Swell Matrix

Area Forecast Discussion

National Weather Service Honolulu Hi

827 pm hst Sat Jun 24 2017

Synopsis

A mid level trough passing over the islands this weekend, will weaken the trade winds and result in an increase in leeward shower activity. The mid-level trough will shift west of the State by early next week, with high pressure then building back northeast of the islands. This will result in a drier and more stable trade wind shower regime, with the trades increasing into the breezy to locally windy range through much of next week. A more showery trade wind pattern will return late in the work week and next weekend, as an upper level low approaches the islands from the east.

Discussion

Currently at the surface, a 1034 mb high centered around 1600 miles north of Honolulu, is driving light to moderate trade winds across the island chain this evening. Meanwhile aloft, a mid-level 700 mb trough is enhancing some of shower activity over the western islands this evening. Infrared satellite imagery shows partly to mostly cloudy skies in place across the State, with radar imagery showing scattered windward showers and some lingering shower activity over leeward areas as well. Main short term concern revolves around rain chances.

Tonight and Sunday, High pressure will track slowly eastward well to the north of the islands, maintaining a light to moderate trade wind flow across the State. Localized sea breezes will likely develop again Sunday afternoon in the more sheltered leeward areas due to the weakened trade wind flow. Meanwhile aloft, the weak mid-level trough will shift westward, reaching Kauai Sunday afternoon. As for sensible weather details, showers will continue to favor windward and mauka areas through the period as the trades continue to blow across the island chain. In leeward areas, showers should end over the next couple hours as daytime heating is lost, with showers redeveloping again Sunday afternoon in the weakened trade wind regime. There could be a few enhanced showers develop Sunday afternoon, particularly over Kauai in association with the mid- level trough moving overhead.

Sunday night through Thursday, The mid-level trough will exit to the west of the state, while surface high pressure builds to the northeast of the island chain. This will increase the pressure gradient across the islands, resulting in strengthening trade winds Sunday night and Monday, with breezy to locally windy conditions expected Tuesday through the end of the upcoming work week. In addition to the strengthening trades, with the exit of the mid-level trough to the west of the islands, a drier more stable airmass will move into the area. Precipitable water values will drop into the 1.0 to 1.2 inch range, below normal for this time of year, and this will result in a drier than normal trade wind shower pattern. Showers will continue to favor windward and mauka areas, with a stray shower reaching leeward areas from time to time due to the strengthening trades. Rainfall amounts are expected to remain light through the period.

Thursday night through next Saturday, Both the GFS and ECMWF are in good agreement showing an upper level low tracking eastward toward the islands Thursday night and Friday, then lifting northward and away from the islands for the first half of next weekend. High pressure will continue to hold in place north of the islands through the period, with breezy trade winds expected to continue through at least Thursday night. The trades are then expected to weaken Friday through Saturday as the gradient slackens due to the approaching upper level trough. As for sensible weather details, both models show deep layer moisture increasing beginning Thursday night, with this enhanced moisture remaining over the islands through next Saturday. As a result, we should see a more showery trade wind pattern through the period, with showers continuing to favor windward and mauka locales.

Aviation

Light to moderate northeast to east trade winds will continue through Sunday. Expect onshore sea breezes to develop mainly along western slopes of all islands with cloud ceilings building over mountain areas after 20Z. TEMPO showers and isolated MVFR conditions are forecast.

No AIRMET's in effect or anticipated over the next 24 hours.

Marine

Light to moderate trade winds associated with 1034 mb high pressure far north of the state will continue through the rest of the weekend. Fresh to strong trades are expected to return through the day Monday and continue through the week as the gradient tightens over the region. Small craft conditions will be likely through this period over the typically windier channel waters, Maalaea Bay and south of the Big Island.

Surf along east facing shores will build and become rough late Sunday through next week due to a combination of increasing trades and a moderate northeast swell associated with a recent batch of gales off the west coast. Surf will approach advisory levels along east facing shores through this time, especially Monday through Tuesday as the northeast swell peaks.

Although surf along south facing shores will trend down toward average going into the upcoming week, small long-period reinforcing pulses from the southern Pacific will keep things from going flat. A blocking pattern has become established east of New Zealand, which is leading to fetch regions associated with passing storms setting up farther east than normal or more toward French Polynesia. Another pulse from this region out of the south- southeast is forecast to fill in Wednesday across the local waters and bring the surf back up a notch going into the second half of the week. For the long range, nothing significant is anticipated at this time as model guidance continues to support this blocking pattern holding down south.

Up north, mainly wrap into the typically exposed locations from the northeast swell Monday through Tuesday and the increasing trades will be expected for surf. Otherwise, nothing significant is anticipated through the week.

Extreme tides that have been observed over the past several days will trend back toward normal into the upcoming week. As a result, flooding impacts along the coast will diminish.

Watches, Warnings, Advisories

None.

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