Area Forecast Discussion
Issued by NWS Honolulu, HI
Issued by NWS Honolulu, HI
063 FXHW60 PHFO 070142 AFDHFO Area Forecast Discussion National Weather Service Honolulu HI 342 PM HST Mon May 6 2024 .SYNOPSIS... Easterly trade winds will decrease to light to moderate levels through this week. A couple of upper-level disturbances and areas of enhanced low-level moisture will keep wetter than normal conditions in the forecast for the majority of this week and into the weekend. && .DISCUSSION... Current radar imagery shows scattered showers riding into the windward and mountain slopes of all islands this afternoon, with the occasional stronger shower spilling over into the leeward areas. Water vapor satellite imagery continues to show a weakening upper- level low slowly drifting from west to east across the island chain. Despite the instability associated with this feature, we have not yet seen any thunder over interior Big Island, though a slight chance remains into this evening as the low tracks eastward across the state. Strong winds around 600mb have prompted the issuance of a Wind Advisory for the Big Island Summits, in effect through tonight. At the surface, a 1029 mb high far north-northeast of the island chain will continue to weaken and move off to the east. Trade wind speeds will trend down over the next few days as a result, becoming light to moderate through the second half of the week with some sea/land breezes developing over terrain-sheltered leeward areas. Wetter than normal conditions remain in the forecast for all islands through the majority of this week. While Tuesday will trend a bit drier, wetter weather will return for Wednesday into this weekend as a couple of disturbances enter the region. Wednesday night and Thursday, the remnants of an old East Pacific cold front drift into the islands on the trade winds, providing a boost in low-level moisture. While the longer-term forecast is often associated with twiddle-twaddle and global model uncertainty, the GFS and ECMWF are in decent agreement about an upper-level trough diving down over the state Thursday night through Saturday, along with a surface trough reflection. The models still differ on the details, but this would keep enhanced showers in the forecast into the weekend, with the potential for some heavy downpours and slight chance of thunderstorms. Will continue to see how this scenario evolves over the next couple of days and refine the forecast as needed. && .AVIATION... Breezy easterly trade winds will persist through this evening and weaken to become moderate to locally breezy tomorrow. Clouds and showers will continue to favor windward and mauka locations, with MVFR conditions possible in any heavier showers. VFR conditions will prevail elsewhere. Although confidence is low, instability associated with an upper level low passing overhead could lead to an isolated thunderstorm or two over the Big Island early this evening. AIRMET Sierra is in effect for windward portions of each island and while there may be a brief lull in activity early this evening, it will likely continue through tonight. AIRMET Tango is in effect for moderate turbulence aloft (FL280-FL380) as the tail end of an upper level jet streak stretches overhead. Conditions should improve tonight as the upper level jet moves off to the east of the state. && .MARINE... Fresh to strong trade winds will slowly ease through Wednesday as a 1029 mb high 900 nm north northeast of the islands moves off to the northeast. A Small Craft Advisory (SCA) is in effect for the typically windy waters around the Big Island and Maui County, and as winds decline to moderate to locally fresh strength late Tuesday, the SCA should be dropped. A surface trough will form north of the state on Wednesday. As this feature drifts southward near or over the islands Thursday into Saturday, the trade winds could ease further or become disrupted, while chances for heavy showers will likely increase. After mainly surf for the next couple of days, High Surf Advisory (HSA) conditions are favored along south shores on Thursday and Friday. Buoys south of the state and guidance continue to suggest that the current south swell will remain less than 2 feet at 15 to 17 seconds in the short term, keeping south shore surf below seasonal average through Tuesday. Forerunners from a larger and longer lived south-southwest swell will arrive Wednesday, and surf is expected rise near, or to, the HSA level during the peak of the swell Thursday and Friday. This swell will gradually decline through the weekend. A small northwest swell of around 3 feet will continue to build through tonight, peak Tuesday, decline Wednesday. Wind-driven waves of 6 to 7 feet at 8 to 9 seconds will maintain rough east shore surf above the May average tonight. As trade winds decline over the next few days, these seas will gradually decline to around May average by Tuesday, then drop to below average for the rest of the week. && .HFO WATCHES/WARNINGS/ADVISORIES... Wind Advisory until 6 AM HST Tuesday for Big Island Summits. Small Craft Advisory until 6 PM HST Tuesday for Maalaea Bay- Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big Island Southeast Waters. && $$ DISCUSSION...TS AVIATION...Farris MARINE...Wroe