Photo Credit: Tyler Rooke

West Side

Today: Cloudy and windy. Scattered showers until late afternoon, then occasional showers and scattered thunderstorms late in the afternoon. Some thunderstorms may produce gusty winds late in the afternoon. Locally heavy rainfall possible late in the afternoon. Highs around 75. South winds 10 to 15 mph increasing to 10 to 30 mph in the afternoon. Chance of rain 80 percent.

Tonight: Windy. Cloudy with occasional showers and scattered thunderstorms. Some thunderstorms may produce gusty winds. Locally heavy rainfall possible. Lows 65 to 72. South winds 20 to 30 mph. Chance of rain 80 percent.

Monday: Windy. Cloudy with occasional showers and scattered thunderstorms. Some thunderstorms may produce gusty winds. Locally heavy rainfall possible. Highs 73 to 81. Southeast winds 10 to 30 mph becoming east up to 10 mph in the afternoon. Chance of rain 80 percent.

South Side

Today: Cloudy and windy. Scattered showers until late afternoon, then occasional showers and scattered thunderstorms late in the afternoon. Some thunderstorms may produce gusty winds late in the afternoon. Locally heavy rainfall possible late in the afternoon. Highs around 77. Southwest winds 10 to 15 mph shifting to the south 10 to 30 mph in the afternoon. Chance of rain 80 percent.

Tonight: Windy. Cloudy with occasional showers and scattered thunderstorms. Some thunderstorms may produce gusty winds. Locally heavy rainfall possible. Lows 64 to 73. South winds 10 to 30 mph. Chance of rain 80 percent.

Monday: Cloudy and breezy. Occasional showers and scattered thunderstorms early in the morning, then showers likely and scattered thunderstorms in the late morning and afternoon. Some thunderstorms may produce gusty winds. Locally heavy rainfall possible. Highs around 84. North winds 10 to 20 mph decreasing to up to 10 mph in the afternoon. Chance of rain 80 percent.

North Shore

Today: Cloudy. Scattered showers until late afternoon, then occasional showers and scattered thunderstorms late in the afternoon. Some thunderstorms may produce gusty winds late in the afternoon. Locally heavy rainfall possible late in the afternoon. Highs around 76 near the shore to around 64 near 5000 feet. North winds up to 15 mph. Gusts up to 30 mph increasing to 40 mph in the afternoon. Chance of rain 80 percent.

Tonight: Cloudy with occasional showers and scattered thunderstorms. Some thunderstorms may produce gusty winds. Locally heavy rainfall possible. Lows around 69 near the shore to around 49 near 5000 feet. Northeast winds up to 15 mph shifting to the east after midnight. Gusts up to 40 mph. Chance of rain 80 percent.

Monday: Cloudy. Occasional showers and scattered thunderstorms early in the morning, then showers likely in the late morning and afternoon. Some thunderstorms may produce gusty winds early in the morning. Locally heavy rainfall possible. Highs 74 to 80 near the shore to around 64 near 5000 feet. East winds around 10 mph shifting to the southeast in the afternoon. Chance of rain 80 percent.

Central Maui

Today: Cloudy and breezy. Scattered showers until late afternoon, then occasional showers and scattered thunderstorms late in the afternoon. Some thunderstorms may produce gusty winds late in the afternoon. Locally heavy rainfall possible late in the afternoon. Highs around 77. Southwest winds around 10 mph becoming up to 25 mph in the afternoon. Chance of rain 80 percent.

Tonight: Breezy. Cloudy with occasional showers and scattered thunderstorms. Some thunderstorms may produce gusty winds. Locally heavy rainfall possible. Lows 65 to 70. South winds up to 25 mph. Chance of rain 80 percent.

Monday: Cloudy and breezy. Occasional showers and scattered thunderstorms early in the morning, then showers likely and scattered thunderstorms in the late morning and afternoon. Some thunderstorms may produce gusty winds. Locally heavy rainfall possible. Highs 78 to 83. Northeast winds up to 20 mph decreasing to up to 10 mph in the afternoon. Chance of rain 80 percent.

Upcountry

Today: Cloudy. Very windy. Showers likely until late afternoon, then occasional showers and scattered thunderstorms late in the afternoon. Some thunderstorms may produce gusty winds late in the afternoon. Locally heavy rainfall possible late in the afternoon. Highs around 58 at the visitor center to around 53 at the summit. South winds 10 to 45 mph. Gusts up to 80 mph increasing to 100 mph in the afternoon. Chance of rain 80 percent.

Tonight: Very strong winds. Cloudy with occasional showers and scattered thunderstorms. Some thunderstorms may produce gusty winds. Locally heavy rainfall possible. Lows 41 to 53. South winds 10 to 65 mph with gusts to 100 mph. Chance of rain 80 percent.

Monday: Cloudy. Very windy. Occasional showers and scattered thunderstorms early in the morning, then showers likely in the late morning and afternoon. Some thunderstorms may produce gusty winds early in the morning. Locally heavy rainfall possible. Highs around 58 at the visitor center to around 54 at the summit. South winds 10 to 45 mph with gusts to 75 mph decreasing to 10 to 25 mph with gusts to 50 mph in the afternoon. Chance of rain 80 percent.

