Photo Credit: Sebastien Gabriel

West Kaua’i

Today: Mostly cloudy. Breezy. Numerous showers in the morning, then scattered showers in the afternoon. Highs around 82 near the shore to around 70 above 3000 feet. East winds up to 15 mph shifting to the southeast in the afternoon. Chance of rain 60 percent.

Tonight: Cloudy and breezy. Numerous showers in the evening, then showers and isolated thunderstorms after midnight. Locally heavy rainfall possible. Lows 64 to 69 near the shore to around 58 above 3000 feet. East winds 10 to 20 mph. Chance of rain 90 percent.

Tuesday: Showers and isolated thunderstorms. Locally heavy rainfall possible. Highs around 81 near the shore to around 69 above 3000 feet. East winds 10 to 15 mph. Chance of rain near 100 percent.

South Kaua’i

Today: Breezy. Mostly cloudy with numerous showers. Highs 77 to 83. East winds 10 to 20 mph. Chance of rain 70 percent.

Tonight: Cloudy. Numerous showers in the evening, then showers and isolated thunderstorms after midnight. Locally heavy rainfall possible. Lows around 66. Northeast winds up to 15 mph. Chance of rain 80 percent.

Tuesday: Cloudy with showers and isolated thunderstorms. Locally heavy rainfall possible. Highs 76 to 82. North winds up to 10 mph. Chance of rain 90 percent.

Kaua’i Mountains

Today: Breezy. Mostly cloudy with numerous showers. Highs 70 to 76 in the valleys to around 63 above 4000 feet. Southeast winds 10 to 20 mph. Chance of rain 70 percent.

Tonight: Breezy. Showers in the evening, then showers and isolated thunderstorms after midnight. Locally heavy rainfall possible. Lows 59 to 65 in the valleys to around 56 above 4000 feet. Southeast winds 10 to 20 mph decreasing to up to 15 mph after midnight. Chance of rain near 100 percent.

Tuesday: Showers and isolated thunderstorms. Locally heavy rainfall possible. Highs 68 to 75 in the valleys to around 62 above 4000 feet. Southeast winds up to 15 mph. Chance of rain near 100 percent.

North Kaua’i

Today: Breezy. Mostly cloudy with numerous showers. Highs 73 to 81. East winds up to 20 mph shifting to the southeast 10 to 20 mph in the afternoon. Chance of rain 70 percent.

Tonight: Cloudy. Numerous showers in the evening, then showers and isolated thunderstorms after midnight. Locally heavy rainfall possible. Lows 60 to 68. Southeast winds 10 to 15 mph decreasing to up to 15 mph after midnight. Chance of rain 90 percent.

Tuesday: Showers and isolated thunderstorms. Locally heavy rainfall possible. Highs 72 to 80. Southeast winds up to 10 mph. Chance of rain near 100 percent.

East Kaua’i

Today: Breezy. Mostly cloudy with numerous showers. Highs 70 to 81. East winds 10 to 20 mph shifting to the southeast up to 20 mph in the afternoon. Chance of rain 70 percent.

Tonight: Cloudy. Numerous showers in the evening, then numerous showers and isolated thunderstorms after midnight. Locally heavy rainfall possible. Lows 60 to 70. South winds up to 15 mph. Chance of rain 70 percent.

Tuesday: Breezy. Showers and isolated thunderstorms. Locally heavy rainfall possible. Highs 68 to 80. Southwest winds up to 20 mph shifting to the east up to 10 mph in the afternoon. Chance of rain near 100 percent.

Detailed Forecast

Synopsis

The weather pattern will shift towards increasing clouds and showers as two cut off low pressure systems near the islands pull up and consolidate additional deep tropical moisture over the region through Wednesday. High pressure building in from the northeast will spread breezy to windy trade winds across the state this week. Converging east to southeast surface winds and leeside island cloud plumes will produce bands of moderate to heavy showers mainly affecting the western islands of Kauai, Niihau and Oahu into Wednesday. These slow moving shower bands will increase threats for flash flooding over the western islands during this time period. Strong trade winds will eventually push this deep moisture westward away from the islands from Wednesday onward, bringing a return to wet and breezy trade wind weather lasting into the weekend.

