Light to moderate trade winds will prevail for the next few days, followed by increasing trade wind speeds this weekend. Meanwhile, an upper disturbance just to the northwest will enhance showers over the western end of the state for the next few days, and could produce a few thunderstorms in the northwest coastal waters as well. The focus for increased showers will shift to the eastern end of the state this weekend as an area of tropical moisture passes by.
Early this morning, scattered thunderstorms are occurring just outside of the coastal waters within the offshore zone about 60 miles northwest of Kauai. These thunderstorms are being supported by an upper low and its associated mid-level trough northwest of the main Hawaiian Islands. The 12z upper air sounding at Lihue is similar to the previous one from yesterday afternoon, depicting an eroded trade wind inversion, sufficient convective available potential energy (CAPE), and around 42 kt of effective shear, all of which point to an unstable environment over the western end of the state. Although the thunderstorm activity has remained to the west thus far in closer proximity to the upper disturbance, various CAMs show that thunderstorms could creep into the coastal waters near Niihau and waters north and west of Kauai later this morning as the cooler 500mb temperatures edge closer to the islands. Therefore, isolated thunder chances have been introduced for these areas with this morning's forecast package.
The upper low and mid-level through will linger for the next couple of days, enhancing incoming trade wind showers across the western half of the state. The upper low is forecast to lift away to the north on Friday, returning more stable conditions to the area.
Meanwhile, trade winds will remain at only light to moderate speeds for the next couple of days as the remnants of former Tropical Cyclone Genevieve cut off the high pressure to our north, interrupting the trade wind flow. The weaker trades will likely result in localized sea breezes each day, bringing increased cloud buildups and a few showers each afternoon. Showers will be more prominent across Kauai and Oahu due to more available moisture and instability. By the weekend, trade winds will increase back to locally breezy levels.
The forecast becomes more interesting again this weekend as model guidance continues to suggest that a plume of moisture will either move over or very near the eastern end of the state late Saturday into Sunday. Despite most guidance keeping the bulk of the moisture just south of the islands, it is likely that at least some of the enhanced tropical moisture will become embedded within the trade wind flow, increasing showers for Maui and the Big Island. With this morning's forecast updates, rain chances have been adjusted upward to better highlight the increasing potential for showers this weekend over Maui and the Big Island. The finer details should become more clear over the next few days.
A weak high pressure ridge far north of the state will keep gentle trade winds in the forecast. Brief periods of showers will favor windward mountain slopes in the overnight to early morning hours. During the day time a hybrid trade wind with sea breezes along leeward mountain slopes will produce a few stray afternoon to early evening showers over south and west slopes of each island.
AIRMET Sierra in effect for mountain obscuration above 020 for the entire island of Kauai.
AIRMET Tango may be issued later this morning due to upper level turbulence as an upper level low and associated subtropical jet stream approaches Kauai from the northwest.
A west to east oriented stationary front remains anchored well north of the state. This, along with a surface low far northeast of the state will help to limit trade wind speeds across the islands today. As both features dissipate by late this week, building high pressure north of the islands will help strengthen trade winds back to fresh to locally breezy. Small Craft Advisories will be needed for at least the typical windier waters Friday through this weekend.
Isolated thunderstorms will be possible today over the waters west of Kauai as an upper level disturbance lingers west of the state.
Surf along south-facing shores will remain below seasonal averages through late this week. By this weekend, a small, long period south swell is expected to arrive, followed by a series of reinforcing south swells that will boost surf heights to near or above seasonal averages into next week.
Small, medium period easterly swell generated from former Tropical Cyclone Genevieve continues to linger, but will gradually ease today. Choppy conditions will remain, generated by the moderate trade winds.
Surf along north facing and west shores will remain flat to tiny through the forecast period, except for areas exposed to the easterly trade wrap.
None.
