Swell Matrix

Area Forecast Discussion

National Weather Service Honolulu Hi

355 pm hst Mon Nov 28 2022

Synopsis

Pockets of heavy showers will move through the area tonight, especially over Windward Maui and the island of Kauai. Cloudy and humid conditions with the potential for isolated heavy showers will continue into Wednesday. Building trade winds will then bring a return to a more typical trade wind pattern from mid-week through the weekend.

Discussion

Deep moisture remains entrenched over the state supporting humid conditions with dewpoints around 70, overcast skies, and pockets of heavy rainfall. Relatively little has changed since this morning with the slowly evolving scenario unfolding roughly as anticipated.

The band of stratiform rainfall with embedded convective elements continues to produce steady rain over Maui County, but is taking on an increasingly narrow appearance on this regional radar composite. This is occurring as the deepest low-level moisture is transported northwest in the direction of Kauai while right entrance dynamics slowly weaken and lift to the northeast. Notably, however, heavy showers have developed over Windward Maui during the last couple of hours as downwind convergence arising from southwest mid-level flow has given a boost to showers within developing ESE trade winds. These showers are moving quite slowly, but have remained mostly offshore this afternoon. Although the potential exists for these showers to become more focused over the terrain of Windward Maui tonight, radar estimated rain rain rates have have also been rather poor (1"/hr or less). Thus, while a threat for nuisance type flooding cannot be ruled out in this location, confidence in flash flood potential has become very low.

Over the remainder of the island chain, conditions will be slightly out of alignment for any discernible flood threat. As southeast winds bend around the Big Island, only Windward Maui is experiencing the orographic lift characteristic of a typical trade wind pattern. Otherwise, light southeasterly flow in the absence of any mechanism to support mesoscale organization will be insufficient to generate anything more than transient heavy showers, particularly in the vicinity of Kauai. The flood watch has therefore been cancelled.

Through the mid-week period, stability will increase in response to building mid-level ridging, even as the resident moist airmass lingers over portions of the area through Wednesday. High clouds will remain common during this time, but the potential for any prolonged breaks in cloud cover will be worth monitoring as it will not take much instability to touch off heavy showers over the Big Island for the next couple of days. Otherwise, moderate trades will build into the area on Wednesday through the weekend bringing a return to a more typical drier airmass and the standard windward shower distribution.

Aviation

Moisture, along with lingering instability, will continue to generate showers, locally heavy at times, across much of the islands into the overnight hours. A few thunderstorms remain possible. The areal coverage of showers is expected to diminish Tuesday. A mix of VFR and MVFR conditions have been noted across the islands this afternoon, and expect this trend to continue into early Tuesday. MVFR conditions, in heavier showers, will continue the need for AIRMET Sierra for tempo mountain obscuration across a portion of the islands tonight.

AIRMET Tango continues for a period of tempo moderate turb btn fl180 and fl340 across the western and central islands this evening. The threat of moderate upper level turb is expected to diminish tonight as upper dynamics decrease north of the area.

A period of light icing remains possible across the islands tonight, in layered clouds aloft.

Ash from the erupting Mauna Loa volcano seen earlier this morning, continues to drift eastward from the Big Island.

Marine

A surface trough lifting across the state from the southwest will bring enhanced showers through Tuesday. Otherwise, a surface low pressure system passing by far north of the islands will result in weak south to southeast winds across the waters through midweek. Gentle trade winds will build back around Thursday.

The current north-northeast swell will decline this evening, while a new long-period west-northwest swell (310 degrees) is forecast to build later tonight. Surf will come near the advisory criteria along most north and west facing shores of the smaller islands by Tuesday. This swell is expected to gradually lower from Tuesday night through Wednesday. Another slightly larger, west- northwest (310 degrees) swell spreading down across the area Thursday and Friday could produce peak surf heights well above the advisory threshold along most north and west facing shores. Only minimal background surf is forecast along south facing shores this week. Surf along east facing shores will remain small through the next several days as winds remain light.&&

.HFO WATCHES/WARNINGS/ADVISORIES, None.

Watches, Warnings, Advisories

None.

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