West Kaua’i

Tonight: Scattered showers in the evening, then showers and isolated thunderstorms after midnight. Haze through the night. Locally heavy rainfall possible after midnight. Lows around 70 near the shore to around 60 above 3000 feet. East winds up to 15 mph shifting to the southeast after midnight. Chance of rain near 100 percent.

Tuesday: Breezy. Haze in the morning. Showers and isolated thunderstorms in the morning, then numerous showers and isolated thunderstorms in the afternoon. Locally heavy rainfall possible. Highs around 82 near the shore to around 70 above 3000 feet. Southeast winds 10 to 20 mph. Chance of rain near 100 percent.

Tuesday Night: Breezy. Cloudy with frequent showers and isolated thunderstorms. Locally heavy rainfall possible. Lows around 69 near the shore to around 60 above 3000 feet. South winds 15 to 25 mph. Chance of rain 90 percent.

South Kaua’i

Tonight: Cloudy. Showers likely in the evening, then showers and isolated thunderstorms after midnight. Haze through the night. Locally heavy rainfall possible after midnight. Lows around 69. East winds around 10 mph. Chance of rain 90 percent.

Tuesday: Haze in the morning. Cloudy with showers and isolated thunderstorms. Locally heavy rainfall possible. Highs 76 to 82. South winds 10 to 15 mph. Chance of rain 90 percent.

Tuesday Night: Breezy. Cloudy with frequent showers and isolated thunderstorms. Locally heavy rainfall possible. Lows around 69. South winds 15 to 20 mph. Chance of rain 90 percent.

Kaua’i Mountains

Tonight: Showers likely in the evening, then showers and isolated thunderstorms after midnight. Haze through the night. Locally heavy rainfall possible after midnight. Lows 61 to 68 in the valleys to around 57 above 4000 feet. South winds up to 15 mph. Chance of rain near 100 percent.

Tuesday: Breezy. Haze in the morning. Showers and isolated thunderstorms. Locally heavy rainfall possible. Highs 74 to 80 in the valleys to around 63 above 4000 feet. South winds 10 to 20 mph. Chance of rain near 100 percent.

Tuesday Night: Breezy. Cloudy with frequent showers and isolated thunderstorms. Locally heavy rainfall possible. Lows 62 to 68 in the valleys to around 58 above 4000 feet. South winds 15 to 25 mph. Chance of rain 90 percent.

North Kaua’i

Tonight: Mostly cloudy. Scattered showers in the evening, then numerous showers and isolated thunderstorms after midnight. Haze through the night. Locally heavy rainfall possible after midnight. Lows 63 to 70. Southeast winds up to 15 mph. Chance of rain 70 percent.

Tuesday: Breezy. Haze in the morning. Mostly cloudy with numerous showers and isolated thunderstorms. Locally heavy rainfall possible. Highs 73 to 83. South winds up to 20 mph. Chance of rain 70 percent.

Tuesday Night: Breezy. Cloudy with frequent showers and isolated thunderstorms. Locally heavy rainfall possible. Lows 63 to 71. South winds 10 to 20 mph. Chance of rain 90 percent.

East Kaua’i

Tonight: Showers likely in the evening, then showers and isolated thunderstorms after midnight. Haze through the night. Locally heavy rainfall possible after midnight. Lows 63 to 72. South winds up to 10 mph. Chance of rain near 100 percent.

Tuesday: Haze in the morning. Showers and isolated thunderstorms. Locally heavy rainfall possible. Highs 70 to 81. South winds 10 to 15 mph. Chance of rain near 100 percent.

Tuesday Night: Breezy. Cloudy with frequent showers and isolated thunderstorms. Locally heavy rainfall possible. Lows 63 to 73. South winds 15 to 20 mph. Chance of rain 90 percent.

Detailed Forecast

Synopsis

A powerful kona storm is expected to affect the state this week and into the weekend, bringing several potential hazards to the islands. The combination of deep tropical moisture, strong winds, and increasing instability could produce periods of heavy rainfall with significant flash flooding, damaging winds, and strong to severe thunderstorms from Tuesday through at least Saturday. Flooding concerns may begin as early as Tuesday and Tuesday night over Kauai and Oahu as deep moisture spreads into the western end of the state, then expands eastward across the island chain later in the week. The potential for strong to severe thunderstorms is expected to increase late in the week and into the weekend.

