West Kaua’i

Tonight: Partly cloudy in the evening, then mostly cloudy with scattered showers after midnight. Lows around 67 near the shore to around 58 above 3000 feet. East winds up to 15 mph. Chance of rain 50 percent.

Wednesday: Cloudy. Numerous showers in the morning, then occasional showers and slight chance of thunderstorms in the afternoon. Haze in the afternoon. Highs around 80 near the shore to around 68 above 3000 feet. South winds up to 15 mph shifting to the southwest around 15 mph in the afternoon. Chance of rain 90 percent.

Wednesday Night: Mostly cloudy. Occasional showers in the evening, then scattered showers after midnight. Lows around 65 near the shore to around 56 above 3000 feet. Northwest winds 10 to 15 mph. Chance of rain 90 percent.

South Kaua’i

Tonight: Partly cloudy in the evening, then mostly cloudy with scattered showers after midnight. Lows around 67. Light winds. Chance of rain 50 percent.

Wednesday: Cloudy and breezy. Numerous showers in the morning, then occasional showers and slight chance of thunderstorms in the afternoon. Haze through the day. Highs 75 to 82. Southwest winds up to 20 mph increasing to 10 to 20 mph in the afternoon. Chance of rain 90 percent.

Wednesday Night: Mostly cloudy. Occasional showers in the evening, then scattered showers after midnight. Lows around 65. Northwest winds 10 to 15 mph. Chance of rain 90 percent.

Kaua’i Mountains

Tonight: Partly cloudy in the evening, then mostly cloudy with scattered showers after midnight. Lows 59 to 66 in the valleys to around 55 above 4000 feet. South winds up to 15 mph. Chance of rain 50 percent.

Wednesday: Cloudy and breezy. Numerous showers in the morning, then occasional showers and slight chance of thunderstorms in the afternoon. Haze in the afternoon. Highs 70 to 77 in the valleys to around 61 above 4000 feet. Southwest winds up to 20 mph. Chance of rain 90 percent.

Wednesday Night: Breezy. Occasional showers in the evening, then scattered showers after midnight. Lows 57 to 63 in the valleys to around 52 above 4000 feet. Northwest winds 10 to 20 mph. Chance of rain near 100 percent.

North Kaua’i

Tonight: Partly cloudy in the evening, then mostly cloudy with isolated showers after midnight. Lows 61 to 68. South winds up to 15 mph. Chance of rain 20 percent.

Wednesday: Mostly cloudy. Breezy. Scattered showers in the morning, then occasional showers and slight chance of thunderstorms in the afternoon. Haze in the afternoon. Highs 72 to 80. South winds up to 20 mph shifting to the west in the afternoon. Chance of rain 90 percent.

Wednesday Night: Mostly cloudy. Occasional showers in the evening, then scattered showers after midnight. Lows 58 to 67. Northwest winds 10 to 15 mph. Chance of rain 90 percent.

East Kaua’i

Tonight: Partly cloudy in the evening, then mostly cloudy with scattered showers after midnight. Lows 60 to 71. Southwest winds up to 10 mph. Chance of rain 50 percent.

Wednesday: Mostly cloudy. Breezy. Numerous showers in the morning, then occasional showers and slight chance of thunderstorms in the afternoon. Haze in the afternoon. Highs 69 to 81. Southwest winds 10 to 25 mph. Chance of rain 90 percent.

Wednesday Night: Occasional showers in the evening, then scattered showers after midnight. Lows 58 to 68. Northwest winds 10 to 15 mph. Chance of rain near 100 percent.

Detailed Forecast

Synopsis

An approaching cold front will allow flow to veer southerly tonight, becoming breezy to locally windy by mid-day Wednesday. The front is forecast to quickly slide down the island chain Wednesday night into Thursday and bring a line of moderate locally heavy rainfall with it. Seasonally cool and dry weather will return late Friday through the weekend as high press becomes re- established. Another cold front may affect the state by Tuesday or Wednesday of next week.

