Photo Credit: Sebastien Gabriel

West Kaua’i

Today: Mostly cloudy with showers likely and slight chance of thunderstorms. Locally heavy rainfall possible. Highs around 80 near the shore to around 68 above 3000 feet. Southeast winds up to 10 mph. Chance of rain 70 percent.

Tonight: Cloudy with showers. Lows around 68 near the shore to around 57 above 3000 feet. South winds up to 10 mph. Chance of rain 80 percent.

Tuesday: Mostly cloudy with scattered showers. Highs around 82 near the shore to around 70 above 3000 feet. Light winds. Chance of rain 50 percent.

South Kaua’i

Today: Cloudy with showers likely and slight chance of thunderstorms. Locally heavy rainfall possible. Highs around 79. Northeast winds up to 10 mph shifting to the southeast around 10 mph in the afternoon. Chance of rain 70 percent.

Tonight: Cloudy with showers. Lows 64 to 69. Southwest winds around 10 mph. Chance of rain 80 percent.

Tuesday: Mostly cloudy with scattered showers. Highs around 81. Southwest winds around 10 mph in the morning becoming light. Chance of rain 50 percent.

Kaua’i Mountains

Today: Cloudy with showers likely and slight chance of thunderstorms. Locally heavy rainfall possible. Highs 67 to 75 in the valleys to around 62 above 4000 feet. Southeast winds up to 15 mph. Chance of rain 70 percent.

Tonight: Cloudy with showers. Lows 57 to 62 in the valleys to around 51 above 4000 feet. South winds up to 15 mph. Chance of rain 80 percent.

Tuesday: Cloudy with numerous showers. Highs 69 to 77 in the valleys to around 64 above 4000 feet. Light winds. Chance of rain 70 percent.

North Kaua’i

Today: Mostly cloudy with showers likely and slight chance of thunderstorms. Locally heavy rainfall possible. Highs 72 to 79. East winds up to 10 mph. Chance of rain 70 percent.

Tonight: Cloudy with showers. Lows 60 to 69. South winds up to 10 mph. Chance of rain 80 percent.

Tuesday: Mostly cloudy with numerous showers. Highs 74 to 81. Southeast winds up to 10 mph in the morning becoming light. Chance of rain 70 percent.

East Kaua’i

Today: Cloudy with showers likely and slight chance of thunderstorms. Locally heavy rainfall possible. Highs 72 to 79. South winds up to 10 mph. Chance of rain 70 percent.

Tonight: Cloudy with showers. Lows 58 to 70. South winds up to 10 mph. Chance of rain 80 percent.

Tuesday: Mostly cloudy with scattered showers. Highs 74 to 81. Southwest winds up to 10 mph. Chance of rain 50 percent.

Detailed Forecast

Synopsis

A trough of low pressure lifting northward through the islands will bring some muggy and wet weather to the state today and tonight, with many areas receiving some beneficial rainfall. Light to moderate southeast to south winds will develop later today through Tuesday, with a few locally heavy downpours possible. Shower coverage will be on the decline and sunshine will be on the increase by mid week, with a more typical trade wind weather pattern developing Wednesday night and holding in place through the weekend.

