West Side

Today: Partly sunny in the morning then becoming cloudy. Isolated showers. Highs 72 to 82. East winds up to 15 mph shifting to the southwest in the afternoon. Chance of rain 20 percent.

Tonight: Cloudy. Isolated showers in the evening, then showers after midnight. Locally heavy rainfall possible after midnight. Lows 63 to 70. Light winds. Chance of rain 80 percent.

Sunday: Cloudy with showers. Locally heavy rainfall possible. Highs 68 to 80. Light winds. Chance of rain 80 percent.

South Side

Today: Mostly sunny in the morning, then cloudy with isolated showers in the afternoon. Highs 73 to 84. Light winds. Chance of rain 20 percent.

Tonight: Cloudy. Numerous showers after midnight. Locally heavy rainfall possible after midnight. Lows around 67. Light winds. Chance of rain 70 percent.

Sunday: Cloudy with showers. Locally heavy rainfall possible. Highs 70 to 80. Light winds becoming north up to 15 mph in the afternoon. Chance of rain 80 percent.

North Shore

Today: Cloudy. Numerous showers in the morning, then scattered showers in the afternoon. Highs 75 to 80 near the shore to around 64 near 5000 feet. Southeast winds up to 15 mph. Chance of rain 70 percent.

Tonight: Scattered showers in the evening, then showers after midnight. Locally heavy rainfall possible after midnight. Lows around 68 near the shore to around 52 near 5000 feet. Southeast winds up to 15 mph. Chance of rain near 100 percent.

Sunday: Breezy and showers. Locally heavy rainfall possible. Highs 75 to 80 near the shore to around 62 near 5000 feet. Southeast winds up to 20 mph. Chance of rain near 100 percent.

Central Maui

Today: Mostly sunny in the morning then becoming cloudy. Isolated showers. Highs 79 to 84. East winds up to 15 mph. Chance of rain 20 percent.

Tonight: Cloudy. Isolated showers in the evening, then numerous showers after midnight. Locally heavy rainfall possible after midnight. Lows around 67. Light winds. Chance of rain 70 percent.

Sunday: Cloudy with showers. Locally heavy rainfall possible. Highs around 78. East winds up to 15 mph. Chance of rain 80 percent.

Upcountry

Today: Partly sunny in the morning then becoming cloudy. Scattered showers. Highs 49 to 67. West winds up to 15 mph. Chance of rain 50 percent.

Tonight: Scattered showers in the evening, then showers after midnight. Locally heavy rainfall possible after midnight. Lows around 45 at the visitor center to around 42 at the summit. West winds up to 15 mph. Chance of rain near 100 percent.

Sunday: Breezy. Showers in the morning, then numerous showers in the afternoon. Locally heavy rainfall possible. Highs 47 to 65. Southwest winds up to 20 mph shifting to the southeast in the afternoon. Chance of rain near 100 percent.

East Maui

Today: Cloudy. Numerous showers in the morning, then scattered showers in the afternoon. Highs 75 to 80 near the shore to around 64 near 5000 feet. Southeast winds up to 15 mph. Chance of rain 70 percent.

Tonight: Scattered showers in the evening, then showers after midnight. Locally heavy rainfall possible after midnight. Lows around 68 near the shore to around 52 near 5000 feet. Southeast winds up to 15 mph. Chance of rain near 100 percent.

Sunday: Breezy and showers. Locally heavy rainfall possible. Highs 75 to 80 near the shore to around 62 near 5000 feet. Southeast winds up to 20 mph. Chance of rain near 100 percent.

Lanai City

Today: Partly sunny in the morning, then cloudy with scattered showers in the afternoon. Highs 66 to 77. Northeast winds up to 10 mph in the morning becoming light. Chance of rain 50 percent.

Tonight: Cloudy. Isolated showers in the evening, then showers after midnight. Locally heavy rainfall possible after midnight. Lows 58 to 65. Light winds. Chance of rain 80 percent.

Sunday: Cloudy. Showers and isolated thunderstorms in the morning, then showers in the afternoon. Locally heavy rainfall possible. Highs 64 to 75. Light winds becoming east up to 10 mph in the afternoon. Chance of rain 80 percent.

Kaunakakai

Today: Partly sunny in the morning, then cloudy with scattered showers in the afternoon. Highs 65 to 82. East winds 10 to 15 mph shifting to the southeast in the afternoon. Chance of rain 40 percent.

Tonight: Cloudy. Showers after midnight. Locally heavy rainfall possible after midnight. Lows 53 to 69. Light winds. Chance of rain 80 percent.

Sunday: Cloudy. Showers and isolated thunderstorms in the morning, then showers in the afternoon. Locally heavy rainfall possible. Highs 63 to 80. Light winds becoming southeast up to 15 mph in the afternoon. Chance of rain 90 percent.

