A powerful Kona storm will impact the state through the weekend, bringing multiple hazards to the islands including significant flash flooding, damaging winds, and strong to severe thunderstorms. Flooding concerns will continue over Kauai this evening and spread to Oahu late tonight, then gradually expand eastward across the island chain through the week. The potential for strong to severe thunderstorms and more widespread flash flooding will increase late in the week and into the weekend.
A wide band of deep moisture with widespread showers and embedded thunderstorms on the east side of a broad low far northwest of the state will continue to impact Kauai and spread eastward to Oahu late tonight. Rainfall rates have remained manageable so far, with most areas of Kauai receiving only a few hundredths to half an inch of rain since early this morning.
CIMSS MIMIC Layer Precipitable Water imagery shows this moisture band contains precipitable water values ranging from 1.5 to 2.25 inches this afternoon. This plume will gradually spread eastward, reaching Maui County by Wednesday and potentially the Big Island by Wednesday night, where heavy showers and thunderstorms will increasingly affect south and southeast slopes. In addition to the abundant moisture, a shortwave rounding the base of a potent upper-level trough northwest of the state will provide increasing instability, while favorable upper-level jet dynamics support large-scale ascent. These ingredients will support periods of heavy rainfall with embedded thunderstorms and gusty winds. Surface winds will continue to veer southerly across the western half of the state and south-southeasterly across the eastern half tonight, strengthening to breezy levels through Wednesday. This flow will enhance showers along south-facing slopes, with the greatest flooding concerns through Wednesday focused over the western half of the state.
By late week into the weekend, the upper-level trough northwest of the islands will continue to sharpen and take on a negative tilt as a strengthening jet streak develops along its western flank and another shortwave rotates through the base of the trough. Deep tropical moisture will spread across the island chain during this time, with model guidance continuing to show precipitable water values exceeding 2 inches. While thunderstorms are expected tonight through Thursday, the environment will become increasingly favorable for strong to severe storms Thursday night through Saturday morning as instability and wind shear increase. Strong upper-level divergence will further support organized convection. By this time, soils may already be saturated, increasing the likelihood of rapid runoff and dangerous flooding.
In addition to the rainfall and severe thunderstorm threat, southerly winds are expected to strengthen through the week, with the strongest winds likely from Thursday night into the weekend as the pressure gradient tightens. This pattern may produce dangerous winds at Haleakala and the Big Island summits, along with damaging downslope winds across north through east sections of the islands capable of downing trees and power lines. Damaging winds may also occur with stronger thunderstorms. A High Wind Watch has been issued for Haleakala and the Big Island summits from Thursday night through the weekend.
Colder air associated with the upper trough combined with increasing moisture may also bring periods of heavy snow to the Big Island summits later in the week and into the weekend, potentially requiring winter storm headlines.
Long-range guidance suggests unsettled weather could persist into next week. Residents and interests across the state should continue monitoring forecasts as this potentially impactful weather pattern evolves.
Clouds and showers have spread across Kauai today, and are expected to persist through Wednesday. High clouds, along with some lower clouds and showers have moved over Oahu. Additional lower clouds and showers are expected to spread over Oahu overnight and persist through Wednesday. Thunderstorms remain embedded in the clouds to the west of the islands, but are a possibility over the islands tonight and Wednesday.
The clouds and showers will move down the island chain tonight and Wednesday. Some of the high clouds are moving over Maui County this afternoon, with lower clouds and showers expected closer to daybreak Wednesday.
MVFR conditions are expected to prevail in the lower clouds and showers, with brief IFR/LIFR conditions in heavier showers or in any thunderstorms that develop.
AIRMET SIERRA remains in effect for mountain obscuration for all of Kauai, and along the east through south slopes of Oahu. This will likely be expanded to include all of Oahu later tonight. Portions of Maui County will likely need an AIRMET SIERRA late tonight or early Wednesday morning.
AIRMET ZULU remains in effect for light icing over Kauai and Oahu. This will likely need to be expanded to include Maui County later tonight or early Wednesday.
AIRMET TANGO has been issued for moderate turbulence over Kauai as the VAD wind profile shows strong southerly winds have spread across the area. Additional turbulence AIRMETs may be needed later tonight.
A powerful Kona storm is expected to bring heavy rain, thunderstorms, and rough seas to the islands and adjacent waters through at least this weekend. Winds will veer southerly and increase to fresh to locally strong speeds through late this week as the Kona storm approaches. A Small Craft Advisory (SCA) is in effect for the waters around Kauai through Wednesday evening, where winds will be enhanced by a front approaching from the northwest. By Wednesday night, a relative lull in southerly winds is anticipated as the front weakens temporarily and the SCA is likely to drop at that point. Later this week, possibly as early as Thursday night, a reintensification of the front over the islands will cause winds to approach, or possibly reach, gale force for portions of the area. These strong winds will slowly creep eastward heading into the weekend.
A series of small, medium- to long-period west-northwest swells will bring small surf to north and west facing shores through the rest of the week. The first of these will continue to build through tonight and peak on Wednesday, resulting in a slight bump in surf along north and west- facing shores. Additionally, a small, medium- period north swell is also possible late this week.
Choppy surf along east-facing shores will continue to trend downward, falling below seasonal levels on Wednesday as winds veer southerly. With the shift of wind direction, surf along south-facing shores will increase, becoming rough and choppy. Late this week, another round of stronger southerly winds will cause south shore surf to build further, meanwhile a series of small, long- period south swells moves through.
Flood Watch through Saturday afternoon for Niihau-Kauai-
Flood Watch through Saturday afternoon for Oahu-
Flood Watch from Wednesday morning through Saturday afternoon for Lanai Mauka-Kahoolawe-Maui Windward West-Maui Leeward West- Haleakala Summit-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 Watch from Thursday evening through Sunday afternoon for Haleakala Summit-Big Island Summits.
Flood Watch from Wednesday evening through Saturday afternoon for Kona-Kohala-Big Island Interior-Big Island South-Big Island Southeast-Big Island East-Big Island North.
Ashfall Advisory until 5 PM HST this afternoon for Big Island East.
Small Craft Advisory until 6 PM HST Wednesday for Kauai Northwest Waters-Kauai Windward Waters-Kauai Leeward Waters.
Small Craft Advisory until 6 PM HST Wednesday for Kauai Channel.
