West Side

Today: Mostly sunny with isolated showers. Highs around 81. East winds up to 15 mph. Chance of rain 20 percent.

Tonight: Partly cloudy with scattered showers. Lows 67 to 74. Northeast winds around 10 mph. Chance of rain 30 percent.

Monday: Mostly sunny with isolated showers. Highs 78 to 83. East winds around 10 mph. Chance of rain 20 percent.

South Side

Today: Mostly sunny with isolated showers. Highs around 86. North winds up to 15 mph. Chance of rain 20 percent.

Tonight: Mostly clear with isolated showers. Lows 67 to 75. North winds 10 to 15 mph. Chance of rain 20 percent.

Monday: Sunny. Isolated showers in the afternoon. Highs around 86. North winds 10 to 15 mph. Chance of rain 20 percent.

North Shore

Today: Breezy. Partly sunny with scattered showers. Highs 78 to 83 near the shore to around 64 near 5000 feet. East winds 10 to 20 mph. Chance of rain 50 percent.

Tonight: Mostly cloudy with scattered showers. Lows around 71 near the shore to around 52 near 5000 feet. East winds 10 to 15 mph. Chance of rain 50 percent.

Monday: Partly sunny in the morning then becoming mostly sunny. Scattered showers. Highs around 81 near the shore to around 64 near 5000 feet. East winds 10 to 15 mph. Chance of rain 50 percent.

Central Maui

Today: Mostly sunny with isolated showers. Highs 81 to 87. Northeast winds up to 15 mph. Chance of rain 20 percent.

Tonight: Partly cloudy with isolated showers. Lows 68 to 73. Northeast winds around 10 mph. Chance of rain 20 percent.

Monday: Sunny with isolated showers. Highs 81 to 87. Northeast winds 10 to 15 mph. Chance of rain 20 percent.

Upcountry

Today: Mostly sunny. Isolated showers in the morning, then scattered showers in the afternoon. Highs around 60 at the visitor center to around 65 at the summit. East winds up to 15 mph. Chance of rain 40 percent.

Tonight: Partly cloudy with scattered showers. Lows around 47 at the visitor center to around 43 at the summit. East winds 10 to 15 mph. Chance of rain 50 percent.

Monday: Mostly sunny. Scattered showers in the morning, then isolated showers in the afternoon. Highs around 59 at the visitor center to around 64 at the summit. East winds 10 to 15 mph. Chance of rain 50 percent.

East Maui

Today: Breezy. Partly sunny with scattered showers. Highs 78 to 83 near the shore to around 64 near 5000 feet. East winds 10 to 20 mph. Chance of rain 50 percent.

Tonight: Mostly cloudy with scattered showers. Lows around 71 near the shore to around 52 near 5000 feet. East winds 10 to 15 mph. Chance of rain 50 percent.

Monday: Partly sunny in the morning then becoming mostly sunny. Scattered showers. Highs around 81 near the shore to around 64 near 5000 feet. East winds 10 to 15 mph. Chance of rain 50 percent.

Lanai City

Today: Sunny with isolated showers. Highs 72 to 80. Light winds becoming northeast up to 10 mph in the afternoon. Chance of rain 20 percent.

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

Monday: Sunny. Isolated showers in the afternoon. Highs 72 to 80. Light winds. Chance of rain 20 percent.

Kaunakakai

Today: Sunny with isolated showers. Highs 70 to 87. East winds 10 to 15 mph. Chance of rain 20 percent.

Tonight: Partly cloudy. Scattered showers in the evening, then isolated showers after midnight. Lows 59 to 73. East winds 10 to 15 mph. Chance of rain 40 percent.

Monday: Sunny with isolated showers. Highs 70 to 87. East winds around 10 mph. Chance of rain 20 percent.

Detailed Forecast

Synopsis

Light to locally breezy trades with generally drier weather through Monday afternoon. Showers will focus along windward exposed upslope terrain the next couple of days. There are increasing chances for more active weather from late Monday through Thursday with higher probabilities for locally heavy rain with isolated thunderstorms.

