West Kaua’i

Tonight: Partly cloudy with scattered showers. Lows around 74 near the shore to around 63 above 3000 feet. Northeast winds up to 10 mph in the evening becoming light. Chance of rain 20 percent.

Wednesday: Mostly sunny with scattered showers in the morning, then partly sunny with numerous showers in the afternoon. Highs 86 to 92 near the shore to around 77 above 3000 feet. Southeast winds up to 10 mph shifting to the south in the afternoon. Chance of rain 70 percent.

Wednesday Night: Partly cloudy. Scattered showers in the evening, then isolated showers after midnight. Lows 72 to 77 near the shore to around 63 above 3000 feet. Southeast winds up to 10 mph in the evening becoming light. Chance of rain 40 percent.

South Kaua’i

Tonight: Partly cloudy with scattered showers. Lows around 74. Northeast winds 10 to 15 mph decreasing to up to 10 mph after midnight. Chance of rain 20 percent.

Wednesday: Mostly sunny in the morning then becoming partly sunny. Scattered showers. Highs 83 to 91. Light winds becoming southeast around 10 mph in the afternoon. Chance of rain 50 percent.

Wednesday Night: Mostly cloudy with scattered showers. Lows around 75. Light winds becoming east up to 10 mph after midnight. Chance of rain 50 percent.

Kaua’i Mountains

Tonight: Partly cloudy with scattered showers. Lows 65 to 72 in the valleys to around 61 above 4000 feet. Northeast winds up to 10 mph in the evening becoming light. Chance of rain 20 percent.

Wednesday: Mostly sunny with scattered showers in the morning, then mostly cloudy with occasional showers in the afternoon. Highs 76 to 86 in the valleys to around 67 above 4000 feet. Light winds becoming southeast up to 10 mph in the afternoon. Chance of rain 80 percent.

Wednesday Night: Mostly cloudy with scattered showers. Lows 65 to 71 in the valleys to around 60 above 4000 feet. Light winds becoming southeast up to 15 mph after midnight. Chance of rain 50 percent.

North Kaua’i

Tonight: Partly cloudy with scattered showers. Lows 67 to 75. Northeast winds up to 10 mph in the evening becoming light. Chance of rain 20 percent.

Wednesday: Mostly sunny with scattered showers in the morning, then partly sunny with numerous showers in the afternoon. Highs 79 to 88. Light winds becoming northwest up to 10 mph in the afternoon. Chance of rain 70 percent.

Wednesday Night: Mostly cloudy in the evening then becoming partly cloudy. Scattered showers. Lows 67 to 75. Southeast winds up to 10 mph. Chance of rain 40 percent.

East Kaua’i

Tonight: Partly cloudy with scattered showers. Lows 66 to 78. North winds up to 15 mph. Chance of rain 20 percent.

Wednesday: Mostly sunny in the morning then becoming partly sunny. Scattered showers. Highs 76 to 89. Northeast winds up to 10 mph shifting to the southeast in the afternoon. Chance of rain 50 percent.

Wednesday Night: Mostly cloudy with scattered showers. Lows 67 to 79. East winds up to 10 mph. Chance of rain 50 percent.

Detailed Forecast

Synopsis

Showers spread over Windward Big Island and Maui tonight and then transition to locally heavy interior showers over Molokai through Kauai tomorrow afternoon. Locally breezy trades return Wednesday night.

Discussion

Today. Model analysis at 500mb places the Hawaiian Islands under the easternmost mid-level low associated with a mature omega block centered near the International Dateline. This low will will stagnate through the near term and gradually retrograde westward and away from the state during the end of the week. Beneath this upper low resides a strongly sheared and substantially weakened Tropical Storm Kiko which is advancing northwestward within SE low-level flow as upper-level flow simultaneously shears convection off to the north. This afternoon's observed soundings out of Lihue and Hilo indicate ample low-level moisture in the lowest 7kft or so, but still capped by dry mid-levels. Backing ENE to NNE winds over the western end of the state indicate influence from the outer portion of Kiko's envelope of low-level cyclonic flow. This light NNE gradient may yet force sea breeze convergence toward the southern portion of Oahu where limited afternoon showers are possible during the remainder of the afternoon. More robust convection exists on the Big Island where terrain has allowed access unstable mid and upper levels where the bulk of the 1000 j/kg of observed CAPE resides. These showers will diminish with the loss of heating this evening.
Tonight. The trailing band of moisture extending from TC Kiko to waters well east of the Big Island will advance west through the area tonight and Wednesday. This band resides along the convergent lead edge of high pressure that will build into the area during the medium range. A wide coverage of showers over windward portions of the Big Island and Maui can be expected tonight mainly after midnight and mainly over areas exposed to ESE flow. Locales such as western Maui and the Hamakua Coast of the Big Island likely observe little in the way of rainfall while amounts up to a couple of inches will be possible for other windward locations. Weak capping will favor localized heavy downpours, but the general expectation is for rain rates to remain below 1″ per hour.
Wednesday. This band of moisture will be positioned over the central portion of the state during the day Wednesday. Emerging ESE flow poses the primary uncertainty in the forecast as it is modeled to be nearly strong enough to wipe out the sea breeze influence, a necessity for low-level forcing tomorrow. Even so, the GFS, EC and the suite of hi-res models maintain a strong consensus that afternoon convection over the western end of the state, particularly Oahu, could be fairly vigorous during peak heating Wednesday afternoon. Forecast soundings indicate plenty of dry air in the column that will need to be overcome, but areas of preferentially strong convergence or persistent convection could nontehless capitalize on resident elevated instability and produce locally very heavy downpours. This is borne out in explicit depictions from the CAMs as well, which simulate echo tops to over 35kft. Weakly convergent ESE flow through about 700mb also poses the threat for island plume interaction from Molokai which would elevate any flooding threat over Oahu. At this time, will hold off on inclusion of thunder over Oahu and Kauai on Wednesday, but it is within the realm of possibility. All told, an uncertain flooding threat perhaps 6 hours in duration during peak heating will exist over Oahu and Kauai on Wednesday.
Extended. Locally breezy trades then return from Wednesday night onward while building upper riding provides ample stability. Showers focused windward and mauka during this time, mainly during the overnight and early morning hours.

