A powerful and slow-moving Kona low northwest of the islands will bring a prolonged period of hazardous weather to the state through the upcoming weekend. Bands of heavy showers with embedded thunderstorms are currently affecting Kauai and approaching Oahu this evening. The heavy rain will gradually spread eastward across the island chain through midweek, bringing periods of heavy rainfall, potential flash flooding, strong Kona winds, and strong to severe thunderstorms.
A high-impact weather event is beginning to unfold across the Hawaiian Islands as a powerful Kona low deepens northwest of the state. Radar imagery this evening shows widespread moderate to heavy showers with embedded thunderstorms around and across Kauai, with the leading edge of the rain approaching Oahu. Peak rainfall rates within the heaviest bands moving through Kauai this evening have reached around 1 to 2 inches per hour. This marks the beginning of what is expected to be a prolonged period of unsettled weather lasting through the upcoming weekend and possibly into early next week.
The large-scale pattern remains characterized by a broad upper- level low northwest of the islands. The associated upper trough is becoming increasingly negatively tilted as a strong jet streak, with core winds of 100 to 140 kt at 250 mb, digs southeastward along its western flank. As the trough amplifies and expands toward the islands through midweek, the state will increasingly fall beneath the jet streak’s left-exit region, supporting enhanced divergence aloft and large-scale ascent.
At the surface, a broad area of low pressure will persist well northwest of the islands in response to the upper-level height falls. Central pressures are forecast to remain near 990 mb, maintaining a southerly to southeasterly flow across the state. This flow will continue to draw deep tropical moisture northward into the island chain, with PWs remaining around two inches.
Moisture and instability will increase through midweek as the moisture plume spreads eastward across the state. The combination of strong upper-level forcing, deep moisture, and persistent southerly flow will support periods of heavy rainfall with embedded thunderstorms. Repeated rounds of convection are likely as smaller disturbances rotate through the upper trough, raising the potential for training rainbands and locally intense rainfall, particularly in terrain where anchoring often occurs.
Flooding concerns are expected to increase statewide as the week progresses, especially where heavy rainfall persists over the same areas for multiple days. By late week and next weekend, soils may already be saturated, increasing the likelihood that additional rainfall will produce rapid runoff and dangerous flooding conditions.
In addition to the rainfall threat, strengthening south to southwest winds may develop later this week and into the weekend as the pressure gradient tightens between the developing low to the northwest and high pressure to the east. If this materializes, strong and potentially damaging Kona winds will be possible, including localized downslope winds capable of downing trees and power lines.
Environmental conditions will also periodically become conducive for strong to severe thunderstorms, particularly Friday into Saturday. Instability combined with strong deep-layer shear could support storms capable of producing damaging winds, large hail, and possibly a tornado.
For the summits of the Big Island, colder air associated with the upper trough may allow periods of heavy snow with significant accumulations later this week and into the weekend, which could require the issuance of winter storm watches/warnings.
Clouds and showers will continue to increase over Kauai tonight with prolonged periods of MVFR conditions expected with tempo IFR/LIFR conditions possible under the heavier showers through Wednesday. Lower clouds and showers will begin to increase over Oahu tonight with MVFR conditions expected. As the showers become heavier, tempo IFR conditions will also be possible over Oahu by Wednesday. Thunderstorms remain embedded in the clouds to the west of the islands, and the threat of lightning will should increase over the western half of the state by Wednesday morning.
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 towards Wednesday morning.
AIRMET ZULU remains in effect for light icing from Kauai through Maui County.
AIRMET TANGO is in effect for moderate turbulence over Kauai as the VAD wind profile shows strong southerly winds have spread across the area. AIRMET TANGO should spread eastward towards Oahu tonight.
A powerful Kona storm will bring heavy rain, thunderstorms, and rough seas to the islands and adjacent waters through at least this weekend. Fresh to locally strong southerly winds are expected 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. By Wednesday night, a relative lull in southerly winds is anticipated, but later this week, possibly as early as Thursday night, a reintensification 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, reaching near advisory levels. Meanwhile, a series of small, long-period south swells will through.
Flood Watch through Saturday afternoon for Niihau-Kauai Leeward- Kauai Mountains-Waianae Coast-Oahu North Shore-Olomana-Central Oahu-Waianae Mountains-Kauai North-Kauai East-Kauai South-East Honolulu-Honolulu Metro-Ewa Plain-Koolau Windward-Koolau Leeward.
Flood Watch from 6 AM HST Wednesday 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.
Small Craft Advisory until 6 PM HST Wednesday for Kauai Northwest Waters-Kauai Windward Waters-Kauai Leeward Waters- Kauai Channel.
