West Side

Today: Partly sunny with scattered showers in the morning, then mostly sunny with isolated showers in the afternoon. Highs 77 to 82. South winds 10 to 15 mph. Chance of rain 30 percent.

Tonight: Partly cloudy. Isolated showers after midnight. Haze after midnight. Lows 68 to 74. South winds up to 10 mph in the evening becoming light. Chance of rain 20 percent.

Tuesday: Mostly sunny with isolated showers. Haze. Highs 79 to 84. Southwest winds up to 10 mph. Chance of rain 20 percent.

South Side

Today: Mostly sunny with isolated showers. Highs around 84. Southwest winds 10 to 15 mph. Chance of rain 20 percent.

Tonight: Mostly clear. Isolated showers after midnight. Haze after midnight. Lows 67 to 76. Light winds. Chance of rain 20 percent.

Tuesday: Sunny and haze. Highs around 86. Light winds becoming southwest around 10 mph in the afternoon.

North Shore

Today: Mostly sunny with isolated showers. Highs around 82 near the shore to around 65 near 5000 feet. Light winds. Chance of rain 20 percent.

Tonight: Mostly clear. Isolated showers after midnight. Haze after midnight. Lows around 71 near the shore to around 53 near 5000 feet. Light winds. Chance of rain 20 percent.

Tuesday: Mostly sunny with isolated showers. Haze. Highs around 81 near the shore to around 66 near 5000 feet. Light winds becoming southeast up to 10 mph in the afternoon. Chance of rain 20 percent.

Central Maui

Today: Mostly sunny. Isolated showers in the morning. Highs 80 to 85. South winds 10 to 15 mph. Chance of rain 20 percent.

Tonight: Mostly clear. Isolated showers after midnight. Haze after midnight. Lows 67 to 72. South winds up to 10 mph in the evening becoming light. Chance of rain 20 percent.

Tuesday: Sunny and haze. Highs 81 to 87. Light winds becoming southwest up to 10 mph in the afternoon.

Upcountry

Today: Partly sunny with scattered showers in the morning, then mostly sunny with isolated showers in the afternoon. Highs around 61 at the visitor center to around 67 at the summit. Southwest winds up to 10 mph in the morning becoming light. Chance of rain 30 percent.

Tonight: Partly cloudy with isolated showers. Haze after midnight. Lows 44 to 56. Light winds. Chance of rain 20 percent.

Tuesday: Mostly sunny with isolated showers. Haze. Highs around 61 at the visitor center to around 67 at the summit. Light winds. Chance of rain 20 percent.

East Maui

Today: Mostly sunny with isolated showers. Highs around 82 near the shore to around 65 near 5000 feet. Light winds. Chance of rain 20 percent.

Tonight: Mostly clear. Isolated showers after midnight. Haze after midnight. Lows around 71 near the shore to around 53 near 5000 feet. Light winds. Chance of rain 20 percent.

Tuesday: Mostly sunny with isolated showers. Haze. Highs around 81 near the shore to around 66 near 5000 feet. Light winds becoming southeast up to 10 mph in the afternoon. Chance of rain 20 percent.

Lanai City

Today: Partly sunny with scattered showers in the morning, then mostly sunny with isolated showers in the afternoon. Highs 71 to 78. South winds 10 to 15 mph. Chance of rain 30 percent.

Tonight: Partly cloudy with isolated showers. Haze after midnight. Lows around 67. South winds around 10 mph in the evening becoming light. Chance of rain 20 percent.

Tuesday: Mostly sunny with isolated showers. Haze. Highs 72 to 79. South winds up to 10 mph increasing to 10 to 15 mph in the afternoon. Chance of rain 20 percent.

Kaunakakai

Today: Partly sunny with scattered showers. Highs 69 to 84. South winds 10 to 15 mph. Chance of rain 40 percent.

Tonight: Partly cloudy with isolated showers. Lows 58 to 73. South winds up to 10 mph. Chance of rain 20 percent.

Tuesday: Mostly sunny with isolated showers. Haze in the afternoon. Highs 70 to 87. Southeast winds up to 10 mph shifting to the south in the afternoon. Chance of rain 20 percent.

Detailed Forecast

Synopsis

The remnants of a cold front have transitioned to a trough that is draped over the western third of the state. While the trough will be weakening through the day, not providing as much focus as on Sunday, the trough will continue to pull southern moisture- rich air up across the islands through tonight. The next front is approaching Kauai and is expected to absorb this trough around Wednesday. The bonding of these two boundaries will keep chances high for returning showers and possible isolated storms through early Tuesday, especially over Kauai and Oahu. Rain chances will lower going through the middle of the week for Oahu but will remain moderately high for Niihau and Kauai due to the close proximity of the western coastal water stationary boundary.

