Photo Credit: Braden Jarvis

West Kaua’i

Today: Mostly sunny with slight chance of showers in the morning, then partly sunny with scattered showers in the afternoon. Highs 79 to 84 near the shore to around 69 above 3000 feet. Light winds. Chance of rain 30 percent.

Tonight: Mostly cloudy with isolated showers and thunderstorms. Lows 65 to 70 near the shore to around 57 above 3000 feet. Light winds becoming east up to 10 mph after midnight. Chance of rain 20 percent.

Sunday: Mostly cloudy with showers and isolated thunderstorms. Highs around 82 near the shore to around 69 above 3000 feet. South winds up to 15 mph shifting to the southwest in the afternoon. Chance of rain 80 percent.

South Kaua’i

Today: Mostly sunny. Slight chance of showers in the morning, then scattered showers in the afternoon. Highs 78 to 85. Northeast winds up to 10 mph. Chance of rain 30 percent.

Tonight: Mostly cloudy with isolated showers and thunderstorms. Lows around 66. Light winds. Chance of rain 20 percent.

Sunday: Breezy. Mostly cloudy with showers and isolated thunderstorms. Highs 77 to 84. Southwest winds up to 15 mph increasing to 10 to 20 mph in the afternoon. Chance of rain 80 percent.

Kaua’i Mountains

Today: Mostly sunny with slight chance of showers in the morning, then partly sunny with scattered showers in the afternoon. Highs 69 to 76 in the valleys to around 62 above 4000 feet. Light winds. Chance of rain 40 percent.

Tonight: Mostly cloudy with isolated showers. Lows 59 to 65 in the valleys to around 55 above 4000 feet. Light winds becoming south up to 10 mph after midnight. Chance of rain 20 percent.

Sunday: Breezy. Mostly cloudy with showers and isolated thunderstorms. Highs 70 to 77 in the valleys to around 61 above 4000 feet. Southwest winds up to 20 mph increasing to 10 to 25 mph in the afternoon. Chance of rain 80 percent.

North Kaua’i

Today: Mostly sunny with isolated showers. Highs 71 to 81. East winds up to 10 mph. Chance of rain 20 percent.

Tonight: Mostly cloudy with isolated showers and thunderstorms. Lows 60 to 68. Light winds becoming south up to 10 mph after midnight. Chance of rain 20 percent.

Sunday: Breezy. Mostly cloudy with showers and isolated thunderstorms. Highs 72 to 81. Southwest winds up to 20 mph. Chance of rain 80 percent.

East Kaua’i

Today: Mostly sunny with slight chance of showers in the morning, then partly sunny with scattered showers in the afternoon. Highs 70 to 81. Northeast winds up to 10 mph. Chance of rain 40 percent.

Tonight: Mostly cloudy with isolated showers. Lows 59 to 70. Light winds becoming southwest up to 10 mph after midnight. Chance of rain 20 percent.

Sunday: Breezy. Mostly cloudy with showers likely and isolated thunderstorms. Highs 70 to 80. Southwest winds 10 to 20 mph. Chance of rain 70 percent.

Detailed Forecast

Synopsis

Scattered showers will develop with sea breezes over island interiors today with isolated thunderstorms possible over portions of Maui and the Big Island. Showers and thunderstorms will increase in coverage Saturday night through Sunday as southwest winds strengthen. Strong and potentially damaging kona, or southwest, winds and strong thunderstorms will move through the area Sunday night through Monday. Breezy westerlies will then focus showers over leeward slopes into midweek.

