Honolulu (HFO) — the National Weather Service forecast in plain English · Plaincast

Synopsis

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.

Discussion

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.