354 FXHW60 PHFO 060139 AFDHFO Area Forecast Discussion National Weather Service Honolulu HI 339 PM HST Mon Apr 5 2021 .SYNOPSIS... Breezy trade winds will persist into Wednesday as strong high pressure holds far north of the state. Leeward areas will be mainly dry, and showers will be focused along windward slopes, especially on windward Big Island and Maui tonight. Trade winds will decrease late Wednesday and remain at moderate strength through Friday, leading to typical trade wind weather. Trades may diminish this weekend. && .DISCUSSION... Breezy trade winds persist, and somewhat wet conditions continue over windward portions of the Big Island, Maui, and Molokai. A 1041 mb high parked over 1400 miles north of the state is driving the breezy trade flow. A slow-moving and dissipating band of moisture remains hung up on windward Maui County and the Big Island, where about 1 to 2 inches of rainfall has been measured at numerous sites today. A drier and stable air mass has settled over Oahu and Kauai, where precipitable water is below an inch and a building ridge aloft is producing an inversion around 6500 ft. Expect an uptick in windward showers on Kauai and Oahu overnight as a shallow band of moisture reaches the state, while shower activity diminishes on Maui County. Windward Big Island should remain wet as the stubborn moisture in place slowly dissipates. Trade winds will hold at locally breezy strength on Tuesday, and perhaps into Wednesday. Dew points will fall into the mid to upper 50s, and the precipitable water values of less than an inch will spread over the entire island chain. As a result, there will be a cool feel to the breezy trades, and modest showers will become confined to windward terrain as shallow pockets of moisture move along the trade wind flow. Trade winds will likely drop a notch late Wednesday. The high to the north will weaken and a cut off low, currently centered about 1400 miles northeast of the state, will drift westward, causing the decline in winds. Trades will remain at moderate strength through Friday. The GFS and ECMWF show a slow moving mid- to upper-level trough just east of the state and an upper ridge to the west. The islands should remain just far enough away from the trough to produce a stable trade wind flow, with showers over mainly windward areas. There is some uncertainty for the weekend forecast. The GFS and ECMWF both show the cutoff low dissipating at the surface, and remnant moisture moving over the islands in a weakening trade wind flow. The ECMWF shows the nearby upper-level trough deepening and possibly moving over the islands, while the GFS is keeping this feature a little farther east. Heavy rainfall is possible, but it is too far out to tell at this time. && .AVIATION... Surface high pressure north of the state will continue to generate breezy trades across the islands for the remainder of this evening into tonight. Thus, isolated to scattered showers will mainly affect the windward coasts and slopes. This activity will likely bring periods of MVFR ceilings and visibility, especially where these clouds bank up against higher terrain. AIRMET Sierra remains in effect for north thru east sections of all islands above 2500 feet due to tempo mountain obscuration from clouds and showers. AIRMET TANGO remains in effect below 9000 feet for areas over and immediately south through west of mountains due to tempo moderate turbulence. && .MARINE... High pressure far to the north will maintain fresh to strong northeast trade winds into Wednesday before gradually dropping to moderate levels Thursday as the high weakens. A Small Craft Advisory (SCA) is in effect for all coastal waters for winds and seas through Tuesday. As winds begin to weaken Tuesday night, some zones may drop out of the SCA. A gale low centered near 31N146W is expected to retrograde to the southwest over the next couple of days, sending an 11-second north-northeast swell toward the islands. Current buoy observations show this swell is filling in this afternoon. Further increase of swell and surf is expected tonight with a peak Tuesday, followed by a gradual decline into the weekend. Surf heights will be further enhanced by the wind waves generated from the gusty trades. A High Surf Advisory has been issued through Wednesday afternoon for east facing shores exposed to this swell. An increase in surf is also possible for the northern beaches on Lanai and along the shoreline north of Keahole Point on the Big Island. This north-northeast swell is also expected to produce surges in north facing harbors, especially Hilo and Kahului, through at least Wednesday, though likely longer. Breaking waves near harbor entrances will also make for hazardous passage into and out of the port. A Marine Weather Statement is in effect to highlight these hazards. Background, long-period southerly swells will bring small surf to south facing shores through the week. A slight increase in southerly surf is possible by the weekend. && .HFO WATCHES/WARNINGS/ADVISORIES... High Surf Advisory from 6 PM this evening to 6 PM HST Wednesday for east facing shores of Kauai, Oahu, Molokai, Maui, and the Big Island. Small Craft Advisory until 6 PM HST Tuesday for all Hawaiian waters. && $$ DISCUSSION...Wroe AVIATION...Thomas MARINE...TS