Swell Matrix

Area Forecast Discussion

National Weather Service Honolulu Hi

822 pm hst Sat may 25 2019

Synopsis

A moderate and stable easterly trade wind flow will focus clouds and showers across windward slopes through Sunday, with mainly dry conditions prevailing over leeward areas. Trades will ease slightly Memorial Day through Wednesday, leading to increased chances for afternoon sea breezes and a few showers across leeward areas. Trades are expected to strengthen Thursday into next weekend, bringing a return of more typical trade wind weather featuring windward and mauka showers and the occasional leeward spillover.

Discussion

Currently at the surface, a 1027 mb high is centered around 1400 miles northeast of Honolulu, while a weak front remains nearly stationary around 250 miles northwest of Kauai. The resulting gradient is driving moderate trade winds across the state this evening. The airmass remains rather dry and stable, with precipitable water (PW) values between 1.1 and 1.3 inches, and inversion heights between 5.5 and 7.5 kft. Infrared satellite imagery shows a mix of low and high cloud across the island chain, resulting in partly to mostly cloudy conditions. Radar imagery shows a few showers moving into windward areas, with some lingering daytime convective showers over leeward sections of the Big Island. Main short term concern continues to revolve around rain chances the next few days.

Model solutions are in good agreement showing high pressure holding in place to the northeast of the state through the middle of next week, while the stationary front lingers a couple hundred miles northwest of the island chain. Moderate trade winds will hold in place through Sunday, then ease slightly with boundary layer winds shifting around to the east-southeast Memorial Day through Wednesday. This will keep light to moderate east to east- southeast winds in place across windward areas, while sea and land breezes become common in many leeward areas. The stationary front will dissipate by the middle of the week, with high pressure then strengthening and shifting closer towards the state through the end of the work week. This is expected to bring a return of trade winds Wednesday night, with the trades then steadily strengthening to moderate and locally breezy levels Friday into next weekend.

A fairly typical trade wind weather pattern will hold in place through Sunday night, with showers favoring windward and mauka areas and a stray shower reaching leeward communities from time to time. The pattern will change slightly Memorial Day through Wednesday, as sea breeze development in the more sheltered areas will result in a few more showers developing each afternoon across leeward and interior sections. There could be a few heavier showers as well due to the increased moisture provided under the east-southeasterly boundary layer flow. A transition back to more typical trade wind weather is expected Wednesday night into next weekend, with showers favoring windward and mauka areas.

Aviation

A high pressure center far northeast of the Hawaiian Islands will produce a surface ridge north of the state. This surface ridge will keep moderate easterly trade winds in the forecast for most airfields over the next 24 hours. Wind speeds will likely remain just below the threshold for moderate turbulence downstream from mountain ranges.

A stable high pressure ridge aloft will keep lower trade wind inversion heights around 7000 to 8000 feet, limiting shower activity to the isolated to scattered range. Any showers that develop will favor the windward and mountain areas. Mostly VFR flight conditions are expected.

No AIRMETs are in effect and none are currently forecast.

Marine

A ridge just north of the area associated with a high far to the northeast continues to bring in fresh to locally strong trade winds. A Small Craft Advisory continues for the windier areas near Maui County and the Big Island overnight. The ridge is expected to weaken early next week, easing winds to light to moderate levels Sunday, with lighter winds likely Monday into Wednesday. The second half of the week should see the ridge building north of the islands which will allow winds to return to fresh to strong levels.

Surf along south facing shores is on the increase due to low pressure systems in the southern hemisphere. Surf is expected to approach advisory levels Sunday, but will continue to monitor buoy reports overnight before making the final decision on the need for a High Surf Advisory. This swell is expected to peak Sunday and Monday, before declining Tuesday. A new long-period, although slightly smaller, south swell is expected to arrive Tuesday and hold into the middle of the week. Additional small south swells are expected during the second half of the week.

A small northwest swell associated with a gale that crossed the dateline a few days ago is expected to hold into Sunday before slowly easing Monday through Tuesday. Another north-northwest swell is possible Thursday. Surf along east facing shores will remain up through the weekend due to the recent fresh to strong trades upstream of the islands. Expect a downward trend early next week as the trades weaken.

Watches, Warnings, Advisories

Small Craft Advisory until 6 AM HST Sunday for Maalaea Bay- Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big Island Southeast Waters.

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