Photo Credit: Sebastien Gabriel

West Kaua’i

Today: Cloudy. Isolated showers and thunderstorms in the morning, then scattered showers and isolated thunderstorms in the afternoon. Locally heavy rainfall possible in the afternoon. Highs around 84 near the shore to around 74 above 3000 feet. Southeast winds up to 15 mph increasing to 10 to 15 mph in the afternoon. Chance of rain 50 percent.

Tonight: Numerous showers and isolated thunderstorms in the evening, then occasional showers and isolated thunderstorms after midnight. Locally heavy rainfall possible. Lows 68 to 73 near the shore to around 59 above 3000 feet. Southeast winds 10 to 15 mph. Chance of rain near 100 percent.

Wednesday: Mostly cloudy with numerous showers and isolated thunderstorms. Locally heavy rainfall possible. Highs around 84 near the shore to around 74 above 3000 feet. Southeast winds 10 to 15 mph. Chance of rain 70 percent.

South Kaua’i

Today: Cloudy. Scattered showers and isolated thunderstorms in the morning, then numerous showers and isolated thunderstorms in the afternoon. Locally heavy rainfall possible in the afternoon. Highs around 82. South winds up to 10 mph increasing to 10 to 15 mph in the afternoon. Chance of rain 70 percent.

Tonight: Occasional showers and isolated thunderstorms. Locally heavy rainfall possible. Lows 66 to 72. South winds 10 to 15 mph. Chance of rain near 100 percent.

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

Kaua’i Mountains

Today: Isolated showers and thunderstorms in the morning, then occasional showers and isolated thunderstorms in the afternoon. Locally heavy rainfall possible in the afternoon. Highs around 77 in the valleys to around 66 above 4000 feet. South winds up to 15 mph. Chance of rain near 100 percent.

Tonight: Occasional showers and isolated thunderstorms. Locally heavy rainfall possible. Lows around 62 in the valleys to around 54 above 4000 feet. South winds 10 to 15 mph. Chance of rain near 100 percent.

Wednesday: Occasional showers and isolated thunderstorms. Locally heavy rainfall possible. Highs around 77 in the valleys to around 66 above 4000 feet. South winds 10 to 15 mph. Chance of rain near 100 percent.

North Kaua’i

Today: Cloudy. Isolated showers and thunderstorms in the morning, then numerous showers and isolated thunderstorms in the afternoon. Locally heavy rainfall possible in the afternoon. Highs 77 to 83. South winds up to 15 mph. Chance of rain 70 percent.

Tonight: Mostly cloudy with numerous showers and isolated thunderstorms. Locally heavy rainfall possible. Lows 62 to 71. Southeast winds 10 to 15 mph. Chance of rain 70 percent.

Wednesday: Mostly cloudy with numerous showers and isolated thunderstorms. Locally heavy rainfall possible. Highs 77 to 83. Southeast winds 10 to 15 mph. Chance of rain 70 percent.

East Kaua’i

Today: Cloudy. Isolated showers and thunderstorms in the morning, then numerous showers and isolated thunderstorms in the afternoon. Locally heavy rainfall possible in the afternoon. Highs 75 to 81. South winds up to 15 mph increasing to 10 to 15 mph in the afternoon. Chance of rain 70 percent.

Tonight: Occasional showers and isolated thunderstorms. Locally heavy rainfall possible. Lows 60 to 73. South winds 10 to 15 mph. Chance of rain near 100 percent.

Wednesday: Occasional showers and isolated thunderstorms in the morning, then numerous showers and isolated thunderstorms in the afternoon. Locally heavy rainfall possible. Highs 75 to 81. South winds 10 to 15 mph. Chance of rain near 100 percent.

Detailed Forecast

Synopsis

A high pressure ridge north of the islands will continue to weaken in response to a rapidly deepening low pressure system moving into the region. This low will drift south this week and break down the ridge over the western islands, opening the door for deep unstable southerly winds to develop over Kauai and Oahu through Thursday. An upper level trough rotating around the low will produce moderate to heavy shower bands over these western islands with thunderstorms. The eastern islands in Maui and Hawaii counties will see less rainfall activity with isolated thunderstorms possible over the Big Island each afternoon. High pressure will build in north of the region by Friday allowing typical easterly trade wind weather to spread across the region into the first half of next week.