Discussion

A fairly active night rain-wise over Kauai and Oahu where many locations have picked up another 1 to 2 inches since late Sunday evening. The front, or the channel for southern latitude moisture to travel north across the western isles and therefore the impetus to a very wet Sunday, has stalled in the vicinity of the Kauai Channel and is troughing out this morning. While forcing is becoming more benign, a greater than 2 inch precipitable water air mass is still being channeled northward along the boundary from the southern latitudes. This alone, with the weak lift provided by the lower level trough, will keep high chances of mainly light showers in place through the day. The Flood Watch remains in effect through early Tuesday; mainly to account for less rain becoming runoff as it falls on saturated grounds. Moderate to locally heavy rainfall could move up within light southerlies and train over already soggy grounds hence exacerbating minor flooding or ponding of water in lower lying areas.
The next cold front is making its way to Kauai and is packing quite the punch as it crosses 165W as evidenced by the cooling cloud tops and frequent lightning. While this front is expected to progressively reach the far offshore waters by Tuesday morning, it is then forecast to slow up and merge with what is left of the tough laying between Kauai and Oahu this morning. Reasoning is that eastern ridging anchored off of the western Mexico coast will budge very little and the associated upper low will ride up along the ridge's northwest flank. This northeast-exiting low will stretch the frontal boundary, troughing the front out as it merges with this current resident trough. High lower to middle level moisture will still stream up along southwesterly steering flow through a good part of the week. Confidence is highest that the boundary will meander just west of Kauai and hang in there for several days. Placement of this trough is crucial for Kauai weather this week. There will likely be several days of mainly overcast skies and return rain regardless of where the trough decides to camp, but the threat for return flooding all hangs on the eventually settling of the trough over the western nearshore waters.
The majority of the model ensemble members place the boundary along or just west of Niihau and Kauai with just a few members introducing periodic rainfall for Oahu. Thus, it appears the flooding threat will significantly diminish over Oahu by early Tuesday but the threat will still remain over Kauai and Niihau through the middle of the week. The main threat will be flooding with locally heavy 1 to 2 inch per hour rain rates inducing return flooding of both roads and streams. As the upper low advances well northeast of the islands by Friday, western state rain chances will remain moderate high but gradually taper off Thursday and Friday. The Flood Watch may need to be extended into Tuesday or Wednesday for Kauai and Niihau. Light to occasionally gusty southerly winds will prevail through Wednesday as the pesky trough hangs in place over the western third of the island chain. What is left of the trough will lift out by the weekend and, after an extended hiatus, trades will return for the weekend. Trades will usher in a drier air mass and assist in clearing out skies while providing a relatively drier weekend/early next week; typical trade shower regime going into the holiday week.

Aviation

A trough in the vicinity of the western islands of Kauai and Oahu will meander through the day. Moisture riding up from the southwest along the trough will create periodic IFR/MVFR cigs and vsbys within heavier showers. Lingering instability will keep a mention of heightened chances for isolated thunderstorms. Winds along or in the wake of the trough are variable. Light winds will tend to be more southerly downstream of the boundary. Expect statewide southerlies tonight into Tuesday morning as the trough completely washes.
AIRMET Sierra in effect for mtn obsc for Kauai and Oahu and for IFR conditions.

Marine

A surface trough continues to lift northwestward through Kauai Coastal Waters today. Expect periods of showers, locally heavy at times with isolated thunderstorms possible along the trough axis. Elsewhere, expect decreasing shower activity in a gentle southeasterly to southerly wind pattern. Another cold front will approach Kauai from the northwest on Wednesday and Thursday, possibly increasing shower activity and thunderstorms for the northwest waters. A Small Craft Advisory remains in effect through this afternoon for most coastal waters around Kauai and Oahu due to high seas from the passing north-northwest swell.
The current large medium period north-northwest (320-340 degree) swell will peak today and then slowly decline into Tuesday. Swell heights around 9 feet at 13 second periods are being reported at both the offshore and several nearshore buoy locations this morning. This means we will continue to see surf heights at High Surf Advisory (HSA) levels for north and west facing shores exposed to this large north-northwest swell. Surf heights will fall just below HSA levels by tonight for Kauai and Oahu. The HSA was extended in time for north and west facing shores of Molokai and north facing shores of Maui. Predicted surf heights for west facing shores of the Big Island are lower then previously forecast, and will only reach HSA levels, therefore the High Surf Warning was lowered to an advisory starting later tonight. The HSA will likely expire for all islands by early Tuesday morning.
A combination of moderate northwest and north swells will keep surf elevated through the week, possibly reaching HSA levels along north and west facing shores from Wednesday into Thursday.
East facing shores remain tiny to small given the lack of trade winds but shorelines exposed to north swell energy may remain slightly elevated through the week. Surf along south facing shores increases due to short period southerly wind wave chop.

Fire weather

Due to enhanced rainfall over the western two-thirds of the state, conditions will remain below critical fire weather thresholds. A trough settling in between Kauai and Oahu will maintain unsettled weather under light to moderate southerlies while more stable, drier (south)easterly winds continue over Maui and Big Island. Temperature inversion heights will range between 7,000 to 9,000 feet today.

HFO Watches/Warnings/Advisories

Flood Watch through late tonight 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.
High Surf Advisory until 6 PM HST this evening for Niihau, Kauai Leeward, Waianae Coast, Oahu North Shore, Kauai North.
High Surf Advisory until 6 AM HST Tuesday for Maui Windward West, Molokai Windward, Molokai North, Molokai West, Maui Central Valley North, Windward Haleakala.
High Surf Advisory from 6 PM this evening to 6 AM HST Tuesday for Kona, Kohala.
Small Craft Advisory until 6 PM HST this evening for Kauai Northwest Waters, Kauai Windward Waters, Kauai Leeward Waters, Kauai Channel, Oahu Windward Waters.

Data Courtesy of NOAA.gov