First Alert Weather Day: Storm system to bring downpours, dangerous surf this week
HONOLULU (HawaiiNewsNow) - Your Hawaii News Now weather team has declared a low-grade FIRST ALERT WEATHER DAY starting Wednesday afternoon into Thursday as we track a front for Oahu and parts of Maui County bringing disruptive weather at times.
Instability builds in on Thursday with an upper level disturbance and that is when the heaviest of the showers will be picking up on Oahu and parts of Maui County (mainly Molokai and parts of Lanai).
Front itself not bringing significant rain - it will stall out and leave behind some residual moisture— with low lying moisture and then eventually a disturbance will see more sporadic downpours develop into Thursday
Rainfall associated with this main line of showers has exceeded an inch for parts of Kauai as it moved through, which will be possible over portions of Oahu during the next 24 hours. Moderate southwest winds will gradually ease through the day as the front stalls.
Residual moisture lingering near the stalled front, combined with an upper trough moving through and closing off over the eastern half of the state will sustain rainfall chances into the second half of the week. With the trades in place, rainfall will primarily focus over windward and mauka areas.
Downpours, unstable conditions
Lingering residual moisture combined with an upper disturbance over the region will keep rainfall chances around through the second half of the week with sporadic downpours here and there. With the nature of this system, it will be challenging to determine where the pop up storms will develop but it appears the most unstable conditions initially will be over Oahu then it will spread towards Maui County and then by the end of the week over Hawaii Island which could potentially bring the first big snow of the season on the summits. As trade winds gradually return, the focus of rainfall will shift to windward and mauka areas later in the week through early next week.
It is a complicated forecast because most of the most intense rain will be just NORTH of the islands. While most of the upper level dynamics and deeper moisture are expected to remain north of the state, increasing lift and pooling low- level moisture along the boundary will enhance the potential for some much needed rainfall across the islands, particularly over the dry leeward area.
Moderate to breezy east-northeasterly trades will return Thursday through the weekend as high pressure builds north of the state, helping to focus clouds and showers over windward and mauka areas.
However, as a short-wave trough dives southeast across the island chain Thursday night into Friday and forms a cutoff low over the state this weekend, instability associated with this feature along with ample upstream moisture will help to enhance showers across the area.
Instability from the cold pool aloft may lead to a few heavier showers or even isolated thunderstorms, particularly from Oahu to the Big Island later this week as the upper trough moves into the area and begins to close off. The most severe weather will STAY NORTH of the islands but will keep our coastal waters unstable.
For the Big Island summits, this setup could support periods of wintry precipitation from late Thursday through early Saturday.
Warning-level surf
On top of the front and instability, there will be back to back swells over the country bringing warning-level surf to north and west shores through Thursday. The National Weather Service calls for dangerously large surf of 20 to 30 feet along north facing shores this evening, building to 30 to 40 feet tonight and Wednesday. Dangerously large surf of 15 to 20 feet along west facing shores this evening, building to 20 to 30 feet tonight and Wednesday.
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