Forecast Discussion

Synopsis

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

Discussion

A 1038 mb surface high is centered far the north of the state this afternoon driving fresh trade winds across the state. Meanwhile, Tropical Storm Ulika is located around 1000 miles east of Hilo and is moving northwest at 8 mph as of 11 am HST. The Central Pacific Hurricane Center forecast has Ulika dissipated well before reaching Hawaii. Aloft, water vapor imagery shows a broad upper level trough located between the islands and Ulika, with upper ridging to the north and west of the state. Visible satellite imagery shows partly to mostly cloudy skies this afternoon, with the more extensive cloud cover over Kauai, Oahu, as well as the typical cloud build ups across the Kona slopes of the Big Island. Radar shows scattered showers carrying in with the trades concentrated across the windward sides of Kauai and Oahu. Fresh trade winds are expected to remain in place through the weekend as the surface high slowly digs south and weakens, essentially keeping the same pressure gradient over Hawaii. The showers over the western end of the state will push west of the area tonight while a relatively drier airmass is ushered in from the east by the trade winds. Afternoon soundings already show precipitable water (PW) value of 1.09 inches at Hilo where the drier airmass has pushed in. Lihue is a bit wetter at around 1.39 inches this afternoon but is expected to dry out as the airmass progresses westward. The drier trade wind pattern will continue through Friday. A surface trough (the remnants of Ulika) will approach from the east over the weekend and become ill-defined as it passes over or just to the south of the state early next week. Aloft, upper level troughing will develop over the islands Friday night and linger over the state through Saturday night. The upper trough is then expected to cut off to two distinct lows to the northeast and northwest of the state before an upper ridge builds back in from the east by the middle of next week. Meanwhile, the trades will diminish and shift around to the east or east-southeast as the trough associated with the remnant of Ulika passes to the west. As for forecast details, recent model runs have been in good agreement showing deeper moisture arriving Friday night and Saturday as the upper trough developing over the islands. PW values increase to between 1.3 and 1.7 inches, highest over the eastern islands, and this should promote wetter than normal trade wind weather across the state. Showers will continue to favor windward and mauka areas, but with the elevated inversion, some showers will reach leeward locales as well. Most of the models have been showing deep tropical moisture with PW values around or in excess of 2 inches (associated with the remnants of Ulika) will overspread the state from east to west Saturday night and Sunday, and linger over the island chain through early next week. However, this is not set in stone yet, as the models have been showing some run-to-run variability. For example, the 12Z ECMWF run is not as aggressive with the moisture and dissipates the remnants of Ulika far to the southeast of the islands, and keeps the upper trough a little farther west. Even if ex-Ulika does not get involved, the upper trough will be enough to lead to a more showery pattern. There is a greater potential for locally heavy rainfall if ex- Ulika gets closer as some of the other models are showing.

Aviation

AMDAR soundings into Lihue/Honolulu/Kahului show 25 knots of winds at low-levels. The inversion is more pronounced over the eastern half of the state and weak/elevated over the western half. We have not received any pilot reports with more than light turbulence. However, wave clouds over Oahu are becoming increasingly well defined extending downstream from the Koolau Range. With these signals, will hold onto the existing AIRMET for potential moderate turbulence in the lee of the mountains. Otherwise conditions will be primarily VFR. Occasional MVFR ceilings are expected in showers, mainly across windward sections but also briefly passing leeward as well. Showers and associated MVFR ceilings and visibilities will increase tonight, but at this time an AIRMET for mountain obscuration is not anticipated.

Marine

High pressure far to the north of the state will keep locally strong trade winds in place through the weekend. A Small Craft Advisory is in effect for the typically windier areas near Maui and the Big Island through Friday, and will likely be extended through 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 early next week.

HFO Watches/Warnings/Advisories

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