Makakilo / Kapolei / Honokai Hale, HI Weather

Synopsis

Light trade winds will produce rather dry conditions over most areas into tonight. Chances for heavy showers will increase during the day on Tuesday as developing low pressure and deep tropical moisture approach the islands from the southwest. The trough, or possibly a weak low, will pass over the island chain Tuesday night through Wednesday night, bringing the threat of heavy flooding rainfall and thunderstorms. The trough will weaken Thursday and Friday, though humid conditions with a chance of heavy showers may linger. Trade winds may briefly redevelop Saturday, followed by stronger northeast winds Sunday.

Discussion

A stable light trade wind flow is in place this afternoon, but a flood threat looms during the next few days. Trade winds are weak as a broad and deep surface low meandering about 950 miles to the northeast is keeping the local pressure gradient loose. A ridge aloft centered just south of the islands is maintaining stable conditions with a solid inversion. Little rain has fallen from Kauai to Maui, where available moisture has been limited and the sounding revealed an inversion at 6,000 ft. Scattered showers have been observed on Big Island slopes, where a small pocket of shallow moisture remains stalled. Westerly winds on the highest elevations of the Big Island summits will support a Wind Advisory into the evening then briefly ease overnight.

Little will change overnight, though increasing high clouds will signal a developing heavy rain threat. While Tropical Depression Olana will remain far to the west and not directly impact the area, a surface trough originating in the western Pacific and extending through and several hundred miles east of the depression will slowly drift to within 300 miles southwest of Hawaii tonight. Meanwhile, an upper level trough, now centered over 1,000 miles west of Kauai, will advance eastward, tapping moisture aloft from thunderstorms along the surface trough and sending increased high clouds overhead.

Chances for heavy rainfall and thunderstorms will increase during the day on Tuesday. The upper level trough will drift closer to the state. The entrance region of a jet stream aloft along the eastern flank of the feature will steadily erode the ridge aloft early Tuesday and then provide large scale ascent over the region later in the day. This will pull the surface trough toward the islands, spreading a deep tropical airmass with precipitable water increasing to around 2 inches by afternoon. Chances for widespread rain should remain relatively low in the morning as the Flood Watch for all islands goes into effect, but as moisture increases later in the day, the flood threat will slowly increase.

The flood threat will be greatest Tuesday night through Wednesday night. The jet stream aloft will provide its strongest ascent over the region, pulling the surface trough over the state and possibly generating an elongated surface low near or over the islands. A deep tropical airmass with precipitable water in excess of 2.25 inches will accompany this system and provide ample fuel for heavy rainfall. The GFS and ECMWF show the surface trough stretching across the central portion of the island chain, favoring Oahu and Maui County for the greatest rainfall totals. However, all islands will have a heavy rain threat, and slow-moving cells anchoring on terrain could drop very heavy rainfall in a short period of time, leading flash flooding anywhere. If a surface low does form along the trough, breezy east winds could form just north of the low, while gusty southerly winds for to the south. Even at this time, it is too early to tell if the low and the stronger winds will materialize, but another round of strong winds is likely over the high summits of Big Island. To further complicate the scenario, strong thunderstorms with localized damaging gusty winds may develop along and just south of the surface trough on Wednesday.

The threat of heavy rainfall will diminish Thursday and Friday. The jet stream aloft will lift to the north, and weak upper level anticyclonic flow will develop, ceasing the large scale ascent over the islands. A weak surface trough and ample moisture will likely linger, providing some chance for spotty heavy showers that could require the Flood Watch to be extended.

In the extended forecast, a brief period of trade winds may return Friday night and Saturday as the lingering moisture associated with the trough is pushed south of the islands. The GFS and ECMWF are showing breezy northeast winds developing Sunday as a shallow front moves near or over the state.

Aviation

A gentle trade wind pattern will continue this evening and become more light and variable tonight with localized land breezes developing across many areas. An area of moisture lingering around the Big Island will bring some showers throughout this evening with periods of MVFR conditions possible mainly over windward Big Island. Otherwise, VFR conditions should prevail through the night tonight.

A band of deep tropical moisture will arrive from the southwest Tuesday and will likely bring periods heavy rain and isolated thunderstorms. MVFR to IFR conditions should be expected at many areas across the state starting as early as Tuesday afternoon and persisting into Wednesday. The heaviest rain bands will likely arrive Tuesday night into Wednesday, where periods of LIFR conditions will be possible under the heavier rain bands.

No AIRMETs in effect, but we will need AIRMET SIERRA for mountain obscuration over majority of the state as the tropical moisture arrives. AIRMET ZULU for moderate icing will also be possible at roughly between FL180 to FL270 on Tuesday.

Marine

This afternoon, a surface low remains nearly 800 nm northeast of the main Hawaiian Islands, while decaying Tropical Depression Olana is located a little over 1000 nm to the southwest. A trough is forecast to approach the offshore waters by late tonight and reach the local coastal waters on Tuesday. This feature will bring heavy showers with embedded thunderstorms over the next several days. Relatively weak prevailing wind flow will persist through the forecast period, except within any thunderstorms that develop.

The current medium to long period northwest swell that has elevated surf along north-facing shores today will continue to build and bring surf to near advisory levels along north and exposed west-facing shores Tuesday, then will gradually decline. An additional small north-northeast reinforcement should arrive by mid-week.

Overlapping south-southwest swells will keep surf along south-facing shores above average through Tuesday. A slightly larger south swell may bring an additional bump to the surf Thursday and Friday.

Surf along most east-facing shores will be small through the first half of the week, but areas exposed to wrapping northerly swells will experience periods of moderate surf. East-facing shores should also see a bump in surf if the north-northeast swell materializes during the latter half of this week.