West Side

Tonight: Partly cloudy in the evening then becoming mostly cloudy. Scattered showers. Lows 65 to 72. Southeast winds up to 10 mph. Chance of rain 50 percent.

Tuesday: Partly sunny. Scattered showers in the morning, then isolated showers in the afternoon. Highs 76 to 81. East winds up to 15 mph. Chance of rain 40 percent.

Tuesday Night: Mostly cloudy with scattered showers. Lows 65 to 71. East winds 10 to 15 mph. Chance of rain 40 percent.

South Side

Tonight: Partly cloudy. Lows 64 to 74. Northwest winds up to 10 mph in the evening becoming light.

Tuesday: Partly sunny. Highs around 84. North winds up to 15 mph.

Tuesday Night: Mostly cloudy. Lows 63 to 73. North winds 10 to 15 mph.

North Shore

Tonight: Mostly cloudy with scattered showers. Lows around 68 near the shore to around 51 near 5000 feet. Southeast winds 10 to 15 mph. Chance of rain 50 percent.

Tuesday: Breezy. Mostly cloudy with scattered showers. Highs 76 to 81 near the shore to around 65 near 5000 feet. East winds up to 20 mph increasing to 10 to 20 mph in the afternoon. Chance of rain 50 percent.

Tuesday Night: Mostly cloudy with numerous showers. Lows around 68 near the shore to around 50 near 5000 feet. East winds 10 to 15 mph. Chance of rain 70 percent.

Central Maui

Tonight: Partly cloudy in the evening then becoming mostly cloudy. Isolated showers. Lows around 67. East winds up to 15 mph. Chance of rain 20 percent.

Tuesday: Partly sunny. Breezy. Isolated showers in the morning. Highs 80 to 85. Northeast winds up to 20 mph increasing to 10 to 20 mph in the afternoon. Chance of rain 20 percent.

Tuesday Night: Mostly cloudy with isolated showers. Lows around 66. Northeast winds 10 to 15 mph. Chance of rain 20 percent.

Upcountry

Tonight: Mostly cloudy with scattered showers. Lows around 45 at the visitor center to around 41 at the summit. East winds up to 15 mph. Chance of rain 50 percent.

Tuesday: Mostly cloudy with scattered showers. Highs around 60 at the visitor center to around 62 at the summit. East winds up to 15 mph. Chance of rain 50 percent.

Tuesday Night: Breezy. Mostly cloudy with scattered showers. Lows around 45 at the visitor center to around 40 at the summit. East winds 10 to 20 mph. Chance of rain 50 percent.

East Maui

Tonight: Mostly cloudy with scattered showers. Lows around 68 near the shore to around 51 near 5000 feet. Southeast winds 10 to 15 mph. Chance of rain 50 percent.

Tuesday: Breezy. Mostly cloudy with scattered showers. Highs 76 to 81 near the shore to around 65 near 5000 feet. East winds up to 20 mph increasing to 10 to 20 mph in the afternoon. Chance of rain 50 percent.

Tuesday Night: Mostly cloudy with numerous showers. Lows around 68 near the shore to around 50 near 5000 feet. East winds 10 to 15 mph. Chance of rain 70 percent.

Lanai City

Tonight: Mostly cloudy. Scattered showers in the evening, then isolated showers after midnight. Lows 61 to 66. Light winds. Chance of rain 30 percent.

Tuesday: Mostly cloudy. Isolated showers in the morning, then scattered showers in the afternoon. Highs 71 to 78. Northeast winds up to 15 mph. Chance of rain 40 percent.

Tuesday Night: Mostly cloudy with isolated showers. Lows around 63. Northeast winds up to 10 mph. Chance of rain 20 percent.

Kaunakakai

Tonight: Partly cloudy in the evening then becoming mostly cloudy. Isolated showers. Lows 55 to 72. East winds up to 15 mph. Chance of rain 20 percent.

Tuesday: Partly sunny with isolated showers. Highs 67 to 84. East winds 10 to 15 mph. Chance of rain 20 percent.

