Declining trade winds will continue to focus showers over windward areas through early Saturday morning. Winds will continue to weaken and continue to shift to the east to southeast through Saturday, as moisture near the state increases and the atmosphere destabilizes. Showers, some potentially heavy, will increase over windward slopes, while leeward areas experience periods of afternoon showers through at least Sunday. Trade winds will return Tuesday or Wednesday.
No changes to the forecast this evening, but there could be some tweaks based on the latest model data by the morning hours to to fine tune the forecast over the weekend. The overall forecast philosophy remains the same.
High pressure to the northeast of the state continues to weaken and move to the east. As a result, the winds over the islands is weakening, and shifting to the east to southeast. We are already seeing that directional shift in the low clouds near the islands, and we can expect that to become more widespread across the region through Saturday. This is in response to both the high to the northeast, and a surface low developing to the northwest of Kauai.
A more unstable airmass is moving over the islands in part due to an upper level low already near the islands, and the upper level trough associated with the developing surface low to the northwest. Currently moisture is somewhat limited immediately nearly the islands, but we are still seeing some slightly heavier showers with the moisture moving over the islands. Chances for showers, some heavy, increase through Sunday as trade winds decrease further and shift out of the east to southeast. The low to the northwest of Kauai will strengthen as the high to the northeast of Hawaii weakens, causing the decline and shift in the winds, and a broad upper-level trough will stretch across the region, creating some instability. Meanwhile, a mid-level trough, currently just south of the Big Island, will drift over the state. Convergent east-southeasterly flow on the mid-level trough's northern flank will produce an increase in moisture, raising precipitable water, from its current value of around an inch, to 1.3 to 1.5 inches.
Guidance has been showing the deepest moisture developing just north of the islands this weekend. However, there will be enough moisture and instability to increase shower frequency and intensity over windward slopes of all islands. With the weaker winds, afternoon sea breezes will produce clouds and showers over some leeward areas. Some showers may be heavy, and an isolated rumble of thunderstorm cannot be ruled out, however the greatest chance from thunderstorms appears to remain north of the coastal waters. The ingredients still do not push confidence high enough for the issuance of a Flood Watch, though the situation will need monitoring.
The GFS and ECMWF show the chances for rainfall declining by late Sunday or Monday. The upper-level trough over the region will weaken, and the mid-level trough will lift northward, lowering chances for rainfall. The main difference between the models is the strength of the east-southeast winds, which are stronger in the GFS due to a more rapid weakening of the nearby surface low.
A gradual return to a more stable trade wind flow is expected Tuesday or Wednesday.
Moderate to locally breezy trade winds will gradually weaken through Saturday and shift out of the east-southeast. Thus, expect isolated to scattered showers to affect mainly windward areas tonight. By Saturday afternoon, however, localized sea breezes may lead to the development of isolated to scattered showers over leeward locations as well. Brief MVFR ceilings and visibility will be possible in some of the heavier showers.
No AIRMETs are in effect.
High pressure to the northeast will continue to weaken, meanwhile an area of low pressure will drift towards the islands from the west. This setup will help maintain fresh to moderate east to slightly southeast winds through Saturday. A few isolated thunderstorms will be possible tomorrow into Sunday due to the proximity of the low. The low is expected to gradually weaken Sunday as a strong high passes north of state which may generate moderate to locally strong easterly winds over the eastern half of the state that may require a Small Craft Advisory. The western half of the state will remain in moderate to locally fresh east southeast flow. The area of low pressure is expected gradually drift west early next week, allowing winds to shift back to a more typical easterly direction and strengthen to fresh to locally strong speeds statewide by Tuesday night and hold through the second half of next week.
A moderate west northwest (300-320) swell will continue to build and peak tonight before quickly dropping tomorrow. A High Surf Advisory remains in effect for north and west facing shores of Kauai, Oahu, Molokai, and north facing shores of Maui until 6 AM Saturday. This swell will then gradually fade over the weekend. A small to moderate, medium period north northwest swell (340-350) may overlap the fading west northwest swell Sunday night into Monday. In the long range, a series of small, long period northwest swell look to keep surf elevated through much of next week.
Rough choppy surf along east facing shores may drop a notch or two Saturday and hold into early next week. As winds strengthen by mid week next week can expect east facing shore surf to pick up a notch or two.
High Surf Advisory until 6 AM HST Saturday for Niihau-Waianae Coast-Oahu North Shore-Maui Windward West-Kauai North-Molokai Windward-Molokai North-Molokai West-Maui Central Valley North- Windward Haleakala.
