West Side

Tonight: Partly cloudy with isolated showers. Lows 66 to 72. East winds up to 15 mph. Chance of rain 20 percent.

Monday: Mostly sunny with isolated showers in the morning, then partly sunny with scattered showers in the afternoon. Highs 77 to 82. East winds up to 15 mph. Chance of rain 40 percent.

Monday Night: Mostly cloudy with scattered showers in the evening, then partly cloudy with isolated showers after midnight. Lows 65 to 71. Southeast winds up to 10 mph in the evening becoming light. Chance of rain 30 percent.

South Side

Tonight: Mostly clear. Isolated showers in the evening. Lows 64 to 73. Northwest winds up to 15 mph. Chance of rain 20 percent.

Monday: Mostly sunny with isolated showers. Highs around 85. North winds up to 10 mph. Chance of rain 20 percent.

Monday Night: Partly cloudy. Isolated showers in the evening. Lows 65 to 74. Northwest winds up to 10 mph in the evening becoming light. Chance of rain 20 percent.

North Shore

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

Monday: Partly sunny with scattered showers. Highs 76 to 82 near the shore to around 65 near 5000 feet. East winds up to 15 mph increasing to 10 to 15 mph in the afternoon. Chance of rain 50 percent.

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

Central Maui

Tonight: Partly cloudy with isolated showers. Lows around 66. East winds up to 15 mph. Chance of rain 20 percent.

Monday: Mostly sunny with isolated showers. Highs 80 to 86. East winds up to 15 mph. Chance of rain 20 percent.

Monday Night: Partly cloudy with isolated showers. Lows around 66. East winds up to 15 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 up to 10 mph. Chance of rain 50 percent.

Monday: Partly sunny with scattered showers. Highs around 58 at the visitor center to around 62 at the summit. East winds up to 10 mph. Chance of rain 50 percent.

Monday Night: Mostly cloudy with isolated showers. Lows around 44 at the visitor center to around 40 at the summit. Southeast winds up to 10 mph in the evening becoming light. Chance of rain 20 percent.

East Maui

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

Monday: Partly sunny with scattered showers. Highs 76 to 82 near the shore to around 65 near 5000 feet. East winds up to 15 mph increasing to 10 to 15 mph in the afternoon. Chance of rain 50 percent.

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

Lanai City

Tonight: Partly cloudy. Scattered showers in the evening, then isolated showers after midnight. Lows around 64. Northeast winds up to 10 mph. Chance of rain 30 percent.

Monday: Mostly sunny with isolated showers in the morning, then partly sunny with scattered showers in the afternoon. Highs 71 to 79. Northeast winds up to 10 mph. Chance of rain 40 percent.

Monday Night: Mostly cloudy in the evening then becoming partly cloudy. Isolated showers. Lows around 63. Light winds. Chance of rain 20 percent.

Kaunakakai

Tonight: Partly cloudy with isolated showers. Lows 56 to 71. East winds 10 to 15 mph. Chance of rain 20 percent.

Monday: Mostly sunny with isolated showers. Highs 69 to 86. East winds up to 15 mph. Chance of rain 20 percent.

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

Detailed Forecast

Synopsis

Light to moderate trade winds are expected Monday with some showers focused over windward and mauka areas, although some interior showers will also be possible. A deepening low to the far northwest of the state will sink southward over the next several days. This will disrupt the trade flow by Tuesday with southerly winds, and introduce higher chances for showers across the state. The threat for heavy rain and isolated thunderstorms will likely increase over Kauai Tuesday through Wednesday.

Discussion

No changes to the forecast this evening, and not expecting to make any significant changes with the morning package.
An upper level trough shifting away from the state is helping to stabilize the airmass over the islands. While trade winds have filtered back in at the surface due to a surface ridge to the far north, expect lighter trade winds on Monday. This will allow for some localized land and sea breezes to set up, which in turn means a few interior showers Monday afternoon. However the more stable airmass should limit rainfall totals Monday.
A surface low pressure system far north-northwest of the islands has an upper level low associated with it, and this will be the focus for our next weather event. This system is expected to strengthen as it sinks southward over the next few days, bringing with it another round of unstable and wet weather to the islands. There is good agreement with the GFS and ECMWF that both the surface and upper level lows will remain to the west of the islands. This will likely mean that Kauai is most likely to experience the heavy showers and possible thunderstorms that will be associated with the system. However southerly winds on the east side of the system will bring increased moisture over all the islands. There is good agreement with these models that precipitable water (PW) will exceed 1.5 inches, which is well above normal for this time of year. The models are also in good agreement with 500 mb temperatures around Kauai and Oahu reaching -10C, which is normal for this time of year. Despite upper level temperatures near normal, the well above normal PW will help to fuel heavier showers.
With this event, we expect a large gradient in rainfall between the western end and the eastern end of the state. The Island of Kauai will be the closest to the low pressure system and should see the greatest rainfall from this event. Heavy rain will be the main concern and a Flood Watch may be needed for Kauai around the Tuesday through Wednesday time frame. With the latest QPF ensemble guidance coming into better agreement, the main moisture axis will likely stay just west of Kauai. Nonetheless, it would only take a slight shift more eastward for Kauai to be under the main moisture band. Even if the main band of rain stays just west, heavy downpours with isolated thunderstorms should be expected over Kauai during this time. For rest of the state, scattered showers with some moderate rainfall totals will be possible with higher rainfall amounts likely around Oahu. By Friday, the upper level low should lift further north and away from the state which should bring the return of stable conditions, although trade winds may not return until later in the weekend.

Aviation

Moderate trades will persist through this evening and weaken slightly heading into Monday. Low cigs and SHRA expected of mauka and windward locations. Winds will be light enough to support isol sea/land breeze development that will bring additional SHRA to some leeward and interior areas during the afternoon hours through tomorrow and clearing of these areas overnight. MVFR/IFR conds will be possible in any SHRA, otherwise VFR should prevail.
AIRMET Sierra for mtn obsc is in effect for windward sections of Oahu through Maui and for the entire Big Island.

Marine

Moderate to locally fresh easterly trade winds will ease on Monday and veer southeast as a high centered far northeast of Hawaii weakens and a surface trough develops about 500 nm northwest of Kauai. On Tuesday, the trough will deepen into a low about 400 nm northwest of Kauai, and moderate winds will veer out of the south across most waters, while moderate to fresh southeast winds develop around the Big Island and Maui through Thursday. The low will also bring a chance of heavy showers and thunderstorms, mainly over the western end of the island chain. The low will lift northward on Friday, allowing winds to back out of the southeast statewide.
Small surf will prevail along north shores through much of the week. Near shore buoys are show a decline in 14-12 second energy from a small north northwest swell today and will continue to fade into Monday. Monday night, another tiny northwest swell will fill in and maintain small surf into Wednesday. Minimal surf is expected later Wednesday through Friday, followed by another small pulse of northwest swell next weekend.
South shore surf will be near April average through Monday, followed by a larger south-southwest swell on Tuesday. The PacIOOS Kaumalapau buoy has been showing the current swell holding at around 2 feet 15 seconds this afternoon, with little change expected Monday. A larger swell will arrive on Tuesday, pushing surf along south facing shores above summer average. The swell will hold through Wednesday, then decline Thursday and Friday.
Small surf along east facing shores will be near current heights through Monday, then drop on Tuesday as southerly winds develop.

HFO Watches/Warnings/Advisories

None.

Data Courtesy of NOAA.gov