Area Forecast Discussion for Hawaii

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Issued: Aug 29, 2016 3:35 AM HST

Moderate trade winds will continue for the next couple of days as high pressure persists north of the state. A much drier airmass is expected over the islands today and tonight, with shower activity limited primarily to windward and mauka locales. By the middle of the week, a more unsettled weather pattern is expected to overspread the state, as we begin to feel impacts from Hurricane Madeline. Hurricane Lester may affect the islands over the upcoming Labor Day weekend, keeping the conditions unsettled across the Aloha state.

Currently at the surface, a 1027 mb high is centered around 1450 miles to the north of Honolulu, while a weak 1010 mb low is located around 475 miles south of the state. These two features are driving the moderate to locally breezy trade winds across the island chain early this morning. Meanwhile, Hurricane Madeline is located around 700 miles east-southeast of the Big Island, with Hurricane Lester located around 1700 miles east of the state. Closer to home, infrared satellite imagery shows partly to mostly cloudy skies in place across the islands, while radar imagery shows scattered showers drifting into windward and mauka areas. Main short term concern for today revolves around rain chances.

Today and tonight, High pressure north of the state and the weak area of low pressure off to the south, will keep a moderate to locally breezy trade wind flow in place through the period. Both the 00z GFS and 00z ECMWF are in good agreement showing a drier airmass with precipitable water (PW) values between 1.0 and 1.2 inches overspreading the state this morning and remaining in place through tonight. This in combination with inversion heights lowering to 7 kft or less, should keep shower activity limited, with rainfall amounts light and confined primarily to windward and mauka areas.

Tuesday and Tuesday night, Moderate trade winds will continue across the Aloha state as the pressure gradient remains tight between high pressure well north of the area and the approach of Hurricane Madeline from the east. Model solutions are in good agreement showing deep layer moisture associated with Madeline increasing, and this appears on track given the plume of higher PWs seen in MIMIC total PW imagery extending to the north and northwest of the cyclone. As a result, we should see shower activity increase from east to west across the state on Tuesday with a showery trade pattern continuing Tuesday night.

Wednesday through next Sunday, The extended portion of the forecast will be highly dependent on the track and intensity of Tropical Cyclones Madeline and Lester as they move over or very near the Hawaiian Islands. The latest track from the Central Pacific Hurricane Center (CPHC) shows Madeline moving over or near the Big Island Wednesday night, and Tropical Storm or Hurricane watches may be needed for the Big Island later today or tonight. On the heals of Madeline, the latest forecast from the National Hurricane Center (NHC) in Miami, shows Lester approaching the eastern Hawaiian Islands by late in the week. Model solutions have shown a bit more variability with Lester over the weekend, so there is more uncertainty on the eventual track and potential impacts to the state of Hawaii. Residents and visitors are urged to closely monitor the latest forecasts from the CPHC and NHC regarding these tropical cyclones. Overall, it looks like a pretty unsettled weather pattern across the state from the middle of the week through the Holiday weekend with the tropical cyclones moving through.

Trade wind flow will persist through the next twenty four hours and beyond. Bands of showers embedded within the trade wind flow will impact mainly windward and mountain areas. Partly cloudy skies along the Kona Coast of the Big Island this morning will give way to convective buildups in the afternoon and evening hours. Isolated MVFR conditions are expected in passing showers. Otherwise VFR conditions are expected to prevail.

A Small Craft Advisory (SCA) remains in effect for Maalaea Bay and the Alenuihaha and Pailolo channels through Monday night. There may be a slight downward trend in winds on Tuesday, but they should pick up once again by mid week as Hurricane Madeline approaches from the east. Depending on the eventual track of Madeline, a Tropical Storm or Hurricane watch may be needed for portions of the marine area later today.

No significant swells are expected through tonight, although surf along east facing shores will be slightly elevated due to the moderate trades. Winds and seas, as well as surf along east facing shores, will increase from east to west late Tuesday and Tuesday night with the approach of Madeline. The east swell will likely peak at warning levels for east facing shores Wednesday into Wednesday night. We will also start to see a swell from Hurricane Lester beginning Wednesday night or Thursday, with surf heights building into the weekend, possibly to warning levels along east facing shores once again. In addition, surf will build along west facing shores from distant Typhoon Lionrock Wednesday night and Thursday, peaking Thursday night through Friday night just below advisory levels.

Small Craft Advisory until 6 AM HST Tuesday for Maalaea Bay, Pailolo Channel, Alenuihaha Channel.