Photo Credit: Braden Jarvis

West Kaua’i

Today: Windy. Frequent showers and chance of thunderstorms. Locally heavy rainfall possible. Highs around 77 near the shore to around 68 above 3000 feet. Northeast winds 20 to 30 mph decreasing to 10 to 30 mph in the afternoon. Gusts up to 50 mph. Chance of rain near 100 percent.

Tonight: Cloudy. Frequent showers and slight chance of thunderstorms in the evening, then scattered showers after midnight. Locally heavy rainfall possible in the evening. Lows around 68 near the shore to around 57 above 3000 feet. East winds 10 to 15 mph. Gusts up to 35 mph. Chance of rain 90 percent.

Tuesday: Partly sunny with scattered showers in the morning, then mostly sunny in the afternoon. Highs around 79 near the shore to around 71 above 3000 feet. Southeast winds around 10 mph. Chance of rain 50 percent.

South Kaua’i

Today: Breezy. Frequent showers and chance of thunderstorms. Locally heavy rainfall possible. Highs around 78. Northeast winds 20 to 25 mph with gusts to 50 mph. Chance of rain near 100 percent.

Tonight: Cloudy and breezy. Frequent showers and slight chance of thunderstorms in the evening, then scattered showers after midnight. Locally heavy rainfall possible in the evening. Lows 64 to 69. East winds 10 to 20 mph. Gusts up to 40 mph. Chance of rain 90 percent.

Tuesday: Partly sunny with scattered showers in the morning, then mostly sunny in the afternoon. Highs around 80. Southeast winds around 10 mph. Chance of rain 40 percent.

Kaua’i Mountains

Today: Windy. Frequent showers and chance of thunderstorms. Locally heavy rainfall possible. Highs 68 to 73 in the valleys to around 61 above 4000 feet. Northeast winds 25 to 30 mph decreasing to 15 to 25 mph in the afternoon. Gusts up to 50 mph. Chance of rain near 100 percent.

Tonight: Breezy. Frequent showers and slight chance of thunderstorms in the evening, then scattered showers after midnight. Locally heavy rainfall possible in the evening. Lows 57 to 62 in the valleys to around 53 above 4000 feet. East winds 10 to 20 mph. Gusts up to 40 mph in the evening. Chance of rain near 100 percent.

Tuesday: Partly sunny with scattered showers. Highs 70 to 75 in the valleys to around 64 above 4000 feet. Southeast winds 10 to 15 mph. Chance of rain 50 percent.

North Kaua’i

Today: Windy. Frequent showers and chance of thunderstorms. Locally heavy rainfall possible. Highs 68 to 76. Northeast winds 25 to 30 mph decreasing to 15 to 30 mph in the afternoon. Gusts up to 50 mph. Chance of rain near 100 percent.

Tonight: Breezy. Frequent showers and slight chance of thunderstorms in the evening, then scattered showers after midnight. Locally heavy rainfall possible in the evening. Lows 61 to 68. East winds 10 to 20 mph. Gusts up to 40 mph in the evening. Chance of rain near 100 percent.

Tuesday: Partly sunny with scattered showers. Highs 71 to 78. Southeast winds around 10 mph. Chance of rain 40 percent.

East Kaua’i

Today: Breezy. Frequent showers and chance of thunderstorms. Locally heavy rainfall possible. Highs 69 to 78. Northeast winds 20 to 25 mph with gusts to 50 mph. Chance of rain near 100 percent.

Tonight: Breezy. Frequent showers and slight chance of thunderstorms in the evening, then scattered showers after midnight. Locally heavy rainfall possible in the evening. Lows 57 to 68. East winds 10 to 20 mph. Gusts up to 40 mph. Chance of rain near 100 percent.

Tuesday: Partly sunny. Scattered showers in the morning, then isolated showers in the afternoon. Highs 72 to 80. South winds around 10 mph. Chance of rain 50 percent.

Detailed Forecast

Synopsis

A surface trough near Maui County will act as a focus for cloud and shower development over all islands as it moves slowly northwest today. A strong upper-level low to the west is destabilizing the atmosphere, bringing heavy showers that could result in flooding, and a Flood Watch remains in effect for all islands today. The potential for heavy showers will gradually diminish tonight and Tuesday as winds become light for most of the week. Trade winds may return by the end of the week.