Aviation

As Tropical Storm Kiko continues to pass north of the islands through Wednesday, trade winds will remain disrupted in places, and a moist and somewhat unstable air mass will persist. Daytime sea breezes are producing areas of MVFR ceilings and visibility in isolated +SHRA/TSRA over interior Big Island and Maui this afternoon, but so far, AIRMET Sierra for mountain obscuration is not needed. This shower activity is expected to diminish overnight as diurnally driven convection diminishes. A slightly drier north to northeasterly flow over Oahu and Kauai has not been as active with showers and MVFR conditions today. Winds on these islands will become variable overnight, and some MVFR conditions could form along windward and north facing terrain. On Wednesday, expect showers and potential for MVFR conditions in +SHRA to be greatest over Kauai and Oahu, while trades slowly rebuild over Big Island and Maui.

Marine

Tropical Storm Kiko is about 125 NM NE of Oahu this afternoon and continues to track west-northwest. Kiko continues to disrupt the tradewind flow with gentle to moderate winds expected across most of the coastal waters tonight. As Kiko continues to track westward, moderate to locally fresh east to southeast trades will return starting with the Big Island late tonight into Wednesday morning and gradually spread westward.
Kiko continues to produce Tropical Storm conditions across the northern offshore waters. A Tropical Storm Warning remains in effect through tonight but there are still some uncertainty on the intensity of Kiko for Wednesday. Latest forecast from NHC shows Kiko weakening to a Tropical Depression by Wednesday morning. For the coastal waters, due to the swells from Kiko coming in above guidance around Kauai and Oahu, a Small Craft Advisory is in effect for the exposed waters around Kauai and Oahu through tonight. Although no wind impacts are expected over the coastal waters, Small Craft Advisory level winds could return for our typical windy areas around Maui and the Big Island late Wednesday afternoon into Thursday.
The latest buoy observations from Mokapu show the main swell energy focused across the 10-14 second spectral bands this afternoon. This swell will likely peak late this afternoon into the evening across Oahu and peak across Kauai later tonight. The High Surf Warning has been extended for Kauai and Oahu through tonight. Surf heights should drop below warning thresholds by Wednesday morning, but advisory level surf could linger around Kauai through parts of the Wednesday (on a steady decline). Meanwhile over the Big Island, the swell from Kiko did not get as large as anticipated and is on a steady decline this afternoon. Thus, the High Surf Warning was cancelled for the Big Island. For Maui County, surf heights are likely currently peaking at advisory thresholds, thus a High Surf Advisory has been issued through tonight.
A reinforcing swell out of the south-southwest arrived today and will be on a slow decline Wednesday into Thursday. Surf heights should hold near the summer average through Wednesday then drop below average by Thursday. Another small bump from the south is possible on Friday. Additionally, multiple small pules of northwest to north-northwest swell energy will generate small surf along north facing beaches throughout the week and into the weekend. A short- lived northeast swell is possible for select exposures as Kiko passes to our north.

Fire weather

High humidity and light to moderate winds continue for the next couple of days with scattered showers focused over island interiors during the afternoon. Locally breezy trades then return Wednesday night as humidity falls to typical values.

HFO Watches/Warnings/Advisories

High Surf Warning until 6 AM HST Wednesday for Olomana, Kauai East, Kauai South, East Honolulu, Koolau Windward.
High Surf Advisory until 6 AM HST Wednesday for Maui Windward West, Molokai Windward, Molokai Southeast, Windward Haleakala, Kipahulu.
Small Craft Advisory until 6 AM HST Wednesday for Kauai Northwest Waters, Kauai Windward Waters, Kauai Channel, Oahu Windward Waters.

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