Area Forecast Discussion for Hawaii

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Issued: Jun 22, 2017 3:30 AM HST

Synopsis
High pressure far north of the islands will maintain gentle to locally breezy trade winds through Friday. The trade winds will deliver passing clouds and showers, mainly to windward areas. A weak mid-level trough developing near the islands this weekend and early next week may weaken the trades and bring an increase in showers.

Discussion
High pressure cells are centered far NNW and NNE of the islands this morning, with a low and associated stalled front separating the two. The low-level gradient S of the highs is supporting trade wind flow over the islands, with speeds ranging from light in sheltered leeward locations to breezy in the more exposed areas. A low aloft near 25N141W has been triggering high-based thunderstorms (in an area otherwise dominated by low-topped stratus) about 900-1500 miles to the ENE of the islands over the past 24-36 hours, with the low currently tracking NW, and associated thunderstorm coverage diminishing. Overnight soundings continue to highlight a stable island atmosphere, with a strong capping inversion based near 8 kft, and a slight overnight increase in PWAT to near 1.25". Based on radar and satellite observations, the inversion is sufficiently high to allow clouds embedded in the trade flow to produce showers, especially as they interact with island terrain.

Little significant change to the overall weather pattern is expected through Friday, with passing clouds fueling a few showers, mainly over windward areas. A mid-level ridge over the islands will slowly and slightly weaken as the low aloft slowly moves N and gradually dissipates NE of the islands. A trend toward slightly weaker trade winds is anticipated Friday and Saturday as a weakness in the low- level gradient, associated with the dissipating low aloft, moves over the islands from the E. The GFS also indicates increasing low- level moisture associated with this feature moving in on the trades by Saturday.

On Sunday and Monday, a weak and subtle trough in the surface pressure and wind fields is expected to move E to W across the area, potentially leading to another slight decrease in trade wind speeds. The GFS is still depicting a slightly sharper trough than the ECMWF, and depicts a weak surface trough that passes over the islands Sunday, resulting in a veered low-level flow that could turn surface winds to the ESE into Monday. Although the low aloft is expected to dissipate NE of the area, it leaves behind a weakness as a new trough aloft moving in from the N helps to cool the mid-levels over the islands. The combination of the incoming mid-level system and a possibility for veered low level flow may bring increased shower chances this weekend, and previous PoP forecasts have been left unchanged. The best chance for additional showers will be mostly windward and mauka, though possibly some sea breeze or upslope showers as well if the winds weaken enough.

On Tuesday and Wednesday, the high currently to the NNW will move E, and models agree that breezy ENE trades will prevail, delivering a few showers to windward areas, with a ridge aloft building over the area from the NW.

Marine
The current long period SSW swell will gradually decrease. However, this swell will be reinforced by another above average south swell, arriving this afternoon, and peaking late tonight or Friday.

In addition to the new southerly swell, tides have been running about a half a foot above predicted levels. These elevated tide levels will increase the risk for minor coastal flooding when combined with the high surf. For the next few afternoons, the predicted high tides are around 2.5 feet, which does not include the extra 1/2 foot mentioned above.

Other wave trains are affecting Hawaii as well, with a small out-of- season northwest swell diminishing today, a small trade wind- generated swell and a short period southeast swell. The southeast swell will continue into the weekend and will add a small component to the wave heights along south facing shores.

In the longer range, models indicate a northeast swell developing for the beginning of next week. This swell will need to be monitored as it interacts with the trade wind swell, and a High Surf Advisory may eventually be needed for east facing shores if the swell comes in bigger than anticipated.

Moderate to occasionally fresh trade winds may diminish slightly over the weekend, with a Small Craft Advisory now in effect through this afternoon for the typically windier zones around Maui and the Big Island.

For more details on the surf, please refer to the Oahu Collaborative Surf Forecast (SRDHFO) prepared by Pat Caldwell under WMO header FXHW52 PHFO.

Aviation
High pressure far NE of the state will maintain light to moderate trade winds over the islands through the period. Clouds and showers will tend to favor the windward and mountain areas, with some showers introducing brief periods of MVFR. Otherwise, expect VFR conditions. No AIRMETs are expected.

Watches/Warnings/Advisories
High Surf Advisory until 6 PM HST this evening for South facing shores of all islands.

Small Craft Advisory until 6 PM HST this evening for Maalaea Bay, Pailolo Channel, Alenuihaha Channel, Big Island Leeward Waters, Big Island Southeast Waters.


Discussion...Birchard

Marine...Kinel

Aviation...Bedal