Swell Matrix

Area Forecast Discussion

National Weather Service Honolulu Hi

959 am hst Fri Dec 15 2017

Synopsis

Breezy north to northeast winds will continue today, but the winds will weaken from tonight into the weekend. An upper level low and increasing moisture will bring unsettled weather to portions of the state through this weekend. As a result, rain chances will increase, with locally heavy downpours possible and a slight chance of thunderstorms. Widespread rainfall is possible early next week as an upper-level trough pushes toward the islands from the northwest.

Discussion

A 1035 mb surface high is near 42N138W, or about 1800 miles northeast of Honolulu. A surface ridge extends southwest from this high through a point approximately 600 miles north of Kauai. Elsewhere, a north to south oriented surface trough, which is moving slowly westward, is evident about 250 miles east of Hilo. The tight pressure gradient between these features is maintaining fresh to strong north to northeast winds across the state. A Wind Advisory remains in effect for the Big Island Summits this morning. Note that the breezy conditions combined with low dew points (in the 50s) continue to make sensible temperatures feel relatively cool.

Aloft, middle and upper-tropospheric lows are located around 240 miles northeast of Hilo. The surface trough described above is a reflection of these features, which are propagating slowly toward the west-southwest. The close proximity of the lows appears to have already destabilized the atmosphere over the eastern end of the state. The early morning Hilo balloon sounding shows no significant low-level inversion. Satellite imagery and radar reflectivity data this morning indicate mainly broken low clouds with embedded scattered showers moving down from the north and northeast toward the windward and north facing sides of most islands east of Oahu. The upper level lows, as well as the surface trough, will approach the islands today. As a result, the north to northeast winds will gradually weaken tonight. The upper lows are expected to reach a position just northeast of the islands on Saturday, and then slowly pull away toward the northeast on Sunday. By Saturday, with the surface trough in the vicinity of the islands, the background flow will become light and variable across most areas. This will allow the development of local afternoon sea breezes. By Sunday, the surface trough is forecast to be located west of the state, with light southerly winds gradually spreading from east to west across the area.

The cooler temperatures aloft associated with the middle and upper-level lows will be close enough to destabilize the atmosphere today, especially over central and eastern portions of the island chain. In addition, increasing moisture will move in to the islands as the surface trough approaches the state. This will likely usher in an extended period of unsettled weather across the region, with increasing chances for rainfall, and the possibility of locally heavy downpours and isolated thunderstorms through the weekend. In addition, there is a chance of rain mixed with snow on the Big Island Summits. By Saturday afternoon, daytime heating and local sea breezes may provide additional support for deep convection over the islands.

The upper lows will lift away to the north late Sunday, and may provide a brief repite from the active weather on Monday. However, an upper- level trough is forecast to dig down toward the islands from the northwest, with deep tropical moisture pushes up from the southeast Monday night into Tuesday. This may provide widespread showers overspreading the islands. In addition, a couple of cold fronts may sweep across the islands from the northwest later in the week, which may result in an extended period of northwest winds.

Aviation

Locally strong north to northeast winds will continue today, before becoming light and variable Saturday as a surface trough east of the state moves west. AIRMET TANGO for low level turbulence is posted across the state. This will likely be able to be cancelled this evening as winds ease.

An upper level low just northeast of the state will gradually move west over the next day. This low will help to destabilize the atmosphere around the state, and will help to enhanced passing showers showers today. Showers could be locally heavy at times, carrying MVFR to isol IFR conditions. The forecast also introduces a slight chance of thunderstorms from Oahu through Big Island this afternoon. AIRMET SIERRA for mountain obscuration cannot be ruled out today, and will be issued as needed.

Marine

A surface trough northeast through east of the Big Island will be moving westward across the coastal waters over the next several days. The trough will weaken the winds over the eastern waters initially today then over the western waters later tonight. Winds should remain light through early next week.

Observations from the nearshore PacIOOS buoys have been showing a declining northwest swell through the morning. This trend should continue through the rest of today. The nearshore buoys are also showing a short period northeast wave component produced by strong northeasterly winds west of the trough. The northeasterly swell is large enough to warrant a High Surf Advisory for east facing shores. As the winds decrease, the northeasterly swell will decrease and drop below advisory level, possibly by early Saturday morning.

As the winds and seas gradually subside, conditions are expected to drop below Small Craft Advisory criteria, possibly by tonight for all waters through Monday.

Heavy showers and isolated thunderstorms will be possible across parts of the coastal and offshore waters today through at least Saturday due to a nearby upper level trough.

See the Collaborative Nearshore Swell and Wind Forecast for details on expected swell and resulting surf.

Watches, Warnings, Advisories

High Surf Advisory until 6 AM HST Saturday for east facing shores of Kauai, Oahu, Molokai, Maui and Big Island.

Wind Advisory until noon HST today for Big Island Summits.

Small Craft Advisory until 6 PM HST this evening for all Hawaiian waters, except the Big Island Southeast waters.

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