Swell Matrix

Area Forecast Discussion

National Weather Service Honolulu Hi

833 pm hst Mon Mar 25 2019

Synopsis

A weakening high pressure system north of the islands will track southeastward as trade wind speeds decreasing on Tuesday. A cold frontal system approaching the Hawaiian Islands from the northwest on Tuesday will break down the ridge north of the state setting up light winds lasting through the weekend. Clouds and showers from this front will move into Kauai late Wednesday night with unsettled weather slowly spreading southeast down the rest of the island chain from Thursday into Friday. Expect lingering unsettled showers over the windward sections of Maui and the Big Island this weekend.

Discussion

This evenings GOES satellite imagery shows a large band of clouds and showers lingering over and north of Kauai. Periods of unstable showers will continue to move in along windward and mountain slopes of all islands during the overnight period with higher precipitation trends favoring the island of Kauai.

In the big picture, a weakening 1025 MB surface high about 700 miles north of the Hawaiian Islands will continue to track towards the southeast through Tuesday. Decreasing surface pressure gradients will cause trade wind speeds to become light starting tomorrow, allowing modified sea breezes to develop over the leeward slopes of each island by afternoon. Minimal shower activity remains in the forecast grids from Tuesday through Wednesday. By Tuesday night offshore land breezes will form with onshore sea breezes with clouds forming over mountain and interior sections of each island on Wednesday.

A cold front will bring clouds and elevated showers to Kauai by Wednesday night. Clouds and numerous showers along the frontal band will slowly spread southeastward to the rest of the island chain on Thursday and Friday. The diminishing frontal band will stall over the eastern half of the state on Friday with elevated showers mainly affecting windward and mountain slopes of Maui and the Big Island. Cooler northerly winds will develop after frontal passage and island temperatures will run a few degrees colder through Friday.

On Saturday and Sunday, a weak high pressure system will track across the Central Pacific keeping light winds across the island chain. Some unsettled showers associated with the diminished cold front will favor windward slopes of each island through Sunday.

By next week Monday and Tuesday, another shallow cold front moves into the region from the northwest and light winds will continue across the region. Additional clouds and scattered showers along this next frontal band will move into the northwestern islands on Tuesday morning. Stay tuned for changes in the precipitation timing part of the forecast for next week, as a higher degree of uncertainty remains with the long range model guidance with this next frontal passage.

Aviation

High pressure north of Hawaii will maintain a trade wind flow, but as the high moves slowly to the east through Tuesday, winds will gradually weaken. Lingering moisture over Kauai will maintain clouds and showers island wise tonight, with the other islands seeing some trade wind showers over windward slopes. As winds weaken on Tuesday, expect to see some clouds develop over interior and leeward areas mainly during the afternoon hours.

AIRMET Sierra continues for mountain obscuration on Kauai. Anticipating this to continue throughout the night, with VFR conditions expected elsewhere.

Marine

The trade winds have started its weakening trend, and this trend will continue through Tuesday. The winds will finally become light and variable early Wednesday, then change to westerly in the afternoon as a front continue its approach to the islands. The front is expected to reach Kauai Thursday afternoon. The slow moving front will dissipate across the eastern islands between Thursday and Saturday. A moderate to fresh northerly wind flow fills in immediately behind the front, then veer to more typical trade winds over the weekend as a weak high pressure system passes north of the islands. Another front will be approaching the islands from the west late in the weekend, where it will be altering the wind flow around the islands once again.

We currently have a Small Craft Advisory (SCA) for the typically windy near-shore waters around Maui County and waters south of the Big Island to 6 AM HST Tuesday. We expect this SCA to run its course with no extension.

The north-northwest swell from this weekend continues to decline. There is small reinforcement from the north tonight through Wednesday. Then we have series of moderate to large swells for the second half of the week. First, a large northwest swell with a 6 foot swell and a 17 second period arrives Wednesday and Wednesday night. A north- northwest (340 degrees), 7 foot swell, 13 second period, follows in Thursday night through Friday. Then, a more west swell of 300 degrees, 6 feet and 15 seconds, comes in Saturday and Saturday night. The Wednesday swell has the potential of reaching advisory level surf for at least the north facing shores. But we will be monitoring closely for all three of them.

With the dying trades, surf along east facing shores will be trending down as well during the next 24 to 48 hours.

A couple of long period swell from the south will maintain small surf along the south facing shores throughout the period.

Watches, Warnings, Advisories

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

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