Swell Matrix

Area Forecast Discussion

National Weather Service Honolulu Hi

357 am hst Mon Oct 15 2018

Synopsis

A cloud band over Kauai and Oahu will move southeast across Maui County today, then stall and dissipate over the Big Island tomorrow. A ridge will build north of the area tonight and persist through the week.

Discussion

Satellite loops show a 90 mile wide band of showery low clouds extending east northeast to west southwest over the islands. The south edge of the band is over the north shore of Molokai and the north edge is over Kauai. This band is associated with the remnant of a front. Clouds associated with this band cover Niihau, Kauai and Oahu and the coastal waters around Kauai County and Oahu. Clouds are isolated over most areas from Lanai to the leeward side of the Big Island of Hawaii. A trough extends northeast from over the Big Island. Broken to overcast low clouds associated with the trough cover the coast and lower slopes of the Big Island from Upolu Point to South Point and the coastal waters east of the Big Island.

A surface ridge will build eastward north of the islands and moderate northeast trade winds will spread over the islands over the next couple of days. The cloud band will shift southeast slowly and weaken. Clouds and showers in the band will spread over Maui County today and tomorrow and reach the Big Island tonight. The cloud band is rather shallow and the winds are rather light, so the showers in the band will affect mainly windward areas of the smaller islands, with only isolated showers spreading to leeward areas. A trough aloft is making the atmosphere less stable over the Big Island with deeper moisture there. Showers will be more active on the Big Island today with some showers building over interior and leeward slopes during the afternoon. As the trough aloft lifts north and weakens tomorrow, the atmosphere over the Big Island will become more stable.

From Wednesday through next weekend a surface ridge will persist north of the main Hawaiian Islands and maintain moderate to locally fresh northeast to east trade winds over the area. Ridging aloft will keep the atmosphere rather dry and stable, so showers will be focused over mainly windward areas with no heavy rainfall expected. Leeward areas will remain mostly dry.

Aviation

A diffuse frontal band remains across the central islands early this morning. The ragged leading edge of this band has advanced slowly toward the south-southeast during the past few hours. The low-level flow behind this feature is beginning to shift to north-northeasterly as surface high pressure builds northwest and north of the area. As a result, low clouds and showers are moving across north and northeast facing slopes of most of the smaller islands. This is producing brief periods of MVFR conditions over some windward sections of the islands. Elsewhere, mainly VFR conditions prevail. AIRMET Sierra is in effect for mountain obscuration on Kauai. This AIRMET may be lowered by daybreak this morning if conditions begin to improve there.

Abundant moisture and a weak surface trough continue to generate low clouds and showers, with periods of MVFR conditions over parts of the windward Big Island early this morning. AIRMET Sierra remains in effect for mountain obscuration on that island from Honokaa to Volcano.

AIRMET Tango for upper-level turbulence remains in effect over the island chain between FL260 and FL420. This AIRMET will likely remain in effect over the eastern end of the state this morning.

By later today, the stalled front will continue to weaken across Maui County. The low-level flow is expected to become northeasterly during the daytime hours. Low clouds and showers will continue to move into northeastern sections of the central and eastern islands, which may result in brief periods of MVFR conditions through late this afternoon. Conditions may improve slightly over windward Kauai later today as drier air moves down.

Marine

Winds will gradually fill in from a more typical trade wind direction and increase into the moderate to locally strong range as a ridge of high pressure noses eastward to the north of the islands. Some of the guidance is depicting advisory-level winds over the windier areas around Maui County and the Big Island tonight into Tuesday. These winds will remain in place through midweek, then begin to trend down once again as a front passes to the north Thursday.

An active surf pattern from the north and south is due this week. A pulse of shorter period north-northwest swell will push surf to near the October average through the day today. A larger north- northwest swell will arrive on Tuesday. Resulting surf will likely be well above the High Surf Advisory level for north and west facing shores on Tuesday and Wednesday, and combined seas over exposed waters will meet the 10 ft threshold for a Small Craft Advisory a that time. This swell will gradually lower Thursday and Friday and may be reinforced to around October average for the upcoming weekend.

The run of late season south-southwest swells will continue through the week. A new pulse has filled in overnight at the nearshore PacIOOS buoys out of the south, which will support moderate surf slightly above the October average along south facing shores. A gradual downward trend from this source is anticipated Tuesday through midweek. A more significant south- southwest swell is expected to fill in on Friday and produce advisory-level surf through the upcoming weekend.

Surf along east facing shores will remain below October average through the week, though a slight increase is due as winds pick up during the next couple of days.

Watches, Warnings, Advisories

None.

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