Photo Credit: Jason Weingardt

West Kaua’i

Today: Mostly cloudy. Numerous showers in the morning, then scattered showers in the afternoon. Locally heavy rainfall possible. Highs 77 to 82 near the shore to around 69 above 3000 feet. East winds around 10 mph in the morning becoming light. Chance of rain 70 percent.

Tonight: Mostly cloudy with numerous showers. Locally heavy rainfall possible. Lows around 66 near the shore to around 57 above 3000 feet. Southeast winds up to 15 mph shifting to the east after midnight. Chance of rain 70 percent.

Sunday: Mostly cloudy. Breezy. Numerous showers in the morning, then scattered showers in the afternoon. Locally heavy rainfall possible. Highs around 80 near the shore to around 69 above 3000 feet. East winds 10 to 20 mph. Chance of rain 70 percent.

South Kaua’i

Today: Cloudy. Showers in the morning, then numerous showers in the afternoon. Locally heavy rainfall possible. Highs 77 to 82. Light winds becoming east around 10 mph in the afternoon. Chance of rain 90 percent.

Tonight: Showers. Locally heavy rainfall possible. Lows around 65. East winds around 10 mph. Chance of rain near 100 percent.

Sunday: Cloudy. Showers in the morning, then numerous showers in the afternoon. Locally heavy rainfall possible. Highs 76 to 81. East winds 10 to 15 mph. Chance of rain 90 percent.

Kaua’i Mountains

Today: Cloudy with showers. Locally heavy rainfall possible. Highs 69 to 75 in the valleys to around 62 above 4000 feet. Southeast winds up to 10 mph. Chance of rain 90 percent.

Tonight: Breezy and showers. Locally heavy rainfall possible. Lows 58 to 64 in the valleys to around 55 above 4000 feet. Southeast winds up to 20 mph. Chance of rain near 100 percent.

Sunday: Breezy. Cloudy with showers. Locally heavy rainfall possible. Highs 69 to 75 in the valleys to around 61 above 4000 feet. Southeast winds up to 25 mph shifting to the east 10 to 25 mph in the afternoon. Chance of rain 90 percent.

North Kaua’i

Today: Cloudy with numerous showers. Locally heavy rainfall possible. Highs 71 to 79. Light winds becoming east up to 10 mph in the afternoon. Chance of rain 70 percent.

Tonight: Mostly cloudy with numerous showers. Locally heavy rainfall possible. Lows 59 to 68. East winds up to 15 mph. Chance of rain 70 percent.

Sunday: Breezy. Cloudy with numerous showers. Locally heavy rainfall possible. Highs 71 to 79. East winds up to 20 mph. Chance of rain 70 percent.

East Kaua’i

Today: Cloudy with showers. Locally heavy rainfall possible. Highs 69 to 80. Light winds becoming east up to 10 mph in the afternoon. Chance of rain 90 percent.

Tonight: Showers. Locally heavy rainfall possible. Lows 59 to 69. East winds 10 to 15 mph. Chance of rain near 100 percent.

Sunday: Cloudy and breezy. Showers in the morning, then numerous showers in the afternoon. Locally heavy rainfall possible. Highs 69 to 79. East winds 10 to 20 mph. Chance of rain 90 percent.

Detailed Forecast

Synopsis

An upper low southwest of the Hawaiian Islands continues to draw up a large band of deep tropical moisture within converging east to southeast wind flow. This unstable tropical moisture band will keep widespread rainfall in the forecast, heavy at times, with developing thunderstorms across much of the state into Sunday. The low will slowly weaken from Sunday into Monday with decreasing rainfall trends as more stable trade winds begin to build in from the east. Another round of unstable showers may develop by Tuesday and Wednesday as another cut off low drops in west of the island chain. More stable conditions return by next week Thursday as a high pressure ridge builds in from the northeast.

