West Kaua’i

Tonight: Windy. Occasional showers and scattered thunderstorms. Locally heavy rainfall possible. Lows around 68 near the shore to around 60 above 3000 feet. South winds 15 to 25 mph with gusts to 45 mph increasing to 15 to 35 mph with gusts to 55 mph after midnight. Chance of rain near 100 percent.

Friday: Windy. Occasional showers and scattered thunderstorms. Locally heavy rainfall possible. Highs around 80 near the shore to around 69 above 3000 feet. South winds 20 to 35 mph with gusts to 55 mph. Chance of rain near 100 percent.

Friday Night: Windy. Occasional showers and scattered thunderstorms until early morning, then scattered showers and isolated thunderstorms early in the morning. Locally heavy rainfall possible until early morning. Lows around 69 near the shore to around 61 above 3000 feet. Southwest winds 20 to 30 mph with gusts to 55 mph. Chance of rain near 100 percent.

South Kaua’i

Tonight: Windy. Occasional showers and scattered thunderstorms. Locally heavy rainfall possible. Lows around 68. South winds 15 to 20 mph increasing to 15 to 30 mph after midnight. Gusts up to 50 mph. Chance of rain near 100 percent.

Friday: Windy. Occasional showers and scattered thunderstorms. Locally heavy rainfall possible. Highs 74 to 80. South winds 20 to 30 mph with gusts to 60 mph. Chance of rain near 100 percent.

Friday Night: Breezy. Occasional showers and scattered thunderstorms until early morning, then scattered showers and isolated thunderstorms early in the morning. Locally heavy rainfall possible until early morning. Lows around 69. Southwest winds 20 to 25 mph. Gusts up to 55 mph decreasing to 45 mph after midnight. Chance of rain near 100 percent.

Kaua’i Mountains

Tonight: Very windy. Occasional showers and scattered thunderstorms. Locally heavy rainfall possible. Lows 62 to 67 in the valleys to around 58 above 4000 feet. South winds 10 to 30 mph increasing to 10 to 40 mph after midnight. Gusts up to 60 mph. Chance of rain near 100 percent.

Friday: Very windy. Occasional showers and scattered thunderstorms. Locally heavy rainfall possible. Highs 71 to 77 in the valleys to around 62 above 4000 feet. Southwest winds 15 to 40 mph with gusts to 60 mph. Chance of rain near 100 percent.

Friday Night: Very windy. Occasional showers and scattered thunderstorms until early morning, then numerous showers and isolated thunderstorms early in the morning. Locally heavy rainfall possible until early morning. Lows 63 to 68 in the valleys to around 59 above 4000 feet. Southwest winds 15 to 40 mph with gusts to 60 mph. Chance of rain near 100 percent.

North Kaua’i

Tonight: Breezy. Occasional showers and scattered thunderstorms. Locally heavy rainfall possible. Lows 62 to 70. South winds 10 to 25 mph with gusts to 60 mph. Chance of rain near 100 percent.

Friday: Breezy. Occasional showers and scattered thunderstorms. Locally heavy rainfall possible. Highs 72 to 80. South winds 10 to 25 mph with gusts to 65 mph. Chance of rain near 100 percent.

Friday Night: Breezy. Occasional showers and scattered thunderstorms until early morning, then scattered showers and isolated thunderstorms early in the morning. Locally heavy rainfall possible until early morning. Lows 63 to 71. Southwest winds 15 to 25 mph with gusts to 65 mph. Chance of rain near 100 percent.

East Kaua’i

Tonight: Windy. Occasional showers and scattered thunderstorms. Locally heavy rainfall possible. Lows 63 to 72. South winds 15 to 20 mph increasing to 15 to 30 mph after midnight. Gusts up to 60 mph. Chance of rain near 100 percent.

Friday: Windy. Occasional showers and scattered thunderstorms. Locally heavy rainfall possible. Highs 69 to 79. South winds 15 to 30 mph with gusts to 60 mph. Chance of rain near 100 percent.

Friday Night: Breezy. Occasional showers and scattered thunderstorms until early morning, then scattered showers and isolated thunderstorms early in the morning. Locally heavy rainfall possible until early morning. Lows 64 to 73. Southwest winds 15 to 25 mph with gusts to 60 mph. Chance of rain near 100 percent.

