Swell Matrix

Area Forecast Discussion

National Weather Service Honolulu Hi

330 am hst Tue may 17 2022

Synopsis

Southeast winds will generate scattered showers and afternoon sea breeze clouds today. Rainfall will increase across the western and central ends of the state through Thursday in advance of an approaching front, with locally heavy rainfall and possible thunderstorms potentially generating flooding concerns. Conditions should improve by the weekend.

Discussion

Current radar trends continue to show scattered, mainly light, showers lingering across much of the islands this morning, with a majority of showers concentrated near Kauai and along the southeast portions of the Big Island. For the rest of today, southeastly trade winds will generate clouds and showers across favored windward portions of the islands, while sea breeze circulations and clouds are expected across interior sheltered leeward areas this afternoon. Moisture continuing to slowly increase across the western end of the state will increase areal coverage of showers around Kauai into this evening. The rainfall over Kauai through this afternoon will increase soil saturation and introduce the threat for hydrologic concerns and possible river responses to additional rainfall expected late tonight through Wednesday night.

The transition continues towards a wetter and more active pattern expected to expand across the islands from west to east through the remainder of this week. The wetter pattern will be the result of a slow moving surface front and upper low approaching Kauai from the northwest Wednesday. A band of deeper layer moisture in advance of the approaching systems is forecast to begin increasing across Kauai tonight/early Wednesday morning before expanding eastward and increasing across Oahu and portions of Maui counties Wednesday into Thursday. This increasing band of moisture will increase the areal coverage and intensity of rainfall across Kauai and Oahu by early Wednesday. Some increasing mid level instability will enhance the rainfall rate potential and chances for thunderstorms, increasing the threat of possible localized flooding especially over western and portions of the central islands through Thursday. Additionally, the upper blocking pattern in place across the region, and associated slow movement of surface and upper level features approaching the islands, will increase the chance for heavy showers training over similar areas and increasing flooding concerns for portions of Kauai through Thursday. The latest modeled precipitation data forecast continues to support increasing chances for potentially significant rainfall amounts tonight and continuing into Thursday, especially across Kauai. Otherwise, scattered showers can be expected over the eastern half of the state, with some locally heavy downpours possible. Winds around Kauai will trend southerly in advance of the front and increase to moderate levels Wednesday and Thursday.

Guidance continues to show upper support associated with the upper low weakening and slowly lifting northeastward away from the area Thursday night through Friday. A high pressure ridge settles back in north of the state. This pattern shift should generate improving conditions by the weekend.

Aviation

Low level winds will persist from the south over the west end of the island chain and from the east southeast near the Big Island. Cloud cover will increase as daytime heating sets in, but the terminals will remain mostly VFR. Showers over Oahu and Maui County will likely increase this afternoon. No AIRMET's are currently in effect.

Marine

Trade winds have diminished and shifted out of the southeast over the past 24-hrs in response to an upper trough and front approaching the region. This has led to a land and sea breeze regime setting up locally, which will continue as the front draws closer. Guidance does show the southerly winds strengthening over the Oahu and Kauai waters later today through Thursday into the fresh to locally strong category as the front nears/stalls west of Kauai. If this evolves, a Small Craft Advisory can't be ruled out for winds over these waters in later packages. For more information on the deteriorating weather conditions associated with the front see the Discussion section above.

Surf along south facing shores has begun to rise overnight with the arrival of a fresh south swell. Offshore buoys to the south and the nearshore PacIOOS buoy at Barbers reflect this with the peak of the energy registering in the 17 and 19 second bands. Expect the rising trend to continue today, then linger through the second half of the week as a similar reinforcement arrives Thursday. In addition to the long-period energy moving through that is typical for this time of year, southerly winds picking up later today through midweek plus the added rainfall moving in will add to the mix with short period choppy conditions.

Although surf along east facing shores will lower with the loss of the trades, some residual easterly swell from upstream should produce small surf into midweek before completely dropping off.

Surf along north facing shores will remain small through midweek, then trend up Thursday as a small, medium-period northwest swell arrives. Satellite data reflected this potential overnight, which showed a decent sized batch of strong northwest winds focused at the islands within the 300 to 320 degree directional band. A return of summer-like conditions is expected over the weekend as this source moves out.

Watches, Warnings, Advisories

Flood Watch from Wednesday morning through late Wednesday night for Niihau-Kauai-

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