Swell Matrix

Area Forecast Discussion

National Weather Service Honolulu Hi

950 am hst Fri Dec 1 2023


A Kona low west of the islands will bring a continued threat for heavy rain and flooding along with isolated thunderstorms today. Conditions will gradually improve over the weekend with stable trade wind weather expected for the first half of next week.


This morning, a Kona low remains located just over 400 miles west-northwest of Kauai, while a 1028mb surface high is centered roughly 1500 miles to the northeast. Latest satellite and radar imagery shows that a band of enhanced showers and thunderstorms is impacting the western islands from Kauai to Molokai, though the bulk of the shower activity throughout the morning has been focused across the Kauai Channel and the island of Oahu thus far. Radar estimated rainfall rates and surface observations show that the bulk of these showers are light to moderate, though there are pockets of heavier rain with rates over 1 inch per hour embedded within the lighter showers, particularly over the coastal waters south of Oahu. VAD wind profiles and morning 12z soundings show that there is up to around 40-50kts of 0-6km bulk shear across the islands, along with 500mb temperatures ranging from around -8C to -10C, with the coldest temperatures aloft over the western end of the state. This lingering instability will continue a slight chance of thunderstorms statewide today, and a few lightning strikes have been detected on GOES GLM (geostationary lightning mapper) imagery across the coastal and offshore waters as of press time.

Southerly flow will continue to bring deep tropical moisture across the islands today as the Kona low remains west-northwest of the island chain. This will maintain the chance for widespread showers, occasional pockets of heavier rainfall, and a slight chance of thunderstorms today. A Flood Watch remains in effect statewide through this afternoon, though as the previous discussion mentioned, the threat for heavy rain appears to be diminishing across the eastern end of the state with the focus shifting primarily to the western islands for the next 24 hours. A key decision point today will revolve around possibly extending the watch in time for portions of the state, trimming some islands from the watch, or expiring it altogether. That analysis is ongoing and any changes to the headline will be made with this afternoon's forecast updates.

As the previous discussion mentioned, the Kona low will gradually break down over the next 24 hours or so. Models show that by early Saturday, the low aloft will become de-coupled from the surface Kona low and translate eastward as a mid/upper trough, while the surface low weakens into a surface trough and remains northwest of the islands. As this occurs, upper jet dynamics will also become less supportive of large scale forcing for ascent, diminishing the threat for widespread heavy rain on Saturday. With the upper trough lingering near the islands, however, we cannot rule out the slight chance for thunderstorms (particularly over the western islands and the Big Island slopes), though overall impacts are expected to be more localized when compared to the previous few days.

By Saturday night and Sunday, a much more stable airmass will become established over the islands as trade winds begin to return. The trades will strengthen early next week, with stable trade wind weather prevailing through the middle of the week. Isolated to scattered showers will generally favor windward and mauka areas throughout the week, especially during the overnight through morning hours each day. Models indicate that the trades may ease late next week as a new front approaches from the west.


Deep tropical moisture streaming north over the region due to the Kona low west of Kauai will keep the threat in place for periods of heavy rain and thunderstorms. Expect widespread MVFR conditions with local IFR under heavier showers. Conditions are expected to slowly improve across the region starting on Saturday.

Aside from turbulence and icing associated with any TS that develops, AIRMET Tango for mid-level turb will continue due to wind shear in the layer between 100-FL200. Light icing in layered clouds will also remain a threat through Saturday. AIRMET Sierra for mountain obscuration will also persist over some areas into Saturday.


Southeast to south winds will continue across the region today with a Kona low to the west-northwest of the main Hawaiian Islands. Bands of showers, some heavy at times, and isolated thunderstorms will continue. Some of these thunderstorms could produce strong and gusty winds over the coastal waters, and isolated waterspouts can not be ruled out. Thunderstorms are also expected to continue over the offshore waters.

The Kona low will weaken this weekend as the ridge builds in north of the islands. The chance for thunderstorms is expected to decrease over the waters near the Big Island and Maui County tonight, but remain over the waters near Kauai and Oahu, and the northwest offshore waters.

Expect gentle to fresh trade winds to spread across the region from Saturday to Sunday, strengthening to fresh to strong trade winds from Monday onward. A Small Craft Advisory (SCA) remains in effect through today for most coastal waters surrounding Maui and the Big Island. This SCA will likely be cancelled by tonight as wind speeds decrease below advisory thresholds. As trade winds increase early next week, expect another SCA to be posted.

The recent small to moderate, medium period northwest (320 degree) swell will gradually decline through Saturday. The next moderate size, longer period northwest (320 degree) swell will arrive and begin to fill in Saturday afternoon and evening. This swell will peak surf to near High Surf Advisory (HSA) heights by Sunday, then slowly subside Monday. A reinforcing north northwest (330-340 degree) swell arriving by Monday night and may lift surf heights well above HSA thresholds and approach High Surf Warning levels on Tuesday and Wednesday.

Expect the short period wind wave chop to persist along south facing shores. Small surf continues along south facing shores with small background south swell pulses next week Monday and Thursday. Strengthening trades through next week will build surf heights along east facing shores.

Watches, Warnings, Advisories

Flood Watch until 6 PM HST this evening for all Hawaii islands-

Small Craft Advisory until 6 PM HST this evening for Maui County Windward Waters-Alenuihaha Channel-Big Island Windward Waters- Big Island Leeward Waters-Big Island Southeast Waters.

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