Photo Credit: Sebastien Gabriel

West Kaua’i

Today: Sunny with isolated showers. Highs around 89 near the shore to around 77 above 3000 feet. Northeast winds up to 15 mph shifting to the west in the afternoon. Chance of rain 20 percent.

Tonight: Partly cloudy in the evening, then mostly cloudy with isolated showers after midnight. Lows around 72 near the shore to around 63 above 3000 feet. Northeast winds up to 15 mph. Chance of rain 20 percent.

Wednesday: Mostly cloudy with numerous showers and isolated thunderstorms. Locally heavy rainfall possible. Highs around 88 near the shore to around 75 above 3000 feet. Northeast winds up to 15 mph. Chance of rain 70 percent.

South Kaua’i

Today: Breezy. Mostly sunny with scattered showers. Highs 84 to 91. Northeast winds 15 to 25 mph. Chance of rain 30 percent.

Tonight: Mostly cloudy. Breezy. Isolated showers in the evening, then scattered showers after midnight. Lows around 73. Northeast winds 15 to 25 mph. Chance of rain 50 percent.

Wednesday: Cloudy and breezy. Scattered showers and isolated thunderstorms early in the morning, then showers and isolated thunderstorms in the late morning and afternoon. Locally heavy rainfall possible in the late morning and afternoon. Highs 83 to 88. Northeast winds 15 to 25 mph. Chance of rain 90 percent.

Kaua’i Mountains

Today: Partly sunny in the morning then becoming mostly sunny. Breezy. Scattered showers. Highs 75 to 85 in the valleys to around 68 above 4000 feet. East winds up to 20 mph shifting to the northeast 10 to 15 mph in the afternoon. Chance of rain 30 percent.

Tonight: Mostly cloudy with scattered showers. Lows 65 to 70 in the valleys to around 60 above 4000 feet. Northeast winds 10 to 15 mph. Chance of rain 50 percent.

Wednesday: Breezy. Showers and isolated thunderstorms. Locally heavy rainfall possible in the late morning and afternoon. Highs 73 to 83 in the valleys to around 66 above 4000 feet. Northeast winds 10 to 20 mph decreasing to up to 20 mph in the afternoon. Gusts up to 40 mph. Chance of rain near 100 percent.

North Kaua’i

Today: Mostly sunny with scattered showers. Highs 77 to 89. East winds up to 15 mph increasing to 10 to 15 mph in the afternoon. Chance of rain 30 percent.

Tonight: Mostly cloudy. Isolated showers in the evening, then scattered showers after midnight. Lows 66 to 75. East winds 10 to 15 mph. Chance of rain 50 percent.

Wednesday: Breezy. Scattered showers and isolated thunderstorms early in the morning, then showers and isolated thunderstorms in the late morning and afternoon. Locally heavy rainfall possible in the late morning and afternoon. Highs 77 to 88. Northeast winds 10 to 20 mph. Chance of rain near 100 percent.

East Kaua’i

Today: Breezy. Mostly sunny with isolated showers. Highs 75 to 87. Northeast winds 10 to 20 mph. Chance of rain 20 percent.

Tonight: Mostly cloudy. Breezy. Isolated showers in the evening, then scattered showers after midnight. Lows 66 to 78. Northeast winds 10 to 20 mph. Chance of rain 50 percent.

Wednesday: Breezy. Showers and isolated thunderstorms. Locally heavy rainfall possible. Highs 74 to 85. Northeast winds 10 to 20 mph. Chance of rain near 100 percent.

Detailed Forecast

Synopsis

Wind speeds will increase rapidly from today through Wednesday as Tropical Storm Calvin produces strong and gusty winds as it passes near the Big Island and just south of Maui County, Oahu, and Kauai County. Damaging wind speeds are possible across most of the state as Calvin approaches and passes by each island. Heavy showers and flooding conditions are expected due to deep unstable tropical moisture forced up the windward mountain slopes of islands in Maui and Hawaii Counties. Calvin will likely weaken into a Tropical Depression by Wednesday and maintain a westward track moving away from the island chain by Wednesday night. Expect improving weather and wind conditions statewide by Thursday as we transition back to breezy trade wind pattern.

Discussion

A high pressure ridge north of the region will drive Tropical Storm Calvin westward into Hawaiian waters later tonight. Weather conditions will change rapidly today through Wednesday as Calvin, currently around 450 miles east-southeast of Hilo Hawaii, continues to quickly move towards the Big Island. The rapid east to west motion of Calvin will tend to limit Tropical Storm impacts to roughly a 12 to 18 hour time period over any one island.
Wind Impacts: Calvin remains on a high confidence westerly track with the ridge north of the islands providing the main steering flow. This storm will continue to weaken as it approaches the Big Island due to cooler water temperatures, the ingest of stable dry air, and some vertical wind shear. However, Tropical Storm Force winds remain in the forecast as the center of the system passes near the south side of the Big Island tonight through early Wednesday morning. A Tropical Storm Warning remains in effect for the Big Island and surrounding waters due to the island's proximity to the Tropical Storm Force wind speeds surrounding the storm. Wind speeds will also increase statewide today through Wednesday as the storm remains on a track passing south of the smaller Hawaiian Islands. These winds will strengthen due to the increasing pressure gradient between the ridge to the north and the low pressure center associated with Calvin passing to the south. This means that wind speeds over the islands in Maui County will at least reach Wind Advisory thresholds and may exceed High Wind Warning criteria in some areas, even as Calvin weakens and passes to the south. Wind speeds will accelerate over and downwind of mountain slopes with stronger gusts expected. A Wind Advisory remains in effect today for all islands in Maui County, A High Wind Watch was also issued for Maui County starting tonight for stronger winds expected on Wednesday. Additional Wind Advisory and High Wind Watch or Warning products may need to be extended to Oahu on Wednesday as Calvin passes to the south. For the moment, Kauai County will see less wind impacts from Calvin based on the latest storm track and intensity.
Rainfall Impacts: Deep unstable tropical moisture surrounding Calvin will be driven up mountain slopes due to the strong wind forcing as the Tropical Storm passes each island. This means the potential for flash flooding remains elevated even for islands not directly in the path of the Tropical Storm. How far north these heavy rainfall impacts will occur is the forecast challenge with this event. The highest potential for heavy rain and flooding are expected along the windward slopes of the Big Island and windward slopes of the eastern half of Maui. Storm total rainfall amounts in the 4 to 8 inch range should be expected for the eastern slopes of the Big Island from North Kohala to the Hamakua Coast to Hilo to South Point, with 1 to 2 inches of rain forecast over the Kona side of the island. Maui may see between 2 to 6 inches of storm total rainfall from the North Shore near Haiku to East Maui near Hana. Other islands will see lower amounts between 0.25 to 2 inches, with locally higher amounts possible favoring windward mountains and slopes. The Flood Watch remains in effect with the potential for flash flooding starting this evening over the Big Island and Maui County. This Flood Watch was expanded in coverage to include Oahu, Kauai and Niihau this morning, with enhanced flood threats to Oahu forecast to start early Wednesday morning potentially impacting the western islands.
By Thursday, Calvin will continue to weaken and pass quickly west of the island chain. High pressure will build in quickly as the storm exits the region, producing more stable breezy trade wind weather with rapidly decreasing shower trends. This trade wind weather will continue through the upcoming weekend with upward trends in shower activity on Saturday and Sunday, mainly along windward and mountain slopes.

Aviation

Breezy northeast to easterly trade winds will continue this morning, with clouds and showers favoring windward and mountain locations. Periods with MVFR conditions in and around clouds and showers are expected. Winds will steadily rise from east to west as Tropical Storm Calvin approaches and passes over or just south of the Big Island later today through tonight, then south of the smaller islands Wednesday. Conditions will quickly deteriorate from east to west, especially on the Big Island as Calvin moves through.
AIRMET TANGO remains in effect below 9000 ft over and immediately south through west of mountains on all islands due to tempo moderate turbulence.
AIRMET SIERRA for mountain obscuration will likely continue due to bands of low clouds with embedded showers moving through.

Marine

Tropical Storm Calvin, located around 400 nautical miles east- southeast of Hilo, Hawaii, is tracking west at 19 knots. Calvin is forecast to enter the eastern offshore waters today, then track near or over the Big Island and its surrounding waters later today into Wednesday. A Tropical Storm Warning is in effect for the offshore waters. Calvin will continue westward Wednesday as it weakens, while passing south of the smaller islands. A return of a more typical trade wind pattern is expected Thursday through the weekend.
A Tropical Storm Warning is in effect for Alenuihaha Channel and all waters surrounding the Big Island today into Wednesday. The Gale Watch has been upgraded to a Gale Warning for the Pailolo Channel and Maalaea Bay today through Wednesday.
Elsewhere, fresh to strong easterly winds will become strong statewide today through Wednesday as Calvin passes over or near the Big Island tonight into Wednesday. Small Craft Advisory is in effect for rest of the coastal waters today through Wednesday.
Coastal impacts associated with Calvin will include rapidly building surf for exposed windward coasts tonight through Wednesday. The High Surf Warning for the Big Island has been expanded to include exposed east facing shores in Maui County starting tonight through Wednesday. Peak surf heights will be short-lived and will likely occur late tonight into Wednesday morning. Although confidence is low, some minor overwash and erosion can't be ruled out for exposed low- lying coastal areas as the surf peaks. For rest of the state, surf along east facing shores will likely reach advisory thresholds Wednesday. Otherwise, no significant swells are expected through the weekend. &&
.HFO WATCHES/WARNINGS/ADVISORIES… Flood Watch from this evening through Wednesday afternoon for Maui and Hawaii Counties.
Flood Watch from late tonight through Wednesday afternoon Oahu, Kauai and Niihau.
Wind Advisory until 6 PM HST this evening for Maui County.
High Wind Watch from this evening through Wednesday afternoon for Maui County.
Tropical Storm Warning for Big Island.
Tropical Storm Warning for Alenuihaha Channel-Big Island Windward Waters-Big Island Leeward Waters-Big Island Southeast Waters.
High Surf Warning from 6 PM this evening to 6 PM HST Wednesday for Kahoolawe-Maui Windward West-Molokai Windward-Molokai Southeast-Windward Haleakala-Kipahulu-South Haleakala-Big Island Southeast-Big Island East-Big Island North.
Gale Warning until 6 PM HST Wednesday for Maalaea Bay-Pailolo Channel.
Small Craft Advisory until 6 PM HST Wednesday for Kauai Northwest Waters-Kauai Windward Waters-Kauai Leeward Waters- Kauai Channel-Oahu Windward Waters-Oahu Leeward Waters-Kaiwi Channel-Maui County Windward Waters-Maui County Leeward Waters.

Data Courtesy of NOAA.gov