East Maui

Today: Cloudy. Scattered showers until late afternoon, then occasional showers and scattered thunderstorms late in the afternoon. Some thunderstorms may produce gusty winds late in the afternoon. Locally heavy rainfall possible late in the afternoon. Highs around 76 near the shore to around 64 near 5000 feet. North winds up to 15 mph. Gusts up to 30 mph increasing to 40 mph in the afternoon. Chance of rain 80 percent.

Tonight: Cloudy with occasional showers and scattered thunderstorms. Some thunderstorms may produce gusty winds. Locally heavy rainfall possible. Lows around 69 near the shore to around 49 near 5000 feet. Northeast winds up to 15 mph shifting to the east after midnight. Gusts up to 40 mph. Chance of rain 80 percent.

Monday: Cloudy. Occasional showers and scattered thunderstorms early in the morning, then showers likely in the late morning and afternoon. Some thunderstorms may produce gusty winds early in the morning. Locally heavy rainfall possible. Highs 74 to 80 near the shore to around 64 near 5000 feet. East winds around 10 mph shifting to the southeast in the afternoon. Chance of rain 80 percent.

Lanai City

Today: Cloudy and windy. Showers likely until late afternoon, then occasional showers and scattered thunderstorms late in the afternoon. Some thunderstorms may produce gusty winds late in the afternoon. Locally heavy rainfall possible late in the afternoon. Highs 70 to 77. South winds 15 to 20 mph increasing to 15 to 30 mph in the afternoon. Chance of rain 80 percent.

Tonight: Windy. Cloudy with occasional showers and scattered thunderstorms. Some thunderstorms may produce gusty winds. Locally heavy rainfall possible. Lows 61 to 66. South winds 25 to 30 mph decreasing to 15 to 25 mph after midnight. Chance of rain 80 percent.

Monday: Breezy. Cloudy with occasional showers and scattered thunderstorms. Some thunderstorms may produce gusty winds. Locally heavy rainfall possible. Highs 70 to 78. Southeast winds 10 to 20 mph shifting to the east in the afternoon. Chance of rain 80 percent.

Kaunakakai

Today: Cloudy and windy. Showers likely until late afternoon, then occasional showers and scattered thunderstorms late in the afternoon. Some thunderstorms may produce gusty winds late in the afternoon. Locally heavy rainfall possible late in the afternoon. Highs 64 to 77. Southeast winds 15 to 20 mph shifting to the south 15 to 30 mph in the afternoon. Chance of rain 80 percent.

Tonight: Windy. Cloudy with occasional showers and scattered thunderstorms. Some thunderstorms may produce gusty winds. Locally heavy rainfall possible. Lows 55 to 71. South winds 15 to 30 mph. Chance of rain 80 percent.

Monday: Breezy. Cloudy with occasional showers and scattered thunderstorms. Some thunderstorms may produce gusty winds. Locally heavy rainfall possible. Highs 64 to 84. Southeast winds 10 to 25 mph shifting to the east in the afternoon. Chance of rain 90 percent.

Detailed Forecast

Synopsis

A kona low will bring the threat of widespread heavy rainfall and thunderstorms, capable of producing catastrophic flooding, and strong south to southwest winds through the first half of the week. For timing, impacts are expected to begin over the Big Island and Maui County through the day today, then Oahu and Kauai tonight into Monday. Rainfall chances should trend down through the second half of the week as a more typical trade wind pattern returns.

Discussion

An evolving synoptic pattern is in place across the region this morning, featuring a strong pulse aloft diving southward to an area northwest of Kauai. Surface cyclogenesis northwest of Kauai will be the response beneath or near the right entrance region of an impressive jet streak setting up. Tightening pressure gradient between this developing surface feature and high pressure to the northeast will translate to strengthening south to southwest winds beginning today through tonight.
Provided the stable boundary layer air in place early this morning over the smaller islands, due to the recent cool spell (dewpoints were in the 50s) with northerly winds following a front over the past 24- to 36-hrs, expect a period of isentropic upglide as the southerly winds begin to ramp up drawing warm/moist air northward. Dewpoints will quickly respond and near 70 degrees by or come tonight. This moistening boundary layer combined with strong upper height falls and forcing for ascent will support heavy rainfall and thunderstorms – beginning over the eastern end of the state this morning, then progressing westward to the smaller islands later today through tonight.
Expect a prolonged period of heavy rainfall, capable of producing extreme rainfall rates, potentially continuing into Wednesday. These rainfall rates for long durations combined with rivers and streams quickly rising will support the potential for catastrophic flooding. In addition to the threat of flash flooding, thunderstorms capable of generating strong straight-line winds will become a possibility, especially Monday through Tuesday when the deep layer shear, instability, and moisture axis could come into phase.
The south to southwest winds may become strong enough for localized impacts, especially downwind of terrain in and around heavy showers or storms. The Wind Advisory for the lower elevations on the Big Island may need to be expanded to the smaller islands in later packages today or tonight if this materializes.
For the Big Island summits, the Blizzard Warning remains in effect as intense mid-level winds associated with the developing low continue. Moisture will be sufficiently deep during this time to bring periods of heavy snowfall.
On Maui, a High Wind Warning remains in effect for the Haleakala Summit after National Park personnel reported winds near 90 mph leading to a closure of the Summit .
An E to W drying trend is anticipated by Thursday as the low moves away and high pressure to the NE pushes trade winds over the area from the E. Trades could become strong next weekend.