Discussion

A low-level trough persists over the island chain. Short-term radar trends indicate persistent heavy showers impacting the island of Kauai, with a rain gage near Kokee reporting 5″ in rain in the 3 hour period ending at 3 am. Flooding is ongoing or imminent there. Elsewhere, numerous showers are peppering the radar over waters N of Oahu and Maui County, where shower coverage over land has generally diminished over the past couple of hours. Radar and satellite show increasing showers and cold cloud tops near and SE of the Big Island, generally moving N. Hi-res guidance indicates this moisture will impact windward Big Island this morning before moving N of the islands this afternoon. Guidance generally agrees that this feature will continue to move N into tonight, taking the greatest low-level moisture convergence along with it, leading to a reduction in shower coverage over the islands.
Water vapor imagery shows a vigorous closed low aloft just W of Kauai, having become nearly stationary within a sharp N-S oriented trough. Broken to overcast layered clouds cover the islands, with the low providing large-scale lift that has led to an increase in thunderstorm coverage along the longitude of Kauai, mainly near 20N. With high pressure centered NW of the islands, a tight low-level gradient between the high and the trough is bringing strong and gusty NE winds to Kauai, where a Wind Advisory remains posted.
The surface trough will generally move NW today, and is expected to be NW of the islands tonight and Tuesday. The threat for Flash Flooding will remain high statewide through at least this morning, and a Flood Watch (FFAHFO) remains in effect. Periods of heavy rainfall will be possible as the surface trough maximizes low-level convergence and the low aloft supports strong upper level diffluence over the area. In addition, deep moisture and cold temperatures aloft will lead to continued snowfall over the Big Island Summits, where a Winter Storm Warning remains posted.
A trend toward drier weather and relatively light winds is anticipated by Tuesday, with still some uncertainty as to the details. The surface trough is expected to move N of the islands, leading to light winds that will favor an E-SE direction, while the low aloft gradually weakens. Best chance for rain in this regime may be in the afternoon/evening as sea breezes drive cloud and shower formation in a moderately unstable environment. Trade wind weather may return toward the end of the week as the trough dissipates and high pressure builds N of the islands. This high may push moisture associated with a frontal boundary over the islands from the N.

Aviation

An upper level low just west of the state will intensify as it tracks due south. Low level southeasterly flow will continue to pull tropical moisture towards the state. Rainfall is expected to intensify over the next six to twelve hours in response to increasing moisture and instability. There is a slight chance of thunderstorms statewide.
As of 2 am Monday, radar imagery indicated a nearly stationary band of moderate showers extending from Kauai to Windward Maui. A second band of showers could be seen approaching the Big Island from the southeast. AIRMET Sierra for mountain obscuration is now in effect for Kauai, Oahu, Molokai, Maui, and Windward Big Island. Conditions should improve towards evening as the upper low moves away from the state.
AIRMET Zulu for moderate mid-level icing is now in effect from Oahu through Big Island including all of Maui County. These conditions are expected to continue through at least afternoon and may spread to Kauai later this morning. Elsewhere, light rime icing above 14 kft prevails.
AIRMET Tango for low level mechanical turbulence continues for Kauai and Oahu due to gusty north winds in the lower levels. Conditions are expected to gradually improve from the east during the day today. Meanwhile, AIRMET Tango for mid and upper level turbulence will remain necessary statewide through at least early evening due to the proximity of the upper low.

Marine

A broad surface trough over the eastern end of the island chain will drift westward today as strong high pressure holds to the northwest of the state, resulting in fresh to near gale northeasterly winds around Kauai and Oahu and locally strong east winds elsewhere. Heavy showers and thunderstorms will produce locally stronger wind gusts over all waters through Monday. Winds will ease tonight and Tuesday as the trough moves away to the west.
A Small Craft Advisory (SCA) is now in effect for most waters for the stronger winds west of the trough and around Big Island/Maui, as well as the very rough seas created by the combination of large wind waves and a large northwest swell. Winds are no longer expected to reach gale speeds, so the Gale Watch has been dropped. Winds will ease tonight and Tuesday, though lingering elevated seas may require the SCA to be extended a bit further in time for some waters.
The aforementioned, long-period northwest (310 degrees) swell peaked overnight and has begun to lower, but the Hanalei and Waimea PacIOOS buoys are still suggesting swell heights around 7 to 8 feet early this morning. The High Surf Advisory (HSA) remains in effect for most north and west facing shores from Kauai through Maui through this afternoon. The northwest swell will continue to decline through Tuesday, with another, moderate reinforcement arriving midweek. East facing shores of Kauai remain in the HSA through this afternoon as well, but may need to be extended due to the rough northeasterly wind swell wrapping in. A small pulse of south swell is possible around midweek for a bump in south shore surf.

HFO Watches/Warnings/Advisories

Flood Watch through this afternoon for all Hawaiian Islands.
High Surf Advisory until 6 PM HST this evening for North and West facing shores of Niihau Kauai Oahu and Molokai, and for North facing shores of Maui, and for East facing shores of Kauai.
Wind Advisory until 6 PM HST this evening for Niihau and Kauai.
Winter Storm Warning until 6 PM HST this evening for Big Island Summits.
Small Craft Advisory until 6 PM HST this evening 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, Maalaea Bay, Pailolo Channel, Alenuihaha Channel, Big Island Windward Waters.

Data Courtesy of NOAA.gov