Moderate trade winds will prevail through tonight, then increase 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 the southern half of the state (Big Island and Maui) from east to west and bring enhanced showers. A typical stable trade wind pattern will return by Tuesday of next week.
An upper level low to the west-northwest is currently bringing high clouds over the state, while a weak surface trough to the northeast has suppressed trade wind speeds. Both the upper low to the west and trough to the northeast will move north and trade winds will strengthen into the moderate to breezy range Friday into the weekend. Showers will focus mostly over windward and mountain areas and should stay on the lighter side due to a normal summertime inversion height of approximately 6,000 ft.
Current satellite imagery depicts a tropical low with an associated tropical wave approximately 960 nm ease-southeast of Hilo. This tropical feature appears to be separating from the ITCZ and model guidance projects this low weakening to a trough over the next few days as it moves west-northwest. This trough will have a decent moisture profile associated with it when it arrives and it is forecast to reach the Big Island by Saturday evening. Rainfall chances and humidity will increase for the Big Island and Maui County, then to a lesser extend, Oahu and Kauai Sunday into Monday. 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 15000-20000 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. 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 by Tuesday through the end of the week with scattered showers mainly windward and mauka areas during the nights and early mornings.
Moderate trade winds strengthen to breezy Friday. Isolated showers are possible tonight mainly windward and mountain areas.
Across windward Big Island, increasing showers are also possible Friday during the day which could create intermittent MVFR conditions.
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. A moderate, long period south swell will peak near 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.