Aviation

High clouds have started to thin over most of the islands in the overnight hours, with most of the high clouds positions over the Big Island in the pre-dawn hours. Some thinner high clouds are found over Kauai. A recent pass from the VIIRS polar satellite shows pockets of low level clouds. AIRMET Sierra for mountain obscuration remains in effect for Kauai, Maui and the Big Island. all areas have seen some improvement overnight, however all remain on the cusp of additional clouds and showers moving in, so the AIRMET remains in effect. While widespread VFR conditions are being reported as of 3 am, there are pockets of MVFR over Maui and the Big Island. Expected conditions to worsen as the day wears on, which will likely require AIRMET Sierra to be expanded to other areas.
South to southwest winds are seen across the state, with generally light winds. Winds are a bit stronger over the lower elevations of Maui and the Big Island. Additionally, strong winds continue over the summits of Maui and the Big Island. As a result AIRMET Tango remains in effect for low level turbulence to the north thru east of the mountains, and that is expected to continue today, and likely into tonight. AIRMET Tango is also in effect for the strong winds over the summits on the Big Island and Maui, which will also likely remain in place through tonight. Winds at the lower elevations of the Big Island and Maui, along with winds on the remaining islands are expected to increase today, which will likely lead to the AIRMET Tango for low level turbulence to be expanded.
Thunderstorms associated with an upper level low to the northwest of the islands support an SIGMET to the northwest of the islands. As this upper level features moves towards the islands, the SIGMET may cover portions of the islands and adjacent waters. Additional thunderstorms are possible in the clouds and showers over the southern end of the island chain today and tonight.

Marine

A kona low has formed over the offshore waters northwest of Kauai. The low will be drifting westward in the few days. The low has also changed the surface wind flow across the main Hawaiian Islands, from westerly to southerly. This southerly winds will be turning strong as well. Furthermore, there will be heavy showers and thunderstorms, of which some of the storms will severe Sunday through Tuesday.
A Small Craft Advisory (SCA) is in effect for all Hawaiian waters as the southerly winds increases to 23 knots or higher, which is good to until late Monday afternoon. There is the possibility the SCA will be extended into Monday night. The SCA is also covering for seas of 10 feet or higher from a large northwest to north swell. This swell has peaked, and a gradual decline is to follow through Sunday night. A High Surf Warning remains in effect for most north facing shores, including the west facing shores of Big Island north of Kailua- Kona Town. This High Surf Warning is good to until 6 am HST Monday morning. A High Surf Advisory is covering the west facing shores of most islands, including the west facing shores of Maui’s Leeward near Honolua Bay. There are no significant swells from the northwest or north through the rest of next week.
The combination of the large surf and astronomical high tide may lead to wave runup along some coastal areas of the north facing shores this morning. High tide along the north shore of Oahu is at 4 am HST with a height of 2.16 feet. Further more, this north swell will have the potential of causing significant harbor surges in both Kahului and Hilo harbors, and waves breaking near harbor entrances.
As the north swell continues to slowly subside, the locally generated south swell will becomes dominant as the southerly winds strengthens to strong later today. Should this occur, there is the potential for surf rising to near advisory level of 10 feet for the south facing shores by Monday.
Moderate surf will grace the east facing shores with the mix swells from the north and south.
The weather is set to begin improving Tuesday night, with the southerly winds giving way to trades on Wednesday, along with subsiding seas.

HFO Watches/Warnings/Advisories

Flood Watch through Tuesday afternoon for all Hawaiian Islands.
High Surf Warning until 6 AM HST Monday for Niihau, Oahu North Shore, Olomana, Maui Windward West, Kohala, Kauai North, Koolau Windward, Molokai Windward, Molokai North, Maui Central Valley North, Windward Haleakala, Big Island East, Big Island North.
High Wind Warning until 6 AM HST Monday for Haleakala Summit, Big Island Summits.
Blizzard Warning until 6 AM HST Monday for Big Island Summits.
Wind Advisory until 6 AM HST Monday for Kona, Kohala, Big Island Interior, Big Island South, Big Island Southeast, Big Island East, Big Island North.
High Surf Advisory until 6 AM HST Monday for Kauai Leeward, Waianae Coast, Maui Leeward West, Molokai West.
Small Craft Advisory until 6 PM HST Monday for all Hawaiian waters,

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Data Courtesy of NOAA.gov