Photo Credit: Brendan Stephen

West Kaua’i

Today: Sunny early in the morning then becoming mostly sunny. Breezy. Highs around 82 near the shore to around 73 above 3000 feet. Northeast winds 10 to 15 mph becoming up to 25 mph.

Tonight: Windy. Mostly cloudy with isolated showers. Lows around 68 near the shore to around 57 above 3000 feet. Northeast winds up to 30 mph. Chance of rain 20 percent.

Sunday: Windy. Partly sunny with scattered showers and thunderstorms. Highs around 82 near the shore to around 73 above 3000 feet. Northeast winds up to 30 mph increasing to 15 to 30 mph in the afternoon. Chance of rain 50 percent.

South Kaua’i

Today: Partly cloudy. Breezy. Isolated showers early in the morning. Highs around 82. Northeast winds 10 to 15 mph increasing to 10 to 25 mph. Chance of rain 20 percent.

Tonight: Mostly cloudy. Windy. Isolated showers in the evening, then scattered showers after midnight. Lows 64 to 70. Northeast winds 20 to 30 mph. Chance of rain 40 percent.

Sunday: Windy. Cloudy with scattered showers and thunderstorms. Highs around 82. Northeast winds 25 to 30 mph decreasing to 15 to 30 mph in the afternoon. Chance of rain 50 percent.

Kaua’i Mountains

Today: Breezy. Mostly cloudy with scattered showers. Highs 73 to 78 in the valleys to around 66 above 4000 feet. Northeast winds up to 20 mph shifting to the north in the afternoon. Chance of rain 50 percent.

Tonight: Cloudy and breezy. Scattered showers in the evening, then numerous showers after midnight. Locally heavy rainfall possible after midnight. Lows around 59 in the valleys to around 52 above 4000 feet. Northeast winds 10 to 25 mph with gusts to 45 mph. Chance of rain 70 percent.

Sunday: Windy. Mostly cloudy with scattered showers and thunderstorms. Highs 73 to 78 in the valleys to around 66 above 4000 feet. Northeast winds 15 to 30 mph. Chance of rain 50 percent.

North Kaua’i

Today: Breezy. Mostly cloudy with scattered showers. Highs 73 to 81. Northeast winds 10 to 25 mph. Chance of rain 50 percent.

Tonight: Windy. Cloudy with scattered showers. Lows 60 to 68. Northeast winds 10 to 30 mph. Chance of rain 50 percent.

Sunday: Windy. Mostly cloudy with scattered showers and thunderstorms. Highs 73 to 81. Northeast winds 15 to 30 mph with gusts to 50 mph. Chance of rain 50 percent.

East Kaua’i

Today: Mostly cloudy. Breezy. Scattered showers early in the morning, then isolated showers in the late morning and afternoon. Highs 74 to 82. North winds 10 to 20 mph. Chance of rain 50 percent.

Tonight: Breezy. Cloudy with scattered showers. Lows 57 to 69. North winds 15 to 25 mph. Chance of rain 50 percent.

Sunday: Windy. Cloudy with scattered showers and thunderstorms. Highs 74 to 82. North winds 15 to 30 mph. Chance of rain 50 percent.

Detailed Forecast

Synopsis

A surface trough near Maui County will support wet weather across portions of the state, especially from Oahu to Maui, through the weekend. Enhanced moisture may expand to Kauai as well by Sunday afternoon. An upper-level disturbance will destabilize the atmosphere, keeping locally heavy rainfall and a few thunderstorms in the forecast through Monday, especially over windward areas. Gusty winds are expected from Kauai to Oahu through early Monday. A generally wet trade wind pattern is expected to prevail from Tuesday through the remainder of next week.

