Photo Credit: Braden Jarvis

West Kaua’i

Today: Partly sunny. Breezy. Scattered showers in the morning, then scattered showers and slight chance of thunderstorms in the afternoon. Highs around 79 near the shore to around 68 above 3000 feet. Northeast winds 10 to 20 mph. Chance of rain 50 percent.

Tonight: Mostly cloudy with scattered showers. Lows around 66 near the shore to around 56 above 3000 feet. Northeast winds 10 to 15 mph shifting to the east after midnight. Chance of rain 50 percent.

Thursday: Partly sunny with scattered showers. Highs around 80 near the shore to around 69 above 3000 feet. Southeast winds 10 to 15 mph. Chance of rain 40 percent.

South Kaua’i

Today: Partly sunny. Breezy. Scattered showers in the morning, then showers likely and slight chance of thunderstorms in the afternoon. Highs 75 to 80. North winds 10 to 20 mph. Chance of rain 60 percent.

Tonight: Mostly cloudy with showers likely. Lows around 65. Northwest winds around 10 mph shifting to the east after midnight. Chance of rain 70 percent.

Thursday: Mostly cloudy with scattered showers. Highs 76 to 81. East winds around 10 mph. Chance of rain 40 percent.

Kaua’i Mountains

Today: Breezy. Frequent showers and slight chance of thunderstorms in the morning, then showers likely and slight chance of thunderstorms in the afternoon. Highs 67 to 73 in the valleys to around 59 above 4000 feet. North winds 10 to 20 mph. Chance of rain near 100 percent.

Tonight: Mostly cloudy with scattered showers. Lows 57 to 64 in the valleys to around 54 above 4000 feet. North winds around 10 mph shifting to the southeast after midnight. Chance of rain 50 percent.

Thursday: Mostly cloudy with scattered showers. Highs 68 to 75 in the valleys to around 62 above 4000 feet. Southeast winds 10 to 15 mph. Chance of rain 50 percent.

North Kaua’i

Today: Mostly cloudy. Breezy. Frequent showers and slight chance of thunderstorms in the morning, then showers likely and slight chance of thunderstorms in the afternoon. Highs 69 to 79. North winds 10 to 25 mph. Chance of rain 90 percent.

Tonight: Mostly cloudy with scattered showers. Lows 60 to 67. North winds 10 to 15 mph shifting to the southeast after midnight. Chance of rain 50 percent.

Thursday: Partly sunny with scattered showers. Highs 71 to 79. Southeast winds 10 to 15 mph. Chance of rain 50 percent.

East Kaua’i

Today: Breezy. Mostly cloudy with showers likely and slight chance of thunderstorms. Highs 67 to 78. North winds 10 to 20 mph. Chance of rain 70 percent.

Tonight: Mostly cloudy with showers likely. Lows 59 to 69. North winds around 10 mph shifting to the southwest after midnight. Chance of rain 70 percent.

Thursday: Partly sunny in the morning then becoming cloudy. Scattered showers. Highs 69 to 79. Southeast winds around 10 mph. Chance of rain 50 percent.

Detailed Forecast

Synopsis

A kona low about 300 miles north of the Big Island will move steadily toward the west over the next several days, strongly affecting island weather. Locally breezy and cool north winds will gradually weaken today, then shift to the south tonight. A significant moisture surge will move over the islands Thursday with southeast winds, and remain in place through at least Saturday. This moisture will fuel what is expected to be a prolonged period of rainfall, with heavy showers and thunderstorms potentially affecting both windward and leeward areas of all islands, with impactful flooding possible. A Flood Watch is in effect from Thursday morning through Saturday afternoon.

Discussion

The center of a kona low is about 300 miles N of the Big Island this morning – after having moved very little overnight. Widespread layered clouds with embedded showers and thunderstorms prevail to the N and E of the low's center, while scattered to broken showery low clouds prevail S and W of the center, and upwind of the islands from Kauai to Maui. The gradient between the low and a strong surface high about 1000 miles NW of Kauai is supporting moderate to locally breezy N to NE winds over Kauai and Oahu, while winds are diminishing over Maui County and the Big Island. Morning soundings show that the island atmosphere remains unstable, with relatively low PWAT between 1 and 1.2″. Radar is again peppered with scattered to numerous showers, especially N and NE of Kauai and Oahu, with some heavier rain cores embedded. These showers have been persistently dampening large portions of Kauai and Oahu overnight, especially windward, but showers spilling over the Koolau have been affecting urban Honolulu as well as other leeward areas. Fewer showers are approaching Maui County in a diminishing NW to N flow. Winds over the Big Island have weakened, which has allowed land breezes and mostly clear skies to develop in most areas. However, some briefly heavy showers and an isolated thunderstorm formed overnight along weak eddies in the near shore waters S and W of the island, and some soaking showers managed to move ashore near Hawaiian Ocean View Estates and Milolii.
The short-term forecast anticipates light and variable winds over the Big Island today as the kona low passing to the N cuts off the winds. A mostly sunny morning will be followed by afternoon sea breezes that spawn upslope clouds and showers, and possibly some thunderstorms. Light to moderate winds will shift to the SE and S tonight, beginning what is expected to be an extended period of deep- layer moisture advection over the area, with a significant surge arriving from the SE early Thursday morning.
The same general pattern with respect to the wind flow and moisture advection will play out on the other islands as the low moves W, with some timing differences. Moderate to breezy N to NE winds on Oahu (Kauai) will diminish this morning (evening), becoming light and variable by the afternoon (evening). This could allow sea breeze circulations to drive heavy shower and thunderstorm formation over the interior and/or S portions of Oahu and Maui County this afternoon. The atmosphere will remain unstable due to the mid-level reflection of the passing kona low bringing steep low- and mid-level lapse rates. By tonight, the low will be passing N of the Kauai, and SE to S winds will be developing over all islands. Moisture will surge NW over the smaller islands on Thursday.
As the kona low moves W and further away from the islands, model guidance has been persistently indicating that a slow-moving low- level convergent band will set up over the island chain, affecting all islands from Thursday until at least Saturday. Strong E to SE winds will feed deep moisture into the band, with the potential for heavy rain to become terrain-anchored, or repeatedly train over the same areas. Difficult to know how windy it will get over land in this regime, as the smaller islands typically lie in the shadow of the Big Island in SE flow, but there could be periods of gusty winds, especially in showers.
The mid-level low and associated instability will shift W of the islands Friday and Saturday, but strong divergence aloft and PWAT surging to nearly 2″ means that there will be a slight chance for thunderstorms, which could also bring strong and erratic winds. A Flood Watch remains in effect for all islands starting Thursday morning. Now is the time to prepare and/or adjust outdoor plans as heavy rainfall could affect both leeward and windward areas.
Deep moisture and cold temperatures aloft mean wintry weather for the Big Island summits and upper slopes, and a Winter Storm Watch is in effect starting Thursday. Periods of freezing rain, sleet, heavy snow and gusty winds will produce hazardous driving conditions and dangerously low visibility for several days.
Long range guidance indicates that the convergent band will be slow to move through the weekend, with the potential for another kona low to develop early next week. This would keep the islands in an extended period of unsettled weather although there may be some breaks.

Aviation

A low pressure system north of the state will slowly track westward over the next 24-hours. This feature will bring unsettled weather to the islands for the next several days.
Moderate north northeasterly trades will give way to mainly light winds by this afternoon and allow for the development of sea breezes.
This pattern will likely bring scattered shower and isolated thunderstorm to all islands. MVFR ceilings and visibility may accompany some of the more robust shower and storm activity due to low cloud bases and reduced visibility.
AIRMET Tango remains in effect for moderate turbulence between FL250 and FL350 across the state due to the subtropical jet over Hawaii. The jet will remain over the islands until at least Thursday.
AIRMET Sierra remains in effect for the north through east sections of Maui, Oahu, Kauai, and Molokai above 2500 feet due to low clouds and showers.

Marine

A kona low with gale force winds, currently about 220 nm northeast of Maui, will slowly pass over offshore waters north of the state through Wednesday night, then drift to the west Thursday and Friday. Fresh to strong northeasterly winds, which have already eased around the Big Island and Maui County, will lower around Oahu today, and Kauai tonight as the low passes to the north. A Small Craft Advisory (SCA) remains in effect for waters around the Maui County Windward Waters, Oahu, and Kauai due to a combination of the winds and large seas. The large seas are mainly short period northeast swell from the developing low. This SCA may be dropped Wednesday night as winds and seas briefly diminish. However, as the low moves off to the west and a strong surface high pressure system develops to the northeast on Thursday, fresh to strong southeast winds and large wind waves will develop, likely requiring a new SCA for most waters into the weekend. In addition to the hazardous winds and seas, heavy rainfall and thunderstorms will be possible.
A High Surf Advisory (HSA) remains in effect for east-facing shores of Kauai and Oahu through Wednesday afternoon due to short period northeast swell. East shore surf will briefly subside Wednesday night, but as southeast winds increase Thursday and Friday, large and rough surf will return, possibly reaching the advisory level again. The current, small northwest swell will subside tonight. Another northwest swell will build Wednesday, peak below advisory levels Wednesday night, and decline Thursday. South facing shores will experience minimal surf this week.

HFO Watches/Warnings/Advisories

Flood Watch from Thursday morning through Saturday afternoon for all Hawaiian Islands.
High Surf Advisory until 6 PM HST this evening for east facing shores of Kauai and Oahu.
Winter Storm Watch from Thursday morning through Saturday afternoon for Big Island Summits.
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.

Data Courtesy of NOAA.gov