Discussion

Broad upper ridging across the West Central Pacific has expanded just far enough east, along with a swath of drier air passing by from the east, to influence and stabilize our weekend weather here in paradise. Morning shortwave IR satellite is depicting a lower level moist air mass advancing in on light to moderate trades. This will increase overall precipitation coverage today but most of the light showers will briefly pass over windward communities with the highest precipitation being confined to the upper elevations. A weakened 1022 mb surface high pressure located approximately 1,000 miles northeast of Hilo is slowly settling southeastward. This has sustained a tight enough downstream pressure gradient to support light trades the next couple of days. Local 12Z soundings are still showing a 6-7k ft boundary layer underneath very dry mid to upper level air. These factors equate to another couple of days of primarily dry, stable weather under partially sunny skies and light trades, locally breezy within higher terrain. Light precipitation will again focus along windward upslope mauka. Higher nocturnal rain accumulations will occur within a relatively thicker 8k to 9k ft moistened layer underneath a thick slab of dry air aloft.
A weakness will develop within the upper ridge over the islands Monday. An upper trough and associated cold front will approach the island chain from the northwest and merge with the weakness channel over the state going into Tuesday. It is a bit early to determine the evolution of this trough but it will more than likely pull up moist equatorial air northward over the islands while destabilizing the regional atmosphere. Numerical weather prediction guidance has precipitable water (pwat) values nearing the upper 90 percentile for this time of year being pulled up and over the islands from Monday through Thursday. 500 mb temperatures within the trough or upper low should be cold enough (-10 to -12 C) destabilize the regional atmosphere enough to result in isolated thunderstorms Tuesday roughly centered between the Kauai and Alenuihaha Channels. The bulk of the higher pwat air is currently being modeled to be in the vicinity of Big Island and points east while the greatest instability exists west over the central and western islands. While the greatest moisture and highest instability may be slightly out of phase, confidence is mounting that the highest threat from thunder will anchor in the proximity of Oahu and Maui County while the highest rainfall that could lead to flooding will focus on Big Island. Another element that increases confidence of this potentially impactful, active mid week pattern is the upper divergence forming along the eastern flank of the upper trough…opening up across the island chain. This upper forcing within a very moist air mass and destabilizing cooler mid to upper levels should provide the necessary ingredients to prolong a very wet pattern through Thursday. A drier late week as upper ridging begins to take hold. Re-established surface high pressure to the northeast will reintroduce strengthened trade winds Thursday and Friday. An upper trough and assoicated surface front will approach the state next weekend. This trough may pack enough of a punch to actually push our first cold front across the island chain early next week.
Freezing levels falling to around 12k feet, along with the potential of a saturated column between the -10 and -20 C levels going into Tuesday, implies that a frozen mix or snow showers will occur within freezing surface temperatures atop Big Island summits. Mid level southwesterly winds will also strengthen east of the upper trough axis and this will significantly increase Haleakala and Big Island summit winds. Thus, there is increasing chances for both a Wind Advisory and Winter Weather Advisory for Mauna Kea and Mauna Loa's summits Tuesday and Wednesday.

Aviation

Primarily VFR across all local terminals. Possible brief MVFR cats in both ceilings and visibilities may occur during times of more moderate intensity showers over such windward-exposed air fields as HTO and LIH. Weaker trades suggest that local breezes will become more dominant across wind-sheltered air fields. Brief -SHRA will continue to mainly focus along windward exposures.
No AIRMETs are in effect.

Marine

Trade wind speeds will decrease today and veer from a more E to SE direction from Monday to Tuesday as the high pressure ridge north of the Hawaiian Islands weakens and drifts E in response to a surface trough over the islands and an approaching cold front far NW of the state. Additionally, the cold front may advance into the offshore waters by Tuesday afternoon, bringing locally strong ENE winds behind it before the front diminishes over the NW coastal waters Wednesday. Expect Small Craft Advisories to be issued for most of the Hawaiian coastal waters from Wednesday into Friday due to a combination of strengthening winds and rising seas. Isolated thunderstorms are possible over the Hawaiian waters through much of this week.
Expect multiple overlapping NW swells this week. This will keep surf heights boosted along exposed N and W facing shores. The challenge with this pattern will be in tracking the multiple swell energies from similar directions and similar periods. Wave height model guidance has trended towards combining these different swells for the Monday through Friday time period. Decreased the NW swell heights from model output by a few feet to account for this discrepancy.
Surf along N facing shores remains small today as a small medium period NW (310-320 degree) swell slowly declines into Monday. The next moderate, medium to long period NNW (320-340 degree) swell will then arrive late Tuesday, peaking Wednesday near High Surf Advisory thresholds along exposed N and W facing shores, then declining through the end of the week. Another overlapping small, medium period NNE (010-020 degree) swell arrives from Thursday into Friday. Then another small, long period NW (320-330 degree) swell arrives in Hawaiian waters by Thursday night, peaking late Friday, then slowly diminishing into next weekend.
Surf along E facing shores will decline into Monday due to the weakening of the local and upstream trade winds. As the high pressure ridge moves east later this week, the combination of a long easterly fetch of 15 to 20 kt winds upstream of the islands from Tuesday through Thursday, along with strengthening local winds along the cold front, will produce moderate and choppy surf along E shores. For S shores, periods of tiny background south swell energy will linger.

Fire weather

Recent island wide rain and more overcast skies, along with relatively higher minimum afternoon humidities under light winds, will assist in maintaining a lower fire weather threat. There is a moderately high chance for a return to a wet weather pattern from Monday night onward. This morning's inversion heights range from 6 to 8k feet.

HFO Watches/Warnings/Advisories

None.

Data Courtesy of NOAA.gov