Discussion

A low pressure system roughly 500 miles south of the Big Island will continue to pull up additional moisture into the region from the deep tropics. Another low pressure system, roughly 800 miles northwest of Kauai will drift south and becoming cut off from the midlatitude flow. This cut off low will become stationary west of the island chain on Tuesday and help consolidate the deep tropical moisture over the Hawaii region.
High pressure will also build in from the northeast this week spreading easterly trade winds across the region through Wednesday. Converging east to southeast surface winds and converging leeside island plumes will drive much of the moderate to heavy shower activity over the next two days. The abundant unstable tropical moisture and surface wind convergence will favor more intense shower activity over the western islands of Kauai, Niihau, and Oahu into Wednesday. However, breezy easterly trade winds will also produce orographic showers along eastern mountain slopes of each island during this period. This means that windward and mountain areas may also see potential for slowly moving moderate to locally heavy showers.
Saturated ground conditions from recent rain events will increase potential flooding threats. A Flood Watch may be needed at least for the western islands later today. We decided not to issue a watch with the forecast package this morning due to lower confidence in the timing of potential heavy rainfall with this next weather event.
Breezy easterly trade winds will strengthen over the region from Wednesday onward, pushing the deep and unstable moisture westward away from the island chain. An upper trough will develop over the region will produce enough upper level divergence to lift the trade wind inversion and enhance windward and mountain showers. Breezy to locally windy wet trade winds are forecast to last from Wednesday into next weekend. Strong trade winds may exceed Wind Advisory thresholds during this time period and advisory highlights may be needed for some of the windier island zones.

Aviation

A weakening upper-level low south of the Big Island is moving slowly toward the east-southeast, while a second upper-level low far northwest of Kauai will move slowly south. These systems will maintain relatively unstable atmospheric conditions today. In addition, deep tropical moisture will linger in the vicinity of the state, which will allow layered clouds with embedded showers to move into the islands. Locally heavy rainfall and isolated thunderstorms are also possible today, especially in the vicinity of the Big Island this afternoon. This will likely produce periods of MVFR (and localized IFR) conditions over parts of the state through this evening. AIRMET Sierra, which is currently in effect for tempo mountain obscuration over most islands, will likely be needed at least into this afternoon. In addition, some of the mid- and upper-level clouds may produce periods of light icing in the layer from 15 to 25 kft through this morning.
A broad surface high pressure system centered far north- northeast of the region will build during the next couple of days. This will likely produce locally breezy east to southeast winds for the east and central islands by this afternoon. The strengthening low-level background flow may increase the potential for low-level mechanical turbulence developing over and downwind of the higher terrain. If so, AIRMET Tango for tempo moderate low-level turbulence may be needed for portions of the island chain starting later today or tonight.

Marine

High pressure northeast of the state and troughing to the west will support a continuation of strong to near-gale force east- southeast winds across the waters through the middle of the week. The troughing will shift west Thursday through next weekend, allowing the winds to shift around to a more typical east and eventually east-northeast trade wind direction. A Small Craft Advisory remains in effect for most waters due to a combination of winds and seas through the week.
Surf along east facing shores will remain rough through the week due to persistent and strong east to southeast winds. Surf will likely return to the 10 foot advisory level tonight into Tuesday due to the strong easterly trade winds upstream over the eastern Pacific. East shore surf then appears to hold at or near advisory levels through the weekend.
The current small long-period northwest (310-320 degrees) swell will linger through the first half of the week before easing Thursday. Expect surf to hold at levels well below the seasonal average each day through the week. A new small northwest swell could give north shore surf a small boost next weekend. South shore surf will remain minimal during the next 7 days.

HFO Watches/Warnings/Advisories

Small Craft Advisory until 6 PM HST Friday for all Hawaiian waters except Maalaea Bay,

Data Courtesy of NOAA.gov