Photo Credit: Jason Weingardt

West Kaua’i

Today: Partly sunny with scattered showers in the morning, then mostly sunny with isolated showers in the afternoon. Highs around 88 near the shore to around 76 above 3000 feet. East winds 10 to 15 mph. Chance of rain 50 percent.

Tonight: Partly cloudy with isolated showers. Lows around 71 near the shore to around 61 above 3000 feet. East winds 10 to 15 mph. Chance of rain 20 percent.

Monday: Mostly sunny with isolated showers. Highs around 89 near the shore to around 77 above 3000 feet. East winds 10 to 15 mph. Chance of rain 20 percent.

South Kaua’i

Today: Breezy. Mostly cloudy with showers likely in the morning, then mostly sunny with scattered showers in the afternoon. Highs 83 to 88. East winds 15 to 20 mph. Chance of rain 70 percent.

Tonight: Breezy. Mostly cloudy with scattered showers. Lows 69 to 74. Northeast winds 15 to 20 mph. Chance of rain 50 percent.

Monday: Breezy. Partly sunny with scattered showers in the morning, then mostly sunny with isolated showers in the afternoon. Highs 84 to 89. East winds 15 to 20 mph. Chance of rain 50 percent.

Kaua’i Mountains

Today: Breezy. Cloudy with occasional showers in the morning, then partly sunny with scattered showers in the afternoon. Highs 75 to 83 in the valleys to around 66 above 4000 feet. East winds 10 to 20 mph. Chance of rain 90 percent.

Tonight: Mostly cloudy. Breezy. Scattered showers in the evening, then numerous showers after midnight. Lows 63 to 68 in the valleys to around 58 above 4000 feet. East winds 10 to 20 mph. Chance of rain 70 percent.

Monday: Partly sunny in the morning then becoming mostly sunny. Breezy. Scattered showers. Highs 76 to 84 in the valleys to around 67 above 4000 feet. East winds 15 to 20 mph. Chance of rain 50 percent.

North Kaua’i

Today: Mostly cloudy with occasional showers in the morning, then mostly sunny with scattered showers in the afternoon. Highs 78 to 87. East winds 10 to 15 mph. Chance of rain 90 percent.

Tonight: Mostly cloudy with scattered showers. Lows 65 to 73. East winds 10 to 15 mph. Chance of rain 50 percent.

Monday: Breezy. Partly sunny with scattered showers in the morning, then mostly sunny with isolated showers in the afternoon. Highs 79 to 88. East winds 10 to 20 mph. Chance of rain 50 percent.

East Kaua’i

Today: Cloudy with occasional showers in the morning, then partly sunny with scattered showers in the afternoon. Highs 76 to 86. East winds 10 to 15 mph. Chance of rain 90 percent.

Tonight: Mostly cloudy with scattered showers. Lows 63 to 76. East winds 10 to 15 mph. Chance of rain 50 percent.

Monday: Partly sunny with scattered showers in the morning, then mostly sunny with isolated showers in the afternoon. Highs 77 to 87. East winds around 15 mph. Chance of rain 50 percent.

Detailed Forecast

Synopsis

Tropical Storm Calvin is expected to move close to, or over, the islands from Tuesday night through Wednesday night, bringing the potential for a period of locally strong winds, heavy rain, and high surf. Stay tuned for forecast updates that will provide greater forecast detail. Otherwise, moderate to breezy trade winds will prevail, bringing limited showers to windward areas, especially during nights and mornings.

Discussion

The locally breezy trade wind weather pattern currently in place will undergo some subtle changes through Tuesday, as a 700 mb trough moves from E to W across the area. Wind speeds will remain fairly steady, supported by a persistent ridge well N of the islands. The passage of the 700 mb trough will support a fairly active trade wind shower pattern into tonight, with regional satellite imagery currently showing broken to locally overcast low clouds upstream of the islands. Showers will favor windward areas, but a few will spread leeward on the smaller islands this morning, with some diurnal decrease in shower coverage in most areas this afternoon. Leeward Big Island will see typical afternoon/evening cloud and showers.
Some drying of the trade flow is anticipated Monday and Monday night as a 700 mb ridge builds over the area, and subsidence ahead of Tropical Storm (TS) Calvin moves over the islands from the E.
Thereafter, island weather will be dependent on the track and intensity of TS Calvin as it is forecast to move close to, or over, portions of the island chain from Tuesday night through Wednesday night. While it still remains too soon to nail down specific details as far as impacts on land, it is looking increasingly likely that at least portions of the state will see some impacts. The official forecast from the National Hurricane Center in Miami, Florida has been consistent in anticipating that the center of Calvin will move close to, or over, the Big Island Tuesday night and Wednesday, while producing tropical-storm-force winds in its northern semicircle. Some uncertainty continues to exist with respect to the track and intensity forecast, and it is also important to remember that impactful weather can occur well away from the center.
Calvin's forward speed will be fairly quick when it is near the islands (toward the west at ~19 mph), so the amount of time any one island experiences gusty winds and heavy rainfall will be somewhat limited (probably on the order of 12-18 hours). However, the potential for extreme rainfall rates will likely warrant the issuance of a Flood Watch (FFAHFO), which is usually done with about a 48 hour lead time – which is also the desired lead time for a Tropical Storm Watch. Latest time-of-arrival graphical forecasts indicate that the earliest reasonable arrival time for tropical- storm-force winds on the Big Island is Tuesday afternoon, so these headlines could be issued for at least portions of the state later today or tonight.
Since the public and marine forecasts for Hawaii are highly dependent on the tropical cyclone bulletins issued at 11 am/pm and 5 am/pm HST, our regularly scheduled forecast issuances will be issued shortly after these bulletins are released.
A return to a typical trade wind weather pattern is expected from Thursday into next weekend as the remnants of Calvin move quickly west away from the islands, likely as a trough.

Aviation

Moderate to locally breezy trade winds will continue through the remainder of the weekend, with multiple bands of passing showers favoring windward and mauka areas. Expect mainly VFR conditions to prevail all areas with brief periods of MVFR in showers.
A strong trade wind inversion between 07 kft and 09 kft persists along with occasionally brisk winds at summit level. AIRMET Tango will remain posted for occasional moderate turbulence to the lee of higher terrain.

Marine

A surface ridge far north of the state will maintain moderate to locally strong trade winds into Monday. A Small Craft Advisory (SCA) remains in effect for the typically windier waters adjacent to the islands of Maui County and the Big Island this weekend. This SCA may need to be extended and expanded into other areas from Monday through Wednesday along with a possible Tropical Storm Watch as Calvin approaches Hawaiian waters. All marine interests in the Hawaiian Islands should monitor the latest forecast advisory product issued by the National Hurricane Center in Miami, Florida under WMO header WTPZ23 KNHC.
Based on the latest advisory issued by the National Hurricane Center in Miami, Florida at 5 AM HST Sunday, Calvin has weakened to a Tropical Storm and will likely enter the Central Pacific Hurricane Center's area of responsibility by early Monday morning. Tropical Storm conditions associated with Calvin are expected to reach the eastern Hawaiian Offshore Waters starting on Tuesday and then impact the eastern half of the state including the Hawaiian Coastal Waters from Tuesday night into Wednesday, with potential impacts extending over the western half of the state from Wednesday into Wednesday night. Although there are still some uncertainties regarding the intensity of Calvin as it approaches the state, the forecast track guidance remains in good agreement with the center of Calvin passing very close or over the Big Island by Tuesday night or Wednesday morning.
A small southeast swell and background swell energy from the south- southwest should maintain small surf along exposed south facing shores this weekend. Modest, choppy surf will persist along east facing shores this weekend due to the trade winds. An upward trend in surf is expected along east facing shores early this week due to the arrival of a medium-period easterly swell, which is being generated by the captured fetch associated with Tropical Storm Calvin. Based on the latest track and intensity, surf heights along east facing shores will reach the High Surf Advisory threshold for most islands starting from Tuesday night into Wednesday. Confidence is building for a brief period of warning level surf within a similar time period along east facing shores of the Big Island and possibly Maui. These surf impacts will heavily depend upon the intensity of Calvin as it moves through the Hawaii region.

HFO Watches/Warnings/Advisories

Small Craft Advisory until 6 AM HST Monday for Maalaea Bay, Pailolo Channel, Alenuihaha Channel, Big Island Leeward Waters, Big Island Southeast Waters.

Data Courtesy of NOAA.gov