Swell Matrix

Area Forecast Discussion

National Weather Service Honolulu Hi

334 pm hst Wed Sep 28 2022

Synopsis

Light to moderate trade winds will remain in place with fairly dry and stable conditions expected through Thursday. The exception will be Kauai and possibly Oahu, where increased moisture associated with a dissipating front may bring an increase in windward showers beginning tonight. An increase in windward showers is possible for most islands Thursday night and Friday as moisture arrives from the east. Light and variable winds this weekend may bring increased afternoon showers to island interiors, with generally dry conditions at night. Trades will restrengthen early next week as another front approaches the area from the north.

Discussion

This afternoon, a stalled frontal boundary is located northeast of Kauai with the Northeast Pacific High located far north of the front. Along the front, scattered convection has developed along the band of clouds that stretches from just north of Kauai all the way to the central California coast. South of the front, relatively dry and stable air remains in place upstream and over the main Hawaiian Islands with moderate trade winds prevailing across the island chain. Afternoon satellite and radar imagery depicts relatively few clouds and showers across the state with most of the scattered cloud cover favoring Kauai and the Big Island. Rain gauges have remained mostly dry today, with the vast majority of sites reporting no measurable rainfall so far this afternoon.

Throughout the day, moisture has been increasing near Kauai as the frontal boundary sags further south. This is notable in the observed Lihue upper air sounding, which shows precipitable water values have jumped from 1.49 inches at 12z early this morning to 1.81 inches at 00z this afternoon. While most of the associated cloud cover has remained northeast of the Garden Isle, windward Kauai has experienced far more clouds than Oahu or Maui County islands thus far today. For this evening and tonight, expect an increase in trade wind shower coverage over windward Kauai and possibly Oahu, with drier conditions persisting over the remainder of the state through Thursday. The main forecast challenge in the short term will be how far south the enhanced moisture will get, which will ultimately determine windward shower and cloud coverage for Oahu through Thursday. Model trends favor increased windward showers for Oahu this evening through the early morning hours on Thursday, followed by a somewhat drier period mid- to late morning as the area of enhanced moisture lifts back northward a bit. As a result, the forecast has been trended slightly wetter for windward Oahu overnight but slightly drier for Oahu and its adjacent waters mid- to late Thursday morning.

A low-level trough is expected to approach the state from the east Thursday night into Friday, increasing moisture and enhancing windward clouds and shower activity for much of the state. Guidance keeps the bulk of this moisture north of the Big Island, however. This weekend, a cutoff low far northeast of the state will push another front southward towards the area. As the front approaches the state, the local pressure gradient will weaken, resulting in light easterly background trade wind flow. This will allow for a land and sea breeze regime to modulate weather over the islands. For now, will continue with the current forecast philosophy of sea-breeze induced shower coverage over the interior of the islands during the afternoons with overnight land breezes clearing clouds and showers each night through early morning.

By early next week, increased moisture associated with the approaching front may move further south over Kauai and Oahu as trade winds restrengthen. There remains some disagreement among models regarding how far south the front will make it, with the latest ECMWF depicting considerably less moisture across much of the state as the boundary remains further north while the GFS shows a wetter solution for the islands, especially the western islands, bringing the frontal boundary and its associated moisture further south. Will continue to monitor trends and adjust the forecast accordingly over the next several days.

Aviation

High pressure north of the state will maintain moderate trade winds through the forecast period. A relatively dry airmass will limit cloud and shower activity for the majority of the state through this evening, with VFR conditions prevailing. Moisture associated with an old front just northwest of Kauai may bring an increase in windward showers tonight over Kauai, and possibly Oahu as well.

No AIRMETs are currently in effect. AIRMET Sierra may be needed for mountain obscuration later tonight over windward areas of the western islands if showers become widespread.

Marine

A ridge of high pressure north of the state will produce moderate to fresh trade winds through Thursday. A front dropping south will cause the ridge to shift southeast and closer to the state, which will result in the winds weakening. A light to moderate trade wind pattern is expected Friday through Saturday. Wind speeds for Sunday will depend on how far south the frontal boundary stalls. Latest guidance is suggesting the frontal boundary stalling near Kauai with locally fresh trade winds near Kauai and north of Oahu by Sunday evening. Our current forecast keeps the moderate to locally fresh trades north of the area, but we will continue watch to see if this trend continues.

A series of small north and northwest swells are expected along north facing exposures through Friday. Forerunners from a slightly larger north-northwest swell will fill in Friday and peak over the weekend. In the long range, models are showing a low pressure developing roughly 1500 nm north-northeast of the state Thursday through Saturday with a decent fetch of gales pointed just east of the state. This should produce a moderate north-northeast swell rising as early as Sunday across the state, but definitely by Monday. Depending on how the storm develops the next few days, we could see surf approach the High Surf Advisory thresholds along north facing shores on Monday.

Surf will remain small along south facing shores over the next few days with mainly background energy from the south and southeast. A tiny boost of energy from the south and southeast is possible on Friday. Although still small, a more noticeable long-period south swell is expected to fill in Sunday into Monday then decline through the middle of next week. East facing shores will remain small during the next several days due to the lack of strong trade winds upstream of the state. Areas exposed to the north should see an increase Sunday into Monday as a moderate north-northeast swell wraps into select eastern exposures.

Watches, Warnings, Advisories

None.

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