Photo Credit: Brendan Stephen

West Kaua’i

Today: Breezy. Mostly sunny with isolated showers. Highs 75 to 82 near the shore to around 66 above 3000 feet. Northeast winds up to 25 mph. Chance of rain 20 percent.

Tonight: Breezy. Partly cloudy with isolated showers. Lows around 62 near the shore to around 52 above 3000 feet. North winds 10 to 25 mph. Chance of rain 20 percent.

Tuesday: Mostly sunny in the morning then becoming partly sunny. Isolated showers. Highs 72 to 79 near the shore to around 64 above 3000 feet. Northeast winds up to 15 mph shifting to the west in the afternoon. Chance of rain 20 percent.

South Kaua’i

Today: Mostly sunny. Breezy. Isolated showers in the morning. Highs 74 to 81. Northeast winds 10 to 20 mph. Chance of rain 20 percent.

Tonight: Partly cloudy. Breezy. Lows around 62. Northeast winds 10 to 20 mph.

Tuesday: Mostly sunny in the morning then becoming partly sunny. Highs 70 to 79. Northeast winds 10 to 15 mph.

Kaua’i Mountains

Today: Breezy. Partly sunny with scattered showers. Highs 65 to 72 in the valleys to around 57 above 4000 feet. North winds 10 to 20 mph. Chance of rain 50 percent.

Tonight: Partly cloudy in the evening then becoming mostly cloudy. Breezy. Scattered showers. Lows 52 to 61 in the valleys to around 49 above 4000 feet. North winds up to 20 mph. Chance of rain 30 percent.

Tuesday: Partly sunny. Scattered showers in the morning, then isolated showers in the afternoon. Highs 63 to 70 in the valleys to around 56 above 4000 feet. Northeast winds up to 10 mph. Chance of rain 30 percent.

North Kaua’i

Today: Breezy. Partly sunny with scattered showers. Highs 67 to 77. North winds 10 to 25 mph. Chance of rain 50 percent.

Tonight: Partly cloudy in the evening then becoming mostly cloudy. Breezy. Scattered showers. Lows 55 to 64. North winds 10 to 25 mph decreasing to up to 20 mph after midnight. Chance of rain 30 percent.

Tuesday: Partly sunny. Scattered showers in the morning, then isolated showers in the afternoon. Highs 66 to 75. Northeast winds up to 15 mph becoming around 10 mph in the afternoon. Chance of rain 30 percent.

East Kaua’i

Today: Partly sunny in the morning then becoming mostly sunny. Breezy. Scattered showers. Highs 66 to 78. North winds 10 to 20 mph. Chance of rain 50 percent.

Tonight: Breezy. Partly cloudy with scattered showers. Lows 55 to 66. North winds 10 to 20 mph. Chance of rain 30 percent.

Tuesday: Partly sunny. Scattered showers in the morning, then isolated showers in the afternoon. Highs 64 to 76. North winds 10 to 15 mph. Chance of rain 30 percent.

Detailed Forecast

Synopsis

A weakening front over Oahu will move southeast down the island chain today and tonight. Expect areas of heavy showers and thunderstorms along the front this morning. Breezy and cool northerly winds will develop behind the front, then gradually ease tonight and Tuesday. A potent winter low will affect the state Wednesday night through early Friday, bringing a significant threat of strong to potentially damaging southwest winds, flooding, and strong to severe thunderstorms. Impacts should diminish Friday as the low races away to the northeast.

