Swell Matrix

Area Forecast Discussion

National Weather Service Honolulu Hi

404 am hst Fri Nov 16 2018

Synopsis

Surface high pressure building far north-northwest of the area will cause the trade winds to strengthen across the islands through early next week. An upper-level trough passing near the state may cause increasing instability and wetter conditions as moisture associated with a weakening frontal band moves down across the area through this weekend. The frontal band is expected to push south of the Big Island Sunday night. The trailing end of another front may reach the islands around the middle of next week.

Discussion

A 1026 mb surface high near 34N 166w, or about 950 miles north- northwest of Lihue, is moving toward the east-southeast at about 15 mph. This feature is causing the pressure gradient to tighten across the state early this morning, which is causing the northeasterly trade winds to gradually strengthen. In addition, the surface high is pushing a weakening frontal boundary down toward the state. The diffuse leading edge of the band of broken to overcast low clouds and numerous showers associated with the front is located slightly less than 200 miles north-northwest of Lihue. This leading edge has been moving toward the south- southeast at 5 to 10 mph during the past six hours.

Aloft, a middle tropospheric trough appears to be near the western end of the island chain, while a broad upper tropospheric trough is digging down toward the region from the northwest. Even though the trough aloft near the islands is producing some instability, the mid-layers of the atmosphere remain relatively dry. Low clouds and showers being transported into the islands by the strengthening low-level trade wind flow are affecting windward sides of most islands. A few of these showers are also passing across some leeward sections of the smaller islands. In addition to the low clouds, high clouds continue to stream over the area from the southwest ahead of the digging upper-level trough.

The forecast guidance continues to indicate the trades will become locally breezy by mid to late morning as the surface high builds toward the east-southeast. At the same time, trade showers are forecast to continue over windward sections. The slight instability due to the troughs aloft may produce some brief heavy rainfall. However, the showers will likely be fast morning, so no significant flooding is forecast at this time. We will need to monitor radar and rain gages in case training of echoes unexpectedly occurs. This scenario, if it occurs later today or tonight, might require the issuance of a Flood Advisory or two for localized ponding of water on roads or elevated flow in some streams and drainage systems over some windward sections.

As we head in to this weekend, the surface high will likely push the weakening frontal band down toward the islands. Based on the latest guidance, we expect the remnant band of low clouds and showers will reach Kauai late tonight, and then Oahu Saturday morning. After that, it may move over Maui County Saturday evening and the Big Island Saturday night. The band is expected to move southeast of the Big Island Sunday night. Numerous showers can be expected over windward and mauka areas as this band passes. In addition, the strong trades will likely transport scattered showers over many leeward sections of the smaller islands.

The surface high will gradually weaken early next week. At the same time, a new front is forecast to approach the region from the northwest due to a new high building behind this front. The trailing end of the front will likely reach the islands around the middle of next week, which will produce a wet trade wind weather pattern.

Aviation

High pressure will build northwest of the state resulting in strengthening trade winds. High level clouds along the subtropical jet will gradually shift eastward as an upper level trough moves over the islands. These high clouds range from FL200-300, but may thicken up later today along the western end of the band. Moderate turbulence and light icing are both expected within this cloud band. AIRMET Tango remains posted for tempo moderate turbulence in FL200-300 Kauai to Maui.

The atmosphere will become more unstable as the upper trough approaches. Combined with increasing trade winds, this will cause showers to become more frequent over windward sections, with areas of MVFR conditions and mountain obscuration possible. AIRMET SIERRA may be required for mountain obscuration later today.

No other AIRMETS are in effect.

Marine

Trades are forecast to increase across the island waters and hold in the fresh to strong category through the weekend as high pressure builds north of the state in the wake of a front. Trades may reach the strong category across most waters Monday night through Wednesday with near gales across the typically windier waters between Maui County and the Big Island. Seas are nearing the advisory level (10 ft) across the northwest waters this morning due to a combination of a new northwest swell filling in and locally generated wind seas. As a result, the small craft advisory currently in place will remain up and has been extended through Sunday for the typically windier waters.

Surf along north and west facing shores will quickly rise this morning as a new northwest swell fills in from a system that reached storm-force category Tuesday night into Wednesday as it tracked east of the Date Line around 40N. Buoy observations are already reflecting this new source within the 14-16 second bands (320-330 deg) at the northwest offshore buoys and the nearshore PacIOOS Hanalei buoy. The swell ended up coming in a couple of feet higher than predicted at the offshore northwest buoys this morning, which should be reflected at the exposed nearshore buoys today. Surf is forecast to reach and exceed the advisory thresholds for north and west facing shores of the smaller islands today, then hold through tonight before slowly easing over the weekend. Expect this source to gradually shift out of a more northerly direction through the weekend as it fades.

A reinforcement out of the northwest is expected to fill in late Monday, peak Monday night into Tuesday, then fade through midweek. This source will be from a storm-force low that was depicted in the latest analysis over the far northwest Pacific near the western Aleutians. ASCAT showed a decent sized area of strong- to gale-force westerly winds associated with it focused at the area within the 300-320 directional band. Guidance remains in decent agreement and shows this feature tracking eastward to the Date Line come Saturday. Surf associated with this feature should remain below the advisory levels along north and west facing shores as it fills in and peaks early next week. Surf along east facing shores will gradually rise over the weekend and early next week as the trades increase into the fresh to strong category, especially next Tuesday as trades reach the strong category across most waters locally. Limited fetch upstream of the state, however, should keep the surf below advisory levels for east facing shores through this time.

Small surf will continue along south facing shores with mainly background southern Pacific pulses moving through.

Watches, Warnings, Advisories

High Surf Advisory until 6 AM HST Saturday for north and west facing shores of Niihau, Kauai, Oahu, and Molokai, and for north facing shores of Maui.

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

Small Craft Advisory until 6 PM HST Saturday for Kauai Northwest Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel- Oahu Windward Waters.

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