Light to moderate trade winds will prevail today, followed by increasing trade wind speeds Friday through this weekend. Most shower activity will occur during the nights and early mornings and affect windward and mauka locations. From Saturday afternoon through Monday, an area of tropical moisture will move over portions of the state from east to west and bring enhanced shower activity to mainly Maui County and the Big Island. A typical stable trade wind pattern will return by Tuesday of next week.
This morning's water vapor imagery loop showed the cyclonic spin of an upper low centered around 475 miles west northwest of Kauai. Mid and upper level moisture could be seen streaming over the state from the southwest, ahead of the feature. At the surface, locally moderate trades persisted as a trough from former TC Genevieve continued to pass well to the northeast of the state and a stationary front remained anchored to the north. Showers have been rather isolated this morning with accumulations since midnight only reaching a few hundredths of an inch at select windward sites. The West Wailuaiki Stream rain gauge on the windward slope of Haleakala, however, did pick up around a third of an inch. The 12Z Hilo upper air sounding shows a subsidence temperature inversion height of 6000 feet, which is typical for this time of year. Not much change is expected through Friday with low cloud cover and isolated to scattered showers generally limited to windward and mauka locations.
On Friday, both the surface trough to the northeast and front to the north will have dissipated, allowing high pressure build in their wake. As local surface pressure gradients tighten in response, trade winds should ramp up to moderate to breezy levels.
By early this weekend, global models have been consistent in advecting an area of deeper tropical moisture towards the Big Island from the east. The most recent GFS model time-height cross section depicts the boundary layer inversion weakening, as conditions become more unstable, and moisture deepening up to 15000 feet. Thus, from Saturday afternoon through Monday, embedded pockets of heavier rainfall will be possible for the Big Island and Maui County, especially for windward locations. At this point in time, it appears Oahu and Kauai will only be on the fringes of this passing moisture. While still too early to determine more concrete forecast details, a few inches of rainfall accumulation could fall on some of the wettest windward sites.
Typical moderate trade wind weather will return for Tuesday through the end of the work week with scattered showers mainly windward and mauka areas during the nights and early mornings.
A weak high pressure ridge far north of the state will maintain gentle to moderate trade winds today with slight increasing trends on Friday. Isolated to scattered shower coverage will favor windward mountain areas, mainly in the overnight to early morning hours.
No AIRMETs are in effect.
Moderate trades persist as surface troughing to the northeast maintains a rather weak pressure gradient over the state. Dissipation of the trough to the northeast combined with renewed trough development south of the islands and strengthening high pressure to the northeast will favor fresh to locally strong trades by week's end.
Background medium and long period energy will produce small surf for south facing shores through Friday. A series of larger, medium to long period south swells arrive beginning on Saturday, causing surf to trend up for south and west facing shores this weekend into early next week. Surf may peak close to the High Surf Advisory threshold by the middle of next week.
Along east facing shores, trade wind swell and moderate trades will support surf slightly below seasonal averages. Surf will build this weekend in response to strengthening trade winds. Surf along north shores remains flat to tiny through the period.
None.