Discussion

Currently at the surface, a weakening front is moving through Oahu. A strong area of high pressure is building in behind the front to the distant northwest. Breezy to locally windy northerlies are present immediately behind the front, with light and variable winds to the east of the front. Infrared satellite imagery shows mostly cloudy conditions across the state. Radar imagery shows scattered showers across the state, with some producing locally heavy rainfall and even some thunder in the most intense cells. Main short term focus revolves around the rain and flooding threat today, with longer term attention focused on a potentially significant system later this week.
The weakening front will in the vicinity of Oahu will track eastward into Maui County this morning, then stall out in this area through late this afternoon. Heavy rain and thunderstorms will remain possible overnight this morning, mainly over Maui County, as a strong shortwave trough pushes eastward across the island chain. Conditions will then stabilize across the state this afternoon with the loss of upper level support. The Flood Watch has been cancelled for Niihau, Kauai and Oahu where the flooding threat has diminished. The Flood Watch has been extended through noon today for Maui County.
Breezy northerly winds will overspread Kauai and Oahu today behind the front. The northerly winds will ease to moderate levels and finally make a push through Maui County and the Big Island tonight as a high building to the distant northwest finally drives the weakening front through the island chain. A cooler and drier airmass will gradually build in behind the front with shower coverage and intensity decreasing and the focus shifting around to the favored terrain.
Mostly dry conditions are expected Tuesday and Tuesday night, especially across the smaller islands, while a few more showers may affect the Big Island where moisture associated with the washed out front will linger. Moderate northerly winds on Tuesday will shift back to light to moderate trades Tuesday night as a storm system begins to develop to the west of the state. Winds will shift more southeasterly and begin to increase through the day Wednesday, with shower chances also ticking up as the moisture associated with the old washed out front lifts northward into the islands.
A potent winter low will affect the state Wednesday night through Thursday night, bringing a significant threat of strong to potentially damaging southwest winds, flooding, and strong to severe thunderstorms. Operational runs of the GFS and ECMWF have come into rather good agreement, and the ensemble guidance from these models suggest that an unusually strong storm low will develop several hundred miles west-northwest of Kauai on Wednesday and pass just north of the state Wednesday night through Thursday night. A winter system of this strength at such a low latitude is quite rare in the Central Pacific, and while it is still too early to pin down exact details, probabilities for significant impacts are rising. Southerly winds will ramp up Wednesday night, and as winds shift out of southwest on Thursday, damaging downslope wind gusts could occur near and northeast of terrain, especially across the western half of the island chain. In addition, winds on the high summits of the Big Island and Maui will be very strong. Bands of heavy rainfall could develop as early as Wednesday night and will bring a threat of flash flooding, especially Thursday and Thursday night to the entire island chain. Thunderstorms developing within the gusty southwest flow could also produce isolated areas of wind damage over any location, and confidence in this threat is increasing as well. Also, expect winter weather on the high summits of the Big Island.
Impacts should diminish as the low races away to the northeast on Friday. Additional rainfall, possibly heavy, may persist into Friday, but winds should ease significantly. Drier more stable conditions could move back in next weekend, but given the differences in the model solutions this far out, this is more uncertain.

Aviation

Cold front is producing widespread SHRA and periods of MVFR/IFR cigs and vsbys over the state. SHRA and TSRA are moving down the island chain and should continue through the day before conds improve west to east. With FROPA expect cooler breezy northerly winds.
AIRMET Sierra is in effect for mtn obsc over Kauai to Maui. Conds will improve west to east and at next issuance Kauai will be dropped.
AIRMET Tango is in effect for mod turb btn 130 and FL300.
AIRMET Zulu for light icing will be dropped at next issuance.

Marine

A cold front moving into the islands of Maui County today will continue to bring fresh to strong northerly winds into the coastal waters surrounding Kauai. Periods of heavy showers and thunderstorms with gusty winds will also continue along the frontal band. The Small Craft Advisory (SCA) was extended in time through 6 PM this afternoon for coastal waters around Kauai. A low pressure system moving through the islands from Wednesday night into Friday morning will bring fresh to strong southerly to southwesterly winds, periods of heavy rain, and thunderstorms.
A blocking high over the northwest Pacific will keep the primary swell direction out of the north throughout the week. This will feature a mix of rough, short period wind waves from the strong northerly winds today, and medium period northerly swells. Guidance remains in good agreement for an upward trend in surf along all exposed north facing shores from Tuesday night into the end of the week due to the potential for a gale- to storm-force low forming well north of the Hawaiian Islands. Surf heights could approach or reach the advisory level for north facing shores from Wednesday through Friday.
Surf along the east-facing shores not exposed to northerly swells will remain small each day due to the lack of local trades and upstream swells.
Surf along south-facing shores will remain small due to background south swells. Uncertainty increases late in the week, with the potential for southerly winds to evolve, which could translate to a brief period of rough, short-period surf.

Fire weather

No significant fire weather concerns this week. Breezy and dry northerly winds area expected around Kauai and Oahu on Monday, but critical thresholds are not expected to be met. A significant winter low will affect the state Wednesday night into early Friday. Strong southerly winds are expected, but humidity will be elevated with high chances for rainfall.

HFO Watches/Warnings/Advisories

Flood Watch until noon HST today for Lanai Mauka, Kahoolawe, Maui Windward West, Maui Leeward West, Haleakala Summit, Molokai, Lanai Windward, Lanai Leeward, Lanai South, Maui Central Valley North, Maui Central Valley South, Windward Haleakala, Kipahulu, South Maui/Upcountry, South Haleakala.
Small Craft Advisory until 6 PM HST this evening for Kauai Northwest Waters, Kauai Windward Waters, Kauai Leeward Waters, Kauai Channel.

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