Area Forecast Discussion for Hawaii

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Issued: Sep 02, 2014 10:00 AM HST

Light to moderate trade winds will continue for much of the upcoming week. The trades will focus showers mainly across windward areas, especially during nights and mornings, with some afternoon showers across leeward sections as well.

Water vapor imagery shows an upper level high over/just south of the state, with a shortwave trough far to the northwest. At the surface, high pressure is located far north-northeast of the state, with a ridge passing several hundred miles to our north. Low pressure, former tc Marie, is far northeast of the state and moving generally westward. 12Z soundings from Hilo and Lihue show inversions ranging from 7kft at Lihue to 10kft at Hilo. Precipitable waters also range from 1.4 to 1.7 inches respectively. Early morning mimic total precipitable water imagery shows an area of higher moisture extending up to 250 miles northeast and east of the state, with drier air farther upstream.

The pressure gradient across the islands remains on the light side due to our separation from the high and influence from former Marie. However, indications are that trades are more entrenched than yesterday. The gradient between buoys 51100 northeast of the state and 51004 southeast of the state has risen by over a millibar over the past 24 hours. The ascat imagery from last night showed around 10 knots of wind near Kauai, 10 to 15 knots near Oahu and Maui county, and 15 to 20 knots near the Big Island. The high resolution WRF from last night, as well as the 12Z global models, show the stronger winds spreading westward across the state today/tonight as well. There may be enough of a stronger gradient for trade winds to dominate, with localized sea breezes confined to sheltered leeward areas. Even with more persistent trades, we will likely see increased cloud buildups across interior and leeward sections during the afternoon and early evening.

An area of low clouds is currently located northeast of Maui and the Big Island, and is moving westward at about 10 mph. These clouds are an indicator of the higher moisture just upstream of the state, and will help to enhance trade wind showers mainly across windward sections tonight into Wednesday. Gfs forecast soundings/moisture plots show drier and more stable conditions spreading across the islands during the day on Wednesday, with lower rainfall chances.

By the end of the week, both GFS and ECMWF continue to show a tongue of moisture trailing from former Marie reaching the islands. The GFS is faster/more aggressive than the ECMWF, but both show the area of clouds/moisture currently extending down to 25°N between 140°W/150°W reaching the islands late thursday/friday. Forecast soundings/cross sections show increasing moisture depth back to near 10kft. During this timeframe, winds may also weaken a bit. The remnant of Marie will move westward and weaken as it encroaches upon the ridge. The gradient across the islands may weaken thursday/friday, before strengthening further Saturday into Sunday as the low dissipates and we feel more of a direct impact from the surface high.

Throughout the day, trade winds will gradually begin to build back. Isol MVFR over windward and mountain areas may occur in passing showers. Otherwise, VFR conditions will prevail. There are no airmets in effect.

Elevated surf will continue along S facing shores this week. Although surf is currently below advisory levels, forerunners of a very long period S swell /20-25 seconds' are expected to build today and tonight, although at low heights. Latest observations from the cdip buoy at Barbers Point show initial energy from this swell is beginning to arrive, but the bulk of the swell energy is dominated by the fading shorter period south swell. As this new swell fills in, it will produce surf near advisory levels along S facing shores, likely from Wednesday into Thursday. Will hold off on the issuance of an advisory to see if buoy data shows greater amplitude swell heights, but an advisory may be necessary this afternoon for surf nearing advisory levels on Wednesday.

Forecast models show a fairly large gale low developing NW of the state this week, generating a NW swell that arrives late monday/tuesday. It is too early for specifics as the low has yet to develop, but resultant surf looks to remain below advisory level heights along N and W facing shores.

Winds and seas will remain below small craft advisory thresholds for at least the next several days.