Northeasterly trade winds should slightly strengthen to moderate speeds today then 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 next week. Deep tropical moisture will bring the potential for heavy rain and thunderstorms across the state starting around Tuesday and persisting through at least Thursday.
This morning's satellite imagery shows an old frontal boundary starting to move into Kauai. This should bring an uptick of scattered showers over Kauai today especially windward and mauka areas. No significant impacts are expected, but it will be showery at times especially over north Kauai. Over on the eastern half of the state, we do have some passing showers over windward and mauka areas due to a weak surface trough located just east of the Big Island. This trough is expected to gradually move east and away from the state this weekend, but some enhanced moisture lingering around east Maui and windward Big Island will produce scattered showers.
For next week, while the forecast still remains complicated, global models and its ensemble members have been slowly coming into better agreement. A low pressure system embedded within a monsoon trough is currently located just west of the Dateline near 14N. This low is expected to gradually move eastward along with the monsoon trough that extends over 1000 miles east of the low pressure system. An approaching upper level trough from the northwest Monday into Tuesday will allow the monsoon trough to lift northward and approach the Hawaiian Islands. Deep tropical moisture with precipitable water values exceeding 2.5 inches is expected to move over the state Tuesday into Wednesday. During this time, an upper level short-wave trough will swing through the state and will provide the support and lift (strong diffulence aloft) for thunderstorms to develop. With the ample upper level support, we could see organized thunderstorms such as squall lines or an MCS (Mesoscale Convective System) develop and if they do, they will be capable of producing damaging winds as well as torrential rainfall.
While the threat of heavy rain and flash flooding will exist across the state Tuesday into Wednesday and likely into Thursday as well, the heaviest rainfall period based off of the 00Z Grand Ensemble mean looks to be from Tuesday afternoon through Wednesday. Latest model trends in the ensembles have been placing the heaviest rainfall totals slightly further east with the heaviest precipitation expected to be somewhere between Oahu and the Big Island. The forecast has been trended this way, but please keep in mind, there is still plenty of uncertainty and the forecast will change in the coming days. Flood Watches will eventually get issued for the entire state as we get closer to next week.
In summary, next week looks to be a very unusual weather pattern setting up for Hawaii. While we don't know exactly where the thunderstorms will set up, the greatest probability looks to be somewhere between Oahu and the Big Island at this time. Due to the very high precipitable water values (99th percentile) and ample upper level support from the short-wave trough, extreme rainfall rates will be possible especially around late Tuesday into Wednesday.
A trough to the north of the island chain will move southward across the western portion of the state today. This will lead to an increase in shower activity, which we have already seen early this morning at PHTO. Have included a TEMPO for low MVFR, but a brief drop to IFR cigs/vis is possible. Scattered showers are expected during the afternoon with MVFR cigs possible with heavier showers. The core of the jet stream currently just north of the islands will continue to push east, which should reduce the threat for light-mod turbulence through mid morning. SIGMET Romeo remains in place for possible severe turbulence where the jet is strongest.
AIRMET Sierra remains in effect for mountain obscuration for windward sections of the Big Island.
A surface low far north-northeast of the Hawaiian Islands will dig southward through the weekend and will stall roughly 800 to 900 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 moving over Kauai and Oahu this afternoon into 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.
Small surf along north-facing shores will begin to build rapidly this afternoon and evening as a moderate, medium-period north- northwest swell arrives. Surf will reach near seasonal average during the peak of the swell tonight into Sunday. 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. 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 back to around seasonal average today into early next week. A larger south-southwest swell may push surf 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 late Saturday through Monday. East-facing shores should also see a bump in surf if the north- northeast swell materialized during the latter half of next week.
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