Discussion

The weather regime heading into the evening is expected to be rather typical of a veered east-southeast flow for the eastern half of the state with showers generally favoring the southeast and east facing slopes of the Big Island and Maui. For the western end of the state, showers with embedded thunderstorms will begin to move in from the west.
Forecast confidence continues to increase regarding the wet and unsettled pattern that is expected to develop tomorrow through the rest of the week and weekend. Impacts, including the potential for significant flash flooding, damaging winds, and strong to severe thunderstorms, will initially be felt across the western end of the state and will gradually expand eastward over the next several days. While this is a long duration event, it looks like the overall evolution can be broken into two primary stages.
The initial stage is expected to start tomorrow or tomorrow night and potentially last through Thursday. As a broad low deepens to the northwest of the state, winds will veer southerly for the western half of the state and southeasterly for the eastern half heading into Tuesday. A broad swath of deep atmospheric moisture with precipitable water values (PWats) ranging from 1.5 to 2 inches, which can be seen as an area of showers and thunderstorms roughly 200 miles west-northwest of Kauai this afternoon, will be drawn up in this southerly flow and move over the western end of the state Tuesday into Wednesday. At the same time, a shortwave will round the base of a potent upper level trough to the northwest, providing instability, and upper level jet dynamics will support large scale ascent across the area. Together, these ingredients will support periods of heavy rainfall with embedded thunderstorms and gusty winds. Impacts during this stage are expected to be focused primarily across the western half of the state.
The second stage looks to start on Friday and persist into the weekend. The upper level trough to the northwest will continue to sharpen and take on a negative tilt as a jet streak on its western flank strengthens and another shortwave trough will round the base of this trough. The axis of deep atmospheric moisture will shift eastward and spread out across the island chain, with model guidance showing PWats exceeding 2 inches. By late week and into the weekend, soils may already be saturated, increasing the likelihood that additional rainfall will produce rapid runoff and dangerous flooding conditions, increasing flooding concerns statewide, particularly where heavier rainfall persists over the same areas for multiple days. While thunderstorms embedded within showers are possible midweek as mentioned above, the potential for strong to severe thunderstorms will increase Friday into the weekend as the environment becomes increasingly unstable, moisture increases and the upper level jet dynamics become even more favorable for large scale ascent.
In addition to the rainfall threat, south to southwest winds may strengthen later in the 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 kona winds and localized downslope winds on north through east facing sides of the islands may be capable of downing trees and power lines.
For the Big Island summits, increasing moisture and colder air associated with the upper trough may support periods of heavy snowfall with significant accumulations later in the week and into the weekend, potentially requiring the issuance of a winter storm watch/warning.
While some uncertainty remains regarding the exact timing and placement of the heaviest rainfall and strongest storms, confidence continues to increase that the islands will experience a prolonged period of unsettled and potentially impactful weather this week. Residents and interests across the state should continue monitoring forecasts as details regarding the timing and location of impacts become clearer.

Aviation

A light background southeast wind regime is currently in effect across the state with localized sea breezes this afternoon. Low cigs and some SHRA are possible along southern and windward slopes. MVFR conditions are possible within showers, while VFR prevails elsewhere. A powerful low pressure system northwest of the state is expected to bring significant impacts across the islands this week. Widespread heavy RA, severe TSTMS and damaging winds will be possible. Conditions will range from LIFR to MVFR at times. Kauai is expected to receive impacts first as early as Tuesday afternoon. These impacts will gradually spread across the rest of the state from west to east and potentially last through the weekend.
No AIRMET's are currently in effect. AIRMET Sierra for mountain obscuration may be needed later tonight at least for Kauai due to showers embedded in the southeasterly flow. Light icing may also be possible when the mid-higher level clouds move closer to the islands tomorrow.

Marine

A powerful kona storm is expected to bring heavy rain, thunderstorms, and rough seas to the islands and adjacent waters Tuesday into the weekend. Strong high pressure far northeast of the state will drift further east, causing fresh to locally strong east to east southeast winds to decline tonight. Winds will begin to veer southerly and increase back to fresh to locally strong Tuesday through late this week as a front enters the northwest waters. A new Small Craft Advisory (SCA) has been issued for the waters to the north and west of Kauai beginning Tuesday, where winds will be southerly ahead of the approaching front. The SCA for the Pailolo Channel and Big Island waters has been canceled with a scatterometer pass earlier today showing that winds have fallen below advisory criteria. Later this week, possibly as early as Thursday night, winds along the front will approach or possibly reach gale force for northwestern portions of the marine area. These strong winds will slowly creep eastward with the front heading into the weekend.
Choppy surf along east facing shores will gradually trend downward, falling below seasonal levels by mid-week as trade winds veer southerly through the week. Surf along north-facing shores was tiny today, with very little northwest swell energy reaching the islands. However, a series of small long period west-northwest swells returning tonight will bring small surf to north and west facing shores through the rest of the week. A small medium period north swell is also possible late this week.
Surf along south-facing shores will remain small, with the exception of areas exposed to wind swell, which will increase through the week. Rough and choppy conditions develop Tuesday and continue through late week as southerly winds increase in advance of a front. A series of small long-period south swells also impacts shorelines through late this week.

HFO Watches/Warnings/Advisories

Flood Watch from Tuesday morning through Saturday afternoon for Niihau, Kauai,
Flood Watch from Tuesday evening 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.
Small Craft Advisory from 6 AM Tuesday to 6 AM HST Wednesday for Kauai Northwest Waters, Kauai Windward Waters, Kauai Leeward Waters.

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Data Courtesy of NOAA.gov