Photo Credit: Brendan Stephen

West Kaua’i

Today: Mostly cloudy with scattered showers. Highs around 83 near the shore to around 71 above 3000 feet. Southeast winds 10 to 15 mph. Chance of rain 50 percent.

Tonight: Mostly cloudy. Isolated showers in the evening, then scattered showers after midnight. Lows around 70 near the shore to around 61 above 3000 feet. Southeast winds up to 15 mph in the evening becoming light. Chance of rain 50 percent.

Wednesday: Mostly cloudy. Scattered showers in the morning, then showers likely in the afternoon. Highs around 83 near the shore to around 71 above 3000 feet. Southeast winds up to 10 mph. Chance of rain 70 percent.

South Kaua’i

Today: Mostly cloudy. Scattered showers in the morning, then isolated showers in the afternoon. Highs around 82. East winds 10 to 15 mph shifting to the southeast in the afternoon. Chance of rain 50 percent.

Tonight: Mostly cloudy. Isolated showers in the evening, then scattered showers after midnight. Lows around 69. Light winds becoming northeast up to 10 mph after midnight. Chance of rain 40 percent.

Wednesday: Partly sunny with scattered showers. Highs 79 to 84. East winds up to 10 mph shifting to the south around 10 mph in the afternoon. Chance of rain 50 percent.

Kaua’i Mountains

Today: Mostly cloudy with scattered showers. Highs 75 to 82 in the valleys to around 64 above 4000 feet. Southeast winds up to 15 mph. Chance of rain 50 percent.

Tonight: Mostly cloudy with scattered showers. Lows 63 to 68 in the valleys to around 58 above 4000 feet. Southeast winds up to 10 mph in the evening becoming light. Chance of rain 50 percent.

Wednesday: Mostly cloudy. Scattered showers in the morning, then showers likely in the afternoon. Highs 74 to 80 in the valleys to around 65 above 4000 feet. Light winds becoming south up to 10 mph in the afternoon. Chance of rain 70 percent.

North Kaua’i

Today: Mostly cloudy with scattered showers. Highs 75 to 83. Southeast winds up to 15 mph. Chance of rain 50 percent.

Tonight: Mostly cloudy with scattered showers. Lows 63 to 71. Southeast winds up to 10 mph. Chance of rain 50 percent.

Wednesday: Mostly cloudy. Scattered showers in the morning, then showers likely in the afternoon. Highs 74 to 83. South winds up to 10 mph. Chance of rain 70 percent.

East Kaua’i

Today: Mostly cloudy with scattered showers. Highs 73 to 83. East winds up to 15 mph shifting to the southeast in the afternoon. Chance of rain 50 percent.

Tonight: Mostly cloudy with scattered showers. Lows 64 to 73. Southeast winds up to 10 mph in the evening becoming light. Chance of rain 50 percent.

Wednesday: Mostly cloudy with scattered showers. Highs 73 to 84. South winds up to 10 mph. Chance of rain 50 percent.

Detailed Forecast

Synopsis

Intermittent showers are possible through Wednesday, including over leeward zones where isolated heavier showers may develop at times. As strengthening trades turn southeasterly, blocking by the Big Island will cause winds to locally weaken over the smaller islands. A land and sea breeze pattern will be possible mid to late week with afternoon a few heavier afternoon showers possible each day.

