Area Forecast Discussion for Hawaii

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Issued: Oct 25, 2014 4:00 PM HST

Remnant moisture from an old front will increase clouds and showers over the islands tonight and early Sunday. A weakening ridge north of the state will keep moderate to light trade winds through mid week, with showers remaining focused across windward and mauka areas. A cold front is expected to push through the islands late in the week, bringing another increase in showers Thursday night into Friday. Heading into next weekend, stronger trades, cooler air, and lower humidity will following behind the front.

Water vapor imagery shows an upper level ridge over the state, with a trough far to the east and a low to the west-southwest. At the surface, high pressure is located far northeast of the state, with a ridge passing about 300 miles north of Kauai. A remnant cloud band from an old front is located over the windward coastal and offshore waters, and also extends far east-northeast from the Big Island. 00Z soundings from Hilo and Lihue show weak and elevated inversions near 9-10kft, although precipitable waters range from 1.47 at Lihue to 1.09 at Hilo. This unusually dry air over the eastern half of the state /average PW at Hilo is 1.45 inches in october/ will be short lived, as early afternoon mimic total precipitable water imagery shows higher values associated with the old front located just northeast and east of the state.

Rainfall amounts have been on the low side this afternoon, at least across the eastern half of the state where moisture has been more limited. Even across Kauai and Oahu, rainfall amounts have been about a quarter inch or less over the past 12 hours. Windward clouds have all but dissipated across east Maui and the Big Island. However, the band of unstable showery clouds associated with the old front is located just offshore of the windward coasts, and a linear extrapolation of the cloud motion brings them into the windward sections within the next 1-2 hours. Therefore will start the forecast with high pops this evening as conditions will deteriorate late this afternoon.

This moisture axis will push through most of the area by Sunday afternoon/evening. Forecast soundings and time sections show conditions stabilizing/moisture depth decreasing for Sunday night/monday. Light to moderate trade winds will keep showers focused mainly across windward sections, but rainfall amounts will be on the light side due to the drier and more stable airmass. Conditions start to change by the middle of the week as the low to our southwest moves toward the islands. This low is expected to weaken as it moves into the ridge over and to the east of the state. However, it has the effect of weakening and raising the inversion height by tuesday/wednesday and enhancing trade wind showers.

By the end of the week, a front approaching from the north may bring a noticeable change in airmass. As a new high passes far north of the state, guidance remains in good agreement pushing a cold front through the islands with a band of clouds and showers Thursday night into Friday. Breezy trades and cooler drier air looks to follow behind it for the weekend. The forecast reflects the current timing of the front, with an increase in showers Thursday night/friday, but this may need to be adjusted as timing or position of the expected front changes.

Isolated showers continue to focus over windward and mauka areas, with some blow over to leeward areas. Brief periods of MVFR are likely in showers. An area of enhanced moisture associated with an old front continues to move towards the islands. Expect frequent showers along windward and mauka areas of the Big Island and Maui this evening and over Oahu and Kauai tonight. Airmet for mountain obscuration will likely be posted this evening and spread down the island chain as lower clouds and showers increase.

Large scale trade winds are expected to remain in the light to moderate category, which means locally fresh trades in the windier channels through the middle of the week. Trade winds are expected to increase noticeable Thursday and Friday as a strong high builds in to the north of the state.

A series of small north swells will continue through Wednesday, with a slightly larger north swell expected during the second half of the week. Larger short period easterly swells area expected late in the week as trade winds strengthen.