Swell Matrix

Area Forecast Discussion

National Weather Service Honolulu Hi

401 pm hst Thu Sep 21 2023


A plume of moisture will continue to produce showery conditions over windward areas through Friday morning. Expect breezy trade winds to persist through Saturday then decline to moderate speeds Sunday into early next week. Low clouds and showers carried by the trades will continue to favor windward and mauka areas, especially during the nighttime and morning hours. Cloud buildups and spotty showers will also be possible along the upslope sections of the leeward Big Island each afternoon and evening.


This afternoon's sounding from Lihue shows a noticeable change in wind direction and speeds compared to yesterday. As a result trade winds have picked up this afternoon with breezy trade winds across the state. Satellite imagery shows moisture riding in with the breezy trade winds and is producing more showers than the typical day during the dry season. But due to the strong inversion at around 8000 feet, rainfall amounts have been light with only Puu Kukui on Maui reporting over 0.75 inches of rain during the past 12 hours.

This somewhat showery and breezy trade wind pattern should continue through Friday. Another plume of moisture should move in tonight, with showery conditions expected over windward and mountain areas late tonight through Friday morning. Looking further ahead, breezy trade winds should persist through Saturday then gradually decline on Sunday as the high north of the state weakens and shifts further west. Showers will continue to ride in with the trade winds and at times select windward areas could see some frequent showers especially Saturday evening through Sunday morning. Some deeper moisture is possible during this time due to a weak upper level trough just northeast of the state. Depending on where the deeper moisture ends up, we could see some moderate showers across select windward areas.

For next week, long term models are showing moderate trade winds persisting through the middle of next week. An upper level ridge building over the state will produce stable conditions. Low level moisture will continue to ride in with the trade winds with scattered showers over windward and mauka areas.


Moderate to locally breezy trade winds will continue. Showers will favor east and northeast facing slopes and coasts both day and night, but be most active in the late night and morning hours. VFR conditions will generally prevail, but brief MVFR ceilings and visibilities are likely in passing showers.

AIRMET Tango for moderate low level turbulence to the lee of higher terrain remains in effect. Conditions are likely to persist through twenty-four hours and beyond.


Fresh to strong easterly trade winds will persist through the first half of the weekend. As a result, a Small Craft Advisory (SCA) remains in effect for the typically windier waters of Maui County and the Big Island through Saturday. Model guidance suggests that a ridge far north of the islands will weaken a bit during the latter part of the weekend, which would weaken trade winds back into the moderate to fresh range Sunday and Monday.

Surf along south facing shores will remain around and just below the seasonal average through the weekend as southwest swells move through. The current small long-period south-southwest swell will ease Friday into the weekend, but will be followed by a similar southwest pulse Sunday through early next week. Looking ahead, small surf will persist as a mix of background southeast trade wind swell and leftover medium-period southwest swell move through next week.

Surf along north and west facing shores may increase slightly over the weekend as a medium-period north-northwest swell arrives. A tiny, medium-period north swell is possible Tuesday into Wednesday, followed by a second, small longer-period pulse arriving Thursday. Otherwise, trade wind wrap into some northern exposures will be the only source for surf.

Rough surf along east facing shores will remain essentially unchanged through Saturday, followed by a slight decline Sunday and Monday.

Watches, Warnings, Advisories

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

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