FORECAST DISCUSSION

Synopsis

High pressure north of the state will drive fresh trade winds in through the weekend, with showers focused primarily over windward and mauka areas. Showers are expected to increase in coverage and intensity over the weekend through much of next week, as a moist airmass associated with the remnants of Ulika moves over the state.

Discussion

A 1036 mb surface high is centered far north of Hawaii and is driving fresh trade winds across the area. Meanwhile, former tropical cyclone Ulika is located around 800 miles east of the Big Island and slowly moving to the west. Aloft, a broad trough resides between former Ulika and the hawaii with second, narrow trough northwest of the area. A weak upper ridge is wedged between these two features and is over the islands this morning. Visible satellite imagery and radar over Hawaii show windward clouds and showers moving in with the trade wind flow. A few clouds and showers are blowing over to leeward sides of the smaller islands. 12Z soundings show a typical 6 to 7 kft inversion over the state with precipitable water values of 1.18 to 1.24 inches. MIMIC total PW satellite shows an area of higher PW about 70 miles upstream of the Big Island. This higher moisture will move towards the state today and will likely result in an increase in showers for the eastern end of the state by late this afternoon, then spread westward across the rest of the islands beginning tonight. Until then, anticipating somewhat of a dropoff in shower coverage today. Fresh trade winds will continue into the weekend as the pressure gradient remains tight due to a the surface high northeast of the islands and the approach of former Ulika from the east. Aloft, there are some differences between the GFS and ECMWF, with the GFS indicating more pronounced upper level troughing shifting closer to the islands, while the ECMWF is not as aggressive. Deep layer moisture will increase from east to west across the islands tonight through Saturday morning, with PW values will rising into the 1.5 to 1.8 inch range for Saturday afternoon and Saturday night. Overall, with troughing aloft and an increase in deep tropical moisture, we should see shower coverage and frequency increase across the state, with showers favoring windward and mauka areas. The GFS and ECMWF both close off a pair of upper level lows to the east and west of the state Sunday while moisture from Ulika filters in. With plenty of deep tropical moisture associated with the remnants of Ulika, along with at least some weak troughing aloft, we should see a wetter than normal pattern across the state through the period. Showers will continue to favor windward and mauka areas on Sunday, with the potential for locally heavy rainfall at times. Next week, the trades will weaken slightly and shift slightly east southeast as the upper low to the northeast induces a surface trough just west of the state.

Aviation

A strong surface high pressure area sitting north-northeast of the state is generating moderate to strong and gusty trade winds throughout the islands. Random bouts of low level moisture and clouds riding in with the trades will produce MVFR conditions mainly along the windward sections of the islands today through the weekend. Currently most of the clouds and moisture are affecting Maui and Molokai and could affect Oahu later today. A large cluster of low clouds will also move into the Big Island this afternoon. Cloud tops are mostly around 8000 to 9000 feet. AIRMET TURB for moderate, intermittent, low level turbulence lee of the mountains continues and is likely to remain in effect through Saturday.

Marine

High pressure far to the north of the state will keep moderate to breezy trade winds in place through the weekend. A Small Craft Advisory is in effect for the typically windier waters around Maui and the Big Island through Saturday, and will likely need to be extended through the rest of the weekend. Short period easterly swell resulting from the trade winds will continue through the remainder of the week. A bit of a bump in surf heights is expected for east facing shores over the weekend as the remnants of Ulika approach from the east. Elsewhere, surf heights will remain small through the middle of next week.

HFO Watches/Warnings/Advisories

Small Craft Advisory until 6 PM HST Saturday for Maalaea Bay, Pailolo Channel, Alenuihaha Channel, Big Island Leeward Waters, Big Island Southeast Waters.

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Discussion/Marine...Foster Aviation...Fujii

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