FORECAST DISCUSSION

Synopsis

21 Jun 2018 13:43:00 GMT: Trade winds will continue to strengthen through tonight as high pressure builds northwest of the state, becoming breezy on Friday, and lasting through part of the weekend. The frequency of the trade showers will also be on the rise. 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

21 Jun 2018 13:43:00 GMT: After a couple of days of light winds and dry weather, we will be returning to a more typical trade wind pattern. Trades will be becoming locally strong by sundown, with further strengthening to moderate to strong area wide by Friday night. This comes about as a new surface high strengthens to 1035 MB while passing 1040 miles north of the islands on Saturday and Saturday night. Typically, with this strength of 1035 MB while passing north of the islands would warrant a Wind Advisory for notorious locations such as Lanai, the Kohala area of the Big Island, and perhaps the central valley of Maui. High res wind models are not indicating this of yet. Trades may ease off slightly Sunday but this could premature. Certainly, the trades will scale down winds to locally strong on Monday. An upper level low, approaching the islands from the east, will come close to Maui and the Big Island before scooting off to the north on Sunday. It will affect the depth of the marine layer by raising it from the current 6 to 8k feet to 10k feet over the Big Island and to 8k feet over the smaller islands tonight. The marine layer deepens further to 14k feet over the Big Island and to 12k feet over the smaller islands on Friday. It stays this way to until Sunday where the air mass stabilizes and the marine layer lowers down to 8 to 10k feet. This condition persists through at least Monday. Both the ECMWF and GFS continues to show a broad but light precipitation field upwind of the islands, a sign of plenty trade showers next week.

Aviation

21 Jun 2018 13:43:00 GMT: Mainly dry weather with light background winds will continue across the state through mid-morning. Isolated MVFR ceilings are possible along some windward facing slopes in brief showers. Areas of MVFR visibilities are possible across parts of the Big Island due to volcanic haze/emissions from Halemaumau/Kilauea crater and the East Rift Zone. Otherwise, VFR conditions are expected to prevail. The trade winds will strengthen later today. This may result in a slight increase in the coverage of low clouds and trade showers over windward sections, especially later today and tonight. No AIRMETs are currently in effect, and none are anticipated this morning. However, as the low-level trade wind flow strengthens, we will monitor observations in case we need to issue AIRMET Tango for mechanical turbulence downwind of the higher terrain on the islands starting later today or tonight.

Marine

21 Jun 2018 13:43:00 GMT: The Wednesday evening ASCAT pass only covered the eastern waters, but did find areas of 20 kt winds in the Alenuihaha Channel and south of the Big Island. Trade winds are expected to continue building today as high pressure strengthens far northeast of the state, and a Small Craft Advisory (SCA) has been posted through Sunday for the typically windy areas around the Big Island and Maui County. Trade winds will strengthen further heading into the weekend, with SCA possibly needed for some additional windward zones. Surf will gradually build along east facing shores as trade winds build today 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. The forecasted local surf heights will be refined as the initial swell reaches the buoy near American Samoa during the weekend.

HFO Watches/Warnings/Advisories

21 Jun 2018 13:43:00 GMT: Small Craft Advisory from 6 AM this morning to 6 PM HST Sunday for Maalaea Bay, the Pailolo and Alenuihaha channels, and waters south of the Big Island.

Footer

21 Jun 2018 13:43:00 GMT: Lau/Houston/Jacobson

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