National Weather Service United States Department of Commerce
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Issued: Apr 20, 2018 9:57 AM HST

Synopsis
Wet weather and cloudy skies will remain through today with some partial clearing over Kauai and Oahu this afternoon. Some showers may be heavy at times and a slight chance of thunderstorms remains through today. A typical trade wind pattern will return by Saturday, with showers favoring windward and mauka areas. High pressure north of the state will maintain moderate to breezy trade winds through the weekend, before strengthening to locally windy levels next week.

Discussion
Wet trade wind weather will continue today with gradual clearing from west to east across the state through tonight. The wet trades will linger over windward Maui County and the Big Island before normal trade wind weather sets in across the state Saturday. An upper level trough moving across the state has increased instability, while an old frontal boundary riding the trades has enhanced low level moisture. The wet weather, along with the copious amounts of rain in the recent past, warrants continuation of the flash flood watch through this afternoon. Mostly cloudy to overcast conditions will prevail this morning with some clearing over Kauai and Oahu this afternoon as the upper trough moves slowly east. These layered mid and high clouds are bringing a wintry mix of weather to the Big Island summits, and a winter storm warning remains through tonight.

Current radar shows diminishing showers pushing into windward areas of Kauai, while showers continue over windward areas of Oahu, Maui County and the Big Island. The forecast calls for showers diminishing from west to east today with lingering enhanced moisture over windward Maui and the Big Island into Saturday.

A surface ridge to the north will maintain moderate to locally breezy trade winds through the weekend. High pressure will strengthen Monday bringing trades to locally windy levels through Wednesday. Overall, a fairly typical trade wind pattern is expected through the period, with bands of enhanced clouds and showers pushing through the islands from time to time. Showers will focus over windward and mauka areas with some showers riding the strong trades to leeward locations.

Aviation
VFR conditions prevail at most TAF sites with occasional MVFR conditions being reported at windward terminals such as PHLI and PHTO in low clouds and shra. Expect these conditions to continue through the day at PHTO but improving conditions are likely at PHLI.

AIRMET Sierra is currently posted for mountain obscuration over N through E sections in low clouds and shra. Conditions may improve from W to E this afternoon. AIRMET Tango is currently posted for low level turbulence S through W of mountain ranges due to breezy trade winds. AIRMET Tango is also posted for upper level turbulence as a strong upper level jet moves over the area. AIRMET Zulu is currently posted for moderate rime and ice from Molokai eastward due to a deep layer of moisture. Conditions should gradually end over Maui County later today.

Marine
A surface high north of the area will move northeast through Saturday, and will continue to produce locally strong trade winds over Hawaiian waters. An upper level trough is keeping winds weaker than might be expected, will continue the possibility for a slight chance of thunderstorms with locally heavy downpours today.

A Small Craft Advisory (SCA) continues through early Saturday morning for the strong trade winds in the windier areas near Maui County and the Big Island. A new high building northwest of the area will cause the trade winds to strengthen starting this weekend. The SCA will likely be extended and expanded as this occurs.

Surf is expected to remain below High Surf Advisory (HSA) thresholds along all shores through this weekend. Rough surf will persist along east facing shores through Sunday night due to the trade winds, but heights will remain moderate through the weekend. As trade winds strengthen early next week, surf may approach the HSA threshold.

A moderate northwest swell is expected to build today. A slightly larger swell will peak Sunday and another moderate swell is possible next Thursday.

A series of small swells from the southern hemisphere will maintain modest surf along south facing shores through this weekend. Lows in the southern hemisphere may strengthen and shift east enough to produce some bigger surf along south facing shores by the end of next week.

See the latest Oahu Surf Discussion (SRDHFO) for additional details on surf and swell.

Watches/Warnings/Advisories
Flash Flood Watch until 6 PM HST this evening for Niihau, Kauai, Oahu, Molokai, Lanai, Kahoolawe, Maui, Kona, South Big Island, Big Island North and East, Kohala, Big Island Interior.

Winter Storm Warning until 6 AM HST Saturday for Big Island Summits.

Small Craft Advisory until 6 AM HST Saturday for Maalaea Bay, Pailolo Channel, Alenuihaha Channel, Big Island Leeward Waters, Big Island Southeast Waters.


Morrison...Discussion

Burke...Aviation

M Ballard...Marine