West Side

Tonight: Mostly cloudy with scattered showers in the evening, then partly cloudy with isolated showers after midnight. Lows 65 to 71. East winds up to 15 mph. Chance of rain 40 percent.

Tuesday: Partly sunny with scattered showers. Highs 77 to 82. South winds up to 10 mph. Chance of rain 50 percent.

Tuesday Night: Mostly cloudy. Scattered showers in the evening, then isolated showers after midnight. Lows 65 to 72. South winds around 10 mph in the evening becoming light. Chance of rain 40 percent.

South Side

Tonight: Partly cloudy. Isolated showers in the evening. Lows 65 to 74. Northeast winds around 10 mph in the evening becoming light. Chance of rain 20 percent.

Tuesday: Mostly sunny with isolated showers in the morning, then partly sunny with scattered showers in the afternoon. Highs around 85. South winds 10 to 15 mph. Chance of rain 40 percent.

Tuesday Night: Mostly cloudy with isolated showers. Lows 64 to 74. Southeast winds around 10 mph. Chance of rain 20 percent.

North Shore

Tonight: Mostly cloudy in the evening then becoming partly cloudy. Scattered showers. Lows around 68 near the shore to around 50 near 5000 feet. East winds 10 to 15 mph. Chance of rain 50 percent.

Tuesday: Partly sunny with scattered showers. Highs 77 to 82 near the shore to around 65 near 5000 feet. Southeast winds 10 to 15 mph. Chance of rain 50 percent.

Tuesday Night: Mostly cloudy with scattered showers. Lows around 69 near the shore to around 50 near 5000 feet. Southeast winds up to 10 mph. Chance of rain 50 percent.

Central Maui

Tonight: Partly cloudy. Isolated showers in the evening. Lows around 66. East winds 10 to 15 mph. Chance of rain 20 percent.

Tuesday: Partly sunny. Isolated showers in the morning, then scattered showers in the afternoon. Highs 81 to 86. Southeast winds 10 to 15 mph. Chance of rain 50 percent.

Tuesday Night: Mostly cloudy with isolated showers. Lows around 66. Southeast winds around 10 mph in the evening becoming light. Chance of rain 20 percent.

Upcountry

Tonight: Mostly cloudy with scattered showers in the evening, then partly cloudy with isolated showers after midnight. Lows around 44 at the visitor center to around 40 at the summit. East winds around 10 mph. Chance of rain 50 percent.

Tuesday: Mostly cloudy. Scattered showers in the morning, then showers likely in the afternoon. Highs around 59 at the visitor center to around 62 at the summit. Southeast winds around 10 mph. Chance of rain 70 percent.

Tuesday Night: Mostly cloudy. Scattered showers in the evening, then isolated showers after midnight. Lows around 44 at the visitor center to around 41 at the summit. East winds around 10 mph. Chance of rain 50 percent.

East Maui

Tonight: Mostly cloudy in the evening then becoming partly cloudy. Scattered showers. Lows around 68 near the shore to around 50 near 5000 feet. East winds 10 to 15 mph. Chance of rain 50 percent.

Tuesday: Partly sunny with scattered showers. Highs 77 to 82 near the shore to around 65 near 5000 feet. Southeast winds 10 to 15 mph. Chance of rain 50 percent.

Tuesday Night: Mostly cloudy with scattered showers. Lows around 69 near the shore to around 50 near 5000 feet. Southeast winds up to 10 mph. Chance of rain 50 percent.

Lanai City

Tonight: Mostly cloudy with scattered showers in the evening, then partly cloudy after midnight. Lows around 63. Northeast winds around 10 mph. Chance of rain 40 percent.

Tuesday: Partly sunny. Isolated showers in the morning, then scattered showers in the afternoon. Highs 71 to 79. Southeast winds up to 10 mph shifting to the southwest in the afternoon. Chance of rain 50 percent.

Tuesday Night: Mostly cloudy with scattered showers in the evening, then partly cloudy with isolated showers after midnight. Lows 61 to 66. South winds 10 to 15 mph. Chance of rain 40 percent.

Kaunakakai

Tonight: Partly cloudy with isolated showers. Lows 55 to 71. East winds 10 to 15 mph. Chance of rain 20 percent.

Tuesday: Partly sunny with scattered showers. Highs 69 to 85. South winds around 10 mph. Chance of rain 50 percent.

Tuesday Night: Mostly cloudy with scattered showers. Lows 55 to 71. South winds around 10 mph in the evening becoming light. Chance of rain 50 percent.

