Photo Credit: Jason Weingardt

West Kaua’i

Today: Mostly sunny with isolated showers. Highs around 84 near the shore to around 69 above 3000 feet. Northeast winds around 10 mph shifting to the south in the afternoon. Chance of rain 20 percent.

Tonight: Partly cloudy. Scattered showers in the evening, then isolated showers after midnight. Lows around 66 near the shore to around 55 above 3000 feet. South winds around 10 mph shifting to the east after midnight. Chance of rain 50 percent.

Wednesday: Mostly sunny with isolated showers in the morning, then partly sunny with scattered showers in the afternoon. Highs around 83 near the shore to around 68 above 3000 feet. Southeast winds around 10 mph shifting to the southwest in the afternoon. Chance of rain 40 percent.

South Kaua’i

Today: Partly sunny with scattered showers. Highs 76 to 83. East winds 10 to 15 mph. Chance of rain 40 percent.

Tonight: Mostly cloudy with scattered showers. Lows around 65. East winds 10 to 15 mph. Chance of rain 50 percent.

Wednesday: Partly sunny with scattered showers. Highs 76 to 83. Southeast winds around 10 mph. Chance of rain 40 percent.

Kaua’i Mountains

Today: Partly sunny with scattered showers. Highs 70 to 80 in the valleys to 58 to 63 above 4000 feet. East winds 10 to 15 mph. Chance of rain 50 percent.

Tonight: Mostly cloudy. Numerous showers in the evening, then scattered showers after midnight. Lows 61 to 66 in the valleys to around 53 above 4000 feet. East winds 10 to 15 mph. Chance of rain 70 percent.

Wednesday: Partly sunny with scattered showers. Highs 71 to 79 in the valleys to around 61 above 4000 feet. Southeast winds around 10 mph. Chance of rain 50 percent.

North Kaua’i

Today: Partly sunny with scattered showers. Highs 73 to 83. East winds 10 to 15 mph. Chance of rain 50 percent.

Tonight: Mostly cloudy with scattered showers. Lows 59 to 69. East winds 10 to 15 mph. Chance of rain 50 percent.

Wednesday: Partly sunny with scattered showers. Highs 73 to 82. Southeast winds 10 to 15 mph. Chance of rain 50 percent.

East Kaua’i

Today: Partly sunny with scattered showers. Highs 70 to 80. East winds around 10 mph. Chance of rain 50 percent.

Tonight: Mostly cloudy with scattered showers. Lows 60 to 69. East winds 10 to 15 mph. Chance of rain 50 percent.

Wednesday: Partly sunny with scattered showers. Highs 72 to 81. Southeast winds 10 to 15 mph. Chance of rain 50 percent.

Detailed Forecast

Synopsis

Trade winds will ease today and turn toward the southeast late tonight and Wednesday, fewer showers are expected of the smaller islands. Developing low pressure northwest of the islands will keep winds light and out of the southeast after midweek. The low could also bring the potential for increased showers, some heavy, mainly to Kauai toward the end of the week into the weekend.

Discussion

Latest satellite and radar imagery show scattered light to moderate showers primary focused along windward and mauka regions with some of these showers spilling over to leeward areas at times. An area of enhanced clouds and moisture embedded in the trade wind flow is moving on shores most windward and mauka areas and may reach windward Kauai later today. The high far northeast of the state will maintain breezy trades through today.
The high to the distant northeast will weaken Wednesday as a potent cutoff low develops over distant portions of the Papahanaumokuakea Marine National Monument. The surface reflection of this slow- moving, deep-layer low will have high end gale force winds peaking west of Laysan Thursday. Over the main Hawaiian Islands, trades will veer to the southeast late Tuesday and Wednesday. This flow regime typically places the smaller islands in the wind “shadow” to the lee of the Big Island, resulting in light winds and limited rainfall, especially as a weak ridge builds over the islands the next couple of days. Guidance is also showing strengthening southwesterly winds over the Big Island summits as the sub- tropical jet moves overhead which could produce advisory level winds as early as Tuesday night and peak near high end advisory level Wednesday night into Thursday.
Limited showers will favor windward areas during nights and mornings, and leeward areas may see isolated afternoon showers as localized sea breezes develop, with typical trade wind weather over the Big Island. Having said that, the forecast vertical wind profile has produced terrain- anchored heavy rainfall over Oahu's Koolau range in the past (sometimes referred to as “WOOSEE” rainfall here at the office – “Windward Oahu Orographic SouthEast Effect”), although anticipating its development with much lead time can be problematic. Greatest potential at this time appears to be Wednesday into Thursday. Deep moisture pooling near the Big Island and near freezing temperatures could also produce mixed snow and freezing rain rear the summits Thursday into Friday.
The surface low will gradually weaken while loitering to the distant northwest through Friday, then move slowly northeast into next week, with an associated convergence band lurking close to Kauai from Wednesday night through at least Sunday. This slow- moving convergence band will bring the potential for heavy showers and thunderstorms that may linger into next week, with the flow becoming southerly near Kauai. Initially, Kauai appears to be the most likely to get heavy rainfall, but the potential for localized heavy rain may spread eastward to other areas of the state in about a week as the band drifts east and winds remain light. Given the lead time, confidence in the details of the forecast are low.

Aviation

Strong high pressure north of the state will slowly weaken as it shifts eastward, causing trade winds to decrease. AIRMET Tango remains in effect for moderate low level turbulence over and downwind of terrain of all islands. Conditions are expected to gradually improve from the west today.
MVFR ceilings may occur across windward slopes within showers brought in by the trades. However, VFR conditions will prevail at the TAF sites. AIRMET Sierra for mountain obscuration above 2500 feet remains in effect for north and east sections of the Big Island. This may be cancelled later today.
High clouds are moving eastward away from the islands this morning. This greatly reduces the likelihood for light rime icing and the need for any issuance of AIRMET Zulu within the forecast area.

Marine

A weakening cold frontal trough approaching Hawaii from the northwest direction will weaken wind speeds over the next few days. A cutoff surface low deepens along the trough northwest of Kauai by Wednesday as a strong high pressure system farther to the north, builds eastward across the Central Pacific basin. This cutoff low will cause trade winds to veer from a more southeasterly direction from Wednesday to Saturday. Wind speeds will likely remain near low end Small Craft Advisory (SCA) levels for the windward waters and channels surrounding Maui and the Big Island through much of this week. The current SCA continues through this afternoon.
Surf along east facing shores will continue to slowly trend lower over the next two days as wind waves ease. Surf along north facing shores will trend higher today as a small, medium- to long- period north-northwest swell fills in across the islands. This swell will peak today and hold through Wednesday. Guidance for the second half of the week shows a low developing north of the state that could aim a moderate medium period north-northwest swell towards the islands. This swell would arrive late Friday or Saturday, but confidence remains low at this time.
For south facing shores, the current small long period southwest swell will decline today. The next long period south swell will arrive in the islands late Thursday, peak on Friday, and then slowly decline through the weekend.

HFO Watches/Warnings/Advisories

Small Craft Advisory until 6 PM HST this evening for Pailolo Channel, Alenuihaha Channel, Big Island Leeward Waters, Big Island Southeast Waters.

Data Courtesy of NOAA.gov