Area Forecast Discussion for Hawaii

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Issued: Jul 23, 2016 4:06 PM HST

Synopsis
Darby will bring stormy weather at times this weekend as it tracks northwest, close to the Aloha State. More settled weather will return on Monday as Darby departs, leaving us in very warm and humid southeast flow for a couple of days. Much more pleasant tradewind weather is expected to return for the latter half of the week, as high pressure strengthens far northeast of the islands.

Discussion
Per the Tropical Cyclone Update from CPHC, the second tropical cyclone landfall on the Big Island in three years occurred at about 2 pm this afternoon, roughly 10 miles east-northeast of Pahala.

The moist circulation of Tropical Storm Darby interacting with the Big Island terrain resulted in a considerable amount of wind acceleration today in some spots, with several wind gauges recording wind gusts in excess of 50 mph. As the core of Darby interacts with the Big Island terrain, deep convection has been blowing up over and near the Kau district over the past few hours. That will continue to be the concern overnight, with additional heavy downpours or thunderstorms possible over the Big Island and then later for Maui county as Darby drags northwest. The confidence is certainly lower about how much of this moisture will make it to Oahu on Sunday and Sunday night, but it seems prudent to leave the Flash Flood Watch in effect until we have a better idea of how much of Darby will survive the trek across Mauna Loa.

Winds will continue to be a secondary threat but there can still be localized areas of strong winds or gusts in areas of strong downsloping and accelerated through gaps, as well as with any deep convective bursts that develop. Not everyone is going to see tropical-storm-force winds, but there will at least be some pockets of wind damage associated with Darby.

Once Darby passes, expect muggy southeasterly flow on Monday. Thereafter, warm but gradually less humid trade wind weather should return beginning midweek as Darby moves farther away from the state and high pressure builds far to the northeast.

Aviation
Darby made landfall this afternoon along the Kau Coast. TC SIGMET Whiskey series covers all of the Big Island. Within this area, it will not be unlikely to experience periods of severe turbulence, icing and low level turbulence, especially in the areas where convection is the deepest. Convection flaring up along the southeast coast of the Big Island and the adjacent waters are carrying CB tops upwards of 55 thousand feet. AIRMET Sierra for IFR conditions remains posted for the eastern half of the Big Island. This will likely remain for the next several hours as Darby moves WNW. Winds at PHTO have veered out of the southeast.

Elsewhere, AIRMET Sierra for mountain obscurations is posted for Maui, Molokai, Oahu, and western Big Island and will likely need to expand to Kauai tonight. Conditions over Maui and Molokai have improved slightly since earlier today and the IFR AIRMET was replaced with a mountain obscuration AIRMET. Prevailing MVFR is still expected for most areas into this evening. Conditions may deteriorate again tonight as Darby is expected to pass south of Maui County tonight. Expect windward Oahu to remain engulfed in MVFR conditions tonight and the chance for more prolonged periods of IFR to increase tonight. Expect PHNL and PHJR terminals to see passing showers through the night.

AIRMET TANGO remains posted for low level turbulence over and immediately south through west of the mountains and for strong surface winds associated with Darby over the Big Island, spreading to Maui tonight.

Marine
Winds and seas are going to continue to be hazardous in the vicinity of Darby for the rest of the weekend. The seas will be highest over windward waters where the upstream fetch was much longer, but should gradually be declining by Sunday over the Big Island waters. The seas will be climbing near the smaller islands as Darby starts tracking northwest, though the island terrain will block the highest seas from reaching shadowed leeward waters. The high surf headlines for east facing shores have been extended through Sunday. There will also be pockets of intense rainfall, very low visibilities, and embedded thunderstorms with frequent cloud-to-surface lightning associated with Darby this weekend as it continues toward the west-northwest.

Watches/Warnings/Advisories
Tropical Storm Warning for Oahu, Molokai, Lanai, Kahoolawe, Maui, Big Island.

Flash Flood Watch from 6 PM HST this evening through late Sunday night for Oahu.

Tropical Storm Watch for Niihau, Kauai.

High Surf Advisory until 6 PM HST Sunday for E facing shores of Kauai Oahu and Molokai.

Flash Flood Watch through Sunday afternoon for Molokai, Lanai, Kahoolawe, Maui, Big Island.

High Surf Warning until 6 PM HST Sunday for E facing shores of Maui and the Big Island.

Tropical Storm Warning for Kauai Channel, Oahu Windward Waters, Oahu Leeward Waters, Kaiwi Channel, Maui County Windward Waters, Maui County Leeward Waters, Maalaea Bay, Pailolo Channel, Alenuihaha Channel, Big Island Windward Waters, Big Island Leeward Waters, Big Island Southeast Waters.

Tropical Storm Watch for Kauai Northwest Waters, Kauai Windward Waters, Kauai Leeward Waters.


Discussion/Marine...R Ballard

Aviation...Eaton