Photo Credit: Sebastien Gabriel

West Kaua’i

Today: Mostly sunny with numerous showers and slight chance of thunderstorms in the morning, then partly sunny with scattered showers and slight chance of thunderstorms in the afternoon. Locally heavy rainfall possible in the afternoon. Highs around 79 near the shore to around 67 above 3000 feet. East winds 10 to 15 mph decreasing to up to 15 mph in the afternoon. Chance of rain 70 percent.

Tonight: Mostly cloudy with showers and slight chance of thunderstorms. Locally heavy rainfall possible. Lows around 69 near the shore to around 58 above 3000 feet. East winds 10 to 15 mph decreasing to up to 15 mph after midnight. Chance of rain 90 percent.

Friday: Cloudy with showers and slight chance of thunderstorms. Locally heavy rainfall possible. Highs around 77 near the shore to around 65 above 3000 feet. East winds up to 15 mph shifting to the southeast in the afternoon. Chance of rain 90 percent.

South Kaua’i

Today: Partly sunny with numerous showers and slight chance of thunderstorms. Locally heavy rainfall possible in the afternoon. Highs around 78. East winds 10 to 15 mph. Chance of rain 70 percent.

Tonight: Cloudy with showers and slight chance of thunderstorms. Locally heavy rainfall possible. Lows 65 to 70. East winds 10 to 15 mph. Chance of rain 90 percent.

Friday: Cloudy with showers and slight chance of thunderstorms. Locally heavy rainfall possible. Highs around 76. East winds 10 to 15 mph. Chance of rain 90 percent.

Kaua’i Mountains

Today: Partly sunny with numerous showers and slight chance of thunderstorms. Locally heavy rainfall possible in the afternoon. Highs 66 to 74 in the valleys to around 61 above 4000 feet. Southeast winds up to 15 mph. Chance of rain 70 percent.

Tonight: Breezy. Cloudy with showers and slight chance of thunderstorms. Locally heavy rainfall possible. Lows 58 to 63 in the valleys to around 52 above 4000 feet. East winds 10 to 20 mph. Chance of rain 90 percent.

Friday: Breezy. Cloudy with showers and slight chance of thunderstorms. Locally heavy rainfall possible. Highs 64 to 72 in the valleys to around 59 above 4000 feet. East winds 10 to 20 mph shifting to the southeast in the afternoon. Chance of rain 90 percent.

North Kaua’i

Today: Partly sunny with scattered showers and slight chance of thunderstorms. Locally heavy rainfall possible in the afternoon. Highs 71 to 78. Southeast winds up to 15 mph. Chance of rain 50 percent.

Tonight: Breezy. Cloudy with numerous showers and slight chance of thunderstorms. Locally heavy rainfall possible. Lows 61 to 70. East winds up to 20 mph. Chance of rain 70 percent.

Friday: Breezy. Cloudy with numerous showers and slight chance of thunderstorms. Locally heavy rainfall possible. Highs 69 to 76. East winds up to 20 mph. Chance of rain 70 percent.

East Kaua’i

Today: Partly sunny with numerous showers and slight chance of thunderstorms. Locally heavy rainfall possible in the afternoon. Highs 71 to 78. East winds 10 to 15 mph. Chance of rain 70 percent.

Tonight: Cloudy with showers and slight chance of thunderstorms. Locally heavy rainfall possible. Lows 59 to 71. East winds 10 to 15 mph. Chance of rain 90 percent.

Friday: Cloudy with showers and slight chance of thunderstorms. Locally heavy rainfall possible. Highs 69 to 76. East winds 10 to 15 mph. Chance of rain 90 percent.

Detailed Forecast

Synopsis

A kona low will move west and away from the state during the next day or so, while a strong area of high pressure north of the islands shifts off to the east. This will bring a moist convergent southeasterly boundary layer flow with a tap into the deep tropics over the island chain, with unsettled weather beginning to affect the eastern end of the state later today and the remainder of the islands tonight. Periods of heavy rain and even a rumble or two of thunder will be possible through the weekend. The weather pattern appears to change little next week, with convergent southeasterly boundary layer flow continuing to bring periods of wet and unsettled weather to the island chain.

Discussion

Currently at the surface, a 1002 mb kona low is located around 250 miles west-northwest of Kauai, while a 1037 mb high is centered around 1250 miles north of Honolulu. Light to moderate southeast winds have now developed statewide as the low continues to move westward and away from the island chain. Infrared satellite imagery shows an expansive area of layered clouds in place from Oahu eastward, resulting in mostly cloudy conditions, with variably cloudy skies in place over Kauai. Radar imagery shows a few showers moving onshore in the southeast flow from Oahu eastward to the Big Island, while scattered showers are affecting Kauai. Main forecast concerns over the next several days revolve around heavy rainfall and flooding potential.
The kona low will shift westward and away from the islands during the next 24 to 36 hours, before stalling out and gradually weakening Friday through the weekend. Meanwhile, the high north of the state will shift slowly eastward through the weekend. The positioning of these features will place the islands in a convergent southeasterly boundary layer flow, with a tap into the deep tropics bringing a stream of 1.5 to 2.0 inch precipitable water values into the state beginning later today over the eastern islands, then spreading to the remainder of the state tonight. This in combination with troughing aloft will bring the potential for periods of heavy rainfall and even a few thunderstorms to the islands during the next several days. A Flood Watch remains in effect from 6 AM this morning through 6 PM Saturday, and this watch may need to be extended in time.
Heavy snow is also expected for the highest elevations on the Big Island, generally above 12,500 feet, and a Winter Storm Warning remains in effect from 6 AM Thursday through 6 PM Saturday. Storm total snow accumulations of several feet appear likely during the next few days.
The overall weather pattern appears to change very little Sunday right through the middle of next week. Strong high pressure will remain anchored north and northeast of the state during this time, while a trough of low pressure lingers to the west of the island chain. This will keep the islands in a convergent southeasterly boundary layer flow which will waver back and forth across the area, with 1.5 to 2.0 inch PW's continuing to stream over the state. As a result, we expect a continuation of wet and unsettled weather, with periods of rain and perhaps a rumble of thunder from time to time.

