drifting westward will be between us and the high, separating us from the strongest gradient. It is not until Wednesday that we see the full effect of the high. Although it is beginning to move away to the northeast by this point, Wednesday appears to be the day with the strongest winds, and we have updated the grids accordingly.

The latest runs of the GFS and ECMWF both show the high shifting east and weakening late in the week, as a new front pushes in from the west. There are still differences with respect to this next system, but it remains well to the north and there are minor impacts to sensible weather across the islands. Trade winds are expected to remain in place through the weekend, with showers mainly across windward sections.

Aviation
A strong surface high continues to build in behind a dissipating stationary front located near the Alenuihaha channel. The surface high is providing fresh to locally strong trade winds over Maui through Kauai. Airmet tango for low level turb remains posted for areas below 8 thousand feet south through west of the terrain from Maui to Kauai. Amdar and VAD wind profiles suggest the layer is comprised of mostly 20 to 25 kt sustained winds with occasional winds up to 30 kts.

Winds over and around the Big Island will begin to increase tonight as the front weakens further and the high continues to push east. Airmet tango will likely need to be expanded later.

The stationary front will continue to focus the bulk of the clouds and showers along north through east sections of Maui county. Terminals such as PHMK, PHJH, PHOG will likely continue to see prevailing MVFR conditions with possible brief IFR moments this evening into tonight. Airmet sierra for mountain obscuration is posted over north through east sections of Maui Molokai and Lanai.

Vfr conditions will prevail at terminals on Oahu and Kauai. The elevated trade winds will help push the enhanced moisture associated with the stationary front back towards Oahu and Kauai. This will increase cloud and shower activity along north through east slopes tonight, especially after midnight. Airmet sierra for mountain obscuration may be needed at that time.

Over the Big Island, VFR conditions will prevail. Clouds and showers will focus along north through east sections, with periods of MVFR. Cloud build ups along leeward areas will scatter out tonight.

Marine
The Hanalei and Waimea buoys have continued to report steady readings above wavewatch guidance, and the Pauwela buoy has reached similar values. Surf reports from the north shore of Kauai indicate high surf. Similar conditions are expected through the night and into Tuesday. The current north swell will turn more northeasterly on Tuesday, and the advisory for north facing shores will likely be able to be dropped during the day. The swell will veer more easterly through the week as the high to our north shifts eastward. The large fetch from the strong high will likely keep advisory level surf for east facing shores through mid-week. This strong high will also lead to small craft advisory conditions for most coastal waters. The strongest winds are expected on Wednesday, though small craft winds will likely continue through much of the week for the typically windier areas.

Watches/Warnings/Advisories
High surf advisory until 6 pm HST Tuesday for north facing shores of Niihau Kauai Oahu Molokai and Maui.

High surf advisory until 6 am HST Thursday for east facing shores of Kauai Oahu Molokai and Maui.

Small craft advisory from 6 am Tuesday to 6 am HST Thursday for Big Island leeward waters, Big Island southeast waters.

Small craft advisory until 6 am HST Tuesday for Maui county leeward waters, Big Island windward waters.

Small craft advisory until 6 am HST Thursday for Kauai northwest waters, Kauai windward waters, Kauai leeward waters, Kauai channel, Oahu windward waters, Oahu leeward waters, Kaiwi Channel, Maui county windward waters, Maalaea Bay, Pailolo channel, Alenuihaha channel.


PUBLIC/MARINE...BRAVENDER

AVIATION...EATON