West Kaua’i

Today: Showers early in the morning, then numerous showers in the late morning and afternoon. Highs around 83 near the shore to around 69 above 3000 feet. Southeast winds up to 10 mph. Chance of rain near 100 percent.

Tonight: Mostly cloudy. Numerous showers and isolated thunderstorms early in the evening, then scattered showers and isolated thunderstorms in the late evening and overnight. Locally heavy rainfall possible in the late evening and overnight. Lows around 71 near the shore to around 61 above 3000 feet. East winds up to 10 mph. Chance of rain 60 percent.

Friday: Mostly cloudy. Scattered showers and isolated thunderstorms early in the morning, then isolated showers and thunderstorms in the late morning and afternoon. Locally heavy rainfall possible early in the morning. Highs around 83 near the shore to around 69 above 3000 feet. Southeast winds up to 10 mph. Chance of rain 40 percent.

South Kaua’i

Today: Mostly cloudy. Numerous showers early in the morning, then scattered showers late in the morning. Numerous showers in the afternoon. Highs 76 to 82. East winds up to 10 mph. Chance of rain 60 percent.

Tonight: Mostly cloudy. Numerous showers and isolated thunderstorms early in the evening, then scattered showers and isolated thunderstorms in the late evening and overnight. Locally heavy rainfall possible in the late evening and overnight. Lows around 70. Light winds. Chance of rain 60 percent.

Friday: Mostly cloudy. Scattered showers and isolated thunderstorms early in the morning, then isolated showers and thunderstorms in the late morning and afternoon. Locally heavy rainfall possible early in the morning. Highs 76 to 83. Southwest winds up to 10 mph. Chance of rain 40 percent.

Kaua’i Mountains

Today: Mostly cloudy with numerous showers. Highs 71 to 82 in the valleys to around 63 above 4000 feet. Southeast winds up to 10 mph. Chance of rain 70 percent.

Tonight: Mostly cloudy. Numerous showers and isolated thunderstorms early in the evening, then scattered showers and isolated thunderstorms in the late evening and overnight. Locally heavy rainfall possible in the late evening and overnight. Lows around 67 in the valleys to around 59 above 4000 feet. South winds up to 10 mph. Chance of rain 60 percent.

Friday: Mostly cloudy. Scattered showers and isolated thunderstorms early in the morning, then isolated showers and thunderstorms in the late morning and afternoon. Locally heavy rainfall possible early in the morning. Highs 71 to 83 in the valleys to around 64 above 4000 feet. Southwest winds up to 15 mph. Chance of rain 40 percent.

North Kaua’i

Today: Partly sunny. Scattered showers in the morning, then numerous showers in the afternoon. Highs 74 to 83. South winds up to 10 mph. Chance of rain 70 percent.

Tonight: Mostly cloudy. Numerous showers and isolated thunderstorms early in the evening, then scattered showers and isolated thunderstorms in the late evening and overnight. Locally heavy rainfall possible in the late evening and overnight. Lows 64 to 72. South winds up to 10 mph. Chance of rain 60 percent.

Friday: Mostly cloudy. Scattered showers and isolated thunderstorms early in the morning, then isolated showers and thunderstorms in the late morning and afternoon. Locally heavy rainfall possible early in the morning. Highs 74 to 83. Light winds. Chance of rain 40 percent.

East Kaua’i

Today: Mostly cloudy. Scattered showers in the morning, then numerous showers in the afternoon. Highs 71 to 81. South winds up to 10 mph. Chance of rain 60 percent.

Tonight: Mostly cloudy. Numerous showers and isolated thunderstorms early in the evening, then scattered showers and isolated thunderstorms in the late evening and overnight. Locally heavy rainfall possible in the late evening and overnight. Lows 65 to 73. South winds up to 10 mph. Chance of rain 60 percent.

Friday: Mostly cloudy. Scattered showers and isolated thunderstorms early in the morning, then isolated showers and thunderstorms in the late morning and afternoon. Locally heavy rainfall possible early in the morning. Highs 72 to 83. South winds up to 10 mph. Chance of rain 40 percent.

Detailed Forecast

Synopsis

A kona low system will move in from the west today, expect increasing showers and thunderstorms with southerly winds affecting each island starting tonight and lasting through the weekend. The threats for heavy rain and thunderstorms will become widespread, with each island affected by pulses of showers through the event as multiple surface lows pass just north of the island chain. A combination of low level forcing and passing upper level troughs will drive the smaller scale heavy rain band and thunderstorm formation. Improving weather trends will develop from west to east on Monday as we transition to a wet trade wind weather pattern lasting through the end of next week.

