Hilo

Today: Mostly cloudy with scattered showers. Highs around 83 near the shore to 66 to 71 at 4000 feet. South winds 10 to 15 mph. Chance of rain 50 percent.

Tonight: Mostly cloudy with scattered showers. Lows 64 to 69 near the shore to around 53 at 4000 feet. South winds up to 10 mph. Chance of rain 50 percent.

Thursday: Mostly cloudy with scattered showers. Highs around 83 near the shore to 66 to 71 at 4000 feet. Southeast winds up to 15 mph increasing to 10 to 15 mph in the afternoon. Chance of rain 40 percent.

Kona

Today: Mostly sunny with isolated showers in the morning, then partly sunny with scattered showers in the afternoon. Highs 81 to 86 near the shore to around 66 near 5000 feet. Northwest winds around 10 mph. Chance of rain 50 percent.

Tonight: Partly cloudy in the evening then becoming mostly cloudy. Isolated showers. Lows 67 to 74 near the shore to around 54 near 5000 feet. Light winds. Chance of rain 20 percent.

Thursday: Mostly cloudy. Isolated showers in the morning, then scattered showers in the afternoon. Highs 81 to 86 near the shore to around 66 near 5000 feet. Northeast winds up to 10 mph. Chance of rain 30 percent.

Waimea

Today: Partly sunny. Isolated showers in the morning, then scattered showers in the afternoon. Highs around 82 near the shore to 66 to 77 near 3000 feet. Northeast winds around 10 mph. Chance of rain 50 percent.

Tonight: Mostly clear with isolated showers in the evening, then mostly cloudy with scattered showers after midnight. Lows around 66 near the shore to 54 to 61 near 3000 feet. Light winds. Chance of rain 20 percent.

Thursday: Mostly cloudy with scattered showers. Highs around 82 near the shore to 66 to 77 near 3000 feet. Southeast winds up to 10 mph. Chance of rain 30 percent.

Kohala

Today: Partly sunny. Isolated showers in the morning, then scattered showers in the afternoon. Highs around 82 near the shore to 66 to 77 near 3000 feet. Northeast winds around 10 mph. Chance of rain 50 percent.

Tonight: Mostly clear with isolated showers in the evening, then mostly cloudy with scattered showers after midnight. Lows around 66 near the shore to 54 to 61 near 3000 feet. Light winds. Chance of rain 20 percent.

Thursday: Mostly cloudy with scattered showers. Highs around 82 near the shore to 66 to 77 near 3000 feet. Southeast winds up to 10 mph. Chance of rain 30 percent.

South Big Island

Today: Breezy. Partly sunny with scattered showers. Highs around 84 near the shore to around 70 near 5000 feet. East winds 10 to 25 mph. Chance of rain 50 percent.

Tonight: Mostly cloudy. Breezy. Scattered showers in the evening, then isolated showers after midnight. Lows around 68 near the shore to around 53 near 5000 feet. East winds 10 to 20 mph. Chance of rain 50 percent.

Thursday: Mostly cloudy. Breezy. Isolated showers in the morning, then scattered showers in the afternoon. Highs around 84 near the shore to around 70 near 5000 feet. East winds 10 to 20 mph. Chance of rain 40 percent.

Puna

Today: Mostly cloudy with scattered showers. Highs around 83 near the shore to 66 to 71 at 4000 feet. South winds 10 to 15 mph. Chance of rain 50 percent.

Tonight: Mostly cloudy with scattered showers. Lows 64 to 69 near the shore to around 53 at 4000 feet. South winds up to 10 mph. Chance of rain 50 percent.

Thursday: Mostly cloudy with scattered showers. Highs around 83 near the shore to 66 to 71 at 4000 feet. Southeast winds up to 15 mph increasing to 10 to 15 mph in the afternoon. Chance of rain 40 percent.

Waikoloa

Today: Mostly sunny. Isolated showers in the morning, then scattered showers in the afternoon. Highs around 85 near the shore to 67 to 73 above 4000 feet. North winds around 10 mph. Chance of rain 40 percent.

Tonight: Mostly clear with isolated showers in the evening, then mostly cloudy after midnight. Lows 68 to 74 near the shore to 50 to 57 above 4000 feet. Light winds. Chance of rain 20 percent.

Thursday: Mostly cloudy. Scattered showers in the afternoon. Highs around 85 near the shore to 67 to 73 above 4000 feet. Light winds. Chance of rain 30 percent.

Detailed Forecast

Synopsis

Southerly winds will focus clouds and showers across leeward areas for the next few days. Heavy rainfall will bring the potential for flooding to Kauai through tonight. A few thunderstorms are also possible. For the remaining islands, coverage and duration of heavier showers is expected to be limited.

