Area Forecast Discussion for Hawaii

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Issued: Sep 24, 2017 8:48 PM HST

Synopsis
A gentle to moderate trade wind flow will be with us for the new work week as a surface ridge of high pressure remains north of the islands. The trades will be light enough to bring on daytime sea breezes favoring trade wind shelter areas such as the leeward coast. This will lead to some afternoon clouds over the lee and interior areas. Meanwhile, the trades will carry in a few showers to the windward areas. A boost in the showers is likely Wednesday through Thursday as a surface trough passes through the islands.

Discussion
The trades are expected to become a tad lighter overnight with gentle to moderate strength persisting between Monday through Friday.

The air mass is currently rather stable with a low level inversion sloping from 6k feet over Kauai to 8k feet over windward Big Island. It is not a robust inversion, likely weakened by an upper level trough draped across the area. This weakness in the inversion could be responsible for some the evening and late night showers along the Kona coast. This trough is forecast to fill through the next 24 hours, with upper ridging taking over for the rest of the week.

As the trades bring in a few showers during the next 2 days, the winds will be weak enough to allow for daytime sea breezes, favoring the lee and interior areas of the islands. Stable mass will be enough to ward off any widespread afternoon showers.

On Wednesday, we anticipate a boost in the trades showers as a trough in the trades passes through the islands from east to west. The ECMWF model differs from the GFS where the EC is faster by a half a day, and is the model of choice. The moisture will linger through Thursday before becoming slightly drier on Friday. With a little more moisture, the chances for afternoon showers increases on both Wednesday and Thursday.

Over the weekend, the trades give way to SE winds over the western half of the island chain. This change is brought on by a front stalling 330 miles NW of Kauai, and has the support of an upper level trough. The old EC run has the front further west than the GFS, as well as the upper trough. But should the GFS pan out, the humidity will be near or at uncomfortable level, and we may have vog.

Aviation
VFR will predominate across the island chain. Only brief MVFR conditions are possible over windward and mauka sections in passing showers, being carried by the light to moderate trade winds. Low ceilings and showers are also possible over leeward parts of the Big Island through the evening hours.

No AIRMETs are currently in effect and none are anticipated.

Marine
Light to moderate trades are expected to persist through the first half of the week as a weak surface trough approaches the islands from the east. The wind flow will be light enough for land/sea breezes to setup over some leeward areas through this time. A return of moderate to fresh trades are forecast through the second half of the week as the trough moves through from east to west. Some of the typically windier channel waters may even reach advisory levels Thursday through Friday.

Surf along south facing shores will remain small through the week with mainly a mix of background southeast and southwest swell energy expected. The highly zonal pattern with systems racing eastward across the southern Pacific continues, with no significant sources shown setting up within Hawaii's swell window over the next several days. The next long-period pulse out of the southwest (210-220 deg) from the Tasman Sea should fill in Tuesday night and continue through midweek, which will lead to a slight increase in surf. This source is from a strong- to gale-force low that moved across the Tasman Sea last week.

Small, short period (10-12 seconds), northwest swell energy will continue and should be enough to keep the surf from going flat along north facing shores through the week. The latest surface analysis and satellite data showed a 998 mb low centered around 1200 nm north- northwest of the islands with a pocket of near gale-force winds focused toward the islands within the 330-340 deg band and seas up to around 10 to 13 ft. This reinforcement out of the north-northwest should reach the local waters late Wednesday and hold through Thursday. A compact storm-force low currently east of Japan is forecast to continue eastward over the upcoming few days toward the Date Line around 35N. Although the GFS/WAVEWATCH III solution suggests a favorable setup for a west-northwest (310-320 deg) swell to fill in across the local waters this coming weekend, the ECMWF is not as aggressive with this system. As a result, forecast confidence for the upcoming weekend remains low at this time. Guidance should come into better agreement over the upcoming couple of days.

Surf along east facing shores will remain small through the first half of the week as the trades locally and upstream of the state remain light. Once the aforementioned surface trough moves through from east to west by midweek, small and choppy surf will likely return as the trades fill back in through the second half of the week.

Watches/Warnings/Advisories
None.


Public...Lau

Aviation...Kinel

Marine...Gibbs