Detailed Forecast

Synopsis

A powerful kona storm will near the state producing periods of hazardous weather impacts across the Hawaiian Islands through the weekend. Expect a combination of threats ranging from significant flash flooding, damaging winds, strong to severe thunderstorms, and snow and ice over the highest Big Island summits. Southwesterly winds will be strongest from Friday to Saturday and will produce damaging wind gusts along north and east of the island mountains. Unsettled wet weather conditions with southwesterly winds will continue into early next week. The kona storm begins to lift northward on Monday, and unsettled weather will start to ease across the Hawaii region. High pressure will build back in north of the state allowing trade winds to return from next Wednesday onward.

Discussion

Latest satellite imagery shows a complex surface low 1500 sm northwest of Kauai with bands of widely scattered to numerous moderate thunderstorms along the eastern flank of the complex. One of these bands with isolated moderate thunderstorms is currently moving through the state producing widespread light to moderate showers along the the west Big Island coast moving north over the island of Maui. A brief break in the rain will occur after this band moves through rest of this afternoon with another band setting up right behind this one with a north northeast track. Latest high resolution model guidance shows moderate to heavy showers reorganizing and training from south to north over Kauai early this evening with the passage of this second band of showers. Increasing probability of moderate to heavy showers and thunderstorms will continue into the overnight hours as the kona low sags southeast and upper level dynamics intensifies, leading to showers intensifying and slowly spreading from Kauai to Oahu overnight and early morning Friday.
Expect a well organized band of moderate to heavy showers to spread state wide Friday associated with the kona low. In addition, southwesterly winds will ramp up with winds speed up into the 50 to 70 MPH range around the 5,000 foot level as the kona low inches closer to the state. A High Wind Warning has been issued for the smaller islands and for the summits of Haleakala and the Big Island as strong southwesterly winds develop around Kauai and then spread through Maui County through the day Friday. A High Wind Watch for the lower elevations of the Big Island remains in effect as stronger winds may not arrive till Friday afternoon or evening thus will continue to monitor. Already saturated ground conditions and trees unaccustomed to southwest winds will fall and likely cause blocked roadways and lead to power outages. A Winter Storm Warning has been issued for the summits of the Big Island as increasing chances of precipitation and freezing temperature aloft will generate periods of heavy snow into the weekend.
High levels of instability, wind shear and atmospheric forcing ingredients are all coming into focus over the islands from Friday through Saturday for a potential severe weather event. A Severe Thunderstorm Watch may be needed for Friday through Saturday time period. However, we will likely wait until we see these severe thunderstorm developing over the local area before pulling the trigger on this Severe Thunderstorm Watch with a short lead time measured in hours. Outside of terrain enhancing wind gusts any strong thunderstorms are capable of producing severe hurricane force wind gusts over islands or coastal waters through Saturday.
From Saturday night to Sunday another upper level short wave trough passes over the islands leading to another upward pulse in moderate to heavy shower activity. Thunderstorms will also threaten the islands and become more of a flooding threat as decreasing upper level and surface wind speeds diminish the damaging wind gust threats. The threat for Flash Flooding will continue into the weekend due to both saturated soil conditions that more easily lead to ground runoff and flooding from even more moderate continuous rainfall rates. The highest flood threats across the state will favor southern and western slopes of all islands. These are typically the drier sides of the islands, less accustomed to heavy rain and flooding. Driving under these heavy shower bands will become challenging with ponding of water on area roadways. A Flood Watch continues for all Hawaiian Islands into Saturday, this Flood Watch may need to be extended in time as the heavy rainfall threat may linger into Sunday.
Monday and Tuesday, the large kona storm begins to lift northward drifting slowly away from the Hawaiian Islands. This means wet weather continues, however moderate to heavy rainfall rates will decrease and southwest winds will gradually weaken through the first half of next week.
By Wednesday, the latest long range guidance shows a high pressure system building back into position just north of the island chain, allowing a return to a typical trade wind weather pattern with more typical brief passing showers over windward mountain areas.

