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Menu of Text Products for the Hawaiian Islands and the Tropical Pacific/Atlantic Oceans: Select a text product: Hawaiian Islands Synoptic Discussion and Guidance Hawaiian Islands 7-day Zone Forecast Urgent Weather Message Special Marine Warning Terminal Aerodome Foreasts (TAFS) Area Forecast for Hawaii and Adjacent Coastal Waters AIRMETs for IFR Conditions, Turbulence, and Icing Oahu Surf Observation and Forecast/Coastal Forecast for Hawaiian Waters Offshore Forecast for Hawaiian Waters North Pacific High Seas Forecast South Pacific High Seas Forecast Hawaiian Islands Automated Surface Observation System Hourly Weather Roundup Hawaiian Islands Wind Data Hawaiian Islands Rainfall Summary Daily Hawaii Cooperative Observations

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Hawaiian Islands Synoptic Discussion and Guidance 151 FXHW60 PHFO 120214 AFDHFO Area Forecast Discussion National Weather Service Honolulu HI 414 PM HST Tue Dec 11 2018 .SYNOPSIS... Windy trades will continue through midweek as a series of highs pass to the north of the islands. The trades will slowly ease late in the week into the weekend. A surface trough developing near the islands early next week may give us light winds by Monday. In the meantime, the trades will be wet for some spots, as areas of moisture moving over the islands enhance rainfall, especially for Oahu and Maui. Trade winds are expected to gradually trend drier for the latter half of the week. && .DISCUSSION... An earlier ASCAT pass showed that the background winds around the islands have come down a bit, but still remain locally windy. These trades are due to a strong subtropical ridge extending roughly W-to-E about 700 miles N of Honolulu. Although the trades may ease a little further overnight as the ridge weakens slightly, we could still see some localized spots reaching Wind Advisory criteria. Big Island summit winds are still solidly at Wind Advisory levels as well, due to a tightening pressure gradient between a large mid-level anticyclone to the NW of the islands, and a mid-level low far to the E of the Big Island. Models indicate the summit winds should start to come down by daybreak Wednesday. Another in a series of strong migratory surface highs will pass well N of the islands Thu into Thu night, with another peak in the blustery trades. There's some chance we will need another Wind Advisory for some areas for that time period, though this high looks slightly weaker and a little farther away than the latest high. After that, the winds back off a bit, back to breezy-to- locally windy levels as the ridge weakens and yet another weaker high passes N of the islands about Saturday. Models show the winds weakening even more through the remainder of the weekend and into early next week as the subtropical ridge weakens and pushes S closer to the islands. By Monday, there's decent agreement in the models that a weak surface trough will develop near the islands, although the precise location of this feature is in question. If this holds, we'll see more areas with gentle land- and sea- breezes setting up early next week in a locally breezy ESE background flow across the windward waters. Models are in good agreement that the next cold front starts to come down the chain on Tuesday with lower dewpoints and brisk N or NNE winds. Moisture is rather limited near the islands, with CIMSS MIMIC-TPW showing values near and upstream at or below 1.2 inches. Nevertheless, areas of moisture convergence interacting with our local terrain, along with a relatively weak subsidence inversion, have been wringing out impressive rainfall in some localized spots. Open-celled showery cumulus should continue to affect Kauai and Oahu tonight, while other windward areas should be quite a bit drier. A gradual trend toward somewhat drier trades statewide appears likely over the next few days as the 700 mb temperatures warm (suggesting a stronger subsidence inversion) and upstream moisture decreases further (PW dropping below 1" for most areas by Thu night). The front on Tuesday does not appear to be too wet at this point, with very limited moisture to work with. && .MARINE... Strong high pressure is currently centered far north of the area. This places the islands in a rather strong trade wind environment. Winds are expected to drop off slightly on Wednesday, but increase once again on Thursday as a new strong high builds in to our north. Winds will gradually drop off Friday through Sunday as the high to our north weakens. A Gale Warning is currently in effect for the Alenuihaha and Pailolo Channels as well as Maalaea Bay through tonight. A Small Craft Advisory (SCA) in effect for the rest of the Hawaiian Coastal Waters due to a combination of strong winds and elevated seas. Another round of Gales is possible Thursday and Thursday night for the Alenuihaha and Pailolo Channels as well as Maalaea Bay as the new high builds in. The current northwest swell will continue to lower tonight but a new moderate size northwest swell will gradually fill in overnight. This swell is expected to peak on Wednesday with surf heights expected to be just below High Surf Advisory (HSA) levels. This swell will lower gradually Thursday and Thursday night. Another smaller northwest swell is expected during the Thursday night through Saturday time period. Another moderate size northwest swell is expected to arrive late Saturday and Saturday night, peak on Sunday, then lower gradually into early next week. Advisory level surf is expected with this swell. An even larger northwest swell is expected Monday night and Tuesday with surf heights possibly reaching warning levels. A HSA is posted along east facing shores due to short period choppy surf produced by the strong trades. These conditions are expected to continue through the remainder of the week. && .AVIATION... A strong surface high north northeast of Hawaii will maintain a tight pressure gradient across the region late this afternoon and into tonight, fueling a breezy to windy trade wind pattern. AIRMET Tango remains in effect for all areas below 9000 feet south and west of the mountain ranges due to expected TEMPO moderate turbulence. For Lanai, the Alenuihaha Channel, and water south of the Big Island, surface winds greater than 30 KT are forecast through this afternoon and will likely continue tonight. Moisture caught up in the trade wind flow will bring scattered showers to mainly the windward slopes and mountain areas over the next 24 hours. Expect brief MVFR conditions associated with the passing showers, but mainly for airports on the windward coasts. && .HFO WATCHES/WARNINGS/ADVISORIES... High Surf Advisory until 6 AM HST Friday for east facing shores. Wind Advisory until 6 AM HST Wednesday for Lanai-Kahoolawe-Maui Central Valley-South Big Island-Big Island North and East-Kohala- Big Island Interior-Big Island Summits. Small Craft Advisory until 6 PM HST Friday for Kauai Northwest Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel- Oahu Windward Waters-Oahu Leeward Waters-Kaiwi Channel-Maui County Windward Waters-Maui County Leeward Waters-Big Island Windward Waters-Big Island Leeward Waters-Big Island Southeast Waters. Gale Warning until 6 AM HST Wednesday for Maalaea Bay-Pailolo Channel-Alenuihaha Channel. && $$ R Ballard/Burke/JT Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office

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