Discussion

Layered clouds originating from deep convection 100 miles or so west-southwest of Kauai have spread eastward across Kauai and Oahu this morning. Low level flow persists from the south near Kauai to southeast near the Big Island, thanks to flow deformation ahead of an approaching front far northwest of the main Hawaiian Islands. The associated low near 38N166W is lifting out to the northeast but, as the supporting upper trough digs southward and swings eastward toward Kauai, the probability for adverse weather along and ahead of the front is increasing.
Models show southerly flow across the western islands becoming more moist and unstable as the day wears on. Shower bands will form and intensify within this convergent flow, mostly over and near Kauai. Islands farther to the east may see some of this activity as well, but with less total rainfall. Southerly wind speeds will also increase a bit statewide. The probability for heavy showers on Kauai will steadily increase through tonight.
By midday today, the digging upper trough will begin to pass over Kauai. This, combined with strengthening moist southerly flow, will significantly boost the probability for flooding on Kauai as heavy showers become more abundant and thunderstorms become possible. Although convection will tend to focus over south-facing slopes, the flood threat will encompass the entire island. A Flood Watch remains in effect for Kauai and Niihau through tonight. In addition, there is a very low, but non-zero, probability that there will be enough vertical shear to generate waterspouts in some of the strongest convection this afternoon and evening. Temperatures aloft will probably be too warm and conditions too stable for hail development, but small hail is not entirely out of the question either.
The upper trough will lift northeast and moisture will spread east over Oahu tonight. However, diminishing convective forcing will likely keep shower intensity in check. A few locally heavy showers may still occur on Oahu, but with less abundance than on Kauai. For Kauai, the departure of the upper trough will mark a transition to orographic forcing. Enhanced moisture will remain in place and heavy rainfall may continue there through tonight. If this happens, the Flood Watch may need to be extended into the first half of Thursday.
Models show southerly flow will weaken considerably Thursday into Friday as the distant surface low lifts northeastward. With the remnant moisture band still lingering over the state, any afternoon showers over island interiors would have the potential to produce heavy rainfall. Therefore, heavy rainfall may occur Friday and Saturday as afternoon sea breezes develop. Drier trade wind weather will return by early next week as surface ridging shifts westward and upper troughing near the islands now becomes zonal.

Aviation

Moisture will continue to increase across western and central islands today, as a frontal system slowly approaches Kauai from the west. The latest radar trends show an area of showers expanding in areal coverage just west of Kauai, while additional bands of showers try to organize and stream northward between Oahu and Molokai. VFR conds currently across the region are expected to quickly deteriorate to MVFR, and possibly IFR, between Kauai and Oahu as moisture continues to advect over the state. Further east, limited moisture and forcing will reduce areal coverage and intensity of showers to more favored trade wind and sea breeze locations through tonight. Thunderstorms are also a possibility across Kauai through Oahu. Otherwise, winds will increase from the south across the west end of the island chain, while remaining weaker from the southeast elsewhere.
Mountain obscuration, meeting AIRMET criteria, is expected to affect western and portions of the central islands today. AIRMET Tango for low-level mechanical turbulence is also possible near Kauai as south winds continue to increase. A period of light ice could potentially develop in a layer between 140/FL240 today for mainly western and central islands, as a band of mid and upper lvl moisture continues to stream up toward these islands.

Marine

Southerly winds have become strong over the Kauai waters overnight, which will continue into Thursday as a cold front moves into the area and stalls near or just west of the Kauai waters. Seas have responded and become rough over waters that are normally protected during trade wind conditions between Kauai and Niihau. Expect locally higher winds and seas in and around heavy showers and thunderstorms that develop as the front draws closer today. Southerly winds will trend down Thursday, then shift back to a typical easterly trade wind direction Friday through the weekend as the ridge lifts northward and the front diminishes west of the area.
For the eastern end of the state, fresh to locally strong southeast winds are expected through tonight – strongest being around the typical coastal areas that experience terrain-induced accelerations with this direction (Hamakua coast and waters around South Point of the Big Island). These areas have been included in the Small Craft Advisory this morning.
Surf along south facing shores will remain up into the second half of the week as the south swell that arrived early Tuesday continues to move through and becomes reinforced through the day Thursday. Offshore buoys to the south and the nearshore PacIOOS buoy at Barbers reflect this with the peak of the energy holding in the 15 to 17 second bands. In addition to the long- period energy moving through that is typical for this time of year, strong southerly winds will add to the mix with short period choppy/rough conditions expected through tonight (roughest Kauai where locally strong south winds are possible).
Looking out into the long range (late next week/last weekend of May), lots of activity brewing down south within Hawaii's swell window around New Zealand. GFS and ECMWF both depicting around 960 mb low pressure passing through/near this area later this weekend and into this coming weekend. First phase of this for Hawaii will potentially be out of the southwest direction from the initial setup over the Tasman Sea around next Thursday (the 26th of May) followed by an upward trend through the end of the month. Will monitor the progression of this system later this week/weekend which will lead to more confidence in what to expect locally toward the end of the month.
Surf along north facing shores will remain small today, then trend up Thursday as a small, medium-period northwest swell arrives. Satellite data reflected this potential over the Monday night into Tuesday, which showed a decent sized batch of strong northwest winds focused at the islands within the 300 to 320 degree directional band. A return of summer-like conditions is expected over the weekend as this source moves out.

HFO Watches/Warnings/Advisories

Flood Watch through late tonight for Niihau, Kauai,
Small Craft Advisory until 6 PM HST this evening for Big Island Windward Waters, Big Island Leeward Waters, Big Island Southeast Waters.
Small Craft Advisory until 6 AM HST Thursday for Kauai Northwest Waters, Kauai Windward Waters, Kauai Leeward Waters.

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Data Courtesy of NOAA.gov