FORECAST DISCUSSION

Synopsis

A persistent ridge of high pressure north of the islands will keep trade winds blowing through the next week, with locally breezy levels expected this weekend. A dry and stable airmass will keep showers limited and confined to windward slopes through early next week. An increase in trade wind showers is then expected Tuesday through late next week as upper level troughing sets up over the island chain.

Discussion

Currently at the surface, a 1027 mb high is centered around 700 miles to the northeast of Honolulu, and this is resulting in moderate trade winds across the state this morning. MIMIC total precipitable water (PW) imagery shows a dry airmass overspreading the island chain with PW values at or below 1 inch. This correlates well with the 12z sounding from PHTO indicating a PW value of 0.77 inches, while the PHLI sounding observed a PW value of 1.23 inches in a slightly more moist airmass which has now shifted west of the Kauai. Additionally the 12z soundings showed a stable airmass working in from the east, with a sharp low level inversion around 3kft at PHTO and a weaker inversion around 3 kft at PHLI. Satellite and radar imagery also reflect the dry airmass over the islands, with clear to partly cloudy skies in most areas and little if any shower activity. Main short term concern revolves around the building large northwest swell moving into the islands. Today through Tuesday, A pair of high pressure areas will consolidate northeast of the state through the weekend, leading to an increase in the local pressure gradient across the islands. As a result, moderate to locally breezy trade winds are expected to prevail through Sunday night, with the trade wind speeds then dropping off a notch on early next week as high pressure lifts further northeast and away from island chain. A dry and stable airmass is expected to limit shower activity across the island chain through the period, with PW values remaining around or slightly below 1 inch. Showers will favor windward and mauka areas, primarily during the overnight and early morning hours. Tuesday night through Friday, Model solutions are in good agreement showing high pressure remaining anchored well to the northeast of the state through the period, with a weakening front approaching from the northwest late in the week. Light to moderate trades are expected to prevail through the period, with some decrease in trade wind speeds possible by the end of the week depending on the proximity of the approaching front to the island chain. Aloft, a weakness in the upper level ridge is expected to move overhead during the middle and latter part of next week, and this should de-stabilize the airmass a bit. This in combination with increasing boundary layer moisture resulting from the persistent easterly trades, should lead to an increase in trade wind showers across the islands. Showers will continue to favor windward and mauka areas, with activity most prevalent during the overnight and early morning hours.

Aviation

Locally breezy trade winds will continue. The morning sounding from PHTO shows a strong low level inversion near 3 kft, and the PHLI sounding shows a weaker inversion near 3 kft as well. AIRMET TANGO for low level turb has been issued this morning for areas over and south through west of the mountains. The first few visible satellite images show wave clouds starting to set up along the koolau mountains. Expect prevailing vfr conditions at most terminals today.

Marine

The large west-northwest (WNW) swell approaching the area continues to rise at NOAA buoys 51101 and 51001. Both buoys reported the swell height at 7.5 feet every 19 seconds from the WNW. Forerunners of this swell have reached the Hanalei, Kauai and Waimea Bay CDIP buoys with reports of 20 and 22 second periods respectfully. These observations are in excellent agreement with the forecast models, and therefore no adjustments are needed to the forecast. Surf on Kauai is expected to build to warning levels (25 ft north/20 ft west) during the day, then peak tonight into Sunday morning. Warning level surf should continue through the day Sunday, before gradually subsiding early next week. The more westerly direction of the swell will likely cause Kauai to shadow the other smaller islands. The current forecast continues to hold surf for exposed north and west facing shores of Oahu and Maui County just under warning levels. This will be monitored as more observations come in from the northwest buoys, and a slightly larger or more northwest swell could bring warning level surf to Oahu/Maui County as well. Surf along west facing shores of the Big Island is also forecast to reach low-end warning level (12 ft) tonight through Sunday night. A high surf warning has been posted for north/west facing shores of Niihau and Kauai, and for the west facing shores of the Big Island. A high surf advisory has been posted for north/west facing shores of Oahu and Molokai, and for north facing shores of Maui. Otherwise, small long period south swells will continue through this weekend and into next week, with a slightly larger long period south swell forecast for Wednesday night through Friday. Another large west-northwest swell will be possible from next Friday into the following weekend. The Small Craft Advisory (SCA) remains posted for the typically windier waters around the Big Island and Maui County through Sunday night. High pressure strengthening northeast of the state will provide locally breezy conditions through the weekend. The expansion of the SCA to cover most of the other coastal waters late this morning, and the remaining waters early this evening, due to building seas of the large incoming west-northwest swell is on track.

HFO Watches/Warnings/Advisories

High Surf Advisory from noon today to 6 AM HST Monday for Waianae Coast, Oahu North Shore, Oahu Koolau, Molokai, Maui Windward West, Maui Central Valley, Windward Haleakala. High Surf Warning until 6 AM HST Monday for Niihau, Kauai Windward, Kauai Leeward. High Surf Warning from 6 PM this evening to 6 AM HST Monday for Kona, South Big Island, Kohala. Small Craft Advisory until 6 AM HST Monday for Maalaea Bay, Pailolo Channel, Alenuihaha Channel, Big Island Leeward Waters, Big Island Southeast Waters. Small Craft Advisory from 6 PM this evening to 6 AM HST Monday for Oahu Leeward Waters, Maui County Leeward Waters, Big Island Windward Waters. Small Craft Advisory until 6 AM HST Monday for Kauai Northwest Waters, Kauai Windward Waters, Kauai Leeward Waters, Kauai Channel, Oahu Windward Waters, Kaiwi Channel, Maui County Windward Waters.

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DISCUSSION...Jelsema AVIATION...Eaton MARINE...Evans

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