| Current Conditions | ||
| Temp |  | 4.2 C |
| RH | | 10 % |
| Wind | | NE 7 mph |
| Road | | Closed |
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Hawaiian Islands Synoptic Discussion and Guidance 991 FXHW60 PHFO 171415 AFDHFO Area Forecast Discussion National Weather Service Honolulu HI 415 AM HST Sat Aug 17 2019 .SYNOPSIS... A surface ridge north of the islands will maintain moderate trade winds through today. Winds will decrease Sunday and Monday, with an uptick expected for the middle of next week. Increased shower activity and muggy conditions are expected starting late Tuesday through Thursday. && .DISCUSSION... There is little or no change in forecast philosophy since last evening. A 1029 mb high near 38N143W, far northeast of the main Hawaiian Islands, along with an associated ridge extending through 30N150W and 26N170W, continues to drive moderate trade winds across the area this morning. High clouds along the southern flank of an upper trough cover skies above the Big Island, while low clouds brought in by the trades continue to concentrate along windward and mauka sections of all the islands. Radar shows scattered to numerous moderate showers embedded within trade flow also favoring windward and mauka areas. Models show the upper trough will move east over the next day or so, likely causing the high clouds to shift east of the state by Sunday. The high will shift eastward over the next few days, causing the ridge to our north to elongate and weaken. As a result, trade winds will decrease today through early next week. By late Wednesday, another high will build in from the northwest, allowing trade winds to increase from midweek into next weekend. Models show a weak trough will move past the islands around midweek, likely bumping up available moisture for showers. Details concerning how much moisture will be available and how far up the island chain the additional showers will develop remain to be worked out. No strong upper trough is expected during this time, so at this point we are only forecasting an increase in trade shower coverage from midweek onward. && .AVIATION... High pressure northeast of the state will keep moderate to locally breezy trade winds in place through tonight. Low clouds and showers will favor windward and mauka areas, bringing occasional MVFR cigs/vsbys. Showers will be most prevalent during the pre-dawn and early morning hours, with improving conditions expected by mid-morning. AIRMET Sierra is in effect for mountain obscuration across windward sections of Oahu, Molokai, Maui and the Big Island. These conditions are expected to improve by mid-morning. && .MARINE... A surface high pressure system far northeast of the state will gradually weaken this weekend, which will cause the trade winds to diminish slightly through early next week. Therefore, the Small Craft Advisory (SCA), which was in effect over the typically windier waters adjacent to the islands of Maui County and the Big Island, has been cancelled this morning. A new surface high pressure system building far north of the state by the middle of next week is expected to cause the trade winds to gradually strengthen starting next Wednesday. Small south and southwest swells will maintain background summertime surf along south facing shores this weekend. A large, long-period south-southwest swell arriving Monday is expected to peak Tuesday, then gradually lower through the rest of next week. Surf heights are expected to reach the High Surf Advisory (HSA) criteria along south facing shores from Monday night through Tuesday. The current west swell produced by tropical cyclones in the Northwest Pacific will continue to slowly decline through Monday. Surf heights produced by this lingering swell will be largest along shorelines with a westerly exposure. A small northwest swell will be possible from next Thursday through Friday. Modest surf will persist along east facing shores today, then will slowly subside from Sunday through early next week as the trade winds weaken. For additional details on swells and surf, please see the latest Collaborative Nearshore Swell and Wind Forecast for Oahu (SRDHFO) product. && .HFO WATCHES/WARNINGS/ADVISORIES... None. && $$ DISCUSSION...Powell AVIATION...Jelsema MARINE...Houston Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office |
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