Swell Matrix

Area Forecast Discussion

National Weather Service Honolulu Hi

416 am hst Fri Jan 24 2020

Synopsis

A stable land and sea breeze pattern will continue through this afternoon for most islands under a weak subtropical ridge. A weakening cold front will move into Kauai and Niihau later tonight, spreading clouds and showers over both islands through Saturday. Trade winds also return late Friday afternoon through Saturday time period spreading windward and mountain showers to all islands. Winds shift towards a more southeasterly direction over the western islands on Sunday and Monday pushing the remnants of the front northward away from the state. Land and sea breezes are forecast in a stable light wind pattern from Tuesday into Thursday. Another cold front may bring wet weather trends in the extended forecast for next weekend.

Discussion

Broken clouds over the western islands of Maui and Hawaii Counties are associated with low level wind convergence around the high pressure ridge lingering over the Hawaiian Islands. These clouds are fairly stable under the trade wind subsidence inversion and are not high enough to produce more than isolated rainfall. Low cloud ceilings will linger along the north and windward slopes of Maui and the Big Island today with diminishing coverage elsewhere. A weakening cold front shows up on this mornings satellite imagery roughly 150 miles northwest of Kauai. This frontal zone will move over the island later this evening.

Today through Sunday, a cold front will approach Kauai from the northwest will spread clouds and showers across the Garden Isle later this evening. The forward motion on this front will stall out with clouds and showers lingering over Kauai and Niihau through Saturday. Trade winds will bleed out ahead of this cold front later this afternoon, spreading windward and mauka showers eastward down the island chain through Saturday. Moderate trade winds will continue to blow across the eastern half of the state through Sunday. Wind directions across the western islands will veer towards a more southeasterly direction on Sunday and Monday. These more southerly winds will push the remnant cold frontal clouds and showers northward away from the western islands with improving weather conditions forecast over Kauai County.

Monday through Friday, a large scale trough anchored over the Central Pacific far north of the islands will bring another round of stable light winds with local scale land and sea breezes through the work week. Long range guidance shows another weakening cold front moving into the western islands on Friday. Additional clouds and showers are possible over Kauai and Oahu by next weekend and wet weather trends were introduced in the extended range forecast grids from next Friday onward. However, large differences remain between the American (GFS) and European (ECMWF) models. The GFS model brings the front into Kauai and Oahu on Friday, while the European model keeps the entire frontal boundary north of the island chain. Expect these differences in frontal placement to diminish as the forecast time period grows shorter.

Aviation

This morning, a dissipating cold front approaching the state from the northwest, combined with surface high pressure over the islands, will again allow for light and variable flow. Expect VFR conditions with most cloud bases above 3000 feet. However, a weak moisture push from the southeast will bring a slight chance of showers to the eastern portions of the Big Island.

For this afternoon, sea breeze development appears likely along most coastal regions with mainly dry conditions. As the aforementioned front approaches this evening, weak tradewind flow may return to Kauai and Oahu.

No AIRMETS are anticipated at this time.

Marine

Light and variable winds will continue this morning, before shifting around to the north and northeast by late this afternoon, then increasing to moderate and fresh levels tonight. Locally strong easterly trade winds will then prevail through the weekend. A new front will approach and stall out to the west of the islands early next week, diminishing the winds over the western islands while moderate to locally strong east- southeasterly winds hang on in the unsheltered areas around Maui and the Big Island. Another front approaching from the northwest during the middle and latter part of next week, will diminish winds further, with light and variable winds prevailing across most of the island chain.

Seas have dropped below Small Craft Advisory (SCA) thresholds early this morning, therefore the advisory has been cancelled. Seas are expected to rise to SCA levels across a large chunk of the marine area again tonight and Saturday as a new large northwest swell arrives. Trade winds will also strengthen to SCA levels across portions of the marine area this weekend. SCAs will likely be required for most marine zones through the weekend due to a combination of winds and seas, with some areas holding onto SCA level winds and/or seas into early next week.

Surf will keep pumping along north and west facing shores as a progressive weather pattern across the northern Pacific keeps a series of large northwest swells moving through the islands. The current swell has dropped below High Surf Advisory (HSA) levels, therefore the advisory has been cancelled.

A new large northwest swell will fill in from northwest to southeast down the island chain late tonight into Saturday with surf peaking at warning levels on Saturday. This swell will be on a downward trend Sunday, but may still hold at warning levels during the day before dropping to advisory levels Sunday night. As this swell eases early next week, advisory level surf may hold through Monday before dropping below HSA threshold Monday night and Tuesday. Over the Big Island, this swell will likely bring advisory level surf to exposed west facing shores over the weekend. Another large northwest swell will be possible during the middle to latter part of next week.

Surf along exposed east facing shores will remain small through the period due to the lack of persistent trades locally and upstream across the eastern Pacific.

Surf along south facing shores will remain small through next week. A couple background south swells expected over the weekend and again on Tuesday should be enough to keep things from going flat.

Watches, Warnings, Advisories

None.

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