Photo Credit: Sebastien Gabriel

West Kaua’i

Today: Breezy. Mostly sunny with isolated showers. Highs around 81 near the shore to around 69 above 3000 feet. East winds up to 20 mph. Chance of rain 20 percent.

Tonight: Breezy. Partly cloudy with scattered showers. Lows 64 to 69 near the shore to around 56 above 3000 feet. East winds up to 20 mph. Chance of rain 50 percent.

Monday: Mostly sunny. Scattered showers in the morning, then isolated showers in the afternoon. Highs around 81 near the shore to around 69 above 3000 feet. East winds 10 to 15 mph. Chance of rain 50 percent.

South Kaua’i

Today: Partly sunny. Breezy. Showers likely in the morning, then scattered showers in the afternoon. Highs 77 to 82. Northeast winds 15 to 25 mph. Chance of rain 70 percent.

Tonight: Breezy. Mostly cloudy with showers likely. Lows around 66. Northeast winds 15 to 25 mph. Chance of rain 70 percent.

Monday: Partly sunny. Breezy. Showers likely in the morning, then scattered showers in the afternoon. Highs 76 to 82. Northeast winds 15 to 20 mph. Chance of rain 70 percent.

Kaua’i Mountains

Today: Mostly cloudy. Breezy. Showers in the morning, then scattered showers in the afternoon. Highs 67 to 74 in the valleys to around 60 above 4000 feet. East winds 10 to 25 mph. Chance of rain 90 percent.

Tonight: Breezy. Mostly cloudy with showers. Lows 58 to 65 in the valleys to around 55 above 4000 feet. East winds 10 to 25 mph. Chance of rain 90 percent.

Monday: Mostly cloudy. Breezy. Showers in the morning, then scattered showers in the afternoon. Highs 68 to 75 in the valleys to around 60 above 4000 feet. East winds 10 to 20 mph. Chance of rain 90 percent.

North Kaua’i

Today: Breezy. Partly sunny with scattered showers. Highs 71 to 81. East winds up to 20 mph increasing to 10 to 20 mph in the afternoon. Chance of rain 50 percent.

Tonight: Breezy. Mostly cloudy with showers likely. Lows 60 to 68. East winds up to 20 mph. Chance of rain 70 percent.

Monday: Partly sunny. Breezy. Showers likely in the morning, then scattered showers in the afternoon. Highs 71 to 81. East winds 10 to 20 mph. Chance of rain 70 percent.

East Kaua’i

Today: Mostly cloudy. Breezy. Showers likely in the morning, then scattered showers in the afternoon. Highs 68 to 80. Northeast winds 10 to 20 mph. Chance of rain 70 percent.

Tonight: Breezy. Mostly cloudy with showers likely. Lows 59 to 71. Northeast winds 10 to 20 mph. Chance of rain 70 percent.

Monday: Mostly cloudy. Showers likely in the morning, then scattered showers in the afternoon. Highs 69 to 79. Northeast winds 10 to 15 mph. Chance of rain 70 percent.

Detailed Forecast

Synopsis

An unstable and wet trade wind pattern will bring widespread showers containing locally heavy rainfall and isolated thunderstorms to the area, especially the Big Island and Maui, through early this week. Wet weather will shift to the remaining islands by Tuesday. The potential exists for heavy rain to develop over portions of the area by the end of the week.

