Photo Credit: Braden Jarvis

West Kaua’i

Today: Frequent showers and isolated thunderstorms. Locally heavy rainfall possible. Highs around 80 near the shore to around 68 above 3000 feet. Southeast winds up to 10 mph. Chance of rain near 100 percent.

Tonight: Frequent showers and isolated thunderstorms. Locally heavy rainfall possible. Lows around 68 near the shore to around 58 above 3000 feet. Southeast winds up to 15 mph increasing to 10 to 15 mph after midnight. Chance of rain near 100 percent.

Friday: Frequent showers and isolated thunderstorms in the morning, then numerous showers and isolated thunderstorms in the afternoon. Locally heavy rainfall possible. Highs around 82 near the shore to around 71 above 3000 feet. Southeast winds 10 to 15 mph. Chance of rain near 100 percent.

South Kaua’i

Today: Frequent showers and isolated thunderstorms. Locally heavy rainfall possible. Highs 76 to 82. East winds around 10 mph. Chance of rain near 100 percent.

Tonight: Cloudy with frequent showers and isolated thunderstorms. Locally heavy rainfall possible. Lows around 67. South winds 10 to 15 mph. Chance of rain 90 percent.

Friday: Cloudy. Frequent showers and isolated thunderstorms in the morning, then numerous showers and isolated thunderstorms in the afternoon. Locally heavy rainfall possible. Highs around 81. South winds 10 to 15 mph. Chance of rain 90 percent.

Kaua’i Mountains

Today: Frequent showers and isolated thunderstorms. Locally heavy rainfall possible. Highs 69 to 76 in the valleys to around 60 above 4000 feet. Light winds becoming south up to 10 mph in the afternoon. Chance of rain near 100 percent.

Tonight: Frequent showers and isolated thunderstorms. Locally heavy rainfall possible. Lows 61 to 66 in the valleys to around 57 above 4000 feet. South winds up to 15 mph. Chance of rain near 100 percent.

Friday: Frequent showers and isolated thunderstorms. Locally heavy rainfall possible. Highs 73 to 79 in the valleys to around 64 above 4000 feet. South winds up to 15 mph. Chance of rain near 100 percent.

North Kaua’i

Today: Cloudy with numerous showers and isolated thunderstorms. Locally heavy rainfall possible. Highs 71 to 80. South winds up to 15 mph. Chance of rain 70 percent.

Tonight: Cloudy with numerous showers and isolated thunderstorms. Locally heavy rainfall possible. Lows 62 to 69. South winds up to 15 mph. Chance of rain 70 percent.

Friday: Cloudy with numerous showers and isolated thunderstorms. Locally heavy rainfall possible. Highs 74 to 83. Southeast winds up to 15 mph. Chance of rain 70 percent.

East Kaua’i

Today: Frequent showers and isolated thunderstorms. Locally heavy rainfall possible. Highs 68 to 80. Southeast winds up to 10 mph. Chance of rain near 100 percent.

Tonight: Frequent showers and isolated thunderstorms. Locally heavy rainfall possible. Lows 62 to 72. South winds 10 to 15 mph. Chance of rain near 100 percent.

Friday: Frequent showers and isolated thunderstorms in the morning, then numerous showers and isolated thunderstorms in the afternoon. Locally heavy rainfall possible. Highs 72 to 82. South winds 10 to 15 mph. Chance of rain near 100 percent.

Detailed Forecast

Synopsis

Deep tropical moisture surging northward over the islands ahead of an upper disturbance settling southeastward into the area will lead to southeast winds and humid conditions with increasing rainfall chances through the second half of the week. The best chance for widespread heavy rainfall and thunderstorms will favor the western end of the state beginning this morning, then potentially over the rest of the state from later today through early Saturday. Southeast winds will linger into the weekend, with a transition back to easterly trade winds and drier conditions late Saturday through early next week.

Discussion

Short-term guidance remains in good agreement through the second half of the week and shows a strong pulse aloft diving southward and pinching off into an upper low west of Kauai through the day today. Deep southerly flow ahead of this approaching feature will lead to deep tropical moisture surging northward over the region. This influx of moisture, coupled with enhanced lift and added instability associated with upper height falls, will translate to an increasing likelihood for widespread heavy rainfall with embedded thunderstorms.
The timing and spatial coverage of this activity will be mostly influenced by the progression of the aforementioned upper low forming and moving through. Based on the current high- resolution guidance, the rainfall is expected to quickly ramp up through the first half of the day today over and around Kauai, mostly likely holding over the waters south and southwest of Kauai through daybreak today where the best low-level convergence is most pronounced. As the scenario evolves and upper height falls expand eastward, widespread rainfall chances will increase over and around Oahu by this afternoon, then the remainder of the state tonight through Friday night. Although the guidance suggest the highest rainfall totals focusing over Oahu and Kauai (where a Flood Watch exists), localized flooding can't be ruled out over the eastern end of the state tonight through Friday night. In addition to the rainfall potential, the threat of a wintry mix is anticipated over the Big Island Summits tonight through early Saturday as the cold pool aloft associated with the upper low moves through. Although this environment will support a few thunderstorms, the threat of strong to severe storms should remain on the low side due to the level of instability and limited deep flow. Another limiting factor could be the extend of the cloud coverage, which will reduce boundary layer heating/instability through this period.
A drying trend is expected over the weekend through early next week as the weakening upper low departs and upper heights rise. This will shift the deep moisture in the area westward with surface high pressure building to northeast. This pattern shift will result in a more typical easterly trade wind set up with showers returning to windward and mauka locations each day.

Aviation

An upper level trough combined with a surface trough will approach the islands today and bring unsettled conditions through tomorrow. Mid and high clouds which are currently moving in from the southwest will increase in coverage through the morning. Showers will also be increasing from west to east. MVFR conditions with embedded IFR conditions due to the heavy showers could start on Kauai around or by mid morning, and spread east to Oahu this afternoon. Scattered showers are likely for the rest of the islands. Thunderstorms will also be possible over and around Kauai and Oahu as well as the Big island slopes this afternoon.
An AIRMET Zulu for tempo light icing is in effect statewide in LYR 150-FL210 and will likely continue throughout the day.

Marine

Moderate to locally fresh easterly trade winds will veer southeasterly across most waters today and may shift out of the south around Kauai tonight as a surface trough stalls about 250 nm west of the state. Thunderstorms will be possible around Kauai, Oahu, and Molokai today, and the threat will expand to all waters Friday. The surface trough will dissipate on Saturday, leading to a return of easterly trade winds and Small Craft Advisory conditions around the Big Island and Maui. Trades may strengthen early next week.
A small, short-period north-northeast swell will build today then decline Friday. Some of this swell will wrap into exposed eastern shores, but otherwise, east shore surf will be below May average as the nearby trade wind flow will be weaker than normal into Saturday. A small northwest swell is possible early next week, while returning trade winds will increase east shore surf.
A mix of small southwest and south-southeast swell may produce inconsistent surf near seasonal average through Friday. Expect south shore surf to be at summertime background heights this weekend through the middle of next week.

HFO Watches/Warnings/Advisories

Flood Watch from 6 AM HST this morning through Friday afternoon for Niihau, Kauai Leeward, Kauai Mountains, Waianae Coast, Oahu North Shore, Olomana, Central Oahu, Waianae Mountains, Kauai North, Kauai East, Kauai South, East Honolulu, Honolulu Metro, Ewa Plain, Koolau Windward, Koolau Leeward.
Wind Advisory until 6 AM HST Friday for Big Island Summits.

Data Courtesy of NOAA.gov