Discussion

Deep cloud cover continues to develop over the Hawaii region this morning in response to a low pressure system moving in from the west with upper level troughing and sub tropical jet stream support. Winds will veer more southerly tonight as shower activity slowly increases across the state, pulling up additional moisture from the deep tropics, ahead of the approaching kona low. Two weak surface lows, supported by upper level troughing, will drift through the islands from later tonight through Sunday. The large scale forcing will move the heaviest showers to different islands in pulses of activity throughout this event.
A Flood Watch remains in effect for most of the Hawaiian Islands for this event. This watch may be expanded statewide as run to run model guidance continues to show differences in where and when the heaviest showers may develop.
The first pulse of increasing moderate to heavy showers and thunderstorms will move in ahead of the first surface low passing just north of the island of Kauai from tonight into Friday. The latest weather model consensus shows the best forcing for increasing moderate to heavy shower activity develops along a wind convergence band favoring the islands in Maui County and Oahu. The next pulse shifts into gear from Friday night into Saturday as the next surface low moves northeastward passing just north of Kauai. The band over Maui County shifts northward to Kauai, Niihau and Oahu during this time period. Stronger upper level dynamics from a passing trough, bulk wind shear from 35 to 50 knots and instability suggests stronger rotating thunderstorms will likely develop near the western islands. Kauai County may see increasing chances for heavy showers and thunderstorms from Friday night into Saturday.
By Sunday, the surface low rapidly moves away from the Hawaiian Islands towards the northeast direction. Cool northerly to northeasterly winds begin to filter into the western islands of Niihau, Kauai and Oahu with drying trends by Sunday night. Western islands in Maui County and the Big Island will continue to see periods of moderate to heavy showers.
On Monday, the heavy shower and thunderstorm threat ends as we quickly transition back to a northeasterly trade to last into Thursday with wind pattern. Cooler temperatures are expected to last into Thursday with enhanced showers due to a weak upper level troughing pattern, showers will favor the typical windward and mountain areas into next weekend.

Aviation

Conditions will gradually deteriorate through the day as an upper-level disturbance and surface low approach from the west. Expect increasing chances for widespread rainfall across portions of the state. The low-level flow will gradually veer from the southeast today to south-southwest by Friday, with speeds increasing into the moderate range at exposed terminals. Expect periods of MVFR/IFR CIGS/VSBYS, especially in showers.
AIRMET Sierra for mountain obscuration can also be expected for much of the area by tonight. AIRMET Tango is in effect for moderate turbulence for FL200-350. These conditions are expected to persist today. AIRMET Zulu for light icing in layer 150-FL250 is also anticipated.

Marine

Light to locally fresh east to southeast winds will slowly strengthen to moderate speeds and veer south by Friday as a ridge strengthens northeast of the state and a broad area of low pressure develops west of the state. The low will strengthen over or near the far northwest offshore waters on Friday before lifting northeast on Saturday. Winds will shift out of the south southwest during this time at moderate speeds. On Sunday, winds will become light and variable for a brief time as the low exits to the northeast of the state while a trailing trough remains draped over the state. A strong high then builds north of the state by Monday, allowing moderate to locally strong northeast winds to develop during the first half of next week.
A moderate, medium period north swell will continue to trend down today, with moderate surf along north facing shores becoming small by this afternoon. The swell continues to gradually decline on Friday as it shifts out of the north-northeast, but experiences a small reinforcing pulse as a strengthening low passes to the north of the islands on Saturday. A new small to moderate long period west- northwest swell is expected to arrive late Saturday and hold through early next week, which will keep at least small surf along west facing shores.
South shores will see continued moderate surf just below advisory criteria as a long period south swell holds on today before diminishing tonight. The next small, long period south swell will fill in Saturday and peak Sunday, providing a small boost in surf along south facing shores before fading into early next week. Surf along east facing shores will remain well below average due to the lack of trades locally and upstream through the weekend. The exception will be for east facing shores exposed to north-northeast swells through the weekend. Choppy surf along east facing shores looks to trend up next week with increasing east to northeasterly winds.

HFO Watches/Warnings/Advisories

Flood Watch from this evening through Sunday afternoon for Oahu, Maui County, and the Big Island of Hawaii.

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Data Courtesy of NOAA.gov