West Kaua’i

Today: Mostly sunny with isolated showers. Highs 86 to 92 near the shore to around 78 above 3000 feet. East winds up to 15 mph. Chance of rain 20 percent.

Tonight: Mostly clear. Isolated showers in the evening. Lows 71 to 76 near the shore to around 63 above 3000 feet. East winds 10 to 15 mph. Chance of rain 20 percent.

Thursday: Sunny. Isolated showers in the morning. Highs 87 to 92 near the shore to around 79 above 3000 feet. East winds 10 to 15 mph. Chance of rain 20 percent.

South Kaua’i

Today: Partly sunny. Numerous showers in the morning, then scattered showers in the afternoon. Highs 83 to 91. Northeast winds 10 to 15 mph. Chance of rain 70 percent.

Tonight: Mostly cloudy with scattered showers. Lows around 75. Northeast winds 10 to 15 mph. Chance of rain 50 percent.

Thursday: Breezy. Partly sunny with scattered showers in the morning, then mostly sunny with isolated showers in the afternoon. Highs 83 to 91. East winds 10 to 20 mph. Chance of rain 40 percent.

Kaua’i Mountains

Today: Occasional showers in the morning, then numerous showers in the afternoon. Highs 75 to 85 in the valleys to around 66 above 4000 feet. East winds up to 15 mph increasing to 10 to 15 mph in the afternoon. Chance of rain near 100 percent.

Tonight: Mostly cloudy. Numerous showers in the evening, then scattered showers after midnight. Lows 65 to 71 in the valleys to around 60 above 4000 feet. East winds 10 to 15 mph. Chance of rain 70 percent.

Thursday: Partly sunny in the morning then becoming mostly sunny. Breezy. Scattered showers. Highs 77 to 86 in the valleys to around 68 above 4000 feet. East winds 10 to 20 mph. Chance of rain 50 percent.

North Kaua’i

Today: Partly sunny. Numerous showers in the morning, then scattered showers in the afternoon. Highs 78 to 89. East winds up to 15 mph. Chance of rain 70 percent.

Tonight: Mostly cloudy in the evening then becoming partly cloudy. Scattered showers. Lows 66 to 75. East winds 10 to 15 mph. Chance of rain 50 percent.

Thursday: Mostly sunny with isolated showers. Highs 79 to 89. East winds 10 to 15 mph. Chance of rain 20 percent.

East Kaua’i

Today: Mostly cloudy. Numerous showers in the morning, then scattered showers in the afternoon. Highs 75 to 88. Northeast winds 10 to 15 mph. Chance of rain 70 percent.

Tonight: Mostly cloudy with scattered showers. Lows 67 to 79. East winds 10 to 15 mph. Chance of rain 50 percent.

Thursday: Partly sunny with scattered showers in the morning, then mostly sunny with isolated showers in the afternoon. Highs 76 to 88. East winds 10 to 15 mph. Chance of rain 50 percent.

Detailed Forecast

Synopsis

A stable and relatively dry pattern will persist through Thursday, along with moderate to locally breezy trade winds, bringing limited clouds and showers to windward and mauka areas. Increased shower activity is expected Thursday night into the weekend. A return to a more stable trade wind pattern is expected early next week.

Discussion

No substantial changes to the forecast with the morning forecast package. Some clouds and showers are riding in on the trades, focusing activity to the windward and mauka sections of the smaller islands. Another areas of clouds and showers upstream of the Big Island will move onshore later this morning.
High pressure is expected to remain north of the islands into the weekend, so expect trade winds to remain in place. However, a decaying surface trough far east of the islands and the tail end of a weakening front draped roughly along 30 to 35N will be pinched off beneath the strengthening high pressure. The moisture from these features will then be carried in on the trade winds to the islands. The ECMWF and GFS continue to agree with precipitable water (PW) values reaching 2 inches near the islands, which is well above average for September.
At the same time, a mid- to upper-level low is forecast to set up near Kauai, bringing 500 mb temperatures down to around -8 to -9 C which is also colder than normal for September. Forecast soundings show the trade inversion lifting to near 10 kft or higher as deeper moisture arrives. Despite the lack of a significant surface feature, those PW and 500 mb temperatures could very well spark some thunderstorm activity, and there is the chance for locally heavier showers. Based on the global models, the main focus for this would be the western half of the state Friday into Saturday. Any heavier cells should remain fast moving within the persistent trades, limiting the duration of heavy rainfall at any one location.
Thunderstorms have not yet been added to the forecast as we await to see some of these model depictions start to appear in the observed data.
Showers will gradually diminish late Sunday into early next week as upper ridging reestablishes and drier air filters back across the islands.

Aviation

Locally breezy trade winds will continue through the period. Some clouds and showers riding in on the trade winds will favor windward and mauka areas. VFR conditions are expected to prevail with pockets of MVFR within passing showers.
No AIRMETs currently in effect, and not anticipating any to be issued today.

Marine

Surface high pressure will remain anchored north of the state for the next several days and bring moderate to locally strong trade winds across the waters. However, winds are set to decrease slightly by Friday as a front drops south and weakens the high. The latest ASCAT pass depicted winds meeting Small Craft Advisory thresholds over portions of the western waters. To account for this, Small Craft Advisories have been issued for the Kauai Channel and Kauai leeward waters in addition to the typically windy areas around Maui County and the Big Island. These advisories will likely be scaled back to the just the typical windy areas from Thursday onward.
Surf along south facing shores will be tiny to small over the next few days with mainly background south and southeast energy. A small to moderate, long-period, south southwest swell will build Saturday into Sunday and help boost south shore surf heights back up to near or above the September average.
Surf along north facing shores should see a small increase late today into tomorrow as a short period northerly swell fills in. This small bump is expected to peak on Friday out of the north to north- northeast direction, followed by a short period north northwest swell this weekend. Moderate to locally strong trades near and upstream of the area will lead to slightly elevated and choppy conditions along east facing shores through this weekend.

Fire weather

Expect decreasing fire weather risk as humidity gradually increases through the week and into the weekend. Rain chances will increase Thursday night into the weekend as deep pockets of moisture filter in on the trades, helping to potentially further alleviate fire weather concerns. Sustained winds briefly hover near to below the critical fire weather threshold each afternoon through Thursday, then trend slightly lighter for the weekend.

HFO Watches/Warnings/Advisories

Small Craft Advisory until 6 AM HST Thursday for Kauai Leeward Waters, Kauai Channel.
Small Craft Advisory until 6 PM HST Thursday for Maalaea Bay, Pailolo Channel, Alenuihaha Channel, Big Island Leeward Waters, Big Island Southeast Waters.