Detailed Forecast

Synopsis

Trades will dissipate overnight as a deepening low to the far northwest of the state drifts southward Tuesday, lingering west of the state over the next several days. Southerly winds will help to bring increased showers across the state. Unsettled weather conditions are expected Tuesday through Wednesday with scattered showers across the state especially during the afternoons. Heavier showers and isolated thunderstorms will be possible around Kauai late Tuesday into Wednesday.

Discussion

No changes to the forecast package this evening, but anticipate making some tweaks with the morning package to factor in the latest guidance. Don't expect that to change the flavor of the forecast.
Monitoring radar closely tonight as winds are beginning to veer to the southeast, with precipitable water (PW) values on the increase. This can lead to showers forming over the Koolau mountain range on Oahu that hang up on the mountains. Otherwise, passing showers are expected over the islands tonight.
The complex surface low pressure system is about 900 miles northwest of Kauai this evening, with its developing frontal boundary roughly 500 miles to the northwest of Kauai. Thunderstorms have begun to form along this developing surface feature, with an isolated thunderstorm or two reaching the northern offshore waters this evening. The GFS and ECMWF continue to be in good agreement with the surface system and its associated strengthening upper level low sinking south over the next few days. While both models bring PW values well above normal (1.5 to 1.7) over the western islands, the ECMWF is a little wetter than the GFS. Both models bring 500 mb temperatures of -10 to -12C which is colder than normal. This amount of moisture and instability increases confidence of heavier showers and thunderstorms over the western islands. Models continue to keep the system just west of Kauai, which places Kauai and Niihau in the main focus, and thus the ones under the Flood Watch starting at noon on Tuesday.
For rest of the state, moist southerly flow will produce humid conditions and scattered showers. With the added moisture, some brief downpours are possible especially during the afternoon and evening hours. Widespread rainfall is not expected for rest of the state, but some showery conditions will be possible at times.
Thursday into Friday, the upper level low should lift further north and away from the state which should bring the return of stable conditions, although somewhat unsettle weather may continue near Kauai. Winds will continue to remain light to moderate and out of the southeast direction, which will produce warm and humid conditions through the rest of the week. Trade winds should return sometime during the first half of next week.

Aviation

Light to moderate trades will continue through the evening, ushering in a few SHRA to windward and mauka locations. Winds will remain light enough for land breezes to develop overnight and help to clear out leeward and interior areas.
Tomorrow (Tuesday), winds will veer southerly in advance of a developing low pressure system to the northwest of Kauai. This should focus SHRA over south-facing coasts and slopes, along with some sea breeze development bringing low cigs and SHRA to north and east- facing coasts and slopes. The proximity of the low could bring +RA and isol TSRA to the western end of the state beginning tomorrow afternoon.
MVFR/IFR conds will be possible in any SHRA/TSRA, otherwise VFR prevails.
No AIRMETs are currently in effect.

Marine

High pressure far northeast of Hawaii will gradually weaken as a gale low develops and becomes anchored roughly 500 nm northwest of Kauai Tuesday. In response, moderate to locally fresh easterly trade winds will ease this evening then veer south around the western end of the state and southeast for the eastern end of the state by Tuesday afternoon. Near the Big Island and Maui, southeast winds may increase to moderate to locally strong levels that could necessitate a Small Craft Advisory (SCA) Wednesday through Friday. The low will also bring a chance of heavy showers and thunderstorms, mainly over the western end of the island chain. The low is expected to move north Friday into next weekend allowing winds to gradually ease and shift more easterly.
A tiny, medium period northwest swell is currently moving through the state as band energy in the 12-14 second range has been steadily increasing since this afternoon based on the Hanalei near shore buoy. This swell will provide a small boost in surf along north and west facing shores as it peaks Tuesday before slowly subsiding into Wednesday. Minimal surf is expected later Wednesday through Friday. A slightly larger small, long period northwest swell is expected next weekend.
South shore surf will remain small through this evening. Long period forerunners of a small, long period south-southwest swell is expected to arrive late tonight into early Tuesday morning, pushing surf along south facing shores above summer average. The swell will is expected to peak Tuesday night then hold through Wednesday, before declining Thursday and Friday.
Surf along east facing shores will remain near or below average due to the lack of strong trade winds over or upstream of the islands. East shore surf may see some side shore chop from the southeasterly wind swell that will develop next week.

HFO Watches/Warnings/Advisories

Flood Watch from Tuesday afternoon through Wednesday afternoon for Niihau, Kauai,

Data Courtesy of NOAA.gov