Aviation

The air mass over the state will remain moist and unstable due to the proximity of a strong low passing west and away from the state. The low is pulling in deep tropical moisture out of the southeast towards the state. Precipitation coverage will increase in both coverage and intensity throughout the day.
Locally heavy showers with isolated embedded thunderstorms are expected to spread into Windward Big Island this morning, Maui and Leeward Big Island in the afternoon, then continue westward across the remainder of the state by late evening. Widespread MVFR conditions will likely necessitate the issuance of AIRMETs for mountain obscuration later today.
In the upper levels, moderate to severe turbulence will remain an issue. SIGMET Victor is currently in effect for occasional severe turbulence just north and northeast of the state. However, conditions could spread over the western half of the state later today. Otherwise, AIRMET Tango for moderate turbulence above fl200 will remain in effect statewide.

Marine

In sharp contrast to the recently strong N to NE winds over Kauai and Oahu waters, a moderate to fresh SE to S wind flow prevails over all waters this morning. This is due to the passage of a kona low to the N of the islands, with the westward-moving low now centered about 200 nm WNW of Kauai. The center of the low passed very close to NDBC buoys 51001 and 51101 overnight, impressively producing frequent wind gusts in excess of 50 kt (peak wind of 56 kt), maximum seas near 25 feet, and a minimum pressure near 998 mb (22 mb drop in 24 h at the buoys).
Over the next day or two, the kona low will move W and SW away from the area, then become nearly stationary and gradually dissipate far W of the islands over the weekend. At the same time, high pressure will become nearly stationary to the distant NE, and strengthen to near 1040 mb. This will result in strong to near-gale E to SE winds developing near and upwind of the islands, with much lighter winds to the W. Little overall change to this pattern is expected into next week, with this convergent low-level flow supporting widespread heavy shower development in Hawaiian waters. Embedded thunderstorms will bring a myriad of marine hazards, including low visibility, cloud-to-surface lightning strikes and locally higher winds and seas.
Near shore PacIOOS buoys indicate a variety of swell spectra in island waters this morning, with highest seas on the W end of the island chain much larger than anticipated. The Hanalei buoy just reached 15 feet, dominated by medium-period NE swell generated by the kona low about 12-24 hours ago. A Small Craft Advisory (SCA) remains posted (primarily due to seas > 10 feet) for most windward waters, and all waters around Kauai. While seas are expected to briefly trend down today, the strengthening E-SE winds will bring SCA winds and increasing seas to most areas starting tonight, and the SCA will expand to nearly all waters at that time.
Primary swells this morning are a moderate-sized relatively long- period NW swell, the aforementioned medium-period NE swell generated by the kona low (with this fetch primarily aimed at Kauai and Oahu waters), and a diminishing short-period E trade wind swell that peaked several days ago. A High Surf Advisory (HSA) for E facing shores of Kauai and Oahu has been extended through today due to the NE swell, with the N facing shores of Kauai County added due to recent observations. This NE swell should diminish fairly rapidly this afternoon as the kona low continues to move away. The NW swell will peak below HSA level heights today, then diminish thereafter, with a small long-period WNW swell expected over the weekend. Building seas and surf along windward shores will likely prompt a HSA for E facing shores of nearly all islands on Friday. Surf along exposed E facing shores may peak near High Surf Warning levels of 15 feet over the weekend, and the upcoming early morning high tides may bring some seawater onto vulnerable coastal roadways.

HFO Watches/Warnings/Advisories

High Surf Advisory until 6 PM HST this evening for Niihau, Olomana, Kauai North, Kauai East, Kauai South, Koolau Windward.
Flood Watch from 6 AM HST this morning through Saturday afternoon for the entire state of Hawaii.
Winter Storm Warning from 6 AM this morning to 6 PM HST Saturday for Big Island Summits.
Small Craft Advisory until 6 PM HST Saturday for Kauai Northwest Waters, Kauai Windward Waters, Kauai Leeward Waters, Kauai Channel, Oahu Windward Waters, Kaiwi Channel, Maui County Windward Waters.
Small Craft Advisory from 6 PM this evening to 6 PM HST Saturday for Maalaea Bay, Pailolo Channel, Alenuihaha Channel, Big Island Windward Waters, Big Island Leeward Waters, Big Island Southeast Waters.

Data Courtesy of NOAA.gov