West Side

Today: Mostly sunny in the morning then becoming partly sunny. Isolated showers. Highs 77 to 88. East winds up to 15 mph. Chance of rain 20 percent.

Tonight: Partly cloudy. Isolated showers in the evening. Lows 67 to 75. East winds 10 to 15 mph. Chance of rain 20 percent.

Wednesday: Breezy. Mostly sunny in the morning, then partly sunny with scattered showers in the afternoon. Highs 77 to 89. East winds 10 to 20 mph. Chance of rain 40 percent.

South Side

Today: Mostly sunny. Isolated showers in the afternoon. Highs 79 to 90. North winds up to 15 mph. Chance of rain 20 percent.

Tonight: Mostly clear. Lows 68 to 73. North winds up to 15 mph.

Wednesday: Sunny in the morning, then partly sunny with scattered showers in the afternoon. Highs 79 to 90. Northeast winds up to 15 mph. Chance of rain 50 percent.

North Shore

Today: Breezy. Partly sunny with scattered showers. Highs around 84 near the shore to around 69 near 5000 feet. Southeast winds up to 20 mph shifting to the east 10 to 20 mph in the afternoon. Chance of rain 50 percent.

Tonight: Mostly cloudy with scattered showers. Lows around 72 near the shore to around 55 near 5000 feet. Southeast winds up to 15 mph. Chance of rain 50 percent.

Wednesday: Mostly cloudy. Breezy. Scattered showers in the morning, then numerous showers in the afternoon. Highs 82 to 87 near the shore to around 68 near 5000 feet. East winds 10 to 20 mph. Chance of rain 70 percent.

Central Maui

Today: Breezy. Mostly sunny with isolated showers. Highs 84 to 90. Northeast winds up to 20 mph increasing to 10 to 20 mph in the afternoon. Chance of rain 20 percent.

Tonight: Partly cloudy. Lows 68 to 73. East winds 10 to 15 mph.

Wednesday: Breezy. Sunny in the morning, then partly sunny with scattered showers in the afternoon. Highs 84 to 90. East winds 10 to 25 mph. Chance of rain 50 percent.

Upcountry

Today: Sunny with isolated showers. Highs 54 to 70. East winds up to 10 mph. Chance of rain 20 percent.

Tonight: Mostly clear. Scattered showers in the evening, then isolated showers after midnight. Lows around 47 at the visitor center to around 44 at the summit. East winds up to 15 mph. Chance of rain 50 percent.

Wednesday: Breezy. Sunny with isolated showers. Highs 54 to 71. East winds up to 25 mph. Chance of rain 20 percent.

East Maui

Today: Breezy. Partly sunny with scattered showers. Highs around 84 near the shore to around 69 near 5000 feet. Southeast winds up to 20 mph shifting to the east 10 to 20 mph in the afternoon. Chance of rain 50 percent.

Tonight: Mostly cloudy with scattered showers. Lows around 72 near the shore to around 55 near 5000 feet. Southeast winds up to 15 mph. Chance of rain 50 percent.

Wednesday: Mostly cloudy. Breezy. Scattered showers in the morning, then numerous showers in the afternoon. Highs 82 to 87 near the shore to around 68 near 5000 feet. East winds 10 to 20 mph. Chance of rain 70 percent.

Lanai City

Today: Mostly sunny in the morning, then partly sunny with scattered showers in the afternoon. Highs 72 to 82. Northeast winds up to 15 mph. Chance of rain 30 percent.

Tonight: Partly cloudy. Lows 61 to 68. Northeast winds 10 to 15 mph.

Wednesday: Mostly sunny in the morning, then partly sunny with scattered showers in the afternoon. Highs 72 to 82. Northeast winds 10 to 15 mph shifting to the east in the afternoon. Chance of rain 50 percent.

Kaunakakai

Today: Mostly sunny in the morning then becoming partly sunny. Isolated showers. Highs 73 to 90. East winds up to 15 mph increasing to 10 to 15 mph in the afternoon. Chance of rain 20 percent.

Tonight: Mostly clear. Isolated showers in the evening. Lows 58 to 75. East winds 10 to 15 mph. Chance of rain 20 percent.

Wednesday: Breezy. Sunny in the morning, then partly sunny with isolated showers in the afternoon. Highs 72 to 89. East winds 10 to 20 mph. Chance of rain 20 percent.

Detailed Forecast

Synopsis

An upper level low pressure system continues to glide south towards the Hawaiian Islands this morning. This unstable system will combine forces with low level cloud bands and produce extended periods of wet weather across the islands from Wednesday into the weekend. Moderate to breezy trade winds during this time period will help to lift showers up over windward island mountains where the highest rainfall amounts are expected. Showers will likely become locally heavy in terrain favored locations with potential for thunderstorm activity from Wednesday night through Friday afternoon. Forecast projections for this weekend show the unstable upper low drifting westward away from the state with improving weather trends in the forecast.

Discussion

The latest water vapor satellite imagery this morning shows a low pressure system with two embedded low centers roughly 300 to 500 miles north of the Hawaiian Islands. Looking into the infrared satellite channel we see an area of numerous thunderstorms continues to fire surrounding the northern-most low center. This unstable cold core upper low will continue to drift south towards the islands over the next two days. Roughly 500 miles east of the Big Island a long unsettled cloud band is riding towards the islands on the easterly trade winds. This cloud band is the convergent remnants of an old Eastern Pacific cold front. Based on the latest short range forecast model guidance, the upper low and remnant low level convergent cloud band will combine forces bringing periods of wet weather to the entire island chain from Wednesday night through Friday.
The short range forecast calls for increasing shower trends as the unstable upper low approaches the islands from the north. Currently only a few bands of showers are moving through the islands in a hybrid light to moderate easterly trade wind and leeward land and sea breeze pattern. Morning 2 AM HST (12Z) upper air balloon soundings from Hilo and Lihue show subsidence temperature inversion heights in the 7,000 to 9,000 foot range respectively. Although the Lihue sounding is likely elevated a bit due to a passing shower band at the time of observation. The large scale downward motions (subsidence) will weaken as the low center drifts into a position near Kauai and Oahu by Wednesday afternoon, lifting the subsidence temperature inversion in the process.
By Wednesday morning expect inversion heights around the 8,000 foot level or higher across the state, leading to periods of increasing wet weather trends. At this point, much will depend upon how the smaller scale (mesoscale) weather pattern interacts with the larger upper low. Low level convergence bands and trade winds lifting clouds up over windward island mountains will be the primary drivers for any locally heavy rainfall. While any location may see periods of locally heavy rain, windward areas and mountains will be favored especially near the upper low center, or under any developing thunderstorms. Storm total rainfall estimates for this entire two to three day event may exceed 2 inches in terrain favored locations. For the moment it remains too early to pin down any islands for flooding, however a Flood Watch may be needed at some point as this event unfolds. Any flooding potential may tend to favor the western half of the state as these islands will be closer to the unstable upper low center.
Thunderstorm activity will favor the western half of the state including the islands of Niihau, Kauai, and Oahu from Wednesday night through Friday. The Big Island may also see isolated thunderstorms from late Thursday morning to early evening as the low level convergent band drifts through the island.
Starting from Friday night to Saturday we begin to see another change in the weather with decreasing shower trends as the upper low begins to drift westward away from the Hawaiian Islands. Expect decreasing local area showers this weekend in a moderate to locally breezy easterly trade wind pattern, with some lingering passing showers favoring the windward and mountain areas during the overnight to early morning hours.
Long range weather models are hinting at yet another unstable upper low drifting towards the Hawaiian Islands early next week, potentially triggering yet another round of wet weather across the island chain. Stay tuned.

Aviation

A hybrid moderate easterly trade wind and afternoon sea breeze pattern will continue today. Onshore sea breezes will favor terrain sheltered leeward areas. Shower trends will increase by early Wednesday morning. Brief periods of MVFR conditions are possible in passing showers mainly over windward and mountain areas today.
No AIRMETs in effect.

Marine

A trough north of the state will continue to disrupt trades through the rest of today, supporting gentle to fresh easterly winds prevailing across the coastal waters. This trough is forecast to fade while high pressure builds far to the northeast later this evening. As this occurs, expect trade winds to build back into the region through the rest of the week. Meanwhile, a an upper level low will move over the islands beginning around Tuesday night. This low is already producing thunderstorms near the offshore waters, which will continue through Friday. As the low comes closer to the islands it will destabilize the atmosphere enough to support isolated thunderstorm chances across the coastal waters from Tuesday night through Friday.
Latest buoy observations show that a moderate, long period northwest (320 degree) swell is currently moving through the islands. This swell is originating from a deep low pressure system that produced storm force winds in the far NW Pacific during the second half of last week, then continued to aim gale force winds at Hawaii through early Sunday as it weakened. Additionally, the quick-moving remnants of Post-Tropical Cyclone Halong also produced a closer, very brief fetch aimed at Hawaii from a slightly more WNW (300 degree) direction late last week. As a result, the current northwest swell is expected to peak this morning, then begin its decline later this afternoon. A High Surf Advisory remains in effect for north and west facing shores of Niihau, Kauai, Oahu, and Molokai, and for north facing shores of Maui through this evening. A Small Craft Advisory (SCA) is in effect for all Kauai Waters, Kauai Channel, Oahu Windward Waters, Kaiwi Channel, Maui County Windward Waters and Big Island Windward Waters due to seas around 10 feet. With winds increasing tonight especially across the windier waters and channels a SCA will go in effect this evening around Maui Co. and the Big Island.
Small, medium period south swell energy will continue through the week with minor pulses of longer periods arriving tonight and again on Saturday. East shores will remain small due to light winds, but should begin to increase beginning Wednesday as trades strengthen.

Fire weather

Wind speeds and afternoon minimum humidity levels will likely remain below critical fire weather thresholds this week. Unstable conditions will produce increasing wet weather trends across the state from Wednesday through Friday. Locally heavy rain and thunderstorms are possible during this time period. Temperature inversion heights near Maui and the Big Island will range from the 6,000 to 7,000 feet elevation range today.

HFO Watches/Warnings/Advisories

High Surf Advisory until 6 PM HST this evening for north and west facing shores of Niihau, Kauai, Oahu, Molokai, and north facing shores of Maui.
Small Craft Advisory until 6 PM HST this evening for Kauai Northwest Waters, Kauai Windward Waters, Kauai Leeward Waters, Kauai Channel, Oahu Windward Waters, Kaiwi Channel, Maui County Windward Waters, Big Island Windward Waters.
Small Craft Advisory from 6 PM this evening to 6 PM HST Wednesday for Maalaea Bay, Pailolo Channel, Alenuihaha Channel, Big Island Leeward Waters, Big Island Southeast Waters.

Data Courtesy of NOAA.gov