Detailed Forecast

Synopsis

A strong high pressure system far north of the region will be disrupted by a developing kona low forming just north of Kauai later tonight. Trade winds will break down today and quickly veer from a more southeasterly to southerly direction by tonight. Deep unstable tropical moisture will produce bands of moderate to heavy showers across all islands. The low center will move southeastward, passing just north of Kauai on Sunday, setting up west of the island chain on Monday. Expect elevated threats for flooding as these shower bands train across each island. Strengthening easterly winds as the ridge briefly builds back into the region will gradually reduce shower trends by Tuesday.

Discussion

A strong 1032 mb surface high pressure system will remain in place far north of the Hawaiian Islands this week. Water vapor satellite imagery this morning clearly shows a deepening upper level low roughly 800 miles north of Kauai, diving quickly south towards the Hawaii region. This upper level system will deepen over the next 12 to 18 hours producing a surface kona low just north of Kauai. The surface low will follow the upper low on a southwesterly track passing just north of Kauai on Sunday, and then setting up west of the state by Monday. Trade winds will initially break down today, then veer from a more southeasterly to southerly direction and strengthen from tonight into Tuesday. Wind speeds may briefly exceed Wind Advisory thresholds for most islands as the wind speeds peak on Monday, more details on this later as the time period grows shorter.
This kona low weather pattern will bring in a wet weather pattern for all islands under the influence of deep unstable tropical moisture across the region. Expect bands of moderate to heavy showers affecting all islands from late tonight through at least Monday. The threat from these shower bands potentially training over each island has caused us to issue a statewide Flood Watch from late tonight through Monday evening. Unstable showers may linger over some islands into Tuesday, which may extend the Flood Watch threat over these islands for one more day. Thunderstorms are likely in this pattern, favoring the western islands of Niihau, Kauai, Oahu, Molokai, and Lanai through Monday. The Big Island may see isolated thunderstorm activity by Tuesday. Island by island impacts will highly depend on how these smaller scale (mesoscale) shower bands form up and interact with island mountains. Large scale lifting under the influence of the upper level cold core low and strong winds lifting these unstable clouds over island mountains will be the primary drivers of heavy rain, thunderstorm and flooding impacts. Take extra caution during this time period, as kona low weather patterns are one of the primary sources for our most severe heavy rainfall events in the Hawaiian Islands.
Deepening moisture will spread clouds and showers to even the highest summits of the Big Island during this event. Snow levels will likely fall to around the 11,000 foot elevation level, bringing periods of snow and freezing rain to the summits of Mauna Loa and Mauna Kea on the Big Island. Our confidence on snowfall and icing amounts remain too low for any winter weather headlines at this time. Stay tuned as roads to the summit may become treacherous due to wintery weather during this time period.
Breezy east to southeast winds will slowly decline Tuesday and Wednesday as the low continues on a westward journey away from Hawaii, and a weak ridge briefly builds back into the region. A weak troughing pattern lingers near Kauai throughout the week, and there is some added uncertainty as to when the enhanced shower activity ends over the Garden Isle.

Aviation

A band of showers continues to move into the windward slopes of the Big Island, Maui, Molokai, Oahu and Kauai this morning. Expect MVFR conditions to continue into the morning hours along the north and east slopes of each island. MVFR conditions should decrease in coverage this afternoon for most islands, with the exception of the windward Big Island slopes where low clouds and showers will linger through much of the day.
AIRMET Sierra is in effect for mountain obscuration for northern through southeastern sections of most islands due to consistent clouds and showers. This AIRMET will continue for most islands this morning then decrease in coverage as clouds lift in the afternoon hours.
No other AIRMETs in effect.

Marine

Trade winds will briefly weaken today and veer southeastward by tonight across the coastal waters as a low pressure system develops north of Kauai and begins to track southwestward. As this low passes just west of the state, strong to near gale-force east to east-southeasterly winds are expected to build Sunday evening, then continue through the first half of next week. The disturbance will bring rough seas, periods heavy rain, and isolated thunderstorms.
A moderate, medium-period north to north-northeast swell (360-030 deg) will continue to produce elevated surf that is just below advisory thresholds along north facing shores today. This swell will then slowly decrease through the rest of the weekend. In addition, this swell will likely produce moderate surges for north facing harbors, especially for Kahului and Hilo. A Marine Weather Statement will remains in effect for maritime interests to watch for these impending harbor surges. Meanwhile, a small, medium- period northwest (310-320 deg) swell will gradually decline through Monday. Another small, long period northwest (310-320 deg) swell is expected to arrive on Tuesday.
Aside from areas exposed to wrapping north-northeast swell, surf along east facing shores is expected to lower this weekend as trade winds weaken. However, a fetch of strong to near gale-force winds will significantly increase easterly wind swell from Sunday night through the first half of the week, which should exceed advisory thresholds for east facing shores.
Surf along south facing shores will remain tiny through the forecast period.
Peak astronomical monthly tides with water levels running roughly 0.5 ft above normal could produce minor coastal flooding through the weekend, and may be enhanced along north facing shores due to the incoming north swell. A Coastal Flood Statement will remain in effect to highlight this flooding potential, especially during the daily peak high tide cycle.

HFO Watches/Warnings/Advisories

Flood Watch from late tonight through Monday afternoon for all Hawaiian Islands.

Data Courtesy of NOAA.gov