West Kaua’i

Rest Of Today: Mostly sunny. Highs 86 to 93 near the shore to around 79 above 3000 feet. South winds up to 10 mph.

Tonight: Partly cloudy. Lows around 76 near the shore to around 66 above 3000 feet. East winds up to 10 mph.

Saturday: Mostly sunny. Highs 88 to 95 near the shore to around 81 above 3000 feet. East winds up to 15 mph.

South Kaua’i

Rest Of Today: Mostly sunny. Isolated showers late in the morning. Highs 81 to 92. East winds up to 10 mph. Chance of rain 20 percent.

Tonight: Partly cloudy in the evening then becoming mostly cloudy. Lows around 76. East winds 10 to 15 mph.

Saturday: Partly sunny in the morning then becoming mostly sunny. Highs 81 to 93. East winds 10 to 15 mph.

Kaua’i Mountains

Rest Of Today: Partly sunny with isolated showers. Highs 77 to 85 in the valleys to around 70 above 4000 feet. Light winds becoming southeast up to 10 mph in the afternoon. Chance of rain 20 percent.

Tonight: Mostly cloudy. Lows around 72 in the valleys to around 64 above 4000 feet. East winds up to 15 mph.

Saturday: Partly sunny in the morning then becoming mostly sunny. Highs 78 to 85 in the valleys to around 76 above 4000 feet. East winds up to 15 mph.

North Kaua’i

Rest Of Today: Mostly sunny with isolated showers. Highs 73 to 89. Light winds becoming northeast up to 10 mph in the afternoon. Chance of rain 20 percent.

Tonight: Mostly cloudy. Lows 68 to 77. Southeast winds up to 10 mph.

Saturday: Partly sunny in the morning then becoming mostly sunny. Highs 75 to 91. East winds up to 15 mph.

East Kaua’i

Rest Of Today: Mostly sunny. Isolated showers late in the morning. Highs 81 to 89. Light winds becoming southeast around 10 mph in the afternoon. Chance of rain 20 percent.

Tonight: Partly cloudy in the evening then becoming mostly cloudy. Lows 70 to 78. East winds up to 10 mph.

Saturday: Partly sunny in the morning then becoming mostly sunny. Highs 81 to 89. East winds 10 to 15 mph.

Detailed Forecast

Synopsis

Strong trades develop this weekend as a tropical system delivers heavy rainfall to windward and southeast slopes of the Big Island. A Flood Watch remains in effect for the Big Island.

Discussion

Trade winds remain light over the western end of the state as surface troughing continues lifting off to the northwest. This trough, the remnant of TC Hernan, has draped a modest band of moisture over Oahu this afternoon where sea breeze convergence has capitalized on the resident weakly stable environment and led to numerous showers over the interior. Prospects for afternoon showers on the other islands remain limited due to diminished moisture and/or stronger trade winds.
Attention then turns to Tropical Storm Moke which is currently positioned about 650 miles southeast of Hilo and is lifting slowly NW. Satellite presentation is characteristic of a storm affected by southerly shear with convection concentrated north of the center of circulation. Moke is also drawing north a band of rich moisture originating from the deep tropics. PWAT values within this moisture plume average about 2.5″ which is what it will be when it reaches the Big Island late Saturday night into Sunday. The forecast quandary for today is seemingly answered by a northward jump in the 18z GFS to much more closely match the 12z EC and the official NHC forecast. This in turn adds confidence to the official forecast which was based on the latter 2 sources and is considerably wetter for the Ka'u and Puna Districts of the Big Island. Instead of straight trades through the lowest 10kft or so as suggested by this morning's GFS, the northern periphery of the envelope of cyclonic flow will envelop the Big Island allowing winds to veer to SE and steer greater moisture content to the SE facing slopes of the Big Island. The forecast for around 10″ in this area, locally higher, remains on track. Rain will begin over SE and Windward Big Island late Saturday night or early Sunday morning, peak in intensity Sunday afternoon/evening and then linger through Monday, particularly over upslope areas where afternoon convection may be fairly robust on Monday afternoon. Strong trades ensure fast-moving showers elsewhere except perhaps Windward Maui where a few inches of rainfall appears increasingly likely. Though trades may be on the wetter side on Monday, prospects for flooding for Molokai, Oahu, and Kauai appear quite limited. The Flood Watch for the Big Island remains in good shape with no adjustments needed.
Secondary forecast concern is wind magnitude. As with the rainfall forecast, the northward jump by the 18z GFS increases confidence in the existing wind forecast. The pressure gradient north of TS Moke will begin strengthening tonight with peak winds expected Sunday and Sunday evening. Low stability and plenty of moisture advection will make for inefficient and limited downsloping. Although the usual suspects will be windy, will shy away from a High Wind Watch for any zones at this time. Instead, this appears to be a solid Wind Advisory (gusts to 50 mph) for at least Maui County and the Big Island and possibly Oahu as well.
Low amplitude high pressure remains anchored along 30N supporting breezy trades continuing through most or all of next week.

