926 FXHW60 PHFO 020146 AFDHFO Area Forecast Discussion National Weather Service Honolulu HI 346 PM HST Wed May 1 2019 .SYNOPSIS... Moderate trade winds will focus clouds and mainly modest showers along windward slopes through tonight, with higher rainfall chances on Kauai. Trades will steadily weaken on Thursday, allowing clouds and spotty showers to develop over leeward and interior sections in the afternoon. An unseasonably strong front will approach the state on Friday, leading to an unstable southerly flow that will produce increasing showers over most islands. The front will move down the island chain late Friday night through Sunday, likely bringing gusty westerly winds, heavy showers, and possibly thunderstorms. Cooler northwesterly winds are expected behind the front into early next week. && .DISCUSSION... A stable trade wind flow will remain in place through tonight. The trades are being driven by a weakening surface high centered nearly 900 miles north of the state, while a ridge aloft is maintaining stable conditions with an inversion holding between 5,500 and 8,000 ft. A band of low level moisture that has been draped over windward Kauai will maintain somewhat wet conditions on the Garden Isle through tonight, while mode rainfall pattern continues across the rest of the island chain. On Thursday, trades will steadily decline, and the mid to upper level ridge overhead will erode. The band of low level moisture should remain over Kauai and near Oahu, keeping shower chances higher over windward Kauai and Oahu. With trades dropping off, afternoon sea breezes could produce spotty showers over leeward and interior sections, especially over the western half of the state. An unstable southerly flow will develop on Thursday night and Friday as an unseasonably strong front approaches the state from the northwest. The upper level trough supporting the front will produce cooling aloft that will wipe out the inversion. The GFS and ECMWF are showing a developing prefrontal trough focusing moisture just west of Kauai, leaving the state under an unstable land and sea breeze regime with near normal precipitable water. Though there could be an isolated downpour at any time, showers over land will favor the afternoon hours. The front will reach Kauai as early as late Friday night then bring unsettled weather down the island chain through the weekend. Future fine tuning will be needed, but the front will likely be near Kauai in the predawn hours on Saturday, reach Oahu midday, move to Maui County Saturday afternoon and evening, then advance to the Big Island by Sunday morning. During this time, a potent upper level low will drop southward into the upper level trough and pass just north of the state. A jet stream along the southern flank of the upper level low should move over the islands and produce forcing that will generate heavy showers along the front. At this time, the greatest amount of forcing appears to be to the northeast of the state, where a surface low could form. However, we will be looking to add thunderstorms to the weekend forecast, and either way, a line of heavy showers or thunderstorms along the front could be capable of producing strong, or even damaging, winds. Another item of note is the potential for gusty downsloping southwest to west winds along and just behind the front. At higher elevations, the high summits on the Big Island and Maui will almost certainly experience warning level winds. Cooler northwesterly winds will build over the state behind the front. Though this flow will generally be drier, the close proximity of the rather strong, late-season upper level low could maintain chances for showers into early next week. && .AVIATION... As a high pressure center passes by north of the islands, trade winds will be in the light to moderate range, with locally breezy periods, through tonight. An extensive area of clouds and showers remains over and upstream of Kauai, so AIRMET Sierra for mountain continues there, but has been dropped for Oahu due to improved conditions. VFR will predominate over the next 24 hours, though isolated areas of MVFR conditions in lower ceilings and passing showers are possible and will be mainly confined to windward and mauka sections state-wide. && .MARINE... A significant change in the island weather pattern is expected to evolve into the weekend, as the trade wind-supporting high to the N will be replaced by a deep-layer low that will push a front across the island chain. Moderate trade winds will be diminishing through Thursday as high pressure to the N moves NE and weakens. Thus, the Small Craft Advisory (SCA) for windier areas around Maui and the Big Island has been cancelled. A surface ridge will shift SE to near the Big Island Friday and Friday night as a developing low (and associated surface front) move toward the islands from the NW, resulting in light SE to S winds. The front is expected to move from NW to SE across the island chain over the weekend, with S to SW winds shifting to the NW after the front passes. Significant uncertainty remains regarding the vigor and placement of the associated convection, the strength of the wind, and (to a lesser degree) the timing of the frontal passage. Potential marine hazards include heavy downpours reducing visibility, and erratic gusty winds, while sustained W to NW winds may briefly reach 25 kt over the weekend, requiring a SCA for some zones. Eventual impacts are highly dependent on the track and intensity of the low. Longer range outlook into next week anticipates weak low pressure centered just N of the islands, with light to moderate W to NW winds prevailing. The developing low will send a short-period swell toward the islands from the NW on Sunday/Monday, with surf expected to remain below advisory levels as the swell peaks Sunday night. Model guidance has not been consistent in the depiction of the surface winds associated with the approaching low, leading to reduced confidence in the surf forecast. In the meantime, distant lows in the NW Pacific have generated small NW swells that will arrive tonight and Thursday, and persist through Saturday. Storminess deep in the S Pacific will send small S and SW swells our way, with an uptick in swell energy expected over the weekend, and maybe again early next week. The just-updated Oahu Surf Discussion (SRDHFO) contains details on the sources of the expected swells. && .HFO WATCHES/WARNINGS/ADVISORIES... None. && $$ DISCUSSION...Wroe AVIATION...TS MARINE...Birchard