Discussion

Visible satellite imagery this afternoon showed scattered low clouds, associated with a dissipated cold font, banking up along windward portions of Maui and the Big Island. Shallow afternoon build ups could also be noted over leeward Big Island due to residual frontal moisture. Aside from some spotty light showers within the areas mentioned above, dry conditions prevailed. Post frontal subsidence aloft brought mostly clear skies this afternoon over the western half of the of the state. Zooming further out on our satellite loop, the frontal band from our next cold front can be seen roughly 850 miles northwest of Kauai, pushing southeastward at a healthy clip.
Tonight, as the front edges closer to the islands, light to moderate easterly flow will begin to veer southeasterly and eventually southerly by Wednesday morning. Southerly and southwesterly winds begin to really ramp up during the late morning hours and peak in the afternoon/evening across the smaller islands. Widespread sustained winds 20 to 25 mph with higher gusts can be expected for most zones of the smaller islands. However, winds will strongest over the ridge tops and downslope of terrain where Wind Advisory criteria will likely be met. In addition to the winds, hi-res model guidance has been consistent over the last several runs with advecting prefrontal moisture with scattered to numerous showers northward over mainly Kauai and Oahu. Maui County and the Big Island, still under subsidence aloft, should remain mostly dry on Wednesday.
Both the GFS and ECMWF are nearly identical in timing, moving the front and associated rain band quickly across Kauai Wednesday evening, Oahu and Maui County Wednesday night into Thursday morning, and through the Big Island by Thursday afternoon. Though some pockets of heavier rainfall may be embedded in the line, the speed of the system should help mitigate the flash flooding threat. That being said, minor flooding or ponding in poor drainage areas remain possible. A slight chance of thunderstorms has been added to the forecast for Kauai and waters northward Wednesday evening and then Oahu and waters northward Wednesday night to account for marginal instability along the frontal rainband. Similar to Monday's system, the front will loose energy as it presses southeastward towards the Big Island. Post frontal, breezy northerly winds develop and advect in a much cooler and drier airmass. Dewpoints will once again drop into the mid 50s with overnight temperatures falling to around 5 degrees below normal for this time of year.
Friday through the weekend, as surface high pressure builds back to the north of the state, flow will veer northeasterly. Both high and low temperatures will remain slightly below seasonal norms. Subsidence aloft and the lack of much moisture in the lower levels will lead to fair weather with limited shower activity. Long range guidance shows potentially yet another robust front to move over the state Tuesday or Wednesday of next week. However, there is still some significant difference in the timing and strength between models.

Aviation

Gentle to breezy southeasterly winds expected through tonight. Winds will begin to increase tomorrow and be locally windy and veer southwesterly, west to east as a cold front approaches from the northwest. Dry and stable conds should continue the rest of tonight. Its possible to see some isol SHRA on windward and mauka locations overnight. As the cold front moves in tomorrow expect conds to deteriorate west to east. SHRA and a few isol TSTMs are possible with fropa. VFR conds should prevail through early morning. IFR/MVFR conds will be possible as the front approaches and passes.
AIRMET Sierra is not in effect but may be needed tomorrow for mtn obsc and/or IFR conds.
AIRMET TANGO is in effect over the entire state for mod turb between FL260 and FL360.

Marine

The remnants of a front can be seen just south of the Big Island this afternoon. The next strong cold front will sweep from west to east through the western islands on Wednesday, and once again stall out near the Big Island by Thursday afternoon. A passing high pressure system far north of the islands will bring back easterly trade winds across Hawaiian Waters from Friday through Sunday. Winds weaken and veer from a more southeasterly direction over the northwestern waters by next week Monday as another cold front approaches the islands from the northwest.
The current large, long to medium period northwest (310-330 degree) swell peaked earlier today and will gradually fade through Wednesday, with surf heights expected to remain above advisory levels through Wednesday afternoon along exposed north and west facing shores. The High Surf Warning (HSW) for exposed north and west facing shores has been converted to a High Surf Advisory (HSA) this afternoon and is now in effect through Wednesday afternoon. The HSA for west facing shores of the Big Island has been cancelled. A Small Craft Advisory (SCA) is in effect for the northwest coastal waters and most windward coastal waters due to elevated seas associated with the current northwest swell.
Another extra large, long period northwest (310-330 degree) swell will build into the region by Thursday, likely producing another round of warning level surf along exposed north and west facing shores lasting through Friday, and lingering near advisory levels Saturday before another reinforcing northwest swell briefly boosts surf heights back to near warning levels on Sunday and Sunday night.
Surf along east facing shores will remain small through most of this week due to shifting wind directions from the passing fronts. However, some east facing shores sensitive to northerly swells may experience a slight uptick in surf heights this weekend as the fading northwesterly swell becomes more northerly. No noteworthy swells are expected to impact the state from the south.

HFO Watches/Warnings/Advisories

High Surf Advisory until 6 PM HST Wednesday for Niihau, Kauai Leeward, Waianae Coast, Oahu North Shore, Maui Windward West, Kauai North, Molokai Windward, Molokai North, Molokai West, Maui Central Valley North, Windward Haleakala.
Small Craft Advisory until 6 AM HST Wednesday for Kauai Northwest Waters, Kauai Windward Waters, Kauai Leeward Waters, Kauai Channel, Oahu Windward Waters, Kaiwi Channel, Maui County Windward Waters, Alenuihaha Channel, Big Island Windward Waters.

Data Courtesy of NOAA.gov