Photo Credit: Jason Weingardt

West Kaua’i

Today: Cloudy. Numerous showers in the morning, then occasional showers in the afternoon. Highs around 84 near the shore to around 70 above 3000 feet. Southwest winds around 15 mph. Chance of rain 90 percent.

Tonight: Mostly cloudy. Numerous showers in the evening, then scattered showers after midnight. Lows around 68 near the shore to around 58 above 3000 feet. West winds 10 to 15 mph in the evening becoming light. Chance of rain 70 percent.

Thursday: Partly sunny with scattered showers. Highs around 82 near the shore to around 70 above 3000 feet. Light winds. Chance of rain 40 percent.

South Kaua’i

Today: Numerous showers in the morning, then occasional showers in the afternoon. Highs 77 to 84. Southwest winds around 15 mph. Chance of rain near 100 percent.

Tonight: Mostly cloudy. Numerous showers in the evening, then scattered showers after midnight. Lows around 69. West winds 10 to 15 mph in the evening becoming light. Chance of rain 70 percent.

Thursday: Partly sunny with scattered showers. Highs 77 to 83. Light winds. Chance of rain 50 percent.

Kaua’i Mountains

Today: Numerous showers in the morning, then occasional showers in the afternoon. Highs 74 to 81 in the valleys to around 64 above 4000 feet. Southwest winds 10 to 15 mph. Chance of rain near 100 percent.

Tonight: Mostly cloudy. Numerous showers in the evening, then scattered showers after midnight. Lows 61 to 67 in the valleys to around 57 above 4000 feet. West winds 10 to 15 mph decreasing to up to 10 mph after midnight. Chance of rain 70 percent.

Thursday: Mostly cloudy with scattered showers. Highs 70 to 76 in the valleys to around 63 above 4000 feet. Northwest winds up to 10 mph. Chance of rain 50 percent.

North Kaua’i

Today: Mostly cloudy. Scattered showers in the morning, then numerous showers in the afternoon. Highs 74 to 84. Southwest winds 10 to 15 mph. Chance of rain 70 percent.

Tonight: Mostly cloudy. Numerous showers in the evening, then scattered showers after midnight. Lows 62 to 70. West winds 10 to 15 mph in the evening becoming light. Chance of rain 70 percent.

Thursday: Mostly cloudy. Numerous showers in the morning, then scattered showers in the afternoon. Highs 72 to 82. Light winds. Chance of rain 70 percent.

East Kaua’i

Today: Numerous showers in the morning, then occasional showers in the afternoon. Highs 73 to 84. Southwest winds 10 to 15 mph. Chance of rain near 100 percent.

Tonight: Mostly cloudy. Numerous showers in the evening, then scattered showers after midnight. Lows 63 to 72. Southwest winds 10 to 15 mph in the evening becoming light. Chance of rain 70 percent.

Thursday: Mostly cloudy with scattered showers. Highs 72 to 82. West winds up to 10 mph shifting to the south in the afternoon. Chance of rain 50 percent.

Detailed Forecast

Synopsis

A kona low developing about 600 miles north of Kauai today will intensify and meander around well northwest of the islands into next week. The low will turn our local winds southerly, pulling copious moisture from the deep tropics over the islands. A slow moving band of downpours and a few thunderstorms is expected to develop near Oahu and possibly Maui county Wednesday night into Thursday, then slowly shift westward toward Kauai by Friday. Excessive rainfall and flash flooding will be a significant concern. Other areas away from the main band could see spotty downpours and localized flooding problems as well. The weather over the Aloha State this weekend and beyond will depend highly on the later evolution and movement of the kona low far to our northwest.