Discussion

Beneath a cold upper low, an unstable trade wind pattern is poised to bring wet weather and an active pattern to the islands for the balance of the coming week. Widespread showers containing locally heavy rainfall will mainly focus over Windward Big Island and Maui for the next 48 hours before shifting to the remainder of the island chain from Tuesday onward. Uncertainty regarding the evolution of the aforementioned low will play a key role in the medium range forecast when the potential exists for a period of heavier to rainfall to impact a portion of the island chain by week's end. Occasional nuisance flooding will be common during the next several days as showers focus over windward terrain. Support for flash flooding is ill-defined and somewhat nebulous at this time but will be a focus of the forecast moving forward.
For today, the latest model analysis places the greatest instability over the western end of the state where exceptional lapse rates on the order of 8C/km reside in the 500-300mb layer. For the most part, convection has failed to capital's on this elevated unstable layer, though convergence in the leeward waters of the Big Island has clearly been able to do so as evidenced by persistent thunderstorms there. At the very least, this is a strong indicator that diurnal heating over the Big Island will bring a thunderstorm threat to interior and leeward zones this afternoon. As the upper low pivots westward, northwesterly flow around its western side will feed steeper lapse rates to the eastern end of the state. The mid-level cold pool, characterized by 500mb temperatures around -14 to -15C, will also impinge on the islands this afternoon as instability expands further down the column. Isolated thunderstorms are therefore also a reasonable bet over Windward Big Island and potentially Windward Maui, especially by this evening. The remaining islands will experience comparatively decreased shower coverage, but brief moderate to heavy showers can be expected, including over leeward areas. Little change is then expected heading into Monday as heavy rain remains focused over Maui and the Big Island. Rain rates of 1 to 2 inches per hour will be common during this time. Strong background trades will maintain a progressive nature to the showers, but the high coverage of rainfall will nonetheless lend itself to rises in streams and rivers.
The resident upper low, representing the southern component of a stable rex block over the central Pacific, will move very little during the week. Strong consensus exists among the global models that it will slowly drift westward during the next few days bringing with it the high coverage of moderate to locally heavy trade wind showers. Rain will become increasingly focused over windward portions of Maui County by Monday evening and the remaining islands no later than Tuesday morning. At the same time, an emerging surface surface trough will reduce or eliminate trades in the vicinity of the Big Island resulting in a transition to a sea breeze pattern with afternoon convection followed by clearing overnight.
As the west Pacific jet extends eastward, additional shortwave energy is modeled to wrap into the existing upper low Tuesday through Wednesday. Subsequent deepening will cause deep layer winds to veer to southeasterly resulting in considerable northward moisture transport in the presence of strengthening upper support. By Thursday, the EC becomes the preferred solution as the 00z GFS suffers from convective feedback (spurious surface lows, etc). Although flooding may be possible in the meantime, the period from Thursday into the weekend is of particular interest for flooding potential.

Aviation

Expect breezy trade winds blowing across the Hawaii region today. A weak upper level low over the islands will continue to produce periods of showers falling mainly over windward and mountain slopes of each island. The highest shower coverage will develop during the overnight to early morning hours.
Mostly VFR conditions are expected with brief MVFR conditions in showers mainly in the overnight to early morning hours. A few thunderstorms may also develop near the Big Island today due to unstable conditions associated with the upper level low.
AIRMET Sierra remains in effect for mountain obscurations above 2,000 feet over north and east sections of Oahu, Molokai, Maui and the Big Island. This AIRMET may expand to Kauai later this morning and will diminish in coverage this afternoon.
AIRMET Tango remains in effect for moderate turbulence over and immediately south through west of island mountains. This AIRMET will likely continue through this evening.

Marine

Strong to near gale force trades will continue through Monday as high pressure remains anchored northeast of the island chain. A trough of low pressure will develop southeast of the state Monday night, then slowly track westward across the islands Tuesday through Thursday. The trades will shift northeasterly and remain strong around Kauai and Oahu ahead of the trough Tuesday and Wednesday, likely requiring an extension of the SCA. Meanwhile winds will ease elsewhere, likely below SCA strength around Maui and the Big Island by Tuesday, with winds shifting southerly and southeasterly and increasing to moderate and fresh levels once the trough moves through. The moderate to fresh south/southeast flow should overspread the entire island chain by late Thursday as the trough shifts west of the state.
Large and rough trade wind swell will continue along east facing shores through tonight, and a High Surf Advisory (HSA) remains in effect to account for this. Surf will remain above seasonal levels Monday through late in the week due to strong trades over and upstream of the islands. The greatest potential for a continuation of advisory level surf for east facing shores will be around Kauai and Oahu, potentially through late in the week, where the winds over and upstream will remain the strongest.
A pair of northwest swells will push north shore surf to near the advisory level at the peaks this week. The first swell will build today, peak tonight and Monday, then decline Monday night and Tuesday. A second northwest swell will build on Wednesday, peak Thursday, and decline Friday. South facing shores will experience minimal surf this week

HFO Watches/Warnings/Advisories

High Surf Advisory until 6 AM HST Monday for Olomana, Kahoolawe, Maui Windward West, Kauai East, Kauai South, East Honolulu, Koolau Windward, Molokai Windward, Molokai Southeast, Windward Haleakala, Kipahulu, South Haleakala, Big Island Southeast, Big Island East, Big Island North.
Small Craft Advisory until 6 PM HST Monday for all Hawaiian waters,

Data Courtesy of NOAA.gov