Aviation

A surface trough has moved north of Kauai, however light winds remain over the western islands, leading to local land and sea breezes. Clouds and showers formed over the interior of Oahu this afternoon prompting AIRMET SIERRA for mountain obscuration. Anticipate these clouds and showers to dissipate shortly after sunset.
Behind the trough, trades are starting to fill back in, and will gradually strengthen across the island chain from east to west. Trade winds will continue to strengthen on Saturday and AIRMET TANGO will likely need to be issued for low level turbulence.
Addtional pockets of MVFR conditions are possible with passing clouds and showers outside of those on Oahu this afternoon. Otherwise, VFR conditions are expected to prevail.

Marine

A trough in the northwest offshore waters will continue to move northwest as trade winds have begun to fill in across the coastal waters from east to west. A Small Craft Advisory (SCA) has been issued through Saturday afternoon for the windier waters and channels around Maui County and the Big Island. As trade winds continue to strengthen over the weekend as Tropical Storm Moke passes south of the state, the SCA may need to be expanded starting Saturday night through Sunday. Although seas greater than 10 feet will most likely be over the eastern half of the coastal waters. Across the offshore waters, a Tropical Storm warning as been issued as Tropical Storm conditions are expected Saturday night into Sunday across the southern offshore waters.
Surf along south facing shores will remain elevated through the weekend. Another reinforcing swell is expected over the weekend which should boost surf heights near the summer average over the weekend. A series of south swells are expected next week with above average surf likely during the second half of next week potentially up to High Surf Advisory levels.
Surf along east-facing shores remains below average tonight then is expected to build this weekend as the trades restrengthen. Surf along east and south- facing shores will be compounded by a southeast swell generated by Tropical Storm Moke that will build through Sunday and then become south, then southwest through early next week. This short- period tropical swell could cause surf to approach advisory levels along east facing shores of the Big Island Saturday night into Sunday, though forecast confidence is on the lower side as the wind field with this tropical cyclone is expected to be relatively small and the forecast track may shift slightly. North facing shores will remain tiny through tonight with a small medium to long period west north swell from Hurricane Lala this weekend.

Fire weather

Trade winds increase over the weekend and the inversion, currently around 6-7kft, will steadily weaken and eventually erode altogether as deep moisture associated with TS Moke moves into the area late Saturday night into Sunday. Heavy rain is expected over windward and SE facing slopes of the Big Island on Sunday.

HFO Watches/Warnings/Advisories

Flood Watch from late Saturday night through Monday afternoon for Big Island,
Small Craft Advisory until 6 PM HST Saturday for Maalaea Bay, Pailolo Channel, Alenuihaha Channel, Big Island Leeward Waters, Big Island Southeast Waters.

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Data Courtesy of NOAA.gov