Discussion

We continue to watch the strongly digging shortwave trough on water vapor imagery early this morning to the NNW of the islands. The late evening scatterometer pass showed that cyclogenesis may be beginning as expected about 600 miles or so N of Kauai. Both the GOES-derived and CIMSS-MIMIC total precipitable water fields are showing a large wedge of relatively moist air pushing northward across the area surrounding Hawaii, with even deeper ITCZ moisture well to our S beginning to creep N under the influence of the sharply digging trough aloft. A gradual increase in the number of showers has been noted over the local area over the last several hours. The showers over the offshore waters growing stronger with gradually cooling cloud tops. The GLM and ground- based lightning detection networks are showing a few thundershowers over the northern offshore waters.
The convection allowing models suggest that today we should continue to see a gradual increase in the number and intensity of showers moving up from the south, mainly around Kauai and Oahu. Starting tonight, as upper difluence moves over the low level moisture convergence, the high resolution and global models suggest a more organized band of heavy showers and embedded thundershowers will develop over or near Oahu, which could linger into Thu. There's quite a bit of uncertainty among the various models about how intense this band could be, but utilized model consensus and NBM to set up the location and timing of heaviest QPF which continues to seem reasonable.
The Big Island may remain too far E to see much heavy rain from this system, aside from some heavy afternoon showers or thundershowers, especially over the slopes. However, not feeling confident enough yet to consider dropping the Flood Watch for that island and would prefer to wait and see how things evolve over the next 12 to 24 hours, since active convection can sometimes wreak havoc on what might appear to be reasonable CAM solutions.
The global models and CAMs show that the band may break up or weaken a bit by Thu night as the best difluence and stronger shortwave energy shifts NE of the islands. Heavy showers will still be possible but they may become more spotty and less organized for a time before reintensification starting Friday. One of the big questions that remains will be how far W will the band be when it reintensifies under stronger upper level difluence Fri into the weekend. The NBM thunderstorm probabilities show values increasing again near Kauai on Friday before shifting westward to the coastal and offshore waters W of Kauai Fri night. Global model rain rates increase quite a bit within the band this weekend, but by then hopefully the heaviest rain is falling out over the water. Still, the models can change on these details and we'll need to keep an eye on this.
One other thing to note is the possibility of some gusty downslope winds on the normal-windward side of the islands. The models don't seem to be super-stoked about the risk for damaging downslope winds, but some gusts over 30 knots appear possible on Wed into Wed night. Adjusted the wind gust grids to raise gusts in the favorable downslope areas, using the University of Hawaii WRF-NMM 950 mb winds.
The global models show that the middle and eastern islands this weekend will have an odd combination of very moist air, with precipitable water near 2 inches…but also a low level subsidence inversion that may try to keep somewhat of a lid the moisture. Nevertheless, with ESE or SE flow continuing, there should be some fairly prolific afternoon showers in any areas shadowed by the background flow. Kauai remains a big question mark during this period as it will continue to be on the periphery of the deeper moisture and strong dynamics just to the west.
There's considerable uncertainty after the weekend, with lingering moisture gradually diminishing and the kona low far to the north likely weakening, leaving a surface trough behind W of the islands. There should be a continuing trend toward more settled weather statewide, with weak mid-level ridging eventually trying to build in. In the meantime, there will be a daily chance of spotty heavy showers, mainly in the afternoons and evenings.

Aviation

An active weather pattern will continue through the rest of the week, especially as a kona low develops about 600 nm to the northwest of Kauai. Showers and a few thunderstorms will bring the potential for periodic MVFR or lower conditions on and off throughout the period. The main focus for showers and the occasional thunderstorm will likely be around Oahu and Kauai today and tonight as the kona low taps into some tropical moisture to the south and pulls it up through the western half of the state. Moderate southerly winds are expected today with higher gusts possible within/near heavy showers or thunderstorms, then winds will become light once again overnight.
AIRMET Sierra is in effect for all of the smaller islands and windward Big Island for tempo mountain obscuration. Sierra may be needed for IFR conditions and Tango for turbulence as the kona low gets closer and conditions deteriorate.

Marine

Low pressure developing north of Kauai will usher a cold front toward the state from the northwest today. The front will advance eastward until stalling over Oahu late Thursday and weakening. The remnant boundary will then drift west during the remainder of the week. Heavy showers and isolated thunderstorms are expected along and ahead of the front.
A series of overlapping, long period S swells will maintain near to below average surf along S facing shores through the near term. Renewed energy out of the southerly quadrant will then fill in late tomorrow night bringing above average surf by Thursday. Surf remains elevated in through the weekend.
The existing small medium period NW swell will peak today. By tonight, a short period fresh NW swell associated with the developing low will spread into the area. Bumped the official forecast up slightly higher than guidance given the propensity for the guidance to undersimulate the magnitude of fresh swell. The forecast peak is near to just below the High Surf Advisory threshold Thursday into Friday.
West facing shores see a bump in surf through the week due to overlapping NW swell and S swells wrapping into exposed areas. Surf along E facing shores remains well below normal through the weekend.

HFO Watches/Warnings/Advisories

Flood Watch through Friday evening for all Hawaii islands,

Data Courtesy of NOAA.gov