Photo Credit: Brendan Stephen

West Kaua’i

Today: Mostly sunny in the morning, then partly sunny with scattered showers in the afternoon. Highs around 82 near the shore to around 70 above 3000 feet. East winds around 10 mph. Chance of rain 50 percent.

Tonight: Partly cloudy with isolated showers. Lows around 67 near the shore to around 58 above 3000 feet. East winds around 10 mph. Chance of rain 20 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 71 above 3000 feet. East winds around 10 mph. Chance of rain 40 percent.

South Kaua’i

Today: Partly sunny. Scattered showers in the morning, then isolated showers in the afternoon. Highs 78 to 84. East winds 10 to 15 mph. Chance of rain 50 percent.

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

Wednesday: Partly sunny with scattered showers. Highs 79 to 85. East winds 10 to 15 mph. Chance of rain 50 percent.

Kaua’i Mountains

Today: Mostly cloudy. Numerous showers in the morning, then scattered showers in the afternoon. Highs 72 to 77 in the valleys to around 63 above 4000 feet. East winds 10 to 15 mph. Chance of rain 70 percent.

Tonight: Mostly cloudy with numerous showers. Lows 60 to 65 in the valleys to around 56 above 4000 feet. East winds 10 to 15 mph. Chance of rain 70 percent.

Wednesday: Mostly cloudy with scattered showers. Highs 72 to 79 in the valleys to around 63 above 4000 feet. East winds 10 to 15 mph. Chance of rain 50 percent.

North Kaua’i

Today: Partly sunny with scattered showers. Highs 72 to 82. East winds 10 to 15 mph. Chance of rain 50 percent.

Tonight: Mostly cloudy with scattered showers. Lows 61 to 69. East winds around 10 mph. Chance of rain 50 percent.

Wednesday: Partly sunny with scattered showers. Highs 74 to 83. East winds around 10 mph. Chance of rain 50 percent.

East Kaua’i

Today: Partly sunny with scattered showers. Highs 71 to 81. East winds 10 to 15 mph. Chance of rain 50 percent.

Tonight: Mostly cloudy with scattered showers. Lows 61 to 72. East winds around 10 mph. Chance of rain 50 percent.

Wednesday: Partly sunny with scattered showers. Highs 72 to 82. East winds 10 to 15 mph. Chance of rain 50 percent.

Detailed Forecast

Synopsis

A surface ridge northeast of the islands will keep gentle to locally breezy trades blowing across the state today. Winds will turn toward the southeast Wednesday into Thursday as an upper- level low develops west of the state and heads our way. This low will spread increasingly moist and unstable air over the islands during the second half of the week, helping to enhance showers and possibly bring a few thundershowers.

Discussion

The forecast remains largely unchanged with the morning package, and uncertainly remains with the upcoming weather feature.
The surface ridge remains about 400 miles to the northwest of Oahu early this morning. A relatively dry and stable airmass is over the region, along with sea level dewpoints in the low to mid 60s. The precipitable water (PW) from the overnight PHLI sounding actually crept up to 1.02 inches, while the PHTO sounding was 1.25. This matches well with the satellite derived PW which shows low PW across the area. Despite these low PW values, there have been a fair amount of clouds impacting mostly the windward portions of the islands overnight, however showers have been limited. The batch of clouds over windward Maui and the Big Island last evening have thinned over the last several hours. At the same time, clouds have increased north of Kauai as was indicated in some of the models yesterday afternoon.
The surface ridge is expected weaken today, causing winds to weaken and become more easterly. The dry airmass over the islands will limit overall shower activity today. With the winds weakening today, interior and leeward areas which have cleared out overnight will likely see some afternoon cloudiness today, but again with limited moisture overhead, showers will likely be limited.
A surface low far northwest of the islands will sink southward over the next 24 hours or so and weaken. An upper level trough digging near 170W will approach the islands, and is the feature we are watching closely. Both the GFS and ECMWF develop a lot along the trough. The GFS continues to be stronger than the ECMWF. The latest run of the GFS is starting to show a southerly direction as the upper low approaches the islands, which is what the ECMWF has been showing for a few runs. Both of these depictions in the latest run are still differing on the placement and strength.
As this upper level feature moves closer to the islands tomorrow, a gradual increase in shower coverage and intensity is expected, and then to persist into the end of the week. Instability increases, but given the placement and strength differences in the models, uncertainty remains for the timing, placement and extent of thunderstorms.
The Thursday night and into Friday time period when a surface reflection of the upper level trough is just west of the islands, we can expect southerly winds for the Kauai and Oahu. A blended approach still seems prudent for the inclusion of a chance of heavy thundershowers around Kauai Thursday night, when there's the most agreement among the various solutions. If things pan out more to the GFS solution, we'd also have to increase southerly wind speeds, possibly to near advisory levels Friday.
As the mid-level feature moves to the east early next week, surface winds should begin to back toward trades, along with much drier and more stable air returning in the wake.

Aviation

Light to moderate northeast trade winds will gradually shift out of the east today as the ridge remains positioned to the north. The stable stratiform cloud deck that has been impacting much of the Big Island and windward Maui has begun to diminish early this morning. If this trend continues through daybreak, AIRMET Sierra, that is currently in place for mountain obscuration, will be cancelled. Otherwise, clouds and showers will favor windward and mauka areas, except over the sheltered leeward locations where sea breezes develop through the afternoon today.
Additionally, moderate turb between FL270 and FL370 has been reported in the vicinity of the islands and is expected to persist through the day. A weakening trend is anticipated over the state later tonight as the area of moderate turb shifts eastward.

Marine

An elongated, weak ridge axis positioned north of the islands will maintain moderate to locally fresh winds through mid-week. Background winds are forecast to gradually weaken today therefore, the Small Craft Advisory (SCA) has been canceled. Typical windy waters around Maui county and the Big Island may reach SCA criteria briefly on Wednesday before a developing trough west of the islands will cause winds to veer to a more southeasterly direction on Thursday into the weekend. The location of the trough to local waters will dictate if winds become light and variable or gives way for the potential for an SCA over the eastern waters. There is also a chance for thunderstorms in the vicinity of Kauai Thursday through Friday morning.
The current north swell is slowly declining and will continue to do so through Wednesday night. A new moderate short-to-medium period northeast (020-030 deg) swell will build Wednesday night and peak on Thursday providing a boost to surf along north and select east facing shores. A series of south swells will maintain small surf along south-facing shores into next week. The ongoing small, long period southwest (220 deg) swell be mixed by a long period southeast (150 deg) swell by Wednesday night. Into the weekend, another small, long period south (200 deg) swell will continue to keep surf slightly below the summertime average.

HFO Watches/Warnings/Advisories

None.

Data Courtesy of NOAA.gov