Discussion

A strengthening surace low northwest of the islands is lifting northward along the frontal zone stalled west of Kauai. This low will remain anchored to the front, remaining well west of the area, until it occludes tonight. Building trades will strengthen as they respond to the sharpening pressure gradient, but will also tend to veer southeasterly with time. ESE surface winds are modeled to reside below a deep layer of southeasterly flow which will direct the lion's share of moisture northwestward and parallel to the island chain. This in turn makes maintenance of a typical trade wind distribution of showers unlikely, although showers will still tend to favor windward zones overall.
Last evening's and this morning's radar composites represent the best visual of expected shower distribution during the next 12-18 hours. SE flow aloft will tend to foster island interaction/downwind convergence with downstream shower behavior modulated by backed ESE low-level flow and the shallow moisture field evident in recent soundings out of Lihue. Transient pockets of disorganized and locally heavy showers will favor the southern half of Oahu and possibly Kauai through this morning and potentially into tonight. Meanwhile, ESE trade winds will favor occasional showers over windward slopes of Molokai through the Big Island.
As the surface low deepens and lifts northward, deep layer flow will veer to southerly tonight into early Wednesday. This will bring an end to the brief window for any showers developing due to island interactions. Mid-level instability will bleed into the western third of the forecast area as the envelope of cyclonic flow around the parent low expands eastward. Warm advection within southerly flow aloft will serve as a poor organizing mechanism for convection, and access to instability aloft will be limited by a stubborn mid-level dry layer. For this reason, widespread heavy rain (or widespread regular rain) is not expected. Instead, potential for heavy showers will initially be limited to areas where orographic forcing in this pattern will be maximized, namely Windward Maui and windward & southeast-facing slopes of the Big Island. Then, as the low lifts northeast and away from the islands during the latter half of the week, the upstream surface trough will be ushered eastward and the gradient will diminish over western portions of the island chain. Emerging sea breezes would then give rise to interior clouds and showers each afternoon Wednesday through Friday, with the highest confidence on Thursday. The presence of high clouds will tend to limit interior convection through Wednesday. By Thursday and Friday, any particularly vigorous convection could feasibly punch through the weakening dry/stable layer and produce localized heavy rainfall before diminishing in the evening. It is worth a mention that the numerous waves of heavy rain modeled by the EC to move over Kauai during mid to late week are entirely spurious.
Consensus is strong among the long range guidance in the general idea of another round of troughing developing northwest of the islands early next week. This would serve to maintain the background of southeasterly flow and increased potential for light winds over the smaller islands.

Aviation

A surface low will deepen northwest of the state through the remainder of today and help to increase east or east- southeasterly flow. Expect a plethora of mid and upper level clouds to stream over the small islands from southwest to northeast through the forecast period. Isolated to scattered showers may disproportionately affect Oahu and Kauai, when compared to other islands, as they are closer to the moisture/instability from the frontal boundary and low. Brief MVFR ceilings and visibility may accompany some of the more robust showers. Isolated showers elsewhere, especially along the windward portions of the remaining islands.
There are currently no AIRMETs in effect.

Marine

As expected no significant changes to the forecast with the morning package. A low pressure system has formed along the stationary front to the northwest of the islands. This low is expected to lift to the north over the next 24 hours or so. Gales that had been in the northern offshore waters have moved north, so expect to drop the Gale Warning for the offshore waters with the 6am issuance. Meanwhile, moderate southeasterly winds continue over the coastal waters, with Small Craft Advisory (SCA) level winds impacting some of the coastal waters near Maui County and the Big Island. The SCA is in effect through Wednesday afternoon, but winds are likely going to remain elevated through the end of the week, before weakening some over the weekend.
A medium size, short-period north (350-010 degree) swell, generated by above mentioned gale, will quickly decline today. A mix of small northwest swells will maintain modest surf along many exposed north and west facing shorelines Wednesday into Friday. The north swell may wrap around and subtly increase surf along east facing shores with more northern exposures today. Increased easterly winds around midweek will result in choppy wind waves along east facing shores as well. A 2-foot moderate period south (190 degrees) swell will help to boost surf a bit along south facing shores into Thursday morning.

HFO Watches/Warnings/Advisories

Small Craft Advisory until 6 PM HST Wednesday for Maui County Windward Waters, Pailolo Channel, Alenuihaha Channel, Big Island Windward Waters, Big Island Leeward Waters, Big Island Southeast Waters.

Data Courtesy of NOAA.gov