Photo Credit: Jason Weingardt

West Kaua’i

Today: Breezy. Cloudy with showers and isolated thunderstorms. Highs around 84 near the shore to around 72 above 3000 feet. Southwest winds 15 to 25 mph. Chance of rain 90 percent.

Tonight: Cloudy with showers and isolated thunderstorms. Lows around 64 near the shore to around 58 above 3000 feet. Southwest winds 10 to 15 mph. Chance of rain 90 percent.

Tuesday: Mostly cloudy. Showers and isolated thunderstorms early in the morning, then scattered showers in the late morning and afternoon. Highs around 84 near the shore to around 72 above 3000 feet. West winds 10 to 15 mph. Chance of rain 80 percent.

South Kaua’i

Today: Breezy. Cloudy with showers and isolated thunderstorms. Highs around 81. Southwest winds 15 to 25 mph. Chance of rain 90 percent.

Tonight: Breezy. Cloudy with showers and isolated thunderstorms. Lows around 64. Southwest winds 15 to 20 mph. Chance of rain 90 percent.

Tuesday: Mostly cloudy. Breezy. Showers and isolated thunderstorms early in the morning, then scattered showers in the late morning and afternoon. Highs around 81. West winds 15 to 20 mph. Chance of rain 80 percent.

Kaua’i Mountains

Today: Windy. Cloudy with showers and isolated thunderstorms. Highs 71 to 78 in the valleys to around 62 above 4000 feet. Southwest winds 15 to 30 mph. Chance of rain 90 percent.

Tonight: Breezy. Cloudy with showers and isolated thunderstorms. Lows 57 to 62 in the valleys to around 55 above 4000 feet. Southwest winds 15 to 25 mph. Chance of rain 90 percent.

Tuesday: Partly sunny. Breezy. Scattered showers and isolated thunderstorms early in the morning, then scattered showers in the late morning and afternoon. Highs 71 to 78 in the valleys to around 62 above 4000 feet. Southwest winds 10 to 20 mph. Chance of rain 50 percent.

North Kaua’i

Today: Breezy. Cloudy with showers and isolated thunderstorms. Highs 74 to 83. Southwest winds 15 to 25 mph. Chance of rain 90 percent.

Tonight: Cloudy and breezy. Showers and isolated thunderstorms early in the evening, then numerous showers and isolated thunderstorms in the late evening and overnight. Lows 59 to 65. Southwest winds 10 to 25 mph. Chance of rain 90 percent.

Tuesday: Breezy. Partly sunny with numerous showers and isolated thunderstorms early in the morning, then partly sunny with isolated showers in the late morning and afternoon. Highs 74 to 83. West winds 10 to 20 mph. Chance of rain 70 percent.

East Kaua’i

Today: Breezy. Cloudy with showers and isolated thunderstorms. Highs 74 to 82. Southwest winds 20 to 25 mph. Chance of rain 90 percent.

Tonight: Breezy. Cloudy with showers and isolated thunderstorms. Lows 55 to 66. Southwest winds 10 to 25 mph. Chance of rain 90 percent.

Tuesday: Partly sunny. Breezy. Showers and isolated thunderstorms early in the morning, then scattered showers in the late morning and afternoon. Highs 74 to 82. Southwest winds 10 to 20 mph. Chance of rain 80 percent.

Detailed Forecast

Synopsis

A cold front continues to approach the Hawaiian Islands from the northwest today. This system will cause winds to strengthen as wind directions are becoming more southerly to southwesterly, pulling up additional unstable tropical moisture, and spreading unstable showers from west to east across the state through Tuesday. Expect a wet weather pattern with periods of heavy showers at times and isolated thunderstorms. Gusty winds and potential for flooding remains a threat for all islands, with the strongest impacts expected along the cold frontal band as they pass through each island. Southern slopes of the Big Island will also see enhanced rainfall activity into Wednesday. A brief period of improving weather trends will then spread from west to east after the frontal band passes each island. A second frontal system brings another round of wet weather swiftly through the island chain from Thursday through Friday. Weather conditions will improve just in time for the upcoming weekend.