Discussion

Satellite imagery this morning continues to show an upper level low southwest of the Hawaiian Islands drawing up a deep unstable plume of tropical moisture across the state. A surface trough with converging east to southeast winds under strong divergence aloft will produce continued wet weather statewide into Sunday. Moderate to heavy rainfall and thunderstorms are likely threats with this low pressure system for all islands. Southeasterly wind flow near the Big Island has lifted this deep tropical moisture up the southeastern mountain slopes producing widespread rainfall accumulations of 5 to 10 inches of total rainfall over the past 24 hours.
A Flood Watch remains in effect for all Hawaiian Islands through this afternoon due to continued moderate to heavy rainfall on already saturated soil conditions, elevating the threat for flash flooding. This Flood Watch may need to be extended into Sunday as threats for flooding may continue for some islands through the weekend. A Winter Storm Warning also remains in effect for the Big Island summits until 6 PM HST this evening due to heavy snow and freezing rain. This warning will likely be extended into Sunday.
In the short term, the upper level low will slowly weaken through Sunday, decreasing the rainfall impacts over the next 24 to 36 hours. The wind directions will also shift to easterly trade winds over the eastern islands of Maui and the Big Island by Sunday as a high pressure ridge builds in from the northeast, potentially pushing the tropical moisture plume further westward, and decreasing rainfall trends for these islands. These more stable easterly trades appear to spread to the western islands, with a brief decrease in shower intensity statewide from Monday into early Tuesday.
In the longer range forecast, the American (GFS) model continues to show another cut off low dropping into the region west of the state. This next system may pull up additional tropical moisture northward into the islands from Tuesday into Wednesday producing another brief period of enhanced shower activity. The latest trends are hinting that the heavier showers in this band may favor the western islands of Kauai, Niihau and Oahu. However, slight changes to the position and intensity of the next upper low may produce significant changes to the island by island rainfall impacts. The 18/00Z European (ECMWF) model keeps drier trade wind trends in the forecast for the eastern half of the state for the Tuesday to Wednesday time period with wetter trends for the western islands. Our rainfall forecast for this time period is based on a blend of both solutions along with enhanced showers over windward areas in a breezy trade wind weather pattern. Stay tuned as the forecast weather and rainfall conditions for each island will likely change as the time period grows shorter and the weather pattern evolves.
Both models agree that by late next week a high pressure ridge will build in from the northeast, producing more stable conditions as breezy to windy trades push the unstable tropical moisture band further westward away from the islands.

Aviation

A deep tropical airmass will remain anchored over the islands through the forecast period. MVFR with periods of IFR will be common over windward and southeast-facing slopes of the Big Island. Elsewhere, light rain with intermittent heavier showers will bring transient reductions to flight category. Abundant low- level moisture will also tend to keep low clouds anchored along terrain. Light to moderate southeast winds will become breezy out of ESE during the latter half of the day.
AIRMET Sierra for mountain obscuration is in effect for Oahu through Maui.
AIRMET IFR is in effect for portions of the Big Island.
AIRMET Tango for moderate turbulence in the FL180/FL420 layer is in effect for the entire area. SIGMET series Victor covers an area of severe turbulence over the western half of the area.
AIRMET Zulu for moderate icing in the FL140/FL250 layer is in effect for Oahu through Big Island. Poor saturation in the favored icing layer is evident on the 12z Lihue sounding and warranted cancelling Zulu for the immediate Kauai area.

Marine

Strong high pressure well northeast of the state will keep fresh to strong east to southeast winds in place across the windward waters through the weekend, with lighter winds continuing over most leeward waters. Winds will strengthen and become more easterly next week, with strong to near-gale force winds gradually expanding to more marine zones as the week progresses. A Small Craft Advisory (SCA) remains in effect through 6 PM Saturday for all windward waters due to a combination of winds and seas. This SCA will likely need to be extended for most windward waters through the middle of next week.
Surf will remain elevated and well above normal levels along east facing shores during the next 7 days due to persistent fresh to strong east to southeast winds over and upstream of the island chain. A High Surf Advisory remains in effect for these areas until 6 PM this evening. East shore surf should fall below the advisory threshold briefly tonight and Sunday, but will likely build back to advisory levels Sunday night through late next week. No other significant swells are expected through late next week, with only small surf expected along north, west, and south facing shores.

HFO Watches/Warnings/Advisories

Flood Watch through this afternoon for all Hawaiian Islands.
High Surf Advisory until 6 PM HST this evening for for east facing shores of Kauai, Oahu, Molokai, Maui, and the Big Island.
Winter Storm Warning until 6 PM HST this evening for Big Island Summits.
Small Craft Advisory until 6 PM HST this evening for Kauai Northwest Waters, Kauai Windward Waters, Kauai Channel, Oahu Windward Waters, Kaiwi Channel, Maui County Windward Waters, Pailolo Channel, Alenuihaha Channel, Big Island Windward Waters, Big Island Southeast Waters.

Data Courtesy of NOAA.gov