| Current Conditions | ||
| Temp |  | 6.8 C |
| RH | | 24 % |
| Wind | | NNE 19 mph |
| Road | | Closed |
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Hawaiian Islands Synoptic Discussion and Guidance 327 FXHW60 PHFO 180218 AFDHFO Area Forecast Discussion National Weather Service Honolulu HI 418 PM HST Sat Aug 17 2019 .SYNOPSIS... The high pressure ridge north of the Hawaiian Islands will weaken today with lighter trade winds forecast on Sunday and more variable winds expected on Monday. Trade wind speeds will trend higher by the middle of next week as the high pressure ridge builds north of the state. Additional moisture will arrive mainly affecting the windward slopes of the southeastern islands with elevated humidity levels and enhanced shower activity as two low level troughs pass south of the state. Moderate to locally breezy trade winds will continue into next weekend. && .DISCUSSION... Only subtle changes were introduced to the forecast since the mornings grids were issued. The trade wind speeds were increased slightly for tomorrow due to a slightly stronger ridge. Precipitation chances for next week were decreased, mainly over the islands just west of Maui as the tropical disturbance appears to track further south than previous guidance. A mid-latitude trough passing through the Central Pacific will weaken the high pressure ridge north of the islands decreasing trade wind speeds each day from Sunday to Monday. Onshore sea breezes caused by day time surface heating will develop over each island with offshore land breezes each night, especially along the more sheltered west facing slopes. Drier trends are forecast for the next few days under the subsidence from a strong upper level ridge. The trade wind subsidence inversion heights will range from 5000 to 7000 feet through Monday afternoon. By Monday evening the weather pattern begins to change as additional low level troughs move into the Hawaii region. The first of these trough arrives from Monday night through Tuesday morning with some enhanced showers developing mainly along windward slopes of each island. A second tropical disturbance is now forecast to pass south of the Big Island on Tuesday night with potentially less impacts on rainfall and humidity levels across the islands by Wednesday. Trade wind speeds will increase starting on Tuesday, as the ridge builds north of the islands and this trough passes to the south of the state. Moderate to locally breezy trades are forecast from Wednesday onward. Higher chances for elevated moisture levels and showers will linger over the Big Island from Tuesday to Thursday with lower precipitation impacts for the rest of the state. We will probably trend precipitation chances downward, in the afternoon forecast grids, across the western islands for the Wednesday through Thursday time periods. Moderate to locally breezy trades are forecast to continue on Friday and next Saturday as the high pressure ridge builds in north of the islands. An upper level cut off low drops in from the north likely enhancing shower regional shower activity especially in the overnight and early morning hours. && .AVIATION... A 1028 mb surface high far to the northeast of the state will maintain light to moderate easterly trade winds through this evening. This flow will help generate isolated showers along the windward slopes and coastal areas, which are then forecast to become scattered after sun down. Brief MVFR conditions will remain possible tonight due to reduced visibility and mountain obscuration from low clouds and showers. No AIRMETs are currently in effect. && .MARINE... A surface high pressure system far northeast of the state will gradually weaken tonight, which will cause the trade winds to diminish slightly through early next week. 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. We may see the return of Small Craft Advisory winds for the typical windy areas around Maui and the Big Island during the second half of next week. Currently, we have mainly background energy from the south and southeast along south facing shores. A small bump is expected Sunday as a small southwest swell fills in. A large, long-period south-southwest swell arriving Monday is expected to peak Tuesday, then gradually lower through the rest of next week. Latest observations from the Pago Pago buoy is indicating the swell running higher than the WaveWatch 3 guidance with energy similar to the July 14th high surf event. This suggests surf heights may near High Surf Warning levels during its peak along south facing shores on Tuesday. The current west swell produced by tropical cyclones in the Northwest Pacific will continue to slowly decline through early next week. 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...Bohlin AVIATION...JT MARINE...Kino Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office |
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