Discussion

The satellite imagery this morning continues to show a low pressure system and cold front approaching Kauai from the northwest. Southerly to southwesterly winds ahead of this frontal band will strengthen today across the state. Convergence shower bands are already developing in the warm unstable air ahead of this cold front and will bring periods of shower activity to the western islands of Kauai and Oahu this morning, then spreading eastward to the islands in Maui County later this afternoon. Stronger shower bands are currently moving into Kauai and Oahu this morning with isolated thunderstorms detected.
The forecast guidance shows a wet and unstable weather spreading from west to east across the state lasting through Tuesday as a cold front sweeps through the island chain. The cold front will slowly weaken as it travels eastward down the island chain. The forward motion on this trough will stall as the low pressure system moves northward away from the state and the boundary diminishes near the Big Island from late Tuesday into Wednesday. Island by island impacts are starting this morning over Niihau, Kauai and Oahu as southerly wind speeds strengthen into the moderate to breezy range. These stronger southerly to southwesterly winds and showers will likely produce both convective wind gusts, and downsloping wind gusts along north and east slopes of island mountains. While some of the stronger convective wind gusts may become damaging to property, we believe the potential for damaging wind gusts with this system will be more isolated and therefore decided not to issue a wind advisory with the morning weather package. Showers may become heavy at times and thunderstorms are expected especially along the frontal band as it passes through each island. A Flood Watch remains in effect through Tuesday afternoon for all Hawaiian Islands due to the enhanced flooding threat.
The Big Island will likely see some enhanced rainfall initially along the southern slopes as the wind directions begin to shift from southeasterly to southerly flow today. These showers could also become heavy at times with the potential for thunderstorms lasting through Tuesday night. Additional Big Island threats are possible for wind and snow over the summits of Mauna Kea and Mauna Loa. We will evaluate the potential for high wind and winter weather products over the summits later tonight through Tuesday.
Both the American (GFS), European (ECMWF) and ensemble guidance continues to show some thunderstorm activity passing through each island mainly associated with the passing cold frontal cloud band. Low level wind speeds increase into the 30 to 40 knot range at the 850 mb level (around 5,000 feet above sea level) near Niihau, Kauai and Oahu through Tuesday afternoon. Stronger wind shear aloft may produce stronger thunderstorm activity during this time period favoring these western islands. Heavy rainfall and wind gusts will be strongest near any developing thunderstorms.
Brief improving weather trends are expected to spread from west to east after the cold front passes each island from late Tuesday near Kauai to Wednesday for Oahu and Maui County. The Big Island will see lingering showers on Wednesday with improving trends from Wednesday night into Thursday.
Another cold frontal system swiftly moves into the islands from the northwest direction with showers sweeping from west to east down the island chain from Thursday through Friday. Improving weather trends are forecast statewide from Friday night into next weekend.

Aviation

Moderate to breezy south to southwesterly winds will be in place today through Tuesday morning as a cold front progresses from northwest to southeast down the island chain. Currently radar is showing numerous showers over Kauai and Oahu with embedded heavy showers. Bands of clouds and heavy showers will move through the island chain as the front progresses. Some of the more intense showers will have gusty winds and there also is a slight chance for thunderstorms.
Conditions will deteriorate for Maui County and the Big Island later this morning into the afternoon. The front will continue bringing widespread MVFR ceilings and visibilities within showers, along with isolated IFR conditions within heavy showers.
Moderate upper level turbulence is expected to be a concern today as an upper level trough moves over the area. AIRMET Tango for moderate turbulence between FL220 and FL420 is in effect. AIRMET Tango for low level turbulence below 090 is also in effect downwind of the terrain. AIRMET Sierra for tempo mountain obscuration is in effect for Kauai and Oahu and will likely be expanded to include other islands today. Light icing is also possible in layer 130 to FL220. Aviation interests are encouraged to monitor for forecast updates throughout the next several days as conditions evolve.

Marine

Increasing S to SW winds are expected today as a front associated with a deepening surface low approaches from the NW. Pre-frontal heavy showers and a few thunderstorms over Kauai and Oahu waters are in the process of spreading to Maui County waters. As the deepening low passes just N of the Hawaiian Offshore Waters through Tuesday, fresh to strong S to SW winds will develop, and a Small Craft Advisory (SCA) is now posted for practically all Hawaiian waters. Winds will be especially gusty in and near heavy showers and thunderstorms.
The prefrontal heavy showers and thunderstorms will spread E to Maui and Hawaii Counties through Tuesday night, with rainfall gradually diminishing over Kauai and Oahu. The actual front (with its attendant wind switch to the NW) will not reach the islands until Wednesday and Wednesday night, by which the time the active weather with the prefrontal band will have shifted E of the islands and weakened. Uncertainty increases late Thursday and Friday, but forecast guidance indicates that another surface low may pass much closer to the islands, again raising chances for gusty winds and thunderstorms.
A small NNW swell (~3 feet 13 seconds) will continue to slowly decline today, but a new (relatively small) longer-period NW swell will gradually build. NDBC buoys 51001/51101 NW of Kauai have been detecting steadily increasing energy in the 15-17 second bands overnight that should result in increasing surf heights along exposed shores on Kauai and Oahu today.
The slow-moving low responsible for the front is expected to develop a fetch of ~40 kt NW-N winds aimed at the islands over the next day or so, with the fetch especially aimed toward the W end of the island chain. The relative proximity of this source will lead to what is expected to be a large, short-lived, medium period NNW (330-340 degrees) swell that will build (mainly across the W end of the chain) tonight into Tuesday. Surf will likely exceed the High Surf Advisory level along N facing shores of Kauai and Oahu, and the elevated seas will likely exceed the SCA criteria of 10 feet in W waters. Forecast confidence decreases later in the week as there is uncertainty in the location and intensity of another surface low that could develop closer to the islands, potentially producing a large, short-to medium-period N swell.
A fetch of persistent trade winds E of the islands is sending short- to medium-period seas/swell of 5-6 feet at 9 to 12 seconds that will gradually diminish through Tuesday. While the source for this E swell will fade, some E shores will experience periods of larger surf as the N swell wraps in. Surf and seas along S facing shores/near shore waters will be unusually choppy until winds shift to the W Tuesday night and Wednesday.

HFO Watches/Warnings/Advisories

Flood Watch through Tuesday afternoon for all Hawaiian Islands.
Small Craft Advisory until 6 PM HST Tuesday for most Hawaiian waters, except Maalaea Bay and Big Island Southeast Waters.

Data Courtesy of NOAA.gov