Waikapū, HI Weather

Synopsis

Northeasterly trade winds should gradually weaken Sunday into Monday. Brief passing showers will drift into the islands on the trade winds, favoring windward and mountain areas during the overnight to early morning hours. A significant change to our weather pattern is expected early next week. Deep tropical moisture will bring the potential for heavy rain and thunderstorms across the state starting Monday night and persisting through at least Thursday.

Discussion

Radar and satellite showed typical trade wind showers and clouds moving into windward and mauka areas this afternoon. Winds were averaging 10-15 mph with the typical windy areas gusting 20-25 mph.

The upper level maps show a large closed low about 1000 miles to our north-northeast, with one ridge to our northwest and another to our east-southeast. At the surface, a low is centered beneath the upper level low, with a ridge to either side. The eastern ridge is helping to keep the trade winds blowing today and this will continue into Monday. Through Monday night, the low to our north will drift slowly southeast, both aloft and at the surface. At the same time, two new surface lows will form to our west- southwest. A trough will connect all three lows...the one that drifted into a position to our northeast along with the two to our west-southwest. Southerly winds will bring deep tropical moisture northward as this trough moves north toward and then through the state Monday night through around Thursday. Precipitable water values will get up over 2 inches, and max out around 2.5 inches. This is exceptionally high (99th percentile).

Heavy rain and thunderstorms will be possible through this period over most of the state, but models are slow to converge on area(s) of maximum concern. Therefore, we have issued a Hydrologic Statement describing the concern for flooding. A Flood Watch may eventually be issued if models continue to show the potential for heavy rain. Upper level winds may be just strong enough to enable the development of organized convection in the form of squall lines or MCSs. If these form, the chance of extreme rainfall rates goes up. Finally, ensembles indicate that Tuesday afternoon through Wednesday is the most likely time of heavy rain and thunderstorms.

Winds will remain generally easterly through Tuesday, but after then there is great uncertainty. Some models show centers of low pressure forming along the trough as it moves over the state. These lows would have a cyclonic circulation, of course, and give different islands very different winds. The location and timing of these lows, as depicted by various models, are quite different. This makes for a very low-confidence wind forecast. Regardless of exact timing/location of any additional lows that may form, at this time, it does not appear that strong winds will impact the state with this overall system. However, showers and any thunderstorms will have the potential for brief gusty winds.

Aviation

SIGMET Romeo remains in effect just north of the state for the possibility of severe turbulence between 25,000 and 38,000 feet.

Otherwise, expect increasing shower chances tonight over mainly windward and mountain areas where MVFR ceilings and visibilities are possible in any shower that passes through. The moderate to locally breezy trade wind pattern will start to weaken Sunday.

Looking ahead, widespread MVFR/IFR conditions in heavy showers and low ceilings are possible beginning late Monday night; see the main discussion for details on the upcoming heavy rain and flooding potential.

No AIRMETs in effect. However, mountain obscuration is possible tonight.

Marine

A surface low far northeast of the Hawaiian Islands will continue to dig southward and stall roughly 800 nm northeast of Oahu on Monday, keeping the subtropical ridge just north of the area rather weak. Trade winds will be at gentle to moderate strength during much of this time, though a brief period of fresh northeast winds may accompany a shallow band of showers as it makes its way down the island chain through tonight. An area of low pressure is expected to move over the islands from the southwest Tuesday and Wednesday, bringing variable winds and the potential for heavy showers and thunderstorms.

Surf along north-facing shores is steadily building this afternoon as a moderate, medium-period north-northwest swell fills in. The Waimea buoy has been showing a steady rise in energy in the 14-16 second band since mid-morning. This north-northwest swell is expected to peak overnight into early Sunday, with surf reaching near or slightly above seasonal average. As this swell declines on Monday, a long-period northwest swell will arrive. This swell will generate moderate surf along north- and west- facing shores through Wednesday. Then a small north-northeast swell will be possible during the second half of next week.

A series of small overlapping south-southwest swells will push surf along south-facing shores back to around seasonal average through early next week. A larger south-southwest swell may push surf slightly above seasonal average late next week.

Surf along most east-facing shores will be small through early next 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 next week.