Photo Credit: Braden Jarvis

West Kaua’i

Today: Showers and isolated thunderstorms. Locally heavy rainfall possible. Highs around 83 near the shore to around 72 above 3000 feet. South winds up to 15 mph shifting to the west in the afternoon. Chance of rain near 100 percent.

Tonight: Mostly cloudy. Showers and isolated thunderstorms in the evening, then scattered showers after midnight. Locally heavy rainfall possible in the evening. Lows 65 to 72 near the shore to around 55 above 3000 feet. West winds up to 15 mph shifting to the northwest after midnight. Chance of rain 90 percent.

Monday: Partly sunny with scattered showers in the morning, then mostly sunny with isolated showers in the afternoon. Highs around 80 near the shore to around 69 above 3000 feet. Northwest winds 10 to 15 mph. Chance of rain 40 percent.

South Kaua’i

Today: Breezy. Cloudy with showers and isolated thunderstorms. Locally heavy rainfall possible. Highs around 80. Southwest winds up to 20 mph shifting to the west in the afternoon. Chance of rain 90 percent.

Tonight: Mostly cloudy. Showers and isolated thunderstorms in the evening, then scattered showers after midnight. Locally heavy rainfall possible in the evening. Lows 62 to 71. West winds 10 to 15 mph. Chance of rain 80 percent.

Monday: Breezy. Partly sunny with scattered showers. Highs around 77. West winds 10 to 20 mph. Chance of rain 50 percent.

Kaua’i Mountains

Today: Breezy. Showers and isolated thunderstorms. Locally heavy rainfall possible. Highs 71 to 77 in the valleys to around 63 above 4000 feet. Southwest winds up to 20 mph. Chance of rain near 100 percent.

Tonight: Mostly cloudy. Breezy. Showers and isolated thunderstorms in the evening, then scattered showers after midnight. Locally heavy rainfall possible in the evening. Lows 54 to 62 in the valleys to around 50 above 4000 feet. West winds up to 20 mph. Chance of rain 90 percent.

Monday: Partly sunny with scattered showers. Highs 68 to 74 in the valleys to around 60 above 4000 feet. West winds up to 15 mph. Chance of rain 50 percent.

North Kaua’i

Today: Breezy. Cloudy with showers and isolated thunderstorms. Locally heavy rainfall possible. Highs 73 to 83. West winds up to 20 mph. Chance of rain 90 percent.

Tonight: Mostly cloudy. Showers and isolated thunderstorms in the evening, then scattered showers after midnight. Locally heavy rainfall possible in the evening. Lows 59 to 70. West winds up to 15 mph increasing to 10 to 15 mph after midnight. Chance of rain 90 percent.

Monday: Breezy. Partly sunny with scattered showers. Highs 70 to 80. West winds 10 to 20 mph. Chance of rain 50 percent.

East Kaua’i

Today: Breezy. Showers and isolated thunderstorms. Locally heavy rainfall possible. Highs 72 to 82. Southwest winds up to 20 mph. Chance of rain near 100 percent.

Tonight: Mostly cloudy. Showers and isolated thunderstorms in the evening, then scattered showers after midnight. Locally heavy rainfall possible in the evening. Lows 55 to 73. West winds up to 15 mph. Chance of rain 80 percent.

Monday: Breezy. Partly sunny with scattered showers. Highs 69 to 79. Northwest winds up to 20 mph. Chance of rain 50 percent.

Detailed Forecast

Synopsis

The potential for heavy rainfall and thunderstorms associated with a front will continue through Monday, initially over the western end of the state through tonight, then over the eastern end on Monday. Conditions will gradually improve from west to east through the first half of the week as high pressure builds to the north in the wake of the front. Breezy southerly winds ahead of the front will hold today, then ease into the light to moderate range as they shift to the north, then to a more typical trade wind direction around midweek. Expect a more typical trade wind pattern to return with showers favoring windward and mauka locations through the second half of the week.

Discussion

The short-term guidance remains in good agreement and depicts the active weather pattern persisting through Monday due to an upper low to the northwest and its attendant surface front forecasted to slowly advance down the island chain. A shortwave pulse rotating through the base of the upper-level trough to the northwest has triggered the next round of heavy showers and storms that is spreading northward over Niihau and Kauai. Provided the saturated soils and elevated streams from the recent heavy rainfall over the past few days, it won't take much for them to respond this morning. Expect this threat to reach Oahu later this morning, then to Maui County later today through Monday. As a result, the Flood Watch has been expanded and now includes all of the smaller islands.
In addition to the expected heavy rainfall, a few thunderstorms could become strong provided the instability and shear profiles, particularly over Kauai and Oahu today. Also, breezy southerly winds could translate to strong downsloping winds over windward areas through this time, especially in the heavy showers/storms moving through.
A drying trend is anticipated from west to east, beginning over Kauai late tonight, then over Oahu through the day Monday as the front advances eastward to Maui County and the Big Island. Conditions should steadily improve over the eastern end of the state Tuesday through Wednesday as the upper level trough exits or shifts northeastward and away from the state. The low-level flow will return to a more typical trade wind direction Wednesday through the second half of the week as surface high pressure builds north of the state.

Aviation

The tail end of a Central Pacific cold front will strengthen as it passes over the western Hawaiian Islands today. An organized band of intense showers associated with with the leading edge of this front can be seen on late night radar slowly approaching western Oahu. These showers, which are accompanied by isolated embedded thunderstorms, are pressing quickly eastward even as they expand to the south.
Surface winds will veer from southeast to west-southwest with the frontal passage. Ahead of the front, light showers will favor south and southeast facing slopes and coasts. Behind the front, moderate showers and embedded thunderstorms will favor west and southwest facing slopes and coasts.
We anticipate issuing AIRMET Sierra for mountain obscuration for Kauai and Oahu later this morning. It will likely need to be expanded to included Molokai and Maui sometime this afternoon.
AIRMET Tango remains for moderate upper level turbulence as been canceled, but conditions are likely to redevelop later today as a new upper level trough approaches from the northwest.
AIRMET Zulu remains in effect for moderate rime icing in cloud, in precipitation, between FL230 and FL280 due to upper level layered clouds. This AIRMET will likely be needed through today and into tonight.

Marine

South to southeast winds will strengthen ahead of an approaching front today. A Small Craft Advisory remains posted for coastal waters west of and including the Kaiwi Channel due to winds. The front will slowly push southeast late today through Monday, before stalling out and slowly dissipating near the Big Island Tuesday and Wednesday. Light to moderate west to northwest winds will fill in behind the front. Winds will become rather light by Monday night and continue through Wednesday, with moderate trades potentially returning for the latter part of the week.
North shore surf will remain small today. Surf will trend up tonight and Monday as a medium-period northwest swell arrives. Surf could reach low end advisory levels along north facing shores Monday night and Tuesday as a second, more north-northwest pulse arrives. This swell will then decline Wednesday through late in the week. Surf along east facing shores will continue to slowly decline today and Monday, with only small very small surf expected through the remainder of the upcoming work week. Small, medium to long period south swell will keep some small surf rolling into south facing shores through late in the week.

HFO Watches/Warnings/Advisories

Flood Watch through late tonight for Niihau, Kauai,
Flood Watch through Monday afternoon for Oahu and Maui County.
Wind Advisory from noon today to 6 PM HST Monday for Haleakala Summit.
Winter Weather Advisory until 6 AM HST Monday for Big Island Summits.
High Wind Warning from noon today to 6 PM HST Monday 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, Oahu Leeward Waters, Kaiwi Channel.

Data Courtesy of NOAA.gov