603 FXHW60 PHFO 041935 AFDHFO Area Forecast Discussion National Weather Service Honolulu HI 935 AM HST Sun Apr 4 2021 .SYNOPSIS... Breezy trade winds will persist for the next few days as strong high pressure holds far north of the state. A band of moisture will produce active showers along mainly windward portions of most islands today, then drift south of the state on Monday as a drier air mass moves in from the north. Trade winds will decrease on Wednesday and remain at moderate strength into next weekend, leading to typical trade wind weather. && .DISCUSSION... Breezy trade winds are in place, and a band of moisture is focusing rainfall over the central portion of the island chain this morning. A rather strong 1040 mb high parked over 1500 miles north of the state is driving the trades, while a nearly stationary cutoff low sits about 1300 miles northeast of the islands. A band of low-level moisture, the tail end of a dissipated front extending from the low, is draped over Oahu and Maui County and is creating persistent showers over windward areas. Expect this feature to creep southward today and lead to a decrease in rainfall on Oahu late this afternoon and an increase in showers on the Big Island, where a passing upper-level trough may trigger an afternoon thunderstorm over leeward and interior slopes. Showers have greatly diminished on Kauai, where a drier air mass is moving in from the north. Trade winds will hold at locally breezy strength during the next few days as the band of moisture creeps southward. Expect wet conditions for windward Maui and the Big Island tonight, where the passing upper-level trough could trigger a few heavy showers along with winter weather on the high Big Island summits. Reduced shower activity is due on Oahu and Kauai. As the moisture band sags southward on Monday, showers will linger on windward Big Island, while a somewhat dry and stable air mass settles over the rest of the island chain. Dew points will fall into the mid to upper 50s, and precipitable water will drop below an inch. As a result, there will be a cool feel to the breezy trades, and modest showers will become confined to windward terrain. Trade winds will likely drop a notch on Wednesday and remain at moderate strength into next weekend. The GFS and ECMWF show a slow moving mid- to upper-level trough just east of the state and a ridge to the west through Friday. The islands should remain just far enough away from the trough to produce a stable trade wind flow. Shallow bands of moisture will ride in on the trade winds and produce showers over mainly windward areas. && .AVIATION... Shower coverage has increased across the central islands due to a band of clouds that has moved into the area overnight. This band combined with breezy northeast trade winds is focusing most of the showers over windward and mountain locations, with some making it into leeward sections. MVFR CIGS/VSBYS can be expected in and around these passing showers, especially through the morning hours. Expect this general trend to continue tonight through Monday morning. AIRMET SIERRA for mountain obscuration remains in effect for windward sections of Maui, Molokai, and Oahu above 2000 ft due to clouds and showers. AIRMET TANGO will continue for moderate turbulence below 9000 ft for Maui and the Big Island and may need to be expanded to include the western end of the state later today. && .MARINE... Large high pressure north of the state is producing a tight enough southern pressure gradient down across the local waters to maintain moderate to fresh trade winds. A Small Craft Advisory (SCA) is now in effect through 6 PM HST Tuesday to account for locally strong trades across those typically windier areas around Maui and the Big Island. The very subtle strengthening and southern movement of the northern Pacific high will likely produce more areawide fresh trades later today into tonight. The combination of this boost in trades and an incoming higher northeast swell will result in elevated seas early in the week. Thus, to account for stronger trades and resultant rough seas, the ongoing SCA will likely need to be expanded westward to encompass more Hawaiian coastal waters. Mid week trades will begin to exhibit minor weakening with agitated seas lowering to under SCA heights from Thursday into next weekend. Recent ASCAT satellite passes have confirmed that a surface low about 1,500 nautical miles northeast of the islands is producing near gale to gale winds along its northern flank. Gales around the northwest to west periphery of this low as it moves northeast away from the area is generating a fetch region of higher swell that will be directed toward the state. Thus, the current 4 to 5 foot, low period north swell will gradually veer more northeast and build to greater than 7 feet the next couple days. North and east facing shore surf will respond with a steady rise with east facing surf likely reaching High Surf Advisory (HSA) thresholds by late Monday. East coast surf should then hold at HSA levels through Thursday with a slow decline expected from Friday into the weekend. A series of Southern Hemispheric gales will push several south swells towards the islands through the week. These small, long period swells will reach the islands through mid to late week and maintain near seasonal average surf along southern coastlines. && .HFO WATCHES/WARNINGS/ADVISORIES... Small Craft Advisory until 6 PM HST Tuesday for Maalaea Bay- Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big Island Southeast Waters. && $$ DISCUSSION...Wroe AVIATION...Gibbs MARINE...Blood