Discussion

Today. Cold WNW flow aloft will maintain relatively steep lapse rates and a state of elevated instability over the islands into early next week. As seen yesterday evening and overnight, this superposition of instability aloft and a weakly capped boundary layer will create favorable conditions for pockets of heavy showers and periods of thunderstorms. Whereas land breezes were sufficient to sustain several hours of nocturnal thunderstorms off the coasts of Oahu and the Big Island overnight, emerging sea breezes this afternoon will similarly serve as convective triggers. For Kauai and Oahu, a subtle increase in mid-level subsidence and dry advection at/above 700mb is expected to limit afternoon convective depths and inhibit inland thunderstorm development. However, afternoon clouds and showers can be expected over interior Kauai and Oahu with a few locally heavy showers possible. The push of dry air will not reach Maui County and the Big Island, however, leaving in place the same airmass that has supported sporadic thunderstorm activity since yesterday. Daytime heating will provide renewed destabilization over land while southwesterly flow at 850mb favors downwind convergence over windward areas. Thunderstorms have therefore been added to Windward Maui and Big Island for this afternoon.
Tonight through Sunday afternoon. The push of dry air into the western end of the state will be short lived as mid-level winds quickly back to southwesterly early tonighting ample mid-level moisture and PWATs around 1.50″ to all areas by midnight. This period will represent the onset of broad and gradually intensifying large scale ascent in advance of the long-advertised rapidly intensifying trough approaching the area. The reservoir of elevated instability supplied by steady WNW aloft will remain established throughout this time and will give rise to an increasing coverage of heavy showers and thunderstorms over the waters to the southwest of the state tonight. This activity will initially be held at bay by land breezes, but convective showers and isolated thunderstorms will readily move onshore as early as the predawn hours Sunday as southwest winds finally materialize at the surface. The remainder of the day Sunday will be characterized by an increasing coverage of showers and isolated to scattered thunderstorms, mainly over the western end of the state. Often a recipe for stratiform rainfall as warm advection stabilizes the column, the instability source aloft will be key to Sunday's character as a fairly active convective day. 0-1km MLCAPEs increasing to well above 1000 j/kg over the western end of the state by Sunday afternoon will contribute to a greater likelihood for thunderstorms capable of intense heavy rainfall, but not much else as the deep layer wind field remains unfavorable for strong or severe thunderstorms through this time. Additionally, given expectation for convective cells to be progressive and widely scattered within a broad corridor of ascent (as opposed to focused along a synoptic or mesoscale boundary), widespread flooding is not expected to be an issue at this time. However, the potential for a flood watch for Sunday or Sunday evening will be evaluated as the character of the day becomes more clear.
Sunday afternoon through Monday. A defined threat for impactful weather exists during this period. The primary threat is strong, damaging winds, especially for windward zones and areas prone to local acceleration within southwest flow. Secondary threats include thunderstorms, some of which may be strong or severe, heavy rain, and at least a low potential for flooding. The threat for significant weather is greatest very late Sunday night through Monday morning over the western half of the state and Monday into Monday evening for the eastern half.
Showers and embedded thunderstorms will continue tracking northeastward within increasingly strong southwest flow as the deepening trough approaches from the northwest. The wind field will strengthen considerably Sunday afternoon through Sunday night, peaking around 45 kts at 850mb to go along with 25-30kts sustained southwest/kona winds at the surface. A wind advisory is likely for this period with focus initially given to areas over and downwind of terrain. A damaging wind event featuring gusts of 58 mph or higher in these locations is within the realm of possibility, but downsloping potential will be somewhat compromised by the lack of an antecedent inversion. Even in the absence of downsloping, the potential for showers to mix down strong winds right off the deck may itself warrant a high wind warning beginning late Sunday.
The cold front reaches Kauai during the predawn hours Monday morning and is modeled to have exceptional vertical structure as a mature jet-front system sustained by an intense left exit region and outstanding dynamic support. Strong mixing within the frontal zone as it moves through the area during Monday has potential to produce wind gusts well in excess of 50 mph as it will be an extremely dynamic feature. Of greater concern during this period will be the potential for a line of strong or severe thunderstorms along the front. The strong unidirectional wind field will readily lend itself to bowing segments moving northeastward bringing wind gusts up to 60 mph. The greatest uncertainty with respect to severe thunderstorm potential will be the extent to which antecedent rainfall stabilizes the boundary layer or even overturns the mid-levels in a scenario of unexpected widespread thunderstorms well in advance of the front. With that in mind, supplemental 6-hourly soundings launched by the DCOs in Lihue and Hilo beginning tonight should provide essential observational clarity leading into the event.
Well-mixed northwesterly flow fills in behind the front bringing gusty conditions through early Tuesday. As mid-level stability returns to the area, breezy westerlies will focus showers over leeward slopes.
Finally, summit winds are forecast to ramp up extremely quickly on Sunday, from around 40 mph late Sunday afternoon to over 80 mph (Big Island) / 65 mph (Haleakala) after midnight Sunday night. Sustained winds will eventually peak in the neighborhood of 100 mph (Big Island) / 85 mph (Haleakala) with higher gusts on Monday morning. While snowfall is likely on the Big Island Summits, 700mb dewpoints hovering at or slightly above freezing casts doubt on the potential for any snow to reach Haleakala Summit except perhaps within thunderstorms or during the cold front passage on Monday afternoon.

Aviation

Showers and isolated thunderstorms will continue across portions of the islands this morning, as a cold front further weakens north of the state. A mix of VFR/MVFR conditions are expected this morning as precipitation moves across the state. MVFR cigs in heavier showers will continue the potential need of AIRMET Sierra for tempo mountain obscuration.
Otherwise a light wind field anchored over the region, will allow for diurnal land and sea breeze circulations this afternoon, which will enhance interior clouds and showers.

Marine

A seasonably active weather pattern is expected over the next several days, with a potent cold front expected to sweep through the island chain Sunday night and Monday, bringing near-gale to gale force SW to W winds, a very large and long-lived NW swell, and the potential for strong to severe thunderstorms. In the meantime, a large NW swell will continue to produce dangerously high surf today, but gradually diminishing wave heights and period will lead to a much reduced threat for significant shoreline impacts. Therefore, the High Surf Warning (HSW) has been downgraded to a High Surf Advisory (HSA) for exposed north and west facing shores, see CFWHFO for details. With combined seas remaining at or above 10 feet in many zones, a Small Craft Advisory (SCA) has been extended through today, see MWWHFO for details.
Winds will remain light and variable through tonight as weak high pressure passes quickly E to the N of the islands, with SW winds increasing on Sunday as the potent cold front approaches. Near- gale to gale force SW to W winds are expected by Sunday night as the front nears and moves SE across the island chain, with the front also bringing the potential for strong to severe thunderstorms through Monday. The front should be E of all islands by Monday night, but the parent low passing by to the N will keep strong to near-gale W winds in place on Tuesday, with winds expected to weaken rapidly thereafter, become light and variable by Wednesday.
The powerful low sending the cold front over the islands will pass very close to the islands early next week, thereby producing a long-lived and very large NW swell that will impact the islands from Sunday night through Tuesday. A HSW is likely for most N and W facing shores, with a HSA also likely for those N and W shores less exposed to the swell, i.e., west facing shores of Maui and the Big Island, and north facing shores of the Big Island. Another large NW swell is possible around the middle of next week. Small surf will prevail along S and E facing shores.

HFO Watches/Warnings/Advisories

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

Data Courtesy of NOAA.gov