Hilo

Today: Breezy. Mostly cloudy with scattered showers. Highs 79 to 84 near the shore to around 68 at 4000 feet. South winds 10 to 25 mph. Chance of rain 50 percent.

Tonight: Breezy. Mostly cloudy with scattered showers. Lows 65 to 71 near the shore to 55 to 60 at 4000 feet. South winds up to 20 mph. Chance of rain 50 percent.

Thursday: Mostly cloudy. Scattered showers in the morning, then scattered showers and isolated thunderstorms in the afternoon. Highs around 82 near the shore to around 69 at 4000 feet. South winds 10 to 15 mph. Chance of rain 50 percent.

Kona

Today: Mostly sunny. Isolated showers in the afternoon. Highs 82 to 88 near the shore to around 68 near 5000 feet. Southwest winds 10 to 15 mph. Chance of rain 20 percent.

Tonight: Partly cloudy. Lows around 74 near the shore to 50 to 56 near 5000 feet. South winds 10 to 15 mph.

Thursday: Mostly sunny in the morning then becoming partly sunny. Isolated showers. Highs 83 to 90 near the shore to around 70 near 5000 feet. Southeast winds around 10 mph shifting to the southwest in the afternoon. Gusts up to 35 mph. Chance of rain 20 percent.

Waimea

Today: Partly sunny with scattered showers. Highs around 78 near the shore to 67 to 76 near 3000 feet. East winds around 10 mph shifting to the north in the afternoon. Chance of rain 50 percent.

Tonight: Mostly cloudy with scattered showers in the evening, then mostly clear after midnight. Lows 64 to 71 near the shore to 56 to 63 near 3000 feet. North winds up to 10 mph in the evening becoming light. Chance of rain 50 percent.

Thursday: Mostly cloudy. Scattered showers in the morning, then numerous showers in the afternoon. Highs around 80 near the shore to 69 to 79 near 3000 feet. Northeast winds up to 10 mph. Chance of rain 70 percent.

Kohala

Today: Partly sunny with scattered showers. Highs around 78 near the shore to 67 to 76 near 3000 feet. East winds around 10 mph shifting to the north in the afternoon. Chance of rain 50 percent.

Tonight: Mostly cloudy with scattered showers in the evening, then mostly clear after midnight. Lows 64 to 71 near the shore to 56 to 63 near 3000 feet. North winds up to 10 mph in the evening becoming light. Chance of rain 50 percent.

Thursday: Mostly cloudy. Scattered showers in the morning, then numerous showers in the afternoon. Highs around 80 near the shore to 69 to 79 near 3000 feet. Northeast winds up to 10 mph. Chance of rain 70 percent.

South Big Island

Today: Mostly sunny in the morning then becoming partly sunny. Breezy. Scattered showers. Highs around 84 near the shore to around 67 near 5000 feet. East winds 10 to 20 mph. Chance of rain 50 percent.

Tonight: Breezy. Partly cloudy with scattered showers. Lows around 74 near the shore to around 53 near 5000 feet. East winds 10 to 20 mph. Chance of rain 50 percent.

Thursday: Partly sunny. Breezy. Scattered showers in the morning, then isolated showers in the afternoon. Highs around 85 near the shore to around 70 near 5000 feet. East winds 10 to 20 mph. Chance of rain 40 percent.

Puna

Today: Breezy. Mostly cloudy with scattered showers. Highs 79 to 84 near the shore to around 68 at 4000 feet. South winds 10 to 25 mph. Chance of rain 50 percent.

Tonight: Breezy. Mostly cloudy with scattered showers. Lows 65 to 71 near the shore to 55 to 60 at 4000 feet. South winds up to 20 mph. Chance of rain 50 percent.

Thursday: Mostly cloudy. Scattered showers in the morning, then scattered showers and isolated thunderstorms in the afternoon. Highs around 82 near the shore to around 69 at 4000 feet. South winds 10 to 15 mph. Chance of rain 50 percent.

Waikoloa

Today: Sunny with isolated showers in the morning, then partly sunny with scattered showers in the afternoon. Highs 80 to 88 near the shore to around 68 above 4000 feet. West winds around 10 mph. Chance of rain 50 percent.

Tonight: Partly cloudy. Isolated showers in the evening. Lows around 74 near the shore to around 55 above 4000 feet. Northwest winds up to 10 mph shifting to the east after midnight. Chance of rain 20 percent.

Thursday: Mostly sunny with isolated showers in the morning, then partly sunny with scattered showers and isolated thunderstorms in the afternoon. Highs 82 to 88 near the shore to 67 to 73 above 4000 feet. East winds up to 10 mph shifting to the northwest around 10 mph 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,

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Data Courtesy of NOAA.gov