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

Detailed Forecast

Synopsis

The threat for heavy slow moving showers has decreased this evening as deeper convective showers have waned over the islands of Oahu and Kauai. Enough of a flooding threat remains however to keep the Flood Watch in effect through the early Tuesday morning hours. A low pressure system currently west to northwest of the Hawaiian Islands will weaken over the next 24 hours as high pressure begins to build into the region from the north. Expect more stable trends for the western and central islands as easterly trade winds blow in across the region from tomorrow afternoon through the rest of the week. Another cold front approaches the islands this weekend, weakening the ridge and decreasing winds across the region into Monday as a shallow cold front moves from west to east across the state.

Discussion

Looking into a combination of satellite and radar imagery this evening, we see heavy showers and thunderstorms have diminished over the western islands. A weakening upper low remains parked west to northwest of the state. Enough instability lingers in this pattern to justify keeping the Flood Watch out through early Tuesday morning, where it will likely be cancelled with the morning forecast grid package.
The latest forecast guidance shows the low pressure system near the islands will weaken as a migratory high center drifts eastward across the Central Pacific basin. Higher pressure will build in from the north as this high passes by just north of the island chain, resulting in a transition to more stable easterly trade winds on Tuesday, with fairly table moderate to breezy trade winds lasting from Wednesday through Saturday. Trade wind temperature inversion heights will drop to around 5,000 to 7,000 feet for most islands from Tuesday into Saturday. Brief passing showers are possible for most islands, mainly affecting windward and mountain areas in the overnight through early morning hours. The one wrinkle in this forecast concerns a weak wind convergence band setting up just east of the Big Island and East Maui by Wednesday. This convergence cloud band will move up the mountain slopes of both islands, enhancing windward showers in these areas from Wednesday night through Friday morning.
The high pressure ridge north of the state will begin to erode on Saturday as a shallow cold front approaches the islands from the northwest direction. Trade winds will diminish by Saturday night into a more light southeast wind pattern lasting through Sunday afternoon. The latest forecast guidance shows the shallow front moving into Kauai and Oahu by early Monday morning with widespread showers and north to northeast winds developing along and behind the front as it passes through each island. A consensus of model solutions shows the frontal cloud band moving through the islands in Maui and Hawaii Counties from next week Tuesday into Wednesday with enhanced shower activity forecast during this time period, especially along north and east slopes of each island exposed to the northeasterly trade winds.

Aviation

Light to moderate east-southeast winds will linger into early Tuesday, then transition back to a more stable and typical trade wind pattern through the day Tuesday. Any showers that develop will favor windward areas, especially overnight through the early morning period. Although chances are diminishing for any widespread heavy rainfall, an upper disturbance drifting eastward into the area could trigger a few moderate to heavy showers.
AIRMET Sierra is in effect for mountain obscuration for Kauai this evening.

Marine

Low pressure located approximately 500 nautical miles west northwest of Kauai is producing moderate to fresh east southeast winds over the eastern half of the state and light to gentle easterlies over the western half. The proximity of this low will maintain lower end chances for overnight isolated storms primarily in the vicinity of Kauai. The low will weaken and wash out through mid week as surface high pressure north of the state becomes the dominant Central Pacific feature. Water-wide easterly trades will strengthen to moderate to fresh magnitudes by Tuesday night. Small Craft Advisories may be needed for the typically windy areas during the second half of the week.
An ongoing small, medium period north northwest swell (340-350 degree) will fade through Tuesday. A series of similar size northwest (310-320 degree) swells will pass through from Tuesday night through Saturday. These swells will maintain solid head high surf along many north-facing shores, waist to head high surf along better exposed west-facing shores. A significant northwest swell may reach the islands early next week.
East shore chop will likely pick up a bit by the middle of the week as easterlies increase over and upstream of the state. Many south shore surf sets will hold at average waist high as a couple of small, long period south swells pass by this week.

HFO Watches/Warnings/Advisories

Flood Watch until 6 AM HST Tuesday for the islands of Niihau, Kauai and Oahu.

Data Courtesy of NOAA.gov