Photo Credit: Brendan Stephen

West Kaua’i

Today: Cloudy and breezy. Occasional showers and isolated thunderstorms early in the morning, then numerous showers and isolated thunderstorms in the late morning and afternoon. Some thunderstorms may produce heavy rainfall. Locally heavy rainfall possible. Highs around 81 near the shore to around 69 above 3000 feet. Southeast winds 10 to 20 mph. Chance of rain 90 percent.

Tonight: Mostly cloudy. Showers likely and isolated thunderstorms. Some thunderstorms may produce heavy rainfall early in the evening. Locally heavy rainfall possible early in the evening. Lows around 70 near the shore to around 61 above 3000 feet. Southeast winds 10 to 15 mph. Chance of rain 70 percent.

Thursday: Mostly cloudy. Showers likely and isolated thunderstorms early in the morning, then scattered showers in the late morning and afternoon. Highs around 82 near the shore to around 71 above 3000 feet. Southeast winds 10 to 15 mph. Chance of rain 70 percent.

South Kaua’i

Today: Occasional showers and isolated thunderstorms. Some thunderstorms may produce heavy rainfall. Locally heavy rainfall possible. Highs 78 to 83. South winds around 10 mph. Chance of rain near 100 percent.

Tonight: Occasional showers and isolated thunderstorms. Some thunderstorms may produce heavy rainfall early in the evening. Locally heavy rainfall possible early in the evening. Lows around 68. South winds up to 10 mph in the evening becoming light. Chance of rain near 100 percent.

Thursday: Mostly cloudy. Occasional showers and isolated thunderstorms early in the morning, then showers likely in the late morning and afternoon. Highs around 82. South winds around 10 mph. Chance of rain 90 percent.

Kaua’i Mountains

Today: Breezy. Occasional showers and isolated thunderstorms. Some thunderstorms may produce heavy rainfall. Locally heavy rainfall possible. Highs 75 to 80 in the valleys to around 63 above 4000 feet. South winds 10 to 20 mph decreasing to up to 20 mph in the afternoon. Chance of rain near 100 percent.

Tonight: Occasional showers and isolated thunderstorms. Some thunderstorms may produce heavy rainfall early in the evening. Locally heavy rainfall possible early in the evening. Lows 62 to 68 in the valleys to around 58 above 4000 feet. South winds up to 15 mph. Chance of rain near 100 percent.

Thursday: Occasional showers and isolated thunderstorms early in the morning, then occasional showers in the late morning and afternoon. Highs 75 to 81 in the valleys to around 65 above 4000 feet. South winds up to 15 mph. Chance of rain near 100 percent.

North Kaua’i

Today: Cloudy and breezy. Occasional showers and isolated thunderstorms early in the morning, then numerous showers and isolated thunderstorms in the late morning and afternoon. Some thunderstorms may produce heavy rainfall. Locally heavy rainfall possible. Highs 73 to 82. South winds up to 20 mph. Chance of rain 90 percent.

Tonight: Mostly cloudy. Showers likely and isolated thunderstorms. Some thunderstorms may produce heavy rainfall early in the evening. Locally heavy rainfall possible early in the evening. Lows 63 to 71. South winds up to 15 mph. Chance of rain 70 percent.

Thursday: Cloudy. Showers likely and isolated thunderstorms early in the morning, then showers likely in the late morning and afternoon. Highs 74 to 83. South winds up to 15 mph. Chance of rain 70 percent.

East Kaua’i

Today: Occasional showers and isolated thunderstorms. Some thunderstorms may produce heavy rainfall. Locally heavy rainfall possible. Highs 72 to 81. South winds 10 to 15 mph. Chance of rain near 100 percent.

Tonight: Occasional showers and isolated thunderstorms. Some thunderstorms may produce heavy rainfall early in the evening. Locally heavy rainfall possible early in the evening. Lows 63 to 73. South winds 10 to 15 mph. Chance of rain near 100 percent.

Thursday: Mostly cloudy. Occasional showers and isolated thunderstorms early in the morning, then showers likely in the late morning and afternoon. Highs 73 to 83. South winds 10 to 15 mph. Chance of rain 90 percent.

Detailed Forecast

Synopsis

Occasional showers with a few thunderstorms will be possible over the western end of the state today. Showers have the potential to produce heavy rainfall across Niihau, Kauai, and Oahu where a Flood Watch remains in effect through the afternoon. Additionally, a few stronger storms cannot be ruled out, especially near Kauai. Further east, lighter showers will tend to focus over southeast- facing slopes with thunderstorms possible over interior Big Island this afternoon and Thursday afternoon. Winds will remain southeasterly through the weekend, though models indicate that they may return to a more easterly direction early next week.

Discussion

Early this morning, a low pressure system remains situated to the northwest of the islands near 30N170W. This surface low lies beneath a deep upper low that is producing an expansive area of cyclonic flow aloft. Broad but poorly focused forcing for ascent is currently over the western end of the state from roughly Kauai to Oahu. Meanwhile, a stationary front is located a couple hundred miles northwest of Kauai.
A band of convergence ahead of the stationary front has continued to produce numerous strong thunderstorms with prolific lightning northwest of Kauai and Niihau throughout the night. This band of thunderstorms, which has been occurring for the past 24+ hours, has continued to slowly nudge closer to the state overnight as a shortwave and associated mid-level trough axis pivots around the parent low. A few of these storms made their way into the coastal waters just northwest of Niihau throughout the night into the early morning hours, prompting the issuance of a couple Special Marine Warnings during this period.
Moisture embedded within moderate southeasterly surface flow wrapping around the Big Island has also produced showers moving northward towards Oahu throughout the night. So far, these showers have remained disorganized, though have brought periodic showers onshore from the south. Despite gauges reporting brief heavy rain rates at times over Oahu, these showers have been progressing at a pace that has negated any flooding issues thus far as of press time.
For the rest of the morning and into early afternoon, the forecast challenge remains twofold: (1) the potential for multiple rounds of showers, which could be heavy at times, to move over Oahu and Kauai from the south within the southerly flow regime, and (2) the potential for enhanced forcing for ascent/potential vorticity to briefly swing around over the western end of state, brushing Kauai as the low aloft to the northwest rotates, which could spark some thunderstorms over the western islands (especially Kauai) and their adjacent waters. Of note, latest trends in satellite imagery show recent intensification of thunderstorms both northwest of Kauai and further to the southwest near 19N162W. The atmosphere near Kauai is certainly unstable as thunderstorms nudge uncomfortably close to the Garden Isle early this morning. The observed 12z sounding from Lihue shows a fairly significant amount of instability (about 1950 J/kg of MUCAPE), 38kts of 0-6km wind shear, and 700mb-500mb lapse rates of 6.9C/km. In this environment, any thunderstorms that are able to develop over the western end of the state as the upper trough becomes negatively tilted would have the potential to become quite strong, so this will need to be monitored closely over the next few hours. Additionally, latest HREF guidance still highlights Kauai and Oahu as having the potential to receive heavy rainfall rates at times today. As a result, the Flood Watch remains in effect for Niihau, Kauai, and Oahu through this afternoon.
Forcing will diminish this afternoon when shower coverage will once again be modulated by poorly forced, moist southerly flow. Isolated thunder produced by afternoon sea breezes coinciding with daytime heating cannot be ruled out over the Big Island slopes both this afternoon and Thursday afternoon as well. Otherwise, the southeast flow pattern will continue into the weekend, locally breezy around Maui County and the Big Island and light to moderate elsewhere. Increased stability will maintain drier conditions, though the lingering moisture band around Kauai will contribute to increased shower coverage there.
There is some uncertainty in the forecast for early next week as model guidance depicts an upper low or trough approaching the islands from the northwest. Should this materialize, another increase in shower activity will be possible during the first part of the work week.

Aviation

A low pressure system far northwest of the state will continue to produce unstable southerly winds over Kauai and Oahu today and tonight. A stationary surface boundary located a few hundred miles northwest of Kauai is producing a linear band of converging winds with heavy showers and frequent thunderstorms. This band is approaching Kauai from the west this morning. Bands of showers will continue to develop in the unstable southerly wind flow near Kauai and Oahu with periods of MVFR and brief IFR conditions in moderate to heavy rainfall and thunderstorms for both islands.
Lower shower trends are forecast across the eastern half of the state with brief periods of MVFR conditions in showers from Molokai to the Big Island. However, afternoon thunderstorms are possible over the Big Island interior due to instability, abundant moisture and surface heating above the trade wind inversion.
AIRMET Sierra remains in effect for mountain obscurations above 020 for the islands of Kauai and Oahu this morning. This AIRMET will likely continue into this evening.
Tempo light rime icing will also be present in cloud within a layer 130-FL180.

Marine

A broad low pressure system west of the state will keep moderate to fresh southerly winds over the western islands and fresh to strong southeasterly winds over the eastern islands through tonight. Trade winds will begin to slowly build back in from east to west across the islands Thursday through the weekend, with the strongest winds holding over the eastern waters. A Small Craft Advisory is now in effect for most waters surrounding the Big Island through 6 PM Thursday.
No significant swells are expected through the middle of next week. A small pulse from the northwest (320 degrees) will build late this afternoon and tonight, peak Thursday and Thursday night, then slowly decline through Saturday. North shore surf will be nearly flat Sunday through the middle of next week.
Fresh trades upstream of the state will continue to produce near seasonal surf along east facing shores during the next 7 days. Small background energy from the southwest, south and southeast will continue to produce small surf along south facing shores for the foreseeable future.

HFO Watches/Warnings/Advisories

Flood Watch through this afternoon Oahu, Kauai, and Niihau.
Small Craft Advisory until 6 PM HST Thursday for Big Island Windward Waters, Big Island Leeward Waters, Big Island Southeast Waters.

Data Courtesy of NOAA.gov