National Weather Service United States Department of Commerce

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Issued: Jun 21, 2018 7:19 PM HST

Update
Zone package has been updated to reflect current conditions, which is a rather dry first part of the evening. Trade showers should be picking up after midnight, especially windward Maui and the Big Island. Also, left in an isolated shower or two for the Kona coast tonight. We are still on track for a wetter trade pattern for the weekend, including Friday.

Synopsis
Trade winds will continue to strengthen through Friday as high pressure builds northeast of the state and maintaining through the weekend. The frequency of the trade showers will also be on the rise beginning Friday. These showers will be favoring the windward and mountain areas, although the breezy trades will blow some of these showers into the lee areas of the smaller islands from time to time.

Discussion
A 1026 mb high is centered 1700 miles northeast of Hawaii and driving moderate trade winds across the state this afternoon. Visible satellite shows widely scattered low clouds moving in with the trades, with clouds banked against windward slopes of Kauai Oahu and Molokai, and leeward sides of Maui, Lanai and the Big Island where onshore sea breezes have set up. The vog plume is directed westward with poor air quality for most of the Big Island.

Afternoon soundings at Hilo and Lihue show precipitable water values around normal of 1.2 to 1.3 inches with inversions around 7 to 8 kft. Satellite PW shows a bit higher values of 1.4 to 1.5 inches about 100 to 150 miles upstream which is expected to make it's way towards the state later tonight, though the evening should be fairly dry.

The high to the northeast is expected to strengthen resulting in a strengthening of the trade wind speeds through Friday night which will maintain through the weekend. Clouds and showers will continue to track in with the winds resulting in an overall increase in both clouds and showers through the weekend. The focus of the showers will be over windward slopes, but the breezy winds will carry clouds and showers to leeward sides of the smaller islands at times. The high to the northeast will weaken Monday and Tuesday, but move closer to the state, resulting a continuation of the breezy trade winds and windward focused rainfall.

An upper level low will approach from the east and skirt by the islands Friday into this weekend. The passing upper low will raise the inversion level over the eastern half of the state allowing incoming clouds to become taller. Thus, incoming showers may become a bit heavier at times across the Big Island and Maui tomorrow into the weekend. The inversion should lower as the upper low moves away early next week.

Aviation
Trade winds have been increasing as expected but winds are still not quite strong enough yet to produce low-level turbulence. There have been wind gusts over 25kts but no areas have experienced sustained 25kt winds yet and there have been no reports of turbulence. No AIRMETs are in place now but will monitor for the possibility of low-level turbulence later through the period. There should be an increase in showers and clouds along the windward slopes and will watch for the possibility of isolated areas of MVFR conditions with CIG/VIS. The best possibility for shower activity remains on the windward and south areas of the Big Island up to Maui County and even that has only a slight possibility of developing Thursday night into Friday.

No AIRMETS currently in place and none are expected through most of today.

Marine
High pressure will track east across the Central Pacific and strengthen north of the Hawaiian Islands on Saturday. A Small Craft Advisory (SCA) remains in effect through Sunday for the typically windy areas around the Big Island and Maui County. Trade winds will increase further on Saturday, with SCA possibly needed for some additional windward zones.

Surf will gradually build along east facing shores as trade winds strengthen tonight into the weekend. For all other shores, surf will remain small with no significant swells anticipated through the weekend. Overlapping long-period south swells are expected next week with the first swell expected to arrive around Tuesday and the second one around Wednesday or Thursday. The forecasted local surf heights will be refined as the initial swell reaches the buoy near American Samoa during the weekend.

Watches/Warnings/Advisories
Small Craft Advisory until 6 PM HST Sunday for Maalaea Bay, Pailolo and Alenuihaha channels, waters south of the Big Island.


Discussion...Lau/Foster

AVIATION...Chevalier

MARINE...TS/Kino