Aviation

Shower activity continues across the islands this afternoon, before a brief dry spell will take hold this evening. Expect MVFR to IFR conditions under showers, to locally as low as LIFR. Winds will remain primarily out of the south and southeast. Showers are then expected to become more widespread overnight, starting at Kauai and moving eastward. Winds will also become gusty to strong tomorrow out of the south, with widespread gusts up to and over 35 kts expected.
AIRMET Sierra remains in effect for Kauai, Oahu, Molokai, Lanai, and Maui due to mountain obscuration from showers. Some relief is expected this evening as showers decrease in coverage, but current thinking is that this AIRMET will remain in effect so long as shower activity persists across the islands.
AIRMET Tango remains in effect for moderate upper-level turbulence between FL240 and 360. This ongoing kona low should keep turbulence prevailing over the next few days.
AIRMET Zulu is also in effect, as high clouds from these showers present icing concerns between FL120 and 260. Once again, some relief is expected this afternoon, before high clouds envelop the islands once again later this evening.

Marine

A powerful kona storm will continue to bring heavy rain, thunderstorms, and rough seas to the Hawaiian coastal and offshore waters into the weekend. Southerly winds will strengthen tonight into the weekend, with gale-force winds possible. A Gale Warning has been issued for the waters near Kauai and Oahu with gale-force winds forecast Friday and Friday night. Winds are expected to near Maui County Saturday and Saturday night to near gale-force, so a Gale Watch has been posted for those areas during that time. These strong to gale-force winds will last into the weekend in association with a band of heavy showers and thunderstorms pushing eastward across the forecast area associated with the kona storm.
A series of small, short to medium period west-northwest swells will bring small surf to north and west facing shores through the remainder of the week. Kauai will block some of this swell energy from reaching Oahu and Maui, but it will make it into the west facing shores of the Big Island. The next west-northwest swell is expected to build Friday into the weekend. Additionally, a small, medium period north swell originating from Alaska Peninsula gap winds is expected to arrive tonight and continue into the weekend.
Rough and choppy surf will continue along south-facing shores due to the southerly wind direction. These south winds will intensify tonight into Friday, causing south shore surf to build Friday into the weekend, likely reaching advisory levels. Once southerly winds diminish early next week, a small to moderate, long-period south swell generated by a gale force low east-southeast of New Zealand will arrive along south-facing shores and build into midweek.
Choppy surf along east-facing shores will continue to trend downward and remain below seasonal levels as south to southeast winds prevail.

HFO Watches/Warnings/Advisories

Flood Watch through Saturday afternoon for Niihau, Kauai Leeward, Kauai Mountains, Waianae Coast, Oahu North Shore, Olomana, Central Oahu, Waianae Mountains, Lanai Mauka, Kahoolawe, Maui Windward West, Maui Leeward West, Haleakala Summit, Kona, Kohala, Big Island Interior, Kauai North, Kauai East, Kauai South, East Honolulu, Honolulu Metro, Ewa Plain, Koolau Windward, Koolau Leeward, Molokai, Lanai Windward, Lanai Leeward, Lanai South, Maui Central Valley North, Maui Central Valley South, Windward Haleakala, Kipahulu, South Maui/Upcountry, South Haleakala, Big Island South, Big Island Southeast, Big Island East, Big Island North.
High Wind Warning until 6 AM HST Sunday for Niihau, Kauai Leeward, Kauai Mountains, Waianae Coast, Oahu North Shore, Olomana, Central Oahu, Waianae Mountains, Lanai Mauka, Kahoolawe, Maui Windward West, Maui Leeward West, Kauai North, Kauai East, Kauai South, East Honolulu, Honolulu Metro, Ewa Plain, Koolau Windward, Koolau Leeward, Molokai, Lanai Windward, Lanai Leeward, Lanai South, Maui Central Valley North, Maui Central Valley South, Windward Haleakala, Kipahulu, South Maui/Upcountry, South Haleakala.
High Wind Warning until 6 PM HST Sunday for Haleakala Summit, Big Island Summits.
High Wind Watch from Friday morning through late Saturday night for Kona, Kohala, Big Island Interior, Big Island South, Big Island Southeast, Big Island East, Big Island North.
Winter Weather Advisory until 4 PM HST this afternoon for Big Island Summits.
Winter Storm Warning until 6 PM HST Saturday for Big Island Summits.
Gale Warning until 6 AM HST Saturday for Kauai Northwest Waters, Kauai Windward Waters, Kauai Leeward Waters, Kauai Channel, Oahu Windward Waters, Oahu Leeward Waters.
Gale Watch from Saturday morning through Saturday evening for Oahu Windward Waters, Kaiwi Channel, Maui County Windward Waters, Maui County Leeward Waters, Alenuihaha Channel.

Data Courtesy of NOAA.gov