Photo Credit: Sebastien Gabriel

West Kaua’i

Today: Breezy. Cloudy with scattered showers in the morning, then sunny with isolated showers in the afternoon. Highs 74 to 82 near the shore to around 65 above 3000 feet. North winds up to 25 mph. Chance of rain 50 percent.

Tonight: Mostly clear. Breezy. Lows 58 to 63 near the shore to around 50 above 3000 feet. North winds up to 20 mph.

Thursday: Mostly sunny. Highs 72 to 80 near the shore to around 66 above 3000 feet. North winds up to 10 mph shifting to the west in the afternoon.

South Kaua’i

Today: Breezy. Mostly cloudy with scattered showers in the morning, then mostly sunny in the afternoon. Highs 74 to 82. Northeast winds up to 20 mph. Chance of rain 40 percent.

Tonight: Mostly clear. Lows around 60. Northeast winds up to 10 mph in the evening becoming light.

Thursday: Mostly sunny. Highs 73 to 80. Light winds.

Kaua’i Mountains

Today: Breezy. Cloudy with numerous showers in the morning, then partly sunny with scattered showers in the afternoon. Highs 66 to 72 in the valleys to around 58 above 4000 feet. North winds up to 20 mph. Chance of rain 70 percent.

Tonight: Partly cloudy with isolated showers. Lows 50 to 59 in the valleys to around 48 above 4000 feet. Northwest winds up to 15 mph. Chance of rain 20 percent.

Thursday: Mostly sunny. Highs 66 to 73 in the valleys to around 58 above 4000 feet. Light winds.

North Kaua’i

Today: Breezy. Cloudy with numerous showers in the morning, then partly sunny with scattered showers in the afternoon. Highs 67 to 77. Northwest winds 10 to 20 mph. Chance of rain 70 percent.

Tonight: Partly cloudy with isolated showers. Lows 54 to 61. West winds 10 to 15 mph. Chance of rain 20 percent.

Thursday: Mostly sunny. Highs 68 to 75. Northwest winds up to 10 mph.

East Kaua’i

Today: Breezy. Cloudy with scattered showers in the morning, then mostly sunny with isolated showers in the afternoon. Highs 66 to 79. North winds up to 25 mph. Chance of rain 50 percent.

Tonight: Partly cloudy. Breezy. Isolated showers in the evening. Lows 53 to 64. Northwest winds up to 20 mph decreasing to up to 10 mph after midnight. Chance of rain 20 percent.

Thursday: Mostly sunny. Highs 66 to 78. West winds up to 10 mph in the morning becoming light.

Detailed Forecast

Synopsis

A cold front will bring a brief round of moderate to locally heavy showers with slight chance of thunderstorms today into Thursday. This will be followed by a reinforcing shot of cool weather. Trades return briefly Friday into Saturday before they are disrupted again by another front approaching from the northwest.

Discussion

Current radar and satellite shows a band of clouds and showers along a cold front moving onshore Kauai early this morning with some gusty northwesterly winds following behind the front. Rainfall rates are generally moderate below 1 inch an hour with isolated pockets of heaver showers with 1-2 inches per hour. This front will quickly move through Kauai early this morning then move over Oahu during the morning traffic hours. The front is expected to weaken has it moves over Maui County this afternoon before becoming diffused over the Big Island and possibly shifting just east of the state tonight into Thursday. Showers associated with the front are expected to clear out quickly limiting flooding concerns. The moderate to locally breezy northwest winds filling in behind the front may be more of a concern but will also be brief and mainly over the western half of the state this morning. Drier air with dew points dropping into the lower to mid 50s will fill in behind the front limited clouds and showers. There still remains some uncertainty regarding how far south this drier airmass will penetrate with confidence decreasing the further south down the island chain you go, especially as the front weakens and the strong northwest winds ease. Even though the probabilities of thunderstorms remains fairly low, this morning package was updated to include a slight chance thunderstorms and locally heavy rain over Maui County this afternoon. The threat is then kept mostly over the eastern waters of the Big Island tonight as the front weakens and gradually progresses further south.
On Thursday, the western end of the state will be cool and dry while the front dissipates over the Big Island and its remnant moisture lingers over the eastern end of the state, especially near the Big Island. By this point, the upper level low will have deepened and brought height falls and lower temperatures aloft across the island chain. Model guidance shows 500 mb temperatures dropping to -12C to -13C over the Big Island by Thursday afternoon. This instability combined with lingering moisture and diurnal heating may be enough to bring an isolated thunderstorm or two to the Big Island Thursday afternoon and a chance for wintery precipitation at the summits which has been added in this morning package.
Friday, winds will veer and easterly trades will strengthen to moderate levels as high pressure builds just north of the state. At the same time, moisture that was being pulled north across the eastern North Pacific earlier in the week will shift across the island chain in the redeveloping trades. That plume of moisture along with the remnant moisture from the dissipated front will increase the chance of windward and mauka showers Friday into Saturday and help dew points to recover back into the 60s. The pressure gradient will weaken again Saturday into Sunday in response to another approaching cold front from the northwest, disrupting the trades once again and bringing lighter winds across the area. Model guidance shows a non zero chance of thunderstorms Friday through the weekend associated with this pool of moisture and upper level disturbance near the Big Island Friday and Saturday, but left it out of the grids at this time since model trends have been inconsistent with each run.
Forecast confidence decreases Sunday night and beyond due to model discrepancies. The general pattern looks to be similar to what we will see today and Thursday as another trough digs down across the Central Pacific and spawns a surface low that will drag a front into our area. However, the strength of this upper- level trough and resulting surface low is a bit uncertain and model guidance is hinting at an area of prefrontal convergence and shortwave energy that could bring rain to the western end of the state ahead of the next front from lingering moisture and possibly a slight chance of thunderstorms.

Aviation

A cold front will track southeast from Kauai to Maui today, then stall out and dissipate near the Big Island late tonight and Thursday. A band of low clouds and showers will accompany the front bringing some MVFR cigs/vsbys, along with increasing north to northwest winds in its wake. VFR conditions will prevail over the Big Island today, with a few showers and some MVFR cigs/vsbys possible over leeward Big Island tonight.
AIRMET Sierra is now in effect for mountain obscuration on Kauai. This AIRMET will likely expand down the island chain through the day as the cold front moves southeastward.
AIRMET Tango may be needed for low level turbulence over and downwind of the terrain behind the front later today. AIRMET Tango will likely be needed for moderate upper level turbulence this afternoon and tonight across much of the state.

Marine

A cold front over Kauai early this morning will continue to move southeast around 15 to 20 knots. Gentle to moderate southwest to west winds are expected to develop today in advance of the front. The front is expected to continue to move southeast over Oahu waters mid- morning, and Maui County waters this afternoon and evening. The front will likely be weakening and slowing in forward speed after crossing Oahu, dissipating near the Big Island Thursday. The front will bring a relatively brief period of rainfall, near gale force winds, and the potential for thunderstorms and heavy showers, mainly over Kauai and Oahu waters.
A short-lived period of strong northwest winds following the frontal passage (with a longer-lived increase in seas) has prompted the issuance of a Small Craft Advisory (SCA) for many zones this morning into Thursday. Winds will diminish as the front dissipates, with a surface ridge moving over the islands from the west by Thursday night. A surface high will pass north of the islands Friday into Saturday, supporting a moderate east- northeast trade wind flow. Long range guidance indicates the potential for another frontal passage late in the weekend.
A new moderate long period northwest swell will build through the day. PacIOOS buoys located at Hanalei and Waimea Bay have begun to pick up the long period forerunners through the early morning. Surf along north and west facing shores are expected to stay just below High Surf Advisory (HSA) conditions today as the swell heights remain near 5 to 7 feet from the northwest. Surf is expected to be larger Thursday and Friday over north facing shores as a developing storm low continues to intensify due north of the islands near 32N. The mix of 8 to 14 second north- northwest to north swell will make for jumbled surf as it overlaps with the diminishing longer period northwest swell. A HSA for select north facing shores and a SCA over most windward waters may need to be issued coinciding with the new swell. This swell will be slow to decline into the weekend as the low will remain nearly stationary. Additionally, a weaker low north of the islands has generated a short-period (8-10 seconds) northeast swell that will peak today before diminishing Thursday. A new NW swell is possible next Monday, otherwise no other significant swells are expected.

Fire weather

Winds are forecast to remain below the critical levels while a reinforcing shot of dry air causes afternoon relative humidity values to fall to near or slightly below 40 percent Thursday and Friday. In addition rainfall associated with a passing front will help to mitigate the fire weather concern. Critical fire weather concerns are not anticipated through the rest of the forecast period.

HFO Watches/Warnings/Advisories

Small Craft Advisory until 6 PM HST Thursday for Kauai Northwest Waters, Kauai Windward Waters, Kauai Leeward Waters, Kauai Channel, Oahu Windward Waters, Kaiwi Channel.

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