Discussion

Currently at the surface, a trough of low pressure is located around 75 miles south of Honolulu, while a weak ridge is positioned a couple hundred miles north of the state. This has weakened the gradient, with light winds present across the entire island chain. Infrared satellite imagery shows abundant layered clouds moving through, bringing mostly cloudy to overcast conditions to the state. Radar imagery shows numerous to widespread light showers moving through the islands, with the exception of Oahu, where little rain has fallen so far. Main short term focus revolves around the potential for some heavy rainfall and flooding.
Deep tropical moisture with precipitable water values of 2+ inches will overspread the islands today. Rain shower coverage is expected to fill in across areas that have seen so far, with periods of wet weather continuing through tonight as the trough moves northward through the islands. The latest model solutions suggest that the rain associated with this trough will focus more over the central and western end of the state than over the Big Island. There remains enough instability that some locally heavy rainfall may occur at times, and even a rumble or two or thunder can't be ruled out during the day today. Overall, most areas will likely see some beneficial rainfall, particularly over the smaller islands. We will maintain the Flood Watch for the entire state through 6 PM this evening, although the potential for flash flooding is certainly not a slam dunk given the amount of deep layered cloud cover in place limiting the potential for vigorous deep convection. Temperatures remain cold enough for some wintry weather on the Big Island summits, where snow has already fallen, and a Winter Weather Advisory remains in effect here through 10 AM this morning, when temperatures should have risen sufficiently to allow rain to mix in with the snow or change over to rain entirely.
Additionally, the Mauna Loa volcano eruption remains active, so the Ashfall Advisory has been extended through 10 AM this morning.
Considerable differences continue to plague the extended portion of the forecast, with the GFS advertising continue unsettled weather with precipitable water values hovering around 2 inches, while the ECMWF suggests a gradual return toward trade winds. For the moment given the uncertainty, will keep a trade wind pattern reflected in the forecast from Wednesday night onward in favor of the latest 00z ECMWF run, although it should be noted that confidence in how the pattern will evolve remains low given that the 12z run of the ECMWF was very similar to the latest 00z GFS solution. Given the preference for the ECMWF solution, we expect a light wind regime to hold in place Tuesday through Wednesday, with lingering deep moisture keeping rain chances elevated, although we should see a drop off in shower coverage and a gradual decrease in cloud cover each day. By Wednesday night and continuing into next weekend, we will show a return to more typical trade wind weather pattern with trades holding generally at moderate speeds.

Aviation

Deepening moisture has spread layered clouds northward and westward across the entire main island chain and will likely linger over the state through at least Monday evening. Low clouds and showers moving in from the south and southeast are expected to increasingly affect the islands through today and tonight.
AIRMET Sierra may be needed later for mountain obscuration across portions of Maui County and the Big Island as showers increase today. Periods of light icing may occur in layered clouds as well. Ash from the erupting Mauna Loa volcano may affect portions of the Big Island, mainly east thru southeast sections, through today and has been included in AIRMET Zulu.

Marine

Two low pressure systems, north and northwest of the state, will keep light southeast to southerly winds in the forecast through Tuesday night. A high pressure ridge will slowly build in from the north on Wednesday and Thursday as these low pressure systems drift away from the island chain. Light to locally fresh trade winds return from Wednesday through Friday. Tropical moisture moving up from the south will produce rain over all islands with slight chances for thunderstorms through Monday afternoon.
The current north-northeast swell continues to fade and a new long period west-northwest swell (300 degrees) will build into the Hawaii region through the day. The long period (19 to 21 second band) forerunners of this swell reached buoy 51001 last night. Surf from this swell will build through the day and likely exceed High Surf Advisory (HSA) criteria along most north and west facing shores of the smaller islands by Tuesday morning. This swell will slowly lower from Tuesday night through Wednesday. Another slightly larger, west-northwest (310 degrees) swell spreading down across the area Thursday and Friday could produce peak surf heights well above the HSA thresholds along most north and west facing shores. Surf heights along east facing shores will decrease and only minimal background surf is forecast along south facing shores this week.

HFO Watches/Warnings/Advisories

Flood Watch through this afternoon for Niihau, Kauai Leeward, Kauai Mountains, Waianae Coast, Oahu North Shore, Olomana, Central Oahu, Waianae Mountains, Lanai Mauka, Kahoolawe, Maui Windward West, Maui Leeward West, Haleakala Summit, Kona, Kohala, Big Island Interior, Kauai North, Kauai East, Kauai South, East Honolulu, Honolulu Metro, Ewa Plain, Koolau Windward, Koolau Leeward, Molokai, Lanai Windward, Lanai Leeward, Lanai South, Maui Central Valley North, Maui Central Valley South, Windward Haleakala, Kipahulu, South Maui/Upcountry, South Haleakala, Big Island South, Big Island Southeast, Big Island East, Big Island North.
Ashfall Advisory until 10 AM HST this morning for Big Island,
Winter Weather Advisory until 10 AM HST this morning for Big Island Summits.

Data Courtesy of NOAA.gov