Discussion

Early this morning, a surface trough near Maui is serving as a focus for showers and isolated thunderstorms that are streaming from east to west into the island. Latest radar imagery shows an area of moderate to heavy rain with occasional lightning is persisting across the windward slopes of Haleakala over an area where significant rainfall has already occurred over the past 12 hours. Stream gauges from east Maui show that water levels have fluctuated throughout the night, but a number of them remain elevated as showers continue. Impacts from flooding that has occurred across the eastern portion of Maui since yesterday afternoon will linger into today. Meanwhile, the moderate to heavy showers that impacted windward portions of the Big Island during the evening have largely diminished early this morning. Drier conditions have been observed elsewhere with only a few windward showers throughout the night, though a notable increase in shower activity across Molokai has been observed over the past couple of hours.
The surface trough will continue to slowly drift westward over the central islands today as a deep upper trough sharpens west of the island chain. Wet conditions are expected to continue across Maui County, but moisture is also expected to spread as far west as Oahu during the day with showers primarily affecting windward areas. Therefore, the Flash Flood Watch that includes Maui County will expand in area to also include the island of Oahu this morning. The deepening upper trough will likely generate widespread upper clouds and instability that will continue thunderstorm chances across the wetter windward areas from Oahu to the Big Island. Kauai, which will remain west of the trough, will remain much drier.
By Sunday, the upper trough to the west-southwest of the state is expected to transition into a compact low aloft. The surface trough will sharpen as enhanced moisture persists over Maui County, Oahu, and perhaps to Kauai by Sunday afternoon. Widespread heavy rainfall and isolated to scattered thunderstorms appear likely within the moist, unstable airmass over the islands. Shower coverage will also likely increase over the Big Island late Saturday into Sunday, likely bringing snow to Mauna Kea and Mauna Loa summits. In addition, as the trough sharpens, northeast trade winds over Kauai and Oahu will increase, potentially to Wind Advisory levels tonight through Sunday.
Wet conditions will likely linger into Monday and possibly Tuesday as the upper-level low weakens somewhat, suggesting that chances for heavy rainfall will decrease as high clouds thin. Breezy northeast to east winds will diminish and shift to the east- southeast as a surface low and the low aloft move a little further west of the islands. Throughout this period, the bulk of the showers will continue to focus across windward areas. By Tuesday, models favor the return of moderate to locally breezy trade winds. Troughing aloft may linger west of the islands throughout the week, however, which suggests that a generally wet trade wind pattern may continue throughout much of next week.

Aviation

A surface trough over the central islands is helping to bring clouds, showers and thunderstorms to Maui County early this morning, which are expected to continue today. Additional showers and thunderstorms are expected to spread to Oahu today. This trough will remain over the area through the weekend. MVFR, and pockets of IFR, conditions remain over the islands from Oahu to the Big Island. Most of this is focused over the windward sides of the islands. As such, AIRMET Sierra remains in effect for these areas. This AIRMET is expected to remain in place through Saturday, and likely beyond.
AIRMET Zulu is in place for possible rime icing from Molokai to the Big Island. This too is expected to remain today, and may need to be expanded to Oahu.

Marine

A surface high pressure northwest of the state will drift south today and tomorrow. A trough near Maui and Big Island waters will slowly drift westward today, sharpening the pressure gradient over western waters. Moderate to locally fresh northeast winds will strengthen to moderate to locally near gale range by Sunday. The Small Craft Advisory has been expanded to included the Kaiwi Channel and waters westward but may need to be expanded as winds and seas spread.
The current northwest swell will continue to lower today. A series of moderate, medium to short period north swells will move through the local waters today into the first half of next week, helping to keep surf elevated along north facing shores. A long- period, moderate northwest swell (310 degrees) is expected to boost surf heights to advisory levels along north and west facing shores Sunday into Monday, before subsiding Tuesday. Choppy, east shore surf will become increasingly rough as the wind swell picks over the next couple of days and some wrap from the north swells mixes in. Background south swells will keep surf small along south facing shores today and tomorrow, then will gradually pick up early next week as a small, long- period swell fills in.

HFO Watches/Warnings/Advisories

Flood Watch through Sunday afternoon for Oahu,
Flood Watch through Sunday afternoon for Lanai Mauka, Kahoolawe, Maui Windward West, Maui Leeward West, Haleakala Summit, Molokai, Lanai Windward, Lanai Leeward, Lanai South, Maui Central Valley North, Maui Central Valley South, Windward Haleakala, Kipahulu, South Maui/Upcountry, South Haleakala.
Small Craft Advisory until 6 PM HST Sunday for Kauai Northwest Waters, Kauai Windward Waters, Kauai Leeward Waters, Kauai Channel, Oahu Windward Waters, Oahu Leeward Waters, Kaiwi Channel.

Data Courtesy of NOAA.gov