Swell Matrix

Area Forecast Discussion

National Weather Service Honolulu Hi

903 pm hst Fri Oct 20 2017

Synopsis

Passing showers will continue into Saturday, as the trade winds remain strong and gusty. The clouds and showers will stay focused over windward and mauka areas with some brief showers venturing into the leeward areas of the smaller islands. The trades will collapse late Saturday, becoming light and variable with daytime sea breezes on Sunday. An approaching front will bring on southerly winds ahead of it along with an increase in vog and moisture.

Discussion

The weakening of the trades has begun, and will continue through Saturday. Winds will still be rather strong Saturday, but not as strong and gusty as in the past few days. The surface high generating the trades is considered not to be very strong with a 1026 mb center, but the pressure gradient across the area is still rather tight. This gradient should loosen up even more Saturday afternoon.

Change is forthcoming after Saturday. A front is forecast to advance toward the islands from the northwest over the weekend, with the ridge of high pressure running ahead of it. This ridge is expected to be 200 miles north of Kauai Sunday morning, then over Maui County Monday morning. It is not a sharp east to west oriented ridge, but a rounded one that will bring on a southerly wind flow across all of the main Hawaiian Islands. The front is supported by by both the GFS and ECMWF, to reach Kauai Monday evening, Maui County Tuesday morning, and pushed just east of the Big Island by mid morning Wednesday.

The bulk of the weather is slated to occur well ahead of the front. Models are in good agreement in bringing in a low level trough with some deep tropical moisture to the Big Island Sunday afternoon and evening. This moisture will then spread to other islands Sunday night into Monday, along with the southerly winds. The bulk of this moisture is expected to reside across Maui County and the Big island with Oahu at the fringe. The front will be aided by a rather vigorous upper level trough although the bulk of the dynamics will remain north of the islands. With this said, locally heavy showers are possible for Maui County and the Big Island between Monday and Tuesday, along with a slight chance of a thunderstorm. For Oahu, the heavier weather is most likely for Monday night.

As the front approaches Kauai, southerly winds will become rather strong on Monday that may warrant a wind advisory for Kauai. We will see how this transpire. It will be monitored closely as the event nears. As noted, the front cuts through Maui County Tuesday morning, then on to the Big Island Tuesday afternoon and night. In the wake of the front, a north wind will bring in a cooler and drier airmass. On Thursday, the front remains stalled just a few hundred miles east of the Big Island.

In the mean time, satellite imagery shows a layer of broken low clouds streaming through Kauai and Oahu, along with the northeast winds. These clouds will likely be directed away from these two islands later tonight or Saturday as the trades turn a bit more easterly. Molokai is in the clear thus far, while some broken low clouds are and will be affecting the windward shores of Maui and the Big Island's northeast facing coast throughout tonight. There is an isolated shower located just off Kailua Kona town now, and expect them to hang around the coastline through midnight before calling it quits.

The wind advisory for parts of the Big Island, Lanai, and Kahoolawe, has been extended to 4 am HST Saturday.

Aviation

Strong and gusty trade winds will continue to support AIRMET Tango overnight for low level turbulence downwind of the mountains on all islands. In addition, AIRMET Tango is also in effect for upper levels over the islands due to speed sheer between FL260 and FL340.

VFR will predominate, as usual. Brief, to occasionally TEMPO, MVFR in passing trade showers will mainly affect windward and mauka sections over all isles through the overnight hours.

Marine

A dynamic (but seasonable) wind, wave and weather pattern is expected over the next several days as high pressure N of the area is replaced by a low that sends a front through the island chain early next week. After the front, light to moderate N winds are possible by the middle of next week, but forecast confidence is lower than normal.

Strong to near gale force trade winds currently prevail, but will be diminishing over the next 24-48 hours as high pressure N of the islands shifts E. A Small Craft Advisory (SCA) remains posted for all waters through tonight, but winds and seas are expected to diminish below SCA criteria in most zones on Saturday, and in the remaining zones by Saturday night. The SCA will be extended in time for these zones with the evening forecast update. As winds and seas diminish, rough surf along E facing shores is expected to diminish below High Surf Advisory criteria by early Saturday.

Although short-period wind waves will be easing, building NNW and S swells Sunday and Monday are expected to keep combined seas elevated, but below the SCA criteria of 10 feet. A larger, shorter-period NNW swell building on Tuesday will likely lead to SCA in all zones as seas exceed 10 feet once again. The first NNW swell will produce surf near the advisory threshold along exposed N facing shores, and the second swell could produce high surf as well. Advisory-level surf along S facing shores is possible Sunday and Monday, with the swell generated by a distant low in the S Pacific. A small long-period W swell is possible next week, generated by large and slow-moving Typhoon Lan in the W Pacific. See the recently updated Oahu Surf Discussion (SRDHFO) for details on the swell sources affecting the islands.

Watches, Warnings, Advisories

High Surf Advisory until 6 AM HST Saturday for east facing shores of most islands.

Wind Advisory until 4 AM HST Saturday for Lanai, Kahoolawe, Big Island North and East Kohala.

Small Craft Advisory until 6 AM HST Saturday for all Hawaiian waters.

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