Honolulu, Hawaii

An approaching disturbance aloft combined with ample moisture and a surface trough moving over the western end of the state have triggered heavy showers and isolated thunderstorms over Kauai and Niihau and may impact Oahu overnight. The tail end of a front will move down the island chain late Sunday through Monday, bringing breezy northeasterly trades and drier air in its wake. Breezy easterly trades will bring periods of windward and mauka showers through the rest of the week.

Heavy rain continues over the Ko`olau range, especially from around Punalu`u southeastward. Most of the heavy rain has ended over Kauai, but flooding continues as runoff hasn`t ended yet. A large area of showers, some quite strong, is moving east through the Kauai Channel. This area should reach Oahu in the next few hours, but at this time we don`t know if the heavier showers will hit the island or stay south. Some high resolution guidance shows it will, while some shows it remaining south. No changes will be made to the forecast at this time.

Trade wind showers anchored along the eastern slopes of Kauais mountains have persistently produced heavy rain through the day in a moist and unstable environment, as evidenced in this afternoons upper air sounding at Lihue. Rainfall rates and gage stream reports prompted the issuance of a Flash Flood Warning for the entire island, with the main trouble spots including the Hanalei Bridge along the Kuhio Highway and the South Fork of the Wailua River. This warning remains in effect through 5:45 PM this evening and will likely be extended. Heavy showers and isolated thunderstorms firing along a surface trough edging over Niihau and Kauai this afternoon will likely maintain the flood threat for the next couple of hours. Showers have also been persistent across Oahu through the day, but rainfall rates and stream gage responses have been unimpressive so far. However, the heavy showers and isolated thunderstorms currently over Kauai may impact Oahu overnight into Sunday morning as an upper level trough helps drive them eastward in advance of an approaching front, depending on how well the surface trough and the various outflow boundaries hold together.

The next pattern change arrives late Sunday into Sunday night as the tail end of a shallow front pushes down the island chain before dissipating. While this boundary will deliver an uptick in showers to windward and north-facing slopes, its primary impact will be the development of breezy and gusty northeast trade winds spreading from west to east. Deep high pressure building behind the front will usher in a drier, more stable air mass. This will lead to a break from the muggy conditions weve been experiencing this past week as dew points are expected to drop into the lower 60s on Monday and Tuesday.

Breezy and stable easterly trades will persist through much of the workweek as the northern high becomes elongated in response to broad low pressure system tracking across the North Pacific. Dew points will gradually rebound closer to seasonal norms by midweek. While rainfall will remain largely focused along windward slopes with only brief leeward drift, a minor uptick in shower coverage appears possible around midweek as trade moisture interacts with added instability from a passing weak mid-level shortwave trough.

Heading into next weekend, forecast attention shifts back to the tropics. While significant spread exists regarding timing and track details, model guidance continues to hint at potential tropical cyclone development far southeast of the state late in the week, with the system potentially nearing or passing south of the islands over the weekend. While background trades look to hold, questions remain concerning their ultimate strength and how much deep moisture is drawn into the region. The National Hurricane Center continues to monitor this area, maintaining a medium (40 percent) chance of tropical cyclone formation over the next 7 days.

Scattered showers currently between Kauai and Oahu will start to push towards PHNL and PHJR (and the rest of Oahu) over the next couple of hours. Persistent MVFR and some IFR ceilings and visibilities are possible in the heavier showers. If these hold together, they could affect Molokai, Lanai, and Maui by morning.

AIRMET Sierra remains in effect for IFR conditions on Kauai which may need to be extended into Oahu. AIRMET Sierra is also in effect for mountain obscuration on Oahu and windward areas of the Big Island due to low ceilings and reduced visibility in showers.

A High Surf Advisory has been posted for north and west facing shores due to an incoming northwest swell. Additionally, a Small Craft Advisory has been issued for all coastal waters due to increasing winds and seas.

A trough to the northwest of Kauai will sink southward overnight, allowing high pressure to the north to strengthen winds behind the trough over the region. An incoming north-northwest (330 degrees) swell will push seas in excess of 10 feet for exposed waters, and the strengthening winds will bring SCA level winds to additional areas. Moderate to fresh northeast trades are expected to continue through much of the week.

The forerunners from the new moderate, long-period northwest swell are expected to arrive in the next couple of hours, with the swell building quickly overnight. By Sunday, advisory level surf is expected for north and west facing shores, which is expected to continue into Monday. The swell may shift to a slightly more northerly direction on Monday before declining on Tuesday. Depending on the exact direction, a Marine Weather Statement may be issued for potential harbor surges at Kahului and Hilo during that time. Another advisory level northwest swell is possible Thursday into Friday.

The current moderate, medium period south swell will gradually decline through the weekend. Surf above seasonal average returns to seasonal average on Sunday as the south swell energy fades. During the first half of next week, south shore surf will return to below seasonal averages.

East-facing shores will see an increase in trade wind swell as northeast winds return on Sunday. Surf along east-facing shores will increase through Monday night and conditions will be choppy as trades sustain through much of the week.

The anomalously wet conditions during the past few months and the rainfall from the past few days has kept the Keetch-Byram Drought Index (KBDI) below 500 across nearly all of the state. Fire weather concerns will be low through early Sunday as trade winds remain light and humidity elevated. Breezy and much drier trade winds will develop by Monday, but since KBDI will remain suppressed, fire weather conditions will be moderate.

High Surf Advisory from 6 AM Sunday to 6 PM HST Monday for Kauai East-Kauai North-Kauai Southwest-Koolau Windward-Maui Central Valley North-Maui Windward West-Molokai North-Molokai Windward-Niihau-Oahu North Shore-Waianae Coast-Windward Haleakala.

Small Craft Advisory from 6 AM Sunday to 6 PM HST Monday for Alenuihaha Channel-Big Island Leeward Waters-Big Island Southeast Waters-Big Island Windward Waters-Kaiwi Channel-Kauai Channel-Kauai Leeward Waters-Kauai Northwest Waters-Kauai Windward Waters-Maalaea Bay-Maui County Leeward Waters-Maui County Windward Waters-Oahu Leeward Waters-Oahu Windward Waters-Pailolo Channel.