Discussion

A stationary boundary roughly 300 miles northwest of Kauai has shown increasing thunderstorm trends this morning as a low pressure system roughly 700 miles farther to the northwest moves south into the Hawaii region. Southerly winds are developing over the western islands later today drawing up deep unstable tropical moisture over the islands of Kauai, Niihau, and Oahu. The subtropical jet stream south of the islands will be drawn northward over the island chain, enhancing some of the upper level forcing over the region.
Expect increasing shower trends through the day today for these western islands as winds veer to a more southerly direction. Upper level forcing provides the lift to produce moderate to heavy rain bands and thunderstorms moving up from the south. These wet weather conditions will start later this morning over Kauai and then spread to Oahu later this afternoon to evening. Short range weather models continue to show the heaviest showers and most unstable thunderstorm activity along a nearly stationary boundary just west of Kauai. That said, a threat for localized flash flooding will remain elevated for Kauai County and Oahu into Thursday with this developing weather system. Maui County and the Big Island will remain in a more stable southeasterly wind pattern during this time period. One exception to this forecast will be lingering chances for isolated thunderstorms each afternoon over the Big Island triggered by strong surface heating above the trade wind inversion and sea breeze moisture advection.
The main forecast challenge with this changing weather pattern is in determining how close this approaching cut off low will get to the western islands. A fifty to one hundred mile error on the low center position could create stronger island weather impacts. We will closely monitor any heavy shower bands that develop and look for training showers or thunderstorms approaching the western islands from the south. If these heavy shower bands are numerous enough, then we will likely issue a Flood Watch product for the affected western islands. As of this morning, we decided to hold back on issuing a Flood Watch as we feel the strongest heavy rain and thunderstorm dynamics will remain west of Kauai. Please stay tuned to the weather forecast this week as this changing weather pattern will likely cause our forecast and potential for severe short range impacts to evolve over time.
Long range forecast guidance continues to show this deep cut off low northwest of the state will drift northward from Thursday onward. This changing pressure pattern will allow the subtropical ridge to strengthen and build back in north of the Hawaiian Islands, allowing a transition back to more typical easterly trade winds from Friday through the upcoming weekend. Expect periods of windward and mountain showers over each island, favoring the overnight to early morning hours. Long range weather model solutions continue to show a weak upper level low developing near the islands from Saturday through Monday, which could enhance trade wind shower activity into the first half of next week.

Aviation

Early this morning, a stationary boundary is located northwest of Kauai, extending from a deepening low pressure system about 1000 miles northwest of Kauai. Latest satellite and radar imagery this morning shows that thunderstorms have developed along a convergent band that lies ahead of the main frontal boundary in the coastal waters about 300 miles west-northwest of Kauai. Meanwhile, light southeasterly background flow is prevailing across the state with VFR conditions statewide.
As this low nudges southward towards Hawaii, it will break down the ridge to the northeast of the state and increase chances for showers and thunderstorms today, particularly over the western islands. In addition to increases in shower coverage from Kauai to Oahu, locally heavy downpours and isolated thunderstorms will be possible as well. Isolated thunder can also not be ruled out over the Big Island slopes this afternoon, aided by sea breezes and daytime heating.
Although VFR flight conditions are expected to continue across the state through the morning, MVFR conditions with isolated IFR visibilities will be possible within heavier showers or thunderstorms beginning later today. While there are currently no AIRMETs in effect, mountain obscuration could become an issue by early afternoon.

Marine

The pressure gradient produced by a surface high moving south off the U.S. West Coast, along with a stalled front out west, will result in moderate to locally fresh southeast winds. A Small Craft Advisory may be required for the notoriously windier zones around Maui and Big Island by Wednesday. A stationary front just on the fringe of our far western offshore waters will serve as a focus for thunderstorm activity over the next few days. A new surface low forming just west of the offshore waters and slowly moving north, along with the northeast high pressing south, will continue to support areawide moderate to locally fresh southeasterlies, possibly transitioning more south around Kauai by Thursday. Convergence ahead of the stationary boundary will keep the threat for western water isolated to scattered thunderstorms alive through the week.
The small, medium period northwest (310 degree) swell that peaked earlier Monday will be gradually subsiding through the day. A similar size, medium period northwest (320 degree) swell will arrive Thursday. This swell will only pick up north and west-facing shore surf by a foot or two as it peaks Thursday night and then falls through Friday. Elevated wind wave chop along eastern exposures generated from a large source region of moderate trades upstream of the islands will persist the next few days. Very small early to mid week bumps along southwest and southeast shores, from either background energy moving in from the Southern Hemisphere (190-210 degree) or from trade wind swell wrap (040-130 degree), respectively, will keep south-facing shore surf around waist high, at best. The combination of the shorter period trade swell and the longer period background south swell will sustain slightly higher surf heights along many eastern and southern Big Island exposures the next few days.

HFO Watches/Warnings/Advisories

None.

Data Courtesy of NOAA.gov