FORECAST DISCUSSION

Synopsis

Tropical Storm Darby will continue to approach the Big Island from the east overnight, and is expected to affect weather statewide into early next week. A tropical storm warning is in effect for the Big Island and Maui County. Tropical storm conditions are expected to begin late tonight on the Big Island, and on the islands of Maui County on Saturday. While considerable uncertainty remains as to the extent of the wind impacts over the islands, large surf along east facing shores and localized heavy rain, especially on the Big Island and Maui County, are expected during the weekend. Windy and wet weather is expected elsewhere across the island chain. After Darby moves away from the islands early next week, a typical trade wind pattern should resume with passing windward showers.

Discussion

At 11 PM HST, the Central Pacific Hurricane Center (CPHC) estimated the center of Tropical Storm Darby to be near 18.8N 153.0W. This position is about 150 miles ESE of Hilo, and 360 miles ESE of Honolulu. Darby is moving west near 10 mph, with maximum sustained winds near 60 mph. Tropical storm force winds extend out up to 125 miles from the center. Forecast grids and text products have been updated to reflect the 11 PM CPHC advisory. Outer rain bands on the western side of approaching Tropical Storm Darby are beginning to move across the Puna district of the Big Island late this evening, with additional rain bands moving across the coastal waters east of the Big Island. Scattered light to moderate showers are present over some windward areas elsewhere around the state, with dry conditions leeward. A tight pressure gradient between a surface high centered far NNW of the islands and approaching Darby is causing locally gusty trade winds. The strongest winds observed so far this evening have occurred at Kealakomo in Hawaii Volcanoes National Park, where north winds of 25 to 35 mph with gusts up to 52 mph are being caused by a combination of the approaching tropical storm and favorable terrain funneling. Bands of heavy showers and squalls associated with Darby are forecast to impact the windward side of the Big Island starting late tonight. Weather conditions will continue to deteriorate through Saturday as the core of the storm approaches the Big Island coast during the day. It should be noted, as we saw with Tropical Storm Iselle in 2014, damaging wind gusts could occur well away from the core of the system due to unusual local terrain effects like gap winds and channeling between the islands. Heavy rainfall associated with Darby will continue to spread northwest over the other islands during the weekend. As a result, a Flash Flood Watch remains in effect from late tonight through Sunday afternoon for Maui County and the Big Island, and for Saturday evening through Sunday afternoon for Oahu. Once Darby passes, expect muggy southeasterly flow to develop for a brief period early next week. Thereafter, warm and slightly less humid trade wind weather should return beginning on Tuesday as Darby moves farther away from the state and high pressure builds far to the northeast.

Aviation

Tropical Storm Darby is approaching from the east and will move onto the Big Island later tonight, Maui County Saturday and Saturday night and possibly Oahu late Saturday night based on the forecast track from the Central Pacific Hurricane Center. TC SIGMET Whiskey series for Darby will cover all or portions of the Big Island with the next issuance shortly after 09Z. This implies the potential for severe turbulence, icing and low level wind shear. These hazards could spread towards Maui on Saturday. AIRMET Sierra for tempo mountain obscuration for north through east sections of Maui. This will likely remain through the night as clouds and showers ahead of Darby push onto the windward slopes. AIRMET Tango continues for mechanical low level turbulence south through west of the mountains across the islands. This is due to the breezy trades and increasing gradient winds ahead of Darby. Elsewhere around the state tonight, we anticipate VFR conditions to prevail with isolated mountain obscurations for windward locations due to passing clouds and showers in the trade wind flow.

Marine

As Tropical Storm Darby moves across Hawaiian offshore and near shore waters, strong winds and building seas can be expected. A tropical storm warning remains in effect for marine zones from the Kaiwi channel eastward, and a tropical storm watch is in effect for the waters around Oahu. Boaters in those waters should seek safe harbor and secure their craft. A small craft advisory is posted for the waters around Kauai not covered by the tropical watches and warnings. Near shore windward buoys show increasing seas, ranging from 10 to 13 feet as of 11 PM. Increasing winds and high seas will continue spreading westward through the remaining coastal waters into Saturday, then continue to affect the waters through Sunday. As Darby departs the area early next week, seas will diminish and the weather will trend back toward summertime normals, with a typical trade wind weather pattern becoming established by Tuesday. Swells associated with the captured fetch from Darby are already affecting the islands, and a high surf warning is in place for east facing shores of Maui and the Big Island, with a high surf advisory in place for exposed east facing shores of the remaining islands. There is a possibility that we may have to expand the High Surf Warning to include east facing shores of islands currently in the advisory, but this is expected to hinge on the eventual track that Darby takes.

HFO Watches/Warnings/Advisories

Tropical Storm Warning for Molokai, Lanai, Kahoolawe, Maui, Big Island. Tropical Storm Watch for Oahu. Tropical Storm Warning for 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 Channel, Oahu Windward Waters, Oahu Leeward Waters. Flash Flood Watch through Sunday afternoon for Molokai, Lanai, Kahoolawe, Maui, Big Island. Flash Flood Watch from Saturday evening through Sunday afternoon for Oahu. High Surf Warning until 6 AM HST Sunday for east facing shores of Maui and the Big Island. High Surf Advisory until 6 AM HST Sunday for east facing shores of Kauai Oahu and Molokai. Small Craft Advisory until 6 AM HST Sunday for Kauai Northwest Waters, Kauai Windward Waters, Kauai Leeward Waters.

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DISCUSSION/